Wireless Sensor Network, 2010, 2, 612-628
doi:10.4236/wsn.2010.28073 Published Online August 2010 (http://www.SciRP.org/journal/wsn)
Copyright © 2010 SciRes. WSN
A Real-Time-Enabled, Blackboard-Based, Publish/Subscribe
Architecture for Wireless Sensor Nodes
Björn Stelte
Faculty of Computer Science, Universität der Bundeswehr München, München, Germany
E-mail: bjoern.stelte@unibw.de
Received May 6, 2010; revised May 18, 2010; accepted May 25, 2010
Wireless sensor network nodes have only limited resources concerning memory and battery life-time. Mem-
ory can be efficiently used by sharing data, and the life-time of a battery can be extended, when the node has
long power saving sleep-phases. We propose a publish/subscribe architecture that achieves these two aims.
The results of our work are of great interest for sensor application developers, giving them now the opportu-
nity to use our architecture for sharing data among different applications on the node as well as the different
layers of the operating system. We introduce a blackboard which is used for centrally storing published val-
ues, like measured data from a monitored sensor. This makes it possible to share stored data without moni-
toring the sensors once again, which is advantageously concerning power consumption, memory space, and
reaction time. Beside the proposed publish/subscribe method for sensor nodes with its notification possibili-
ties, our architecture fulfills also real-time requirements. We show how the well-known sensor operating
system MANTIS OS can be extended by a real-time enabled, blackboard-based publish/subscribe architect-
ture. This architecture and first of all its implementation is of special interest for cross layer optimization of
sensor applications. Cross-layer approaches benefit from our architecture because the available implementa-
tion can be used as an efficient framework for central storing and managing of shared values.
Keywords: Blackboard, Real-Time, Publish/Subscribe Architecture, Wireless Sensor Network
1. Introduction
Today, sensor nodes are on the one hand small in size and
economically cheap to produce, but on the other hand the
applications on the nodes get more and more complex. So
an application developer has to cope with limited re-
sources, like memory space, power consumption, and
CPU performance. Resource-sharing can moderate the
resource limitations. In this paper the sharing of informa-
tion between program parts is considered, not the com-
munication between devices.
A resource-sharing system enables parts of the system
to share information. A system that uses continuous que-
ries for the resource-sharing is called publish/subscribe
system. The continuous queries are the subscriptions and
the submitter of the queries are the subscribers. If a new
value is published by a publisher, this event triggers an
inquest whether at least one of the queries is matching. So
the system handles all publications and subscriptions as
well as the notification process.
An good example for a publish/subscribe system can be
found at the stock exchange. Because the price of a share
is fluctuating, a profit is possible as long as a prospective
customer buys the share when the price is low and sells it
later when the price has risen. Assumed that the customer
is willing to maximize his total profit, he will observe
more than only one share at the same time. But the more
shares the customer has in its portfolio, the more complex
gets his situation.
At the stock exchange a publish/subscribe notification
system is often used to moderate the complexity. Cus-
tomers submit subscriptions with the name of the share
and a threshold value for the price of the monitored share
to a notification system at the market. This allows to have
more than one subscriber for the same subscription. So
instead of having several customers monitoring a share,
the system does the job for them. If the given price is
reached, then the system tries to immediately inform all
relevant subscribers.
So at the stock exchange a publish/subscribe notifica-
tion system is very useful. The system provides advan-
tages for both the customer and the stock exchange. The
customer displaces the monitoring process to the system
and saves time, because he gets a notification of his sub-
scribed event as requested. The stock exchange benefits
from the publish/subscribe system, because the number of
customer queries is reduced dramatically. Secondly, the
same continuous queries from different submitters can be
summarised to one query for all interested subscribers. To
sum up, resources are spared, if a publish/subscribe sys-
tem is used for sharing stock information.
2. The Publish/Subscribe Paradigm
For a dynamic interaction between components, asyn-
chronous communication models are used, like message
passing, broadcast, and publish/subscribe. Eugster et al.
Describe in [1] that only the publish/subscriber commu-
nication fully assists all three decoupling dimensions,
which are the time, space and synchronization decoup-
Karl and Willig explain in [2] as well as Köpke et al. in
[3] that also a WSN application can benefit from the
publish/ subscribe paradigm. So before analyzing the
suitability of publish/subscribe for sensor node applica-
tions, we should have a deeper look at the publish/
subscribe basics first. A subscriber is not interested in all
events a publish/subscribe service may support. So the
subscriber specifies the events it is interested in and cuts
the amount of all possible events towards its interest.
There are three different types of subscriptions possible,
1) Topic-based
2) Content-based
3) Type-based
At a topic-based publish/subscribe scheme, participants
subscribe to a topic or subject, which is identified, e.g., by
keywords. This is like joining a group, where pub- lished
data for the group is forwarded to all group members. A
mailing list is a good example for this type of pub-
lish/subscribe, where every topic is like an event ser- vice
of its own. In a content-based publish/subscribe scheme,
the decision if a subscriber gets a notification or not, is
based on present keywords in the content of the event. A
subscriber defines filters or constraints, which identify
valid events. If a published value fulfills the subscription
filter, the notification service informs the active sub-
scriber. The other way round, if it is not valid the sub-
scriber gets no notification message. The type- based
publish/subscribe variant enables the subscriber to specify
in which kind of values it is interested in. An event can be
of different subtypes, e.g., a number could be of the type’s
integer or float. Type safeness may be an argument to use
this type of publish/subscribe scheme. The content-based
scheme is an extended form of the topic based one. At the
topic-based publish/subscribe, every publication is for-
warded to every subscriber of the event. The con-
tent-based scheme uses an addition filter, allowing to
notify only all subscribers of the event with a matching
2.1. Publish/Subscribe Architecture for Sensor
Especially for distributed system publish/subscribe ar-
chitectures are widely available. So if such an architecture
is fitting for a sensor node, we can use the architecture for
an implementation. But unfortunately, these classical
publish/subscribe architectures are not suitable to sensor
nodes, because they pay no attention to the before men-
tioned resource limits of sensor nodes.
As Köpke et al. describe in [3], a blackboard can be
used for storing shared values at a central point. Within a
data-centric control interface, setting and accessing data is
possible. In their implementation for TinyOS, they use a
publish/subscribe based control interface. The interesting
part is how they solved the space problem which leads to
the fact that concepts like the well-known Elvin concept is
being unimplementable for WSNs. Köpke et al. use a
blackboard for storing the data. In their implementation,
each published information is defined by a unique number,
a so called notification event number. A publisher writes
its data on the blackboard and then informs the blackboard
about the publication. Because of the unique event num-
ber, the publisher can tell the blackboard the corre-
sponding event for the published value. The blackboard
itself consists of three elements: the index table, the sub-
scriber table and the subscriber flags. The tables are of
constant size, which have to be calculated at compile-time.
The index table consists of two entries for each event,
namely the event offset and the event count. The offset
value states the relative start position of the event at the
subscriber table and subscriber flags. The count value
gives the amount of subscribers for the event. So the index
table has a size of twice of the number of events. The
subscriber table stores the handlers for each subscriber.
The position of the handler can be calculated from the
offset value in the index table. An entry in the subscriber
array has two flags for each subscriber. The first flag is the
published flag and the second one is the subscribed flag.
