Conflicts Resolution and Situation Awareness in Heterogeneous
Multi-agent Missions using Publish-subscribe Technique and Inferential
Reasoning
Sagir Muhammad Yusuf and Chris Baber
University of Birmingham, B15 2TT, U.K.
Keywords:
Multi-agent Reasoning, Bayesian Networks, Multi-agent Learning, Publish-subscribe, Heterogeneous
Multi-agent Coordination, Cooperative Perception.
Abstract:
In this paper, we propose a priority-based publish-subscribe approach to tackle reasoning in beliefs conflicts for
a heterogeneous multi-agent mission. Agents subscribe to other agents’ topics and rank them based on agents’
situation awareness. Bayesian Belief Network (BBN) was used in maintaining agents’ belief and recorded
mission information could be used for the BBN training using conjugate gradient descent or expectation-
maximization algorithms. The output of the training is the learned network for agents’ predictions, estimations,
and conclusions. We also propose an agent’s self presumption inferential reasoning where agents learned
heuristics and used them for future inferences. We test the system by using a team of heterogeneous Unmanned
Aerial Vehicles (UAVs) with different sensor profiles and capacities tasked together to perform forest fire
searching. To verify belief and settle conflicts, agents follow these steps: sequentially assess the prioritized
publish-subscribe topics, inferential reasoning using the learned network, inferential reasoning using logical
propositions, and learning process. From our experiment, the BBN training and prediction perfection grow
up with the increase in the number of training data. Future work focuses on obtaining the optimal number
of samples needed for effective prediction, effective agents’ beliefs merging, communication protocol, and
bandwidth utilization.
1 INTRODUCTION
The heterogeneous multi-agents mission comprises of
a team of different agents with different beliefs (sen-
sor data), roles, and capacities tasked collectively to
achieve a particular goal. It has a potential advan-
tage of categorizing agents’ based on their special-
ization, backup provision, and robustness (Cort
´
es and
Egerstedt, 2017; Setter and Egerstedt, 2017; Khan
et al., 2015; Setter and Egerstedt, 2017; Yanmaz et al.,
2017). For example, consider a team of aerial and
grounds robots tasked to conduct forest fire fight-
ing, agents can divide the task into extinguishing,
fire spreading monitoring, rescuing, and so on. Use
of heterogeneous agents provides a wider opportu-
nity for detecting false alarm due to sensor variation
(Merino et al., 2006). For example, agents carrying
camera sensors may have different beliefs from agents
using object sensors in the vehicle overload detection
system. The agents using facial recognition can de-
tect sitting on each other’s lap passengers and count
them as two, while agents using an object sensor may
count it as one passenger. Another agent may use a
weight balance sensor to count passengers based on
their masses. A challenge arises in merging beliefs
derived from different sensors to make an optimal cor-
rect decision that will support agents’ cost functions.
In every multi-agent mission, energy, mission time,
communication link, and other resources need to be
utilized.
We tackled this problem by applying a priority-
based publish-subscribe technique model using
Bayesian inferential reasoning and Distributed Situa-
tion Awareness (Endsley, 1995; Stanton et al., 2006).
The idea is that each agent has its own beliefs. It
will then subscribe to other relevant topics. Agent
belief variation is sorted out using probabilistic prior-
ity value. The probabilistic priority value rises with
the number of right decision and agents’ situation-
awareness (i.e., how agents currently perceive the en-
vironment). It changes from agents to agents based on
the agents’ environmental adaptation because some
agents perform better than other agents in different
scenes. For example, agents using thermal sensors
846
Yusuf, S. and Baber, C.
Conflicts Resolution and Situation Awareness in Heterogeneous Multi-agent Missions using Publish-subscribe Technique and Inferential Reasoning.
DOI: 10.5220/0009147408460851
In Proceedings of the 12th International Conference on Agents and Artificial Intelligence (ICAART 2020) - Volume 2, pages 846-851
ISBN: 978-989-758-395-7; ISSN: 2184-433X
Copyright
c
2022 by SCITEPRESS – Science and Technology Publications, Lda. All rights reserved
could perform better than the one using camera sensor
in detecting fire during day time because of the pos-
sibility of having fire-like terrains (e.g., dried grass).
