Enabling Agents to Dynamically Select Protocols for Interactions

0.13 MB·English

by  Jose Ghislain Quenum Samir Aknine

Enabling Agents to Dynamically Select Protocols for Interactions
Jose´ GhislainQuenum,SamirAknine
Laboratoired’InformatiquedeParis6
75015Paris,France
{jose.quenum, samir.aknine}@lip6.fr
5
0
0
2 Abstract task.Moreover,thesetofprotocolsusedinmulti-agentin-

n teractions is increasingly enlarging, and a static proto-
a The work achieved in multi-agent interactions design col selection will only consider the protocolsthe designer
J
mostly relates to protocols definition, specification, etc. In knows even if these agents have the capacity of inter-
5
2 thispaperwetackleanewproblem,thedynamicselection preting and executing other protocols unknown at de-
of interaction protocols. Generally the protocols and the sign time. To overcome this limitation, we should enable

] rolesagentsplayinprotocolbasedinteractionsareimposed agents to dynamically select protocols in order to inter-
A
uponthesystematdesigntime.Thisselectionmodeseverely act.
M
limitsthesystem’sopenness,thedynamicbehavioursagents As yet, there have been some efforts [1, 4] to enable
. canexhibit,theintegrationofnewprotocols,etc.Toaddress agentstodynamicallyselecttherolestheyplayduringinter-
s
c thisissue,we developedamethodwhichenablesagentsto actionsusingMarkovDecisionProcesses,planningoreven
[ select protocols themselves at runtime when they need to probabilisticapproaches.However,theydon’tsuitprotocol