The published flag is set if an event is published for the
subscriber. The subscriber flag is set only if the subscriber
is currently subscribed to the event, otherwise the flag is
clear. Subscriptions and unsubscriptions can be done at
run-time, but the storage for the subscriber is reserved at
compile time. The subscriber flags are stored in RAM, all
other tables are transferred into the program structure at
compile time. So the concept uses a minimal amount of
RAM storage. This is effective and safes memory because
of a central storage. But the concept has the disadvantage
of being not flexible. The number of events and sub-
scribers, and also the handler addresses have to be known
Copyright © 2010 SciRes. WSN
at compile time. A modification at run-time is not possible,
because the storage is in ROM. So a subscriber can not
declare it is interest in an event at runtime, this is done at
compile time. The unsubscribe functionality is more like a
pause function, where the subscriber declares its not
willing to get more notifications at this point in time. The
concept uses the publish/subscribe paradigm for decoup-
ling in space and time. In Köpkes concept subscribers can
only make a topic-based subscription for an event. On
every event change they get notified if their subscription
is active. The subscribers are notified by calling the
function whose address is stored at the handler field.
2.2. Static Data Storage
Köpke’s concept allows only topic-based and static sub-
scriptions. Our idea is to extend the given concept to a
content-based system, so that subscribers can set and
modify a subscription filter, like type and limit, as well as
the stored handler address. Instead of storing index and
subscriber table in the structure, these now have to be
stored in RAM too, in order to modify the values at
run-time. Because a WSN node has only limited resources,
especially RAM, the maximum number of events and
subscribers is limited. At compile time, the needed stor-
age size has to be calculated and reserved, since most
operating systems for WSN nodes have no malloc()
functionality available (MANTIS has non).
A developer should know the number of events which
are published by a publisher thread, and subscribers for
the events. Subscriber and events are in a relationship to
one another. Each subscriber has to know the name or id
of the event it wants to subscribe to. So the storage ca-
pacity required by the concept is calculated at compile
time. A test should show at compile time, if enough ad-
ditional space is available to handle the storage of the
Köpke et al. use one published flag per subscriber in the
subscriber flags array. In this concept, we use one publish
flag for each event. There can be several distinct sub-
scribers for one event, with different subscription filters.
The subscriber flags array here is similar to Köpkes ar-
chitecture, within the difference that one publish flag
followed by n subscriber flags and one block flag are used
for one event. The block flag is needed for safeguarding
reasons. To calculate the bit position of the flags, we need
no additional field in the index table. The offset value
used to calculate the handler reference can also be used
here for the subscriber flags. If a publisher sends a new
value for an event to the blackboard, the published flag for
this event is set and in the notification state entered later,
only the events with an activated published flag have to be
processed. In contrast to Köpke’s concept, this pub-
lish/subscribe system is able to handle content-based
notifications. On subscription, the subscriber commits the
event identifier and the subscription filter. This filter
consists of two fields, namely the subscription type and
subscription limit. When a new value is published and the
subscriber is subscripted for this event, the blackboard has
to prove, if the subscription filter is fulfilled or not. On a
positive decision, the subscriber gets a notification. As in
Köpke’s concept, this could be done by invoking a han-
dler. The advantage of this concept over Köpke’s is that it
is more flexible. A subscriber is able to unsubscribe, if a
subscription is not needed anymore, and a second sub-
scriber can later subscribe and can reuse the reserved
storage from the first subscriber. In order to reduce the
amount of storage, the index table can be skipped if the
number of subscribers is fixed, e.g. having fixed the
number of subscriber for each event to 6, the offset in the
subscriber flags is 8. So for each event, only 1 Byte has to
be reserved for the flags per event statically. The problem
here is fragmentation.
In the worst case, assuming at least one subscriber is
available, space for 5 additional subscribers are unused,
but the storage for them is still allocated. If more than 6
subscribers are interested in the event, a dummy sub-
scriber can be created, which publishes to new event on
notification of this dummy subscriber. Additionally sub-
scribers can subscribe to this event, instead of the fully
subscribed root event.
2.3. Dynamic Data Storage
In a static storage, storage reservation is necessary at
compile time. Space for events and subscribers is calcu-
lated and each gets its own part of storage space. Another
concept is to reserve a given size of storage for the noti-
fication system at compile time and reserve space for
events dynamically at run time. On creation of an event, a
data container is created, which holds all relevant data of
the event. The published values as well as the subscriber
filters are outsourced and only accessible by a pointer.
Therefore, the event container holds the pointer addresses
to these data. Beside the pointers, information like the
number of subscribers and the number of publishable
values is also stored in the container. The number values
are needed to calculate the last position of either the pub-
lished value array or the subscriber arrays for subscriber
limits and types. The subscriber flags are stored in the
event container. Unlike the static variant, the subscriber
flags are not limited and can have different sizes for each
event. The number of subscribers can be found in the
container, so the size of the subscriber flags in bits is two
plus number of subscribers. Each event container has a
pointer to the next container, so each container is reach-
able through its predecessor. A direct access on a con-
tainer could be done by using a index table, which holds
all next pointers.
Storing pointers to the values instead of the values in
Copyright © 2010 SciRes. WSN
the container has the advantage that sharing is possible. In
a situation where two different events have the same
subscribers, the pointers for subscriber limits and types
are the same. The individual subscribers of the two events
share the subscriber array and so reduce the amount of
needed space. A modification of the filters will influence
both events at the same time, this means that cycle counts
are reduced for modifying the filters. Secondly, not only
the last published value can be stored, now several values
can been stored externally with the pointer to the first
value in the container. Each event container can handle a
different number of published values. The containers are
linked by a next pointer, so a list of containers arises. This
list can represent a priority order, where a container in the
front has a higher priority as its successor.
The priority order can be changed at run time by
swapping the next pointers. At the notification phase, the
blackboard starts with highest priority containers. As in
the static concept, only event container, whom published
flag in the subscriber flags array is set, need further
processing. If at least one of the subscriber flags is set as
well and the corresponding subscriber filter is true, the
blackboard marks the subscriber to be notifiable. After all
containers are processed, the notify list with all marked
subscribers can be processed. A method like first-come-
first-serve should be adequate here, with the subscribers
of higher prioritized events first. Reordering should be
used for containers which are used often. Reordering can
be done at run time, depending on a situation or event. The
event container is easily expandable for the developer.
Additional values can be stored in the structure by adding
new variables in the event container. The main problem
with the dynamic storage concept is the needed storage
capacity and the needed computational time for modify-
cations. Specially on publishing a value for an event, we
need a index table to get the current pointer address to the
container for the event quickly. The index table is only
needed for publishing. Without the table, we would have
to search for the correct container by hopping from one
container to the next, until the correct container is found.
The index table, holding the id of the event at its pointer to
the container, has also the problem of being static. Since
sensor operating systems like MANTIS OS do not support
malloc() functionality, the storage for the index table has
to be reserved at compile time. So even when the dynamic
storage concept is used, the amount of accepted events is
limited at compile time.
2.4. Combination of Static and Dynamic Storage
Since neither the static nor the dynamic concept (ex-
plained in the sections before) seems to really suit to WSN
nodes, a combination of static and dynamical should be
discussed. The idea is to reduce used space while keeping
The static concept limits the subscriber to define better
filter constraints for their subscription. A subscriber can
only post one filter for each event. It should be possible
for the subscriber to define several rules, which have to be
valid for notification. In Elvin [4], a special subscription
language was used for defining subscription filters. De-
fining a special subscription language is expensive re-
garding storage and computational time. Another idea is
the creation of special subscriber type, used for dummy
subscriptions. When the first subscription filter is valid, a
sub-event is notified by a dummy subscriber with the
second filter rule. The second stage subscription rule is
validated after the notification and the real subscriber can
be informed if this second subscription filter is also valid.
This idea leads to a tree-like subscription filter net. So a
nesting of subscriptions covers the same complexity as a
subscription language, like the one in Elvin.
At the dynamic concept, an index table has to be used
for finding the requested event container or the list of
container has to be gone through to find the correct con-
tainer. In the static variant, no index table is needed, be-
cause the number of subscribers are fixed. The idea now
is to get a combination of the two concepts, but the ques-
tion is how to do this? First, an observation may help.