The reverse could be the case when the agents are op-
erating on higher altitudes where heat could not be
sensed effectively.
We applied Bayesian Belief Network (Pavlin
et al., 2010; Wang and Xu, 2014; Williamson, 2001)
in monitoring the agents’ beliefs and their causal
relationships hard-coded during pre-mission plan-
ning. The BBN’s nodes are categorized into situa-
tion, awareness, and utility nodes in the BBN. The
agents’ situation nodes are the current belief of the en-
vironment. For example, fire present or absenteeism
based on sensor information. Awareness node is the
confirmed situation; that is decided belief that under-
gone verification process from different agents using
different sensors that, fire is present or not at a par-
ticular location. The utility node is a measure of
how welcome the agents are with the proposed be-
liefs. We modelled the agents’ utility as Distributed
Constraint Optimization (DCOP). DCOP is the be-
haviour of the agents towards the optimal assignment
of their variables with the aim of minimizing cost
functions(Fioretto et al., 2018; Fransman et al., 2019;
Maheswaran et al., 2004; Zhou et al., 2018).
Every agent has topics to broadcast to the network
(i.e., its sensor data) and subscribe to other agents’
topics in order to be authenticating its beliefs to a gen-
uine knowledge of the environment. The environment
may keep changing, and communication may be lim-
ited, which is a potential challenge of such architec-
ture. However, we propose a knowledge-base infer-
ential reasoning and Bayesian learning to solve this
issue.
Inferential reasoning allows the agents to predict,
estimate, and draw conclusions based on previous
experience (Fransman et al., 2019; Wang and Xu,
2014; Williamson, 2001). Bayesian inference com-
putes predictions based on the probabilities of the
prior variables using conditional probabilities (Wang
and Xu, 2014). Another mode of inferential reasoning
we want to discuss is the agents’ presumption infer-
ence. In this approach, agents build in their knowl-
edge in the form of if-then logical propositions rules,
then use that knowledge to make predictions and con-
clusions. For example, in multi-agent search and res-
cue missions, if an agent sees its co-agents hovering
over a place instead of usual navigation, it can per-
ceive that something interesting is present in that lo-
cation and act to support that agent.
In this paper, we tackle the problem of agents’
belief variation conflict using the publish-subscribe
technique. The agents’ beliefs are modelled us-
ing Bayesian Belief Network, and agents’ resource
utilization uses DCOP. We propose Bayesian and
heuristic-based inferential reasoning in monitoring
the agents’ belief conflict. We use heterogeneous
multi-agent coordination to conduct wildfire monitor-
ing as the use case.
The rest of this paper was organized as follows.
Section 2 describes the basics background of publish-
subscribe, inferential reasoning, and agents’ reason-
ing towards resource utilization (DCOP). Section 3
describes the related work and summary of our con-
tribution. Section 4 describes the ontology of the sys-
tems using forest fire monitoring as the use case and
experiments with their results. Finally, section 5 de-
scribes the conclusions and future works.
2 BACKGROUND
2.1 Publish-subscribe Multi-agent
Interaction
In a multi-agent system, the publish-subscribe tech-
nique categorizes the agents into senders (publishers)
and receivers (subscribers). Agents subscribe to other
co-agents’ topics (information) if it is helpful and in-
teresting to them (Hackney and Clayton, 2015; Rivera
et al., 2016). Agents might have a maximum number
of subscriptions and topics to be broadcast into the
network. As such, agents prioritize their publishers
based on information saliency and context-awareness
(i.e., current environmental situation). Agents use
sends and acknowledge protocols to monitor mes-
sages delivery and update a list of subscribers (Rivera
et al., 2016). That is, agents may change their pub-
lishers or subscribers over time based on the environ-
mental changes.