interact with one another. Regardingthe conditionswhich based coordination mechanisms. Indeed, as protocols are
2
v holdintheMASagentscaneitherjointlyperformtheselec- partially sorted sequences of pre-formatted messages ex-
6 tionorindividually.Wedefinetheconceptsandalgorithms change,selectingthemtoexecutea taskrequiresthattheir
3 whichenableagentstoperformthisdynamicselection.We descriptionsmatchthatofthecollaborativetask.Thesolu-
0
alsodescribethemechanismswhichhelpagentsanticipate tions proposed so far do not explicitly focus on protocols
1
interactioninconsistencies. and do not check such compliance either. To address this
0
void, we developeda method which enables agents to dy-
5
0 namicallyselectprotocolsandrolesinordertointeract.Our
/ methodputstheusualassumptionsaboutmulti-agentinter-
s
c actionsastepfurther.First,weconsiderthatsomeinterac-
1 Introduction
v: tion protocols can be known only at runtime. Thus, start-
i ingfroma minimalversion,agentsinteractionmodelscan
X Generally, the interaction protocols which support agents
grow up by integrating these protocols from safe and au-
r collaborative tasks’ execution are imposed upon multi- thenticated libraries of interaction protocols when needed.
a
agentsystems (MAS) at design time. This static protocols
Second,weconsiderthatagentsmayhavedifferentdesign-
selection severely limits the system’s openness, the dy-
ers,thereforetheymayencompassdifferentprotocolsspec-
namic behaviours agents can exhibit, the integration of
ification formalisms. Furthermore, an interaction protocol
new protocols, etc. For example, consider a collabora- isatriple{R,M,Ω}whereRisasetofinteractingroles
tive task which can be executedfollowing varied methods
whichcanbeoftwotypes:initiatorandparticipant.Anini-
either by means of a Request protocol [5] (an identi-
tiatorroleistheuniqueroleinchargeofstartingtheproto-
fied agent exhibiting specific skills is requested to per-
colwhereasaparticipantroleisanyroletakingpartinthe
form the task) or a Contract Net protocol (CNP) [6] (a
protocol.Consequently,aninitiatoragentwillbeanyagent
competition holds between some identified agents in or-
playing the initiator role in an interaction while a partici-
der to find out the best one to perform the task). Since
pantagentwillbeanyagenttakingupaparticipantrole.In
the MAS is open and CNP looks for the best contrac- addition,protocolsusedinMAScanbeclassified1 inthree
tor, it will undoubtedly be preferred to Request for such
categories:(1) 1-1 protocols, which are protocolsmade of
a task in absence of constraints such as execution de-
lay.Thus,selectingRequestatdesign-timepreventstheini-
tiator agent from benefiting a better processing of this 1 Acomplexprotocolcanbeacombinationofthesebasiccategories.
2 Problem Description
two roles (initiator and participant) both of them having
only one instance (ex Request); (2) 1-1N protocols, again
protocols made of two roles with several instances of the Our purpose in this research, is to ease protocols defini-
participant(exCNP);(3)1-Nprotocols,whichareprotocols tion,implementationanduse.Thus,weclaimtofreeagents
with several distinct participantroles each of them having programmersfromhardcodingtheprotocolsandtheexact
only one instance (ex an auction protocol with one buyer, roles to use every time their agents have to execute a col-
one seller and one manager). Subsequently, we define the laborativetask.Rather,theyshouldonlymentionintheini-
dynamicprotocolsselectionprocessforeachofthesecate- tiator agent’ssource codethe descriptionof the collabora-
gories. tivetasktoexecute.Then,onceanagentcomesacrosssuch
Rather than explicitly indicating the protocols and the a description, it will launch the dynamic selection process
roles to use for all the agents which will execute the de- implicating potential participant agents. Concretely, given
siredinteraction,wesuggestthatagentsprogrammerssim- a collaborative task tj which is to be executed by a set
ply mention the collaborativetask’s description in the ini- A = {a1,a2,...ak} of agents, the selection problem is
tiatoragent’ssourcecode.Assoonasanagentlocatessuch statedas howanagentcanselectaprotocolandarolein-
adescription,itidentifiessomepotentialparticipantagents sidethisprotocoltoexecutetj? Itconsistsinfindingouta
andthereafterfiresthe dynamicprotocolselection process protocolpandtherolesr1,r2,...rp eachai shouldbeen-
takinguptheinitiatorrole.WeassumethattheMASispro- acting in this protocol in order to get tj executed. We as-
videdwithpotentialparticipantagentsidentificationproce- sumethateachagentisprovidedwithaninteractionmodel
dures.Agentscandynamicallyselectprotocolsintwopos- I = {r1,r2,...rn} containingsomeconfiguredprotocols
sible ways. First, the initiator agent and all the potential (p1...pm)someofwhichcanbeintroducedatruntimeand
participantagentscollectivelyselectaprotocolandassign foreachprotocolpi asetofroles{r1,r2,...rk}thisagent
rolestoeachagentinsidethisprotocol.Thisisthejointpro- canplayduringinteractionsbasedonpi.
tocolselectionmethodwhichassumesthatagentstrustone Inthefollowingtwosections,weelaborateonoursolu-
another and that they don’t dread publishing their knowl- tiontotheselectionproblem.
edgeandpreferences.Ontheotherhand,agentscanindivid-
uallyselectprotocolsandrolesandstartthedesiredinterac-
3 Joint Protocol Selection
tion.Thisistheindividualprotocolselectionmethodwhich
assumesthatagentsdonottrustoneanotherand/orthesys-
Onceanagentlocatesthedescriptionofacollaborativetask,
tem is heterogeneous (several sub systems with different
it finds out a set of protocols needed to execute this task
protocolformalismsarepluggedtogether).Inthismethod,
which it refines to a sub set of protocols whose initiator
as the selected roles may mismatch, agents should antici-
roles are configured inside its interaction model. Moving
pate errors in order to guarantee consistent messages ex-
from collaborative tasks models to protocols’ requires the
change.Wefocusonwrongmessagestructureerrorwhich
agentstoanalysebothmodelsanddetecttheiradequacy.In
indicatesthatsomethingiswrongin themessagestructure
thispaperweassumethatagentsareabletoexaminetasks
(performative,content,language,ontology,etc.)andwrong
and protocols models and relate the first ones to the sec-
message content error which indicates that the message’s
ond ones. After moving from task to protocols the initia-
contentdoesn’tmatchtheexpectedcontentpattern.Wear-
toragentshouldidentifyallthepotentialparticipantagents