Subscriptions are for an event and the event is well known
to the developer. The application developer uses a sub-
scriber for an event with a fixed identifier. So the devel-
oper knows the total number of subscribers and events in
the system. As a result, the developer is able to reserve as
much space as necessary for the notification system at
compile time.
Because the dynamic concept uses storage space
wastefully, a more static concept is needed. Figure 1
shows the combination of static and dynamic storage for
one event. A management frame is used instead of a
container. This frame consists of six parts, which have a
total size of 7 Bytes. The first part contains the well
known subscriber flags. The number of subscribers is
limited in this example to six, so the first management
frame part has exactly a size of 8bit. This is profitable,
because having a 8 bit CPU, flags can be set and cleared
rapidly without being vigilant about byte bounds. The
second part of the management frame consists of eight
1bit flags for describing type and status of the event. We
will discuss the meaning of this field later in this section.
The remaining parts of the management frame are the four
pointers to the published data, the subscriber limit, type,
and handler. A pointer itself has normally a size of 32 bit,
but with the observation in mind that every data or sub-
scriber value has a size of 32 bit, only storing the position
instead of the real pointer address is enough. In the
management frame, each of the positions, which can be
stored, has a size of 10 bits. So for all four positions, we
need 40bits, and with adding the 16bits from part one and
two, we have 56 bits, which are 7 Bytes, in total for a
management frame. As in the static variant, an index table
Copyright © 2010 SciRes. WSN
Copyright © 2010 SciRes. WSN
Figure 1. Effective data storage.
is not needed, because each management frame has the
same size. So calculating the start position of event x is
We have reserved 10 bits for storing the position, in-
stead of the pointer. The question is know how to use this
position for calculating the real pointer and is the size of
10 bits huge enough? First we will show that 10bits are
sufficient by taking into consideration that data is stored
on a sensor node. Secondly the pointer calculation with
position value as an offset value is shown.
With 10 bits representing the position of a value, there
are 210 possibilities. So 10 bits are sufficient to number at
least 1024 values. Each value has a size of 32 bits and so
in total 32 bits × 1024 = 32768 bits or 4096 Bytes are ad-
A sensor node like the Mica2dot has only 4 kByte
RAM, the considered 10bits are adequate for addressing
32bit values. With a start address and the 10bits for the
position, the pointer is calculateable. The position multi-
plied with 32 gives the offset, which has to be added to the
start position. The result is the searched position address
of the value.
When the total size of RAM would be reserved for the
notification system, realistically at most 35 events can be
created with 35 × 6 = 210 possible subscribers. When all
of them store an individual subscription filter and all
events store one published value, than 35 × 7 Bytes + 35 ×
96 Bytes = 3605 Bytes are used. Not surprisingly, the
amount of needed space can be cut, if subscription sharing
is used, like in the dynamic variant. Additionally, unlike
in the dynamic variant, the subscription blocks for all 6
subscribers can be partly shared. As mentioned, no index
table is needed, because of a static size of the management
frame. Therefore, the number of possible subscriptions to
one event is fixed to a number like six. If there are more
than six subscribers for an event, the idea is to create a
new event and make one special subscription at the first
event, which is always true at examination. The new event
is called a sub-event of the first one. There may be at most
six sub-events for each event and also each sub-event
could have its own sub-events. So the number of sub-
scribers for an event is only limited by the available
storage space and not by the architecture. The special
subscription used here needs a significant value in the
subscription type and the position of the sub-event in the
subscription limit field. As seen in the dynamic concept, a
priority based scheme is possible. The subscribers in the
root event have a higher priority than its sub-events. Next
to prioritization, it is possible to create a sequence of
subscriptions, which represents multiple equations of a
For example, a subscriber who would like to get a no-
tification, provided that the published value is larger than
a limit x and smaller than a limit y, defines two subscrip-
tion filters. The first filter, from the type forwarding and
greater with the subscriber limit value of x, only notifies a
sub-event if the greater equation is valid. The subscriber
handler field of this first subscriber filter holds the address
of the sub-event. The second subscriber filter is the filter
of a subscriber of the sub-event. This filter is a normal
subscription filter with a smaller type of subscription
equation and the limit y. If the first equation is valid, the
sub-event is activated by a notification. If also the second
subscription equation is valid, the real subscriber gets a
notification. The positive effect is, that the published
value is stored only once.
Both management frames have the same position stored
for the published value. It is also possible to extend this
strategy for n equations, but than reducing the offcuts is
more and more important. When sub-events are created, a
tree of events is built. In this scenario, it is possible to use
the quenching strategy from Elvin. If no subscriber of a
sub-event is active, all subscriber flags are cleared, than
the subscriber flag for the sub-event should also be
cleared in the subscriber flags array of the root event. This
has the consequence that the blackboard cuts the sub-
event branch of the event tree. Quenching for sub-trees
reduces computational time and should be used as often as
As mentioned before two bits in the management frame
are reserved for the event type. So far there is no sensible
necessity for using the event type. A later possible usage
may be the decision if one of the bits should be used as a
storage flag. This flag than indicates if the blackboard
information is stored in RAM or in the ROM part. A set
storage flag lets the blackboard use for calculating the
correct storage pointers the start address of the RAM part
as a cleared flag means the start address of the ROM part
has to be taken. If more than one publication should be
stored, the difference between the number of the publish
and the subscriber type pointer has to be greater than one.
Than there is enough space reserved for storing more than
one published value. For simplification the front value,
which is in the position of the pointer address should
always been the latest value. So on each publication all
published value has to be pushed 32 bits to the right
within their space reservation. Than the first entry is
overwritten by the new published value.
2.5. Evaluation
In this section, we will compare the memory usage of the
three discussed storage concepts. Since a WSN node has
only limited amount of available memory, one goal is to
use as less as possible memory for storing event and
subscriber values plus minimizing the management
overhead. On the other hand, the storage concept should
be flexible. The flexibility of the dynamic concepts need
more memory than the static one, but the question is how
big this gap is and how is it justifiable? For a comparison,
we need the memory usage of all three concepts. First we
will calculate the number of bits the static concept needs
for storing one event. With the values of Table 1, defini-
tion 1 shows the amount of needed bits for one event with
n subscribers and one published value.
DEFINITION 1. bitsstatic = 97 bits * #subscribers + 2
bits + 32 bits * #value
For the dynamic concept, Table 2 shows the addi-
tional needed space the dynamical concept needs com-
paring to the static one. This additional amount of mem-
ory is statically, so for each event the dynamic concept
needs at least 160 bits more memory than the static con-
cept. Definition 2 shows the total amount of memory in
bits for one event with n subscribers and i published val-
DEFINITION 2. bitsdynamic = 97 bits * #subscribers +
162 bits + 32 bits * #values
The static-dynamically mixed concept needs 56 bits for
the management frame and 32 bits per published value.