Moreover, the agents’ coordination architecture
can be centralized or decentralized. In a centralized
approach, agents are controlled by a server, as such
publishing and subscribing management became eas-
ier. The challenges of this approach is that the cen-
tral server needs much computational power, mem-
ory, communication bandwidth, and other resources
if the number of agents increases (Cort
´
es and Egerst-
edt, 2017; Saicharan et al., 2016; Turpin et al., 2014;
Vasile and Zuiani, 2011). In a decentralized approach,
agents have the full right to control their values and
use two types of communications approaches, that is,
explicit and implicit messaging (Gerkey and Mataric,
2002). In explicit messaging, agents share informa-
tion when they are close to each other. In implicit
communication, agents do not send messages; rather,
Conflicts Resolution and Situation Awareness in Heterogeneous Multi-agent Missions using Publish-subscribe Technique and Inferential
Reasoning
847
they sense other co-agents within a few inches as
in swarm of ants, birds, etc. (Ferrante et al., 2015;
Reynolds, 1987; Saicharan et al., 2016).
We applied a priority-based multi-agents publish-
subscribe model of interactions. Agents subscribe
to other co-agents’ topics and prioritise its belief on
that publisher based on its specialisation, situation-
awareness, or data saliency. For example, in fire
searching missions by a team of heterogeneous UAVs
carrying different sensor profiles. Let us assume the
agents are carrying an infrared, thermal, and visual
camera to detect the fire. If the agents with thermal
sensors subscribe to the agents with infrared and cam-
era sensors, it can be updating their priorities based
on the situation awareness (Endsley, 1995; Stanton
et al., 2006). During day time (sunny time), informa-
tion from the agents with visual sensors could have
higher priority than the agent with an infrared sen-
sor. However, the reverse is the case during night
time. Therefore, Distributed Situation Awareness -
DSA (Stanton et al., 2006) changes the agents’ data
saliency (i.e., their topics priority). The overall archi-
tecture was represented in figure 1.
Figure 1: Agents Subscription Layer.
Figure 1 describes the subscription protocol in a
top-down passion. We assumed that the agents could
access topics and the current environmental condition
(situation awareness of the environment). Based on
this situation, agents reason and assign priority to top-
ics from their publishers against a hard-corded rule.
It means topics priority changes with environmental
change. When an agent needs topics from different
publishers to verify its belief, priority would be used
in making a decision (i.e., making conclusion with the
highest priority topics).
2.2 Inferential Reasoning
We assume that the agents are accessible to the global
belief based on the Bayesian Belief Network (BBN)
of figure 2. They need to verify their beliefs based
on that network. We use forest fire detection using
heterogeneous UAVs mounted with different sensors
as the use case.
Figure 2: Bayesian Belief Network for Multi-agent Mis-
sion.
Figure 2 describes the agents’ Bayesian Belief
Network for forest fire detection. The system has
four different sensors (i.e., dry material, temperature,
colour, and wind sensors); agents carrying these sen-
sors broadcast their sensor information (as topics) of
which other co-agents can subscribe to for their belief
verification. For example, agent using colour (cam-
era) sensor can subscribe to other agents using dry
material, wind, and temperature sensors to be verify-
ing its beliefs against false detection e.g., detecting
object with the same fire colour palettes. It could be
verify simply by using votes technique. Sensor nodes
are the situation node, while other nodes (decision
nodes) are the awareness nodes. Utility nodes could
be attached for DCOP cost function optimization (i.e.,
nodes to be grading how decision nodes favour cost
functions). From figure 2, four types of sensors were
used in the system. The fire has two sensors, that
is, heat sensors and colour (visual sensors). Different
agents are carrying these sensors, and each can raise
a false detection based on the operating environment.
For example, yellowish dried grass can confuse the
agents using a visual sensor; agents can confirm their
belief from topics broadcast by other co-agents using
heat or dried material sensors and prioritise the top-
ics of those agents during daytime (i.e., the concept
of situation awareness). All agents could subscribe to
the services of the agent providing wind direction in
order to know the direction of the spread of the fire.
In centralized systems, agents could easily access
ICAART 2020 - 12th International Conference on Agents and Artificial Intelligence
848
and update their topics on time to time basis. In a
decentralized approach, the agent sends request con-
firmation to their publishers or a medium agent (bro-
ker) to extend their messages. A variation of this ap-
proach is in (Merino et al., 2006). Cases from pre-
vious missions could be gathered and stored as text,
cas, or csv file for network training using machine
learning algorithms. The well-known machine learn-
ing algorithms to be used for the BBN training are
expectation-maximization and gradient descent algo-
rithms because they handle missing and uncertain
data (Romanycia, 2019). The output network is a
well-trained BBN for making predictions, estimation,
avoiding redundant verification (request for verifying
the same belief within a short time), and conclusion.