guethatourmethodintroducesmoreflexibilityinprotocols
forthedeterminedprotocols.Bothstepsprovidetheinitia-
execution,fosters agents autonomy,favours their dynamic
toragentwithasparsematrix:potentialparticipantslinked
behavioursandsuitsMAS’ openness.Inthispaperwe de-
toprotocols.Thesepotentialparticipantagentsarethuscon-
scribe both methods and detail their principles, concepts
tacted whether at the same time or one after the other and
and algorithms. We exemplify them towards a web docu-
arerequiredtovalidateaprotocol.Tocontrastthemessages
ments filtering MAS composed of (1) query agents repre-
exchanged during the joint selection and those exchanged
senting the queries users formulate, (2) document agents
during normal interactions, we proposed some performa-
representingthedocumentsretrievedfromtheweb,(3)and
tiveswhichweinformallydescribeherebellow:
ruleagentscorrespondingto anylinguisticruleinvokedto
compute documents attributes (author(s), content, call-for-collaboration thesenderofthisperfor-
language,etc.). mativeinvitesthereceivertotakepartinaprotocolde-
Thepaperisorganisedasfollows.Section2formallyde- scribedinthecontentfield.
finestheprotocolsselectionproblem.Sections3and4de-
unable-to-select thesenderofthisperformativein-
tailourmethods.Section5discussessomerelatedworkand
forms the receiver that it cannot play a participant
section6drawssomeconclusions.
role in the related protocol.The in-reply-to and
reply-withfieldshelprelatethismessagetoaprior
call-for-collaboration.Thereasonswhyan wherea isoneofthepotentialparticipantagentsformerly
i
agentmayreplythisperformative,thoughidentifiedas identifiedandp oneofthe1-1protocolsdeterminedforthe
j
a potential participantfor the protocol,are (1) its au- currenttask. In the midstof the solution search is the ma-
tonomysinceitmaynotwanttoexecutethisprotocol trix’s exploration. Hence, it behoves the initiator agent to
atthismomentand(2)someerrorsinsomefields. explorethematrixtraversingprotocolsorpotentialpartici-
pantagents.Intheprotocol-orientedexploration,theinitia-
stop-selection the sender of this performative asks
torselectsa protocolanditeratesthroughthesetofagents
thereceivertostoptheselectionprocessthismessage
whichitidentifiedforthisprotocolandretainsonethatfits
islinkedto.
the protocol. As soon as an agent is determinedthe selec-
ready-to-select the sender of this performativeno-
tion process successfully completes. Otherwise, the itera-
tifiesthereceiveroftheparticipantrolesitcanbeen-
tionproceedsuntilthereisnomoreprotocoltoselect.Anal-
acting regarding the protocol description it received.
ogously, in the agent-oriented exploration the initiator se-
All the participant roles in any protocol compatible
lectsanagentanddelvesintoitsprotocols’setlookingfor
with the current one can be listed. This grouping not
onetheycanexecutetogether.Whateverexplorationtheini-
onlyrevealsbyorderofpreferencetherolesthesender
tiatoradopts,itshouldovercomethematrixsparsitybyse-
commits in playing but it also avoid going back and
lectingasnextelement(protocoloragent)theoneholding
forth about protocols sharing the same background.
theleastsparsevector.
Roles of protocols are compatible when they can ex-
Consider, by way of illustration, a query agent q1
ecutesafeinteractionsalbeitthedifferenceintheirre-
which is requested to execute a task t1: “find out a doc-
spectivespecifications.Asanexampletheinitiatorrole
ument exhibiting the following characteristics: (lan-
ofCNPcaninteractwitheithertheparticipantroleof
guage=’English’,content=’plain/text’)”.Protocolsandpo-
CNP or that of Iterated CNP (ICNP [5]). While the
tential participant agents identifications for t1 lead to the
initiatorofICNPcan’tinteractwiththeparticipantof
matrix given in table 1 where a cross in a cell [l,c] in-
CNPbecauseoftheprobableiterations.
dicates that the agent at column c can play a partici-
notify-assignment the sender of this performative pant role in the protocol at line l. This matrix reveals
informsthereceiverabouttherolethelatter hasbeen that q1 has identified IPS (an Incremental Problem Solv-
assigned to in the jointly selected protocol. The as- ing protocol where a problem submitter -initiator- and
signed role is one among those the receiver priorly its solver -the participant- progressively find out a so-
committedinplaying. lution to a given problem) and Request. A diagram-
matic representation of IPS is given in figure 1. In addi-
Whatever protocol category the selection is concerned
tion, q1 identified seven potential participant document
with,wecandescribethejointselectionmessagesexchange
agents d1 ... d7. q1 adopts a protocol-oriented explo-
sequenceasfollows:
1. theinitiatoragentsendsacall-for-collaboration
d1 d2 d3 d4 d5 d6 d7
encapsulatingaprotocol’sdescription.
IPS x x x x x
2. Each participant agent can reply with an Request x x x x
unable-to-select driving the initiator agent to
stop the selection process between both agents by Table1.matrixfortaskt1
sendingastop-selection.
3. Each participant agent can also reply with a
ready-to-select. In this case, the initia- ration and selects IPS (the least sparse vector). There-
toragentparsestheparticipant’sproposalsandadopts fore, it will try to validate IPS with d1, d2, d4, d5
oneofthemsendinganotify-assignmentorre- or d7. If q1 receives a ready-to-select in re-
jectalltheproposalssendingastop-selection. ply to a priorcall-for-collaboration,it explores
thelistofpreferredrolesandassoonasitfindsthepartici-
Intheremainderofthissectionwedetailthejointselec-
pantroleofIPSorRequestitnotifiesitsagreementwith a
tionmethodforeachclassofprotocol.
notify-assignment.If q1 identified more than these
protocolsandaroleofanyofthemispointedoutinthepar-
3.1 InsidetheJoint ProtocolSelection ticipant’s ready-to-select, this protocol will be
adopted. In the case it didn’t find out any role it ex-
3.1.1 1-1Protocols
pects or it received an unable-to-select, q1 replies
In the dynamic1-1 protocols selection, the aim of the ini- withastop-selectionandaslongastherearestillun-
tiatoragentistoearlyfindoutasolution,acouple(ai,pj) explored potential participantagents for IPS, q1 will con-
adopts its correspondinge as the participantagents’
Problem Problem p
Submitter Solver set. The solution is then e and the relevant protocol
p
declare problem r belongsto.
p
refuse to solve
2. Otherwise:
accept to solve
(a) For each e , we compute the difference
ı
with the union of all the sets except e :
ı
ask about processing
difı =(eı−[{e},j 6=ı). Then, we se-
respond