For each event six subscribers are possible and space is
reserved for this six subscribers, which is 6 * 96bits per
event. When more than six subscribers wants to subscribe,
additional events, so called sub-events are created. Each
of them has its own management frame and the additional
space for the subscribers. That published values are ref-
erenced by the value pointer, which is for all subevents the
same. Definition 3 shows the needed space mathematic-
DEFINITION 3. bits concept = 32 bits * #values + (56
bits + (6 * 96 bits)) * (ceil(#subscriber/6)))
On a node, like on any other computer board, the
available memory for the notification system is fixed. In
the following figures a capacity of 4 kByte is assumed. As
Figure 2 shows, the extreme points, where a huge amount
Table 1. Storage for the static event handling.
published value 32 bit
status flags 2 bit + n bit * subscribers
subscriber handlers 32 bit * subscribers
subscriber limits 32 bit * subscribers
subscriber types 32 bit * subscribers
Table 2. Additional Storage for the dynamic event han-
pointers 96 bits
numbers 16 bit
event type 16 bit
next pointer 32 bit
Copyright © 2010 SciRes. WSN
Figure 2. Number of possible events vs subscribers for 4
of subscriber or events are capable, are unusable. In this
example a range of 3 to 20 events seems to be a good
working area. On the one hand a huge amount of sub-
scribers but a less number of events are supported and on
the other hand a huge amount of events but a less number
of subscribers are capable. It is a trade-off between this
two factors. A developer has to decide, for which of them
he wants to optimize his system. When a subscription on
many events should be possible, like up to 20 events in the
example of 4 kByte of memory, the question is how many
subscribers are supported in average. Assuming a uniform
distribution of the subscribers on the events, all events
have the same number of subscribers, all events get the
same quota of memory allocated. All calculations in this
section are for events with six subscribers. In this scenario
the static concept dominates its competitor concepts,
because its overhead per event is small. The dynamic
concept needs only a static additional amount of 160 bits
per event, assuming that all stored values, the published
value and the subscription values, can be stored compact.
This assumption leads to an efficient storage but addi-
tional computational time for shifting the values on add-
ing and removing new subscribers or events. The pre-
sented architecture does not need this additional compu-
tational time but pays its dynamically with additional
needed memory
3. Real-Time-Enabled Publish/Subscribe
A value is published on the blackboard when the publish
function is executed within a thread. The function needs
as parameters the event id, which is unique, and the new
value, which should be published. When the blackboard
thread is activated later, the blackboard works off the
publishing procedure. First it tests if the event is blocked
by another process. If the blocked flag in the subscriber
flags array is set, the publisher has to wait for unblocking
or skip the publishing process for this event. When the
block flag is not set, the blackboard set the block flag and
the publish flag. Now no other process can disturb the
publishing process for this event. The publish flag is set
and so leads to later verification of the subscription. If all
subscriber flag are cleared the published flag should also
be cleared, because when there is no subscription, no
subscriber can get a notification. Also it could be possible
to inform the producer about this situation. This process is
called quenching and instructs the producer to skip further
publishments for the event. When at least one subscription
is made for the event, the producer has to be informed
once again, now to stop the skipping.
In our concept, not the complete management frame
has to be modified, only the first byte needs a modifica-
tion. The first byte consist of the subscriber flags, which
should be notified as described. In order to store the pub-
lished value in the right position, the publish block of the
management frame is read and the value is stored at the
real address. After finalization of the publishing process
the block flag in the subscriber flags array is cleared. A
consumer thread calls the subscribe function of the
blackboard API for creating a subscription for an event.
The event id and the subscription filter values are given to
the function by parameters. The subscriber gets a unique
subscription number, which is the next free number of the
all subscriber flags in the event, back as a result of the
function call. Which this number a later pausing or un-
subscribing can be done. Like in the publishing process,
the block flag is tested and set. If no other subscriber flag
is set before and quenching is used, the producer has to be
informed, that there is at least one subscriber, who is
interested in its published value. The first two bytes of the
management frame have to be modified.
In the first byte, the subscriber flags array, the corre-
sponding subscriber flags has to be set. In the second byte,
the type field, also the corresponding flag is set, standing
for a reservation of the subscription block for the sub-
scriber. Since a structure optimization for reducing offcuts
is not done so far, the data flags block should be a copy of
the subscriber flags. The needed next free subscriber
number, is the first position of a subscriber flag, which is
cleared and which representing flag in the data block is
not set. If no space is available in the management frame
for an additional subscriber, a new event is created, called
sub-event. One of the subscribers is copied to the new
event, meaning coping subscription filter values and set-
ting the subscriber and data flags in the new sub-event. In
the gap of the old subscriber in the root event, a new
subscriber is created with a subscription filter of a special
forwarding type. This subscription is always true and on
notification processing, the new sub-event gets activated
by notification. Because the root event holds the position
to the published value, the position can be copied to the
position stored in the sub-event.
The subscription filter values are subscription type,
limit and handler. These values are stored at the position,
which stand in the three position blocks of the manage-
Copyright © 2010 SciRes. WSN
ment frame. The first subscriber takes the first 32bit be-
ginning at the given position, the second subscriber the
next 32bits and so on. This procedure is done for all three
values. An unsubscription has to be divided in a pausing
and a real unsubscription request. Pausing means that the
subscriber wants to skip notifications for a short time and
wants to resume later. Unsubscribing means that the
subscriber gives its subscription up and that the reserved
memory space can be used be another subscriber. The
subscriber executes the unsubscribe routine with the pa-
rameter of the event id, the subscriber number and if its
willing to pause or not.
Like in the other processes, the block flag is tested and
set first. If the corresponding subscriber flag is set, the
flag will be cleared. If not pausing, but unsubscribing is
chosen, also the corresponding data flag is cleared. After
all, the block flag is cleared and the unsubscribe process
has finished. If a new value is published and a subscrip-
tion inequality of at least one subscriber is fulfilled, the
subscriber needs a notification. But how to notify the
subscriber? Before possible solutions can been discussed,
the goals should be clear. A notification method should be
1) effective concerning space and time
2) be implementable (Mantis prototype)
3) fulfill the three publish/subscribe constraints
Subscribers can be splitted into two parties. Subscribers,
which are interested only in the notification itself, not in
the published value, are in the first party. These sub-
scribers are namely the alert subscribers. The second party
of subscribers need the value for their further proceeding,
like sending a message with the published value to an-
other node. These subscribers react on a notification like
the alert ones, but also need the value at notification. Each
of these subscribers could be build as an alert subscriber,
which requests the value at notification or the value is
passed to the subscriber with the notification.
Subscribers who are only interested in getting a noti-
fication as an alert have three choices in getting a notify-
cation. First, the subscribers could poll for a notification.
The blackboard needs a notification queue, in which all
subscriber ids are stored which have to be to notified. On a
poll request, the blackboard will give a true back as re-
sponse, when there is an entry for the subscriber in the
queue and false otherwise.
After every valid polling request, the found subscriber
entry is removed form the queue. The problem with the
polling method is, that when its used too often, too much
computational time is wasted, having an active consumer
thread. So the thread with the waiting and polling sub-
scriber disables the operating system to switch to lower
electric consumption state. The time the subscriber waits
for the notification may be used to let the micro-controller
sleep, but with an active polling process this not possible.
In a situation, where the producer of the event is not in the
same thread of the subscriber, it is possible to reduce the
polling to a minimum.
On a reactivation of the thread, only one polling is al-
lowed, because a second request would always return
false. But even in this situation, the polling method in-
hibits an efficient power management. The only positive
fact is, that the subscriber defines the point in time, when
the notification should be processed.
A second idea is the usage of a function call. In some
operating systems a alert function is available, which
starts a function, when a given point in time is reached.
For execution of the function, a reference to the function
is given to the alert process on creation of the alert. The
same could be done here. The function would be executed
in the context and time of the blackboard. So the sub-
scription thread is definitely not blocked and does not wait
or pull for getting a notification. The problem is, that the
function may use some variables from the subscribing
thread. This is not allowed, better not possible, when the
function is executed at the blackboard thread. Only global
variables are reachable for the function in order to publish
the computed new value. Another idea may be, that the
function uses the publish function to make the value
known. But then the whole process chain has to be built as
nested functions. This complicates the process unnec-
essary in context of debugging and implementation in-
The second point is that this function call method
breaks the multi-thread concept. The process chain would
be computed at the blackboard thread. The main scheduler
would be useless in this situation. So a new additional,
inline scheduler must be implemented in the blackboard.