It reduces the communication cost of the interaction
and improves context-based reasoning and optimiza-
tion. Another approach is to use the agents’ self-
presumption inference technique. That is, build the
knowledge of the environment as if-then rules. The
set of conditions are mapped to their various actions.
3 RELATED WORK
Agents’ belief verification is a well-known problem in
multi-agent system. Different approaches and strate-
gies were used in solving the problem. Merino et
al (Merino et al., 2006) describe a five steps (i.e.,
detection, alarming, confirmation, localization, and
evaluation) approach for multi-agent belief verifica-
tion. They used a team of heterogeneous Unmanned
Aerial Vehicles (UAVs) mounted with different sen-
sors to conduct forest fire monitoring in order to
test the model. Agents raise an alarm, then other
agents verify the claim. The agents will mark the
area and evaluate the fire risk. They implemented a
blackboard communication approach for belief veri-
fication. Yanguas-Rojas and Mojica-Nava (Yanguas-
Rojas and Mojica-Nava, 2017) describe a multi-agent
searching problem in which space was segmented into
a set of Voronoi cells. Agents Voronoi size allocation
is based on the agent’s capacity (heterogeneity of the
agents).
Moreover, Lumelsky and Harinarayan (Lumelsky
and Harinarayan, 1997) solve the problem using the
cock-tail party model. In this approach, agents navi-
gate to other agents they want to consult and share in-
formation, a similar approach to the publish-subscribe
technique. In (Gerkey and Mataric, 2002) and (Yan
et al., 2011), a similar approach was used, in which
agent was categorized as buyers and sellers or auc-
tioneers and bidders, respectively. Agents will be
broadcasting their knowledge and beliefs for other
agents to bid (buy) based on assigned protocols.
In this paper, we propose a priority-based publish-
subscribe approach for agents’ belief variation han-
dling. It uses a changing priority technique to
make decisions in order to tackle the problem in
dynamic environment. We also suggested Bayesian
and agents’ self-presumption inferential reasoning for
making predictions, learning, and adaption purposes.
We claim that our approach reduces communication
cost (as agents can make optimal predictions with-
out contacting other agents), can work in a dynamic
environment, and improve belief authentication and
agents adaptation. A potential challenge of this ap-
proach arises in belief fusion and communication fail-
ure management.
4 THE PROPOSE MODEL
Our model tackles the problem of agents’ belief vari-
ation due to sensor differences using priority-based
publish-subscribe, Bayesian inference, and learning
technique. It uses four steps:
Priority-base Publish-Subscribe Technique.
Bayesian Learning
Bayesian Inference, and
Agents self presumption Inference
The task for the agents is to understand differ-
ent knowledge, current context (situation-awareness),
and topics to subscribe to in order to have genuine
knowledge of the environment. Figure 1 describes the
structure of generating the priority value of the topics.
For example, in heterogeneous multi-agent missions
for forest fire searching, during day time, agents with
sensors not affected by sunlight or the nature of the
scene will have higher priority such as those carrying
cameras or heat sensors if the terrain has confusing
scene such as dried grasses with coloured palettes like
a fire. The agents with the camera will have a lower
priority. The priority increase with an increase in the
right decisions made and decrease with an increase in
false information (false data). If agent detect a scene,
it will listen to all its publisher’s interesting topics,
prioritize them, and make a decision by selecting the
one with the highest priority. Equivalence in priority
means both options are the same (have the same effect
on cost function utilization). It made the approach
to be adaptable for the agents. Bayesian Belief Net-
work could be constructed (without filling in the con-
ditional probabilities table) by a human expert e.g.,
using Netica (Romanycia, 2019) and put as an in-built
add-in for the agents’ memory. Simple if then rules
Conflicts Resolution and Situation Awareness in Heterogeneous Multi-agent Missions using Publish-subscribe Technique and Inferential
Reasoning
849
will be used in monitoring the agents’ behaviours dur-
ing the mission. For example, if an agent detects a
fire, it will update its BBN to take the effect. This al-
low an autonomous approach for filling in the BBN
conditional probability tables during the learning pro-
cess. Bayesian learning could be used in obtaining a
well-trained network for making predictions, estima-
tions, and conclusions in the absence of reliable top-
ics or communication. That is, if agents have no up-
dated topics from their publishers, that will be used to
verify their beliefs, they can use the network for pre-
dictions, estimations, and decision-making purposes.