This scheduler schedules all function calls for the black-
board. As we can see, the function call method is tricky
and so arduously to proper implement. A third idea, how
to implement a notification, is the idea of a locked sub-
scriber block in the subscribing thread. At the first sight,
blocking the subscriber is not wished, concerning the
decoupling rules. But here, not the whole subscriber is
blocked, only a part of it, the part which handles the things
to do when a notification arrives, is locked. Instead of
out-sourcing the code in a function, which has to be
executed on notification, here the code is still in the thread,
but locked with a global available semaphore, which the
subscriber has defined. The previous discussed notifica-
tion model with function calls can been transformed in
this model. The code of the notification functions for the
subscribers build subscription blocks in a special sub-
scription thread. Each thread is locked by a semaphore,
which is unlocked if the blackboard thread calls the noti-
fication for the thread. Instead of executing functions at
notification, the semaphore is unlocked and the subscrip-
tion thread is activated once.
The only thing the blackboard has to do, is to unlock the
semaphore. In the subscriber block of the thread, it has
than to be decided, if the semaphore should be relocked
again or not. The positive effect on this method is that first
the subscriber decides the point in time of the notification
Copyright © 2010 SciRes. WSN
in the thread and secondly thread variables are still
available. The problem is, that there could be a problem
with the scheduler, when the subscribing thread is not
activated contemporary. When the blackboard clears a
semaphore for a subscriber, the subscriber could execute
the now unlocked code. In a situation, where a producer
publishes a new value for this event before the subscriber
thread was activated, the blackboard once again tries to
notify the subscriber by unlocking the already unlocked
semaphore and the subscriber only gets the notification
for the last published value. So when a notification
guarantee should be given, than the scheduler has to prior
the thread with an outstanding notification. The needed
modification for the scheduler should be minimal. An
additional queue is needed, where the blackboard writes
all subscriber ids to notify down. The scheduler works off
this list and gives the corresponding subscriber thread the
highest priority. In the next step, the subscriber id is re-
moved form the list, so that on the next scheduler round,
the thread get its old priority back. The problem here, is
that the scheduler must know, which subscriber id belongs
to which thread id. The blackboard can store this infor-
mation at subscription. Each thread knows its id, so this id
could be given as a parameter to the blackboard on sub-
scription and stored in a extra table. Either the scheduler
reads this table to decide which thread to prefer or the
blackboard does this and gives the scheduler the thread id
instead of the scheduler id. The interference in the
scheduler process has effects on the thread order. When
many notifications should be worked off, it could be that
some threads will become stunted, because all notified
threads have a higher priority. Aging is a solution to avoid
this scenario. A second problem is the question how in
which order the high priority threads should be activated.
A normal first-come-first-serve scheduling strategy
should do that work in a normal scenario, but in a real-
time enabled notification system the whole scheduler
must been modified.
4. Concept of a Blackboard for WSN
So far, we have shown how to store events and subscrip-
tions on the blackboard and how producer and consumer
can use the blackboard API. Now we will discuss the
blackboard process, which is settled in an own thread, the
blackboard thread. The assignment of the blackboard
thread is to process all publish and subscribe requests as
well as verifying the subscriptions and start the notifica-
tion process if necessary. The blackboard thread can join
five different states. These states are
1) sleep state
2) modify state
3) prove state
4) notify state
5) optimize state
The sleep state is always the first and last state in the
blackboard process. First, being in this state, it is proved if
at least one producer has published a value. This can be
done, by looking in the publisher queue. If this list has at
least one entry, the blackboard thread switches to the
modify state, else the thread is let sleep and the scheduler
can switch to another waiting thread. The publishing
queue is needed, because the publish/subscribe paradigm
demands a decoupling in time. When a publisher would
access the storage management directly, than one part of
the blackboard assignment would be done by the pro-
But than the producer would be blocked, while it writes
its data to the blackboard storage. To prevent this, the
producer only pushes its data to the blackboard, which
queues this in the publisher queue. Not surprisingly, the
same is done for the subscriber so the queue is extended to
also store subscription requests. On activation of the
blackboard thread, the queues are worked off by switch-
ing to the modify state. In the modify state, each entry in
the queue is worked off in a first-come-first-serve manner.
This is done because of the fact that a subscriber should
only get a notification for an event, which is published
after the subscription. For the unsubscribe request its vice
versa, if a consumer unsubscribes no further notification
should arrive after the unsubscription. If all subscriptions
in the queue would have be done before publishing in the
blackboard, than the resulting process order may be in-
correct. After the processing of each queue entry, the item
can be purged from the queue. When the queue is empty
and at least one publish entry was processed, than the
thread switches to the prove state. Else the blackboard has
finished and switches to the sleep state.
In the prove state, the blackboard process goes over all
management frames, in order to find an activated publish
flag in the subscriber flags array of the frame. If one is
found, it is proved if one or more subscriptions are ful-
filled. For each of them an entry with the subscriber id and
the subscription handler address is stored in the notifica-
tion queue. Next, the publish flag is cleared and the next
management frames are tested. When all frames are
worked off and at least one notification has to be done, the
blackboard thread switches to the notify state. Else, no
notification has to be done and so the sleep state is acti-
In the notify state, all notification assignments, which
are stored in notification queue, have to be worked off in a
first-come-first-serve order. Therefore, all notifications
semaphores, which reference is the subscription handler
address, are cleared. Following, the value of the thread id,
which belongs to the subscriber, is pushed to the sched-
As described in the section before, the scheduler de-
cides, when a thread is activated, When the queue is
processed, the blackboard finishes its process by switch-
ing to the sleep state.
Copyright © 2010 SciRes. WSN
Optional, a optimize state can be insert after the publish
state. The assignment of the optimize state is to reduce
offcuts in the blackboard storage. This state increases the
needed computational time of the blackboard, so it should
only be used, if the fixed available memory for the
blackboard is nearly reached.
5. Integration of Publish/Subscribe into
The interaction style describes how parties communicate
which each other. As Rozanski and Woods describe in
Chapter 11 of [6] the interaction style for a blackboard
depends on referential and non-temporal coupling. The
referential coupling means that the sender has a reference
to the receiver’s address. Concerning the blackboard the
sender is the publisher which publishes an event. The
publisher knows the event identifier, so it has an indirect
reference to all subscribers of the event by the event itself.
An asynchronous data exchange between sender and
receiver is a non-temporal coupling. Exactly this decoup-
ling in time is one of the three main conditions explained
in Section 2. The point in time when the receiver gets the
notification of the publication is not predictable. It could
happen at once after the publication or some time later.
Therefore, setting a deadline is conceivable in order to
isolate the notification point in time. This directly leads to
the idea of an real-time enabled publish/subscribe system.
The presence of deadlines requires the modification of the
operating system scheduler. The scheduling of threads,
which contain deadline afflicted subscribers, leads to the
question if all deadlines could be met. First and foremost,
the used real-time scheduler algorithm determines, if all
deadline expections will be fulfilled. For simplifying the
choice for the correct scheduler algorithm, we will make
two important assumptions in this paper. We will assume
that the system is always schedule-able and that the sys-
tem is never in an overloaded condition. This assumptions
can be made, because the task is to create a real-time
enabled publish/subscribe system and not a real-time
enabled OS.
This makes things different, because in this scenario a
central component, the blackboard, decides the point in
time when the deadline of a thread should be modified and
how the value of the deadline should be modified.
5.1. Overview MANTIS OS
MANTIS OS [1] is built in the classical multi-thread
design. The application threads are separated from the
underlying operating system, which is split into two layers.
The upper layer consists of the system API. The threads
are using the system API, when a radio communication is
started or a sensor is triggered. The system API is based
on the lower layer which consists of the kernel, the com-
munication (COMM) and the devices (DEV) part. The
COMM part handles asynchronous I/O, like radio, serial,
etc. and the DEV part all synchronous I/O e.g. reading
data from the sensor. The COMM part is interrupt-driven
and so no polling is used. For example a received data
packet from the antenna triggers a hardware interrupt,
which pushes a transfer function for storing the packet
into a receive queue. The main part of the base layer is the
part of the operating system kernel. Besides the existence
of mutex, semaphore and alarm functionality, the sched-
uler is the main part of the kernel.
The scheduler decides which of the available and ready
threads to activate. Threads can call a sleep function to
sleep for a defined time. Then the system API gives the
scheduler the opportunity to determine a new schedule
omitting the sleeping threads. If no thread is ready for
activation, a idle thread is activated, which lets the mi-
cro-controller sleep for the rest of the time-slice.
The number of threads in MANTIS OS is limited to at
most 8. This was a design decision with the total available
memory in mind. The problem is that each thread gets a
reserved memory block of its own. These blocks are re-
served statically with the best-fit method. So a dynamical
allocation inside the thread is not possible. MANTIS OS
is built in the classical multi-thread design. The applica-
tion threads are separated from the underlying operating
system, which is split into two layers. The upper layer
consists of the system API. The threads are using the
system API, when a radio communication is started or a
sensor is triggered. The system API is based on the lower
layer which consists of the kernel, the COMM and the
DEV part. The COMM part handles asynchronous I/O,
like radio, serial, etc. and the DEV one all synchronous
I/O e.g. reading data from the sensor. The COMM part is
interrupt driven and so no polling is used. For example a
received data packet from the antenna triggers a hardware
interrupt, which pushes a transfer function for storing the
packet into a receive queue.
The main part of the base layer is the part of the oper-
ating system kernel. Besides the existence of mutex,
semaphore and alarm functionality, the scheduler is the
main part of the kernel.
The scheduler decides which of the available and ready
threads to activate. Threads can call a sleep function to
sleep for a defined time. Then the system API gives the
scheduler the opportunity to determine a new schedule
omitting the sleeping threads. If no thread is ready for ac-
tivation, a idle thread is activated, which lets the micro-
controller sleep for the rest of the time-slice.
5.2. Scheduler
The scheduler is preemptive designed, allowing MANTIS
OS to switch active threads even if they have not finished
their execution. The scheduler is built as a priority-based
Copyright © 2010 SciRes. WSN
Figure 3. Blackboard extension of MANTIS OS.
Round-Robin scheduler with five defined priority levels.
The time slices are 20 msec long and controlled by a
hardwarebased timer interrupt. At the end of a time slice
or if a thread calls an interruption command (like
mos_thread_sleep() ), the running thread is halted and
appended to the end of the ready queue. The dispatcher
function of the scheduler grabs the first thread from the
ready queue and executes a context switch. The ready
queue itself is split into five separated queues. Each pri-
ority level has its own queue, so in total we have five
queues for the ready queue. The implemented Round-
Robin scheduler first looks at the existence of a head
element in the highest priority queue, then in the next
lower prioritized queues. The queues are build as a
chained list with a head and a tail element. The cost for the
search of a defined element in the list has a complexity of
O(n). Beginning the search at the head of the list, at most
all n elements have to be visited once. The round-robin
scheduler always takes the first element, the head of the
list, which it found in O(1). The extended scheduler needs
to purge the ready queue from the thread the EDF sched-
uler has chosen for activation. So if the EDF part is active,
the dispatcher function has to find the thread in the ready
queue and to overwrite the predecessor threads next
pointer with the next pointer of the chosen thread in the
Fortunately, MANTIS OS already has such a function,
which is called mos tlist get thread(). Unfortunately, this
function has a bug not finding the correct thread nor any
thread in the list. The problem here was a incorrectly
defined if clause, not correctly testing the thread id against
the search id. We have bug-fixed this function and use it
in the EDF part of the dispatcher.
As seen in the section before, the implemented com-
bined scheduler always tries to find a EDF schedule-able
thread first and only falls back to the round-robin method
if none is found. To find at least one thread with a dead-
line, all entries for the threads have to be visited once. The
MANTIS OS scheduler uses a thread table for storing all
thread relevant information. Here, we have extend the
thread table by the deadline variable. To set the deadline
for a variable, I have written a new function called set
deadline (thread, deadline). This function can be used to
assign a given deadline to a certain thread. The scheduler
will go through the all thread table entries beginning at the
first and memorizes the thread that is in the ready status
and has the lowest deadline of all ready threads. This
search has a complexity of O(n), with n as the number of
all threads.
If no thread is found, the round-robin part of the dis-
patcher determines a thread to activate. Going through the
different priority queues of the ready queue, this has a
complexity of O(k) with k as the number of queues.
If a thread is found, the EDF part has to remove the
thread from the ready queue. In contrast to the round-
robin part, the priority of the chosen thread is known, so
the search function mos tlist get thread() is used here. This
function needs at least one iteration to find the correct
thread in the queue and in worst case n iterations, which is
the number of elements in the queue. Because only 10
threads are possible in MANTIS OS, the time needed for
searching and removing the element of the list is short.
My extension of the dispatcher function has the same
complexity class of O(n) as the original scheduler of
As well as the real-time ability, activating threads only
when they are needed is also a design goal of the sched-
uler. To do so, a new thread state, called blackboard state,
was created.
A thread in this state cannot be activated by the
Round-Robin scheduler, because the thread is not in the
needed ready state and so it’s not listed in the ready queue.
The EDF scheduler part is able to schedule this kind of
thread. It handles the blackboard state thread like a ready
thread. So if the thread is in the blackboard state and has a
deadline lower than the maximum value for a deadline, it
is schedule-able by the EDF scheduler part. The advan-
tage over the normal scheduling is that the thread is only
activated if it has a deadline. Because only the blackboard
thread sets the deadline for a thread with at least one ac-
tive and valid subscription, a thread in blackboard state
will only be woken up when it is required.
The wake-up process is similar to the sleep method.
The difference between the sleeping and the blackboard
state is that a sleeping thread cannot been awoken when it
sleeps and that the sleeping thread sleeps for a defined
time. After the sleep time, the thread has to compete for a
time slice, processes its commands and sleeps again. A
thread in blackboard state is only activated if the correct
published value is available and is suspended again after
its processing. This new method allows longer micro-
processor sleep and suspend times, because the idle thread
will be activated more often as when only sleeping thread
Copyright © 2010 SciRes. WSN
would be used.
In order to set the blackboard state for a thread, we have
implemented a new function, called mos give up(). This
function changes the current thread state to the blackboard
state by overwriting the state parameter of the thread entry
in the thread table. The Round-Robin scheduler does not
know this new state type and so it does not interfere. The
blackboard thread uses this blackboard state. If for ex-
ample a value is published, the blackboard thread gets a
very low deadline and is activated after the another thread
publishes a value.
When the blackboard thread has finished the processing,
it sets the deadline to infinity and the state type back from
running to the blackboard value. So the blackboard thread
is only active when it is required. No unneeded processing
time is wasted, because the thread is not activated when it
has nothing useful to do.
5.3. Blackboard API
The publish and subscribe process can be done at once by
directly executing the corresponding function or by using
a buffer which the blackboard processes afterward. By
buffering all publish/subscribe commands the sender of
these commands has not to hand over process time for the
publish or subscribe process. When the blackboard thread
is the next activated thread after the buffer is filled, we can
guarantee that the all buffer entries are processed directly
after the thread switching. The blackboard thread proc-
esses all buffer entries the notification process has started.