We conducted and experiments using wildfire moni-
toring to monitor agents data on simulation platform.
From the results, we claimed that the network predic-
tion accuracy grows up with the increase in training
data for both expectation-maximization and conjugate
gradient descent algorithms used, as shown in figure
3, though we are still working on understanding the
nature of the growth of the prediction error perfection
and data utilization.
Figure 3: Prediction error rate reduces by the number of
samples.
Figure 3 describes the network training perfection
using 1, 10, 100, 1000, 10000, 100000, and 1000000
number of samples (mission data). From figure 3, us-
ing 1 sample data, the network gives prediction per-
fection fully (therefore should not be used). The pre-
diction error rate is the number of samples predicted
wrong (Romanycia, 2019).
Bayesian inference: agents could use the network
to predict less important matter or in the case where
communication within the system is limited, or no up-
dated topics. We claimed that for a static environ-
ment mission, the network could make good predic-
tions that are as nice as received information from the
publisher (from our Aerospace Multi-agent Simula-
tion Environment results and netica). In the case of a
dynamic system, the agents use the time-base learn-
ing algorithm such a context-based gradient descent
algorithm (Bottou, 2010; Romanycia, 2019). It con-
siders recent cases with higher priority, as such made
it to handle environment dynamism.
Agents’ self presumption inference is the learned
or in-built set of knowledge (in form of if-then rules)
for agents’ reference and decision making. In order
word it is a tuple P:
P = {A, C, β, α} (1)
Where A is the set of agents, C is the set of condi-
tions or logical proportions, β is the function for map-
ping conditions to actions, and α is the corresponding
actions. Agents update their self presumption on time
bases and learned new rules based on the environmen-
tal interaction and sensor data. Therefore, the agents’
option starts from topic information, inferred value,
and agents’ self presumption. Human expert and sen-
sor information will be responsible for updating the
rules of the system.
5 CONCLUSIONS AND FUTURE
WORK
We propose a multi-agent belief variation handling
architecture using four main steps, priority-based
publish-subscribe, Bayesian learning, Bayesian infer-
ence, and agents’ self-presumption inference based on
in-built or learned parameter. We claimed that (from
our experiment) this approach reduces the communi-
cation cost, system failure, and improve adaptability
based on environmental changes. Because the learned
network could be used to make an optimal predic-
tions, estimations, and conclusions on current and fu-
ture events during the agents mission. This could
remove too much communication and computation
process in other call-and-confirm approaches such as
in (Gerkey and Mataric, 2002; Merino et al., 2010;
Merino et al., 2006; Yan et al., 2011) etc. The learn-
ing process also handles environment dynamism by
prioritising recent cases in learning process over older
cases and instant update of the BBN policies.
Future work focused on investigating the optimal
number of samples for BBN training and nature of
growing of the prediction accuracy of the learned net-
work. Information fusion management also remain
a challenge for our proposed architecture which is
to be solved later. Other challenges are incomplete
information due to communication failure or insuf-
ficient resources, communication protocol manage-
ments, etc.
We are also intended to innovate a way of solving
redundant belief verification. That is, agents to avoid
double belief authentication and deadlock occurrence
solution more especially in a dynamic and uncertain
situation. Finally, we will look at the effective tech-
nique for making decision more especially in a dy-
namic and uncertain situation. This aspect refers to
ICAART 2020 - 12th International Conference on Agents and Artificial Intelligence
850
effective application of game theory in agents belief
verification.
ACKNOWLEDGEMENTS
We appreciate any comments that help in making this
paper a great one. Our thanks also goes to Petroleum
Technology Development Fund (PTDF) of Nigeria
for sponsoring this project.
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Conflicts Resolution and Situation Awareness in Heterogeneous Multi-agent Missions using Publish-subscribe Technique and Inferential
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