We have implemented a queue which is filled with
blackboard commands that the API makes available to the
application threads. On filling the buffer or executing the
publish command, the deadline of the blackboard thread is
The value of the deadline is set to the lowest possible
value for the deadline (the number one). This will lead to
an activation of the blackboard thread instead of any other
one on the next scheduler run. The blackboard is preferred
against the other threads. So when a thread executes a
buffering command or pushes at least one new publication,
the blackboard thread is always executed directly after the
publishing thread.
Each entry in the buffer consists of several components.
The most important one is the identifier, which has the
following values for the different blackboard commands.
1) subscribe
2) modify subscription
3) unsubscribe
4) pause subscription
5) publish
Other stored components are always the number of the
event for that the command is and the current time-stamp.
If a subscription command is called, the buffer has also to
store the subscription parameters subscriber id, type,
value, deadline, and the address of the semaphore of the
notification process. The buffered communication be-
tween the application threads and the blackboard thread is
asynchronous. All commands are buffered first and
processed later. The problem is that it could happen that
an unsubscription is temporally before a publication. So
this unsubscribe command has to be processed before the
publish process to avoid side-effects.
The time when the command is stored in the buffer is
important. All buffered commands have to be processed
in the order of their entering in the buffer. So the imple-
mentation of filling and removing buffer elements is used
in the first-in first-out manner.
Besides using the buffered commands subscribe_ re-
quest() and publish_request(), the direct commands sub-
scribe() and publish()without using the bufferare
also possible. The difference is the point in time when
the command is processed. The direct commands are ex-
ecuted at calling and use up processing time of the cur-
rent thread. On the other hand, the buffered commands
are stored temporary and executed when the blackboard
thread is activated. The processing of the calling thread
is not inhibited, because the command execution is after
the thread was suspended from the scheduler. The second
difference can be found at the error handling. Because
the buffered commands communicate asynchronously
with the blackboard, the blackboard ignores errors. The
calling thread does not wait on the command execution,
so no value is returned as status information. It is differ-
ent for the direct command procedure. Because the
thread executes the command and waits on a result af-
terward, an error return is possible.
5.4. Blackboard Thread
As described before the blackboard thread is only awaken
if at least one element is in the blackboard buffer. The
blackboard thread will process the buffer in the modify
state beginning with the head element of the buffer. Af-
terwards the kind of the element is identified by its id
number. Each kind of element has its own function that is
called for storing the parameters on the blackboard stor-
age and the management frames. This first modify state is
finished when no element is left over. In the first instance
of the second phase it is proven if at least one command
processed in the modify state was a publish command.
Therefore, a flag is set on occurrence of a publication.
In the prove state the test flag can be easily tested on
activation. If no value was published before, the black-
board is finished and resets its deadline afterward. Be-
cause no value was published, no subscriber needs a no-
There is no new value for which a subscriber may wait.
The next thread will be chosen by the scheduler and this
one is not the blackboard thread, because its deadline has
been set back to infinity and its thread state has been set to
Copyright © 2010 SciRes. WSN
Copyright © 2010 SciRes. WSN
dominates the new one. When all active subscriptions are
proven for the event, the next event with an enabled pub-
lish flag is searched. The blackboard process has finished
when all management frames have been visited.
the blackboard value. But if at least one publish command
was processed in the modify state, there is the possibility
that one or more subscription filters are fulfilled and so a
notification process has to be launched.
The elapsed time of the notification process is O(n)
with n events or in worst case 6 * n with n events in total
and all with a set publish flag. Normally a blackboard
thread should be finished within a time slice. If not and the
dispatcher is launched while the thread is active, the
blackboard thread is chosen again by the EDF scheduler
part and because current running thread and new thread
are the same, no context switch is done. So the blackboard
thread always finishes its task before another thread is
chosen for execution.
First the events with possible notifiable subscribers
have to be found. Therefore, the search algorithm goes
through all management frames and stop if it found one
with a set publish flag. This flag is cleared and for all
active subscribers of the event it has to be tested if their
subscription filter matches to the stored publication. If the
test result is positive, the notification of the subscriber is
interpolated. In this notify state, the stored subscription
data is used to clear the semaphore at the given address
and to set a new deadline for the thread with the stored
thread id.
The deadline is the latest point in time when the noti-
fication arrives at the subscriber. For setting the new
deadline, the deadline has to be calculated. The formula
for computing this point in time is t_deadline = t_ pub-
lished + Dt_deadline where Dt_deadline is the value for
the deadline given at the subscription and t_published is
the point in time when the value was pushed from the
publisher thread in the blackboard queue.
6. Validation of the Implementation
First, we will show that if a deadline-enabled notification
for at least one subscriber is available, the deadline for the
subscription thread is set and the thread is activated within
the deadline. We assume that all deadline threads are
schedule-able concerning their deadlines, since this is our
prerequisite as mention before. So the publisher willing to
publish a new value starts the publish request() function.
Inside this function call, the deadline for the blackboard
thread is set to an initial value. This leads to an activation
of the blackboard thread after the currently running thread
is interrupted, because of the initial deadline value. No
other thread could have a lower deadline than the black-
board thread.
In the implementation, the calculation is started as early
as possible or more exactly directly after the positive
subscription test. The calculated deadline time is passed to
the scheduler by setting the new deadline time for the
subscriber thread in the thread table. If this thread has
already a deadline, which is lower or equal than the new
calculated value, the old will not be overwritten. Then the
old deadline is tighter as the new one and so the old value
Figure 4. Blackboard thread process.
Copyright © 2010 SciRes. WSN
The largest period of time that a thread could be acti-
vated is the duration of one time slice, here 20 msec. At
the latest of this time period the blackboard thread is
running, processes the publish request and starts the noti-
fication process. Because the publish/subscribe paradigm
forbids to block the publisher, starting the blackboard
thread directly as the next thread is the earliest permitted
point in time. Beside the activation time of the blackboard
thread, this thread has to be nonpreemptive. This is
guaranteed by the fact that the blackboard thread keeps its
initial deadline value until the blackboard task has fin-
If the scheduler tries to switch the threads, the EDF part
of the scheduler will find the blackboard thread as the one
with the highest priority and will keep running the
blackboard thread. So if the blackboard thread has fin-
ished its task, every mutex for the subscription notifica-
tion has been cleared and all subscription threads have
their deadline. After the blackboard task execution the
scheduler is ready to activate the deadline enabled threads
in their priority or better deadline defined order. Secondly,
we will discuss the problem of mutual exclusions. Shared
resources are normally saved by using the mutual exclu-
sion concept. But by using a real-time scheduler and
mutual exclusion, a so called priority inversion can occur.
This means that if a low priority thread allocates a re-
source by setting a mutex, a later try to allocate a high
priority thread is blocked and has to wait until the mutex is
cleared. So there is the possibility that the blocked thread
misses its deadline, because it has to wait for the lower
priority thread to release the mutex.
Therefore, it is important to activate a high priority
thread early to prevent priority inversion. The first con-
dition, the early activation of the highest priority thread, is
fulfilled by the EDF scheduler, because the highest pri-
ority means that the thread has the lowest deadline of all
ready threads. The second condition needs some modify-
cation of the mutex processing. For our implementation
we have chosen the concept of priority inheritance as
described in [5] for preventing the priority inversion
problem. Therefore a thread that tries to set a mutex an-
other thread with a lower priority has blocked, helps the
mutex owner by passing on its lower deadline to the
thread. The original deadline is preserved within the
mutex structure and reverted when the mutex is released.
The needed modifications in the MANTIS source code
are only marginal. A blocked thread is in the so called
blocked thread state and its deadline is not influenced. So
after releasing the mutex the blocked thread gets back in
the ready state and is - because of its lower deadline - the
first thread that will be activated. Its waiting time abetted
its priority class, now it is even more important to sched-
ule this thread.
The following shows parts of the mutex code. On each
mutex set attempt it is proven if the mutex is free or not. If
not it is tested if a priority inversion situation has occurred
or not. This could only happen if the current thread dead-
line is lower than the deadline of the mutex owner thread.
Than the original deadline of the owner thread is stored in
the mutex structure and afterwards its deadline is over-
written with a copy of the current thread’s deadline. Since
the current thread is blocked, the thread which has set the
mutex is activated afterwards. Only the blackboard thread
could have a sooner deadline as the owner thread, all other
threads have lower priority.
In the code that unlocks the mutex, only one additional
line was added for testing on existence of a stored old
deadline and reverting it. On every successful mutex
unlock, this line is executed before the scheduler function
is called. Because MANTIS OS differs between mutex
and semaphore, mutex and semaphore code is separated.
So also the semaphore code is modified in the same
manner as described for the mutex code.
6.1. A Simple Test Case
This section discusses the use of the implemented pub-
lish/subscribe architecture in MANTIS OS by an intro-
ducing example application. This application consists of
three autonomous threads with two of them periodically
sleeping and one being only activated on notification of
the blackboard. The application is spilt into tasks and each
task has its own thread. One of the threads takes over the
publisher part and the two others are processing the pub-
lished data.
The publisher process, which is in a sensor application
the part which enquires a sensor, publishes a number and
sleeps afterwards for 30 milliseconds by executing the
mos sleep() function of MANTIS OS. The thread is re-
scheduleable after the expiry of the sleeping time. The
only task of this thread is to regularly push data to the
blackboard, which has to react afterwards.
The two threads with the subscribers for the publisher
event are split in a thread, which is only activated on
notification (subscriber1) and one which is periodically
and on notification activated (subscriber2). The second
subscriberthread sleeps for 10 msec after each execution
In each running state of the publisher thread, one value
is published before the state is switched to sleeping. The
blackboard thread is executed afterwards and modifies
the management frame and stores the value. Afterwards,
the blackboard tests all subscriptions of the subscribers
of the published event. If the subscription rule is valid,
the blackboard notifies the thread with the subscriber by
setting a deadline and clearing the correct mutex. Be-
cause subscriber1 has a shorter deadline defined as sub-
scriber 2, we have included debug code so that we could
measure the execution times. The runtime diagram (6.1)
shows the execution times of the MANTIS OS threads
Figure 5. Test case scenario.
for this example. The example implementation was exe-
cuted on a Mica2Dot sensor node. The node was con-
nected to the host PC via JTAG interface. On the host,
the avr-gdm application in combination with the averice
JTAG communication software was used.
Each round-robin time-slice has 20 milliseconds, but
as the diagram (6.1) shows, no thread needs a complete
time-slice of its own. The publisher thread is activated
every 30 msec as supposed and executes within 1 milli-
second. The blackboard thread is launched directly after
the sleeping phase of the publisher thread begins. Then
subscriber1 is activated before subscriber2, because of its
sooner deadline. The diagram also shows long periods of
the idle thread, which means that the micro-controller
has relatively long sleep periods. Two threads, the
blackboard and the subscriber1 thread, are only activated
when its necessary. So a long active idle thread is possi-
ble, which allows a low overall power consumption.
Also, less context switches are the consequence. So be-
sides a better response time, the multi-threading over-
head is reduced by using the blackboard thread state in-
tensely. Blackboard stated threads are executed in a just-
in-time manner.
In this example, the publisher thread issues new values
and than goes to sleep at once. In the next scenario, we
have added a simple waiting routine directly after the
publication command. So it is possible to simulate a
working task, since in a normal application the publish-
ing thread needs some time for monitoring the sensors.
The problem is that the publish/subscribe system
should not block the running task. So it is possible that
the current thread is not interrupted until the time slice is
over, originate in that the developer has not used a inter-
rupt function like the sleep command before the time
slice interrupt occurs. If a defined notification deadline is
sooner as the remaining time of the time slice, then the
deadline is not grantable. In a test series we will show
that a defined deadline is met when possible. In the other
condition it is not predictable how long a subscriber has
to wait on the notification additionally. Figure 6 shows
the results of the test series. The results show the time
the notification of the subscriber needs with reference to
the time the publisher thread processes the waiting task
after the publication. The deadline in this example is set
to 10 msec for the subscriber. As long as the additional
processing time of the publisher is under 10 msec, the
deadline is met. All notifications are transferred within10
msec. But in the scenario when the additional processing
time is over 10 msec, the values fluctuate. As the results
show, the deadline of the notification is fulfilled as long
as the publisher thread is interrupted in good time. If the
developer does not let the thread interrupt after a public-
cation and processed s time-consuming task instead, the
deadline is not met. But the results also show that if the
total additional processing time is over the time slice
value of 20 msec, the time needed to notify the sub-
scriber level off at about 11 msec. In this condition, the
elapsed time slice is responsible for a thread switch. Be-
cause then the blackboard thread has the highest priority,
the blackboard processes the publication and notifies the
subscriber before the interrupted thread is rescheduled
again. This example shows that a developer has to take
care, if the deadline is set to a value under 20 msec, and
the publisher threads task is not interrupted soon enough
after the publication.
If the developer chooses for the subscriber (in this
example) a deadline above 11 msec, the deadline would
be met in the over-load scenario. A deadline of 20 msec
and above would be always met. The problem is that the
developer not always knows how long the blackboard
thread has to wait for the interruption of the publisher
thread. Therefore, the developer should execute a dis-
patch thread() or mos sleep() directly after the publica-
tion. If this is not done, then the ime needed between
publication and interruption has to be stimated and added
to the deadline value. The test results how that it is im-
portant to choose an acceptable value for he deadline. A
solution may be to increase the deadline alue by one unit
every time the deadline is not met. Further esearch
should show if this method is feasible and useful. The
Copyright © 2010 SciRes. WSN
Copyright © 2010 SciRes. WSN
Figure 6. Run-time diagram of the test case example.
Figure 7. Results of the test case measurement.
concept is similar to the deadline aging concept, used y
several real-time scheduler implementations. We will not
urther discuss this problem, because one conceptional
assumption s that the thread is interrupted early enough
when ubscribers have to be notified.
The example shows that the deadline of the subscriber
is met f possible. In the condition above 10mesc, the
point in time hen the subscriber is notified is not pre-
dictable anymore as expected.
7. Conclusions
We have shown in this paper that it is possible to develop
and implement a blackboard-based publish/subscribe
architecture for sensor nodes which is also able to fulfill
real-time requirements. Next to the real-time ability the
introduced architecture flexible concerning the amount of
subscribers, subscriptions and events but on the other
hand memory space is economically used. We have
shown this by comparing our concept against a memory
sparing but static architecture and against a total flexible
but memory wasting dynamic storage concept. Our black-
board-based approach shows a good tradeoff between
memory usage and flexibility. Also, it is shown that the
three publish/subscribe constraints, namley time, space
and synchronization decoupling, are redeemed. Therefore,
sensor application designer have now the possibility to
use our available framework for easily sharing values
between different program parts at least for MANTIS OS
based applications. The development of crosslayer con-
ceptions benefits from our architecture since the central
storing of the published values is managed through our
presented blackboard. All publishing and notifying pro-
cesses are hidden for the application developer. The
needed operating system modifications especially of the
scheduler was shown exemplary for the MANTIS OS.
The postulated real-time attribute of the publish/subscribe
architecture made it necessary to convert the existing
scheduler implementation towards an EDF scheduler. Not
only it was shown that it is possible to make a sensor
operating system real-time enabled but also the shown
architecture benefits from the real-time ability. Subscribe
are able to define deadlines concerning the notification
which enables us to keep track of the system even in an
overload situation where deadlines can be fulfilled any-
8. References
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Copyright © 2010 SciRes. WSN