lectthelargestdif .
ı
stop processing
(b) If nodecisioncouldbemade,we saveallthe e
ı
(only for the highest index) and proceed on our
add information iteration.
Afterasolutionhasbeenfoundforanindexk,wecheck
Figure1.IncrementalProblemSolvingProto- whethersomesets havenotbeensavedfora higherindex.
col If no sets were foundthe final solution is the one at hand.
Otherwise,weselectfromthelattersthesetwhoseintersec-
tionwiththecurrentlyselectedsetisthelargest.Ifnosolu-
tinuecontactingthem.Inabsenceofsolutionwhenthepo- tionhasbeenfoundweiteratethroughtheselectionprocess
tentialparticipants’sethasbeenthoroughlyexploredfora changingprotocols.
protocol,thesameprocessistakenagainuponanotherpro-
tocolifthereisany.Incasenosolutionhasbeenfoundand 3.1.3 1-NProtocols
no more protocoland participantcan be explored, the dy-
A solution to the dynamic 1-N Protocols selection prob-
namic protocol selection fails and the subsequent task
lem is a triple (A,p ,m) where A is the set of par-
remainsnotexecuted. j
ticipant agents, p the protocol to use and m an as-
j
sociative array mapping each agent to the role(s) it
3.1.2 1-1N Protocols
will play in the protocol. Here again, the matrix is ex-
A solution to the dynamic 1-1N Protocols selection prob- plored only in a protocol-oriented way. The initiator
agent a waits for all the participants’ replies and gath-
lem is a couple (A,p ) where A is the set of participant ı
j
erstheready-to-selectmessages.Therolesareclus-
agentsandp theprotocoltouse.Forthiscategoryofpro-
j
tered following the protocols they belong to and the
tocols the matrix is explored only in a protocol-oriented
protocols which have not been identified by the initia-
way since all the identified agents for a protocol are con-
tor agent are eliminated. For each protocol p, a maps
tactedatthesametime.Onceallthecontactedagentshave ı
each role r to a set of agents which candidated for it:
replied,theinitiatoragentshouldselectacommonprotocol 
for the agents (generallyfor a partof them) which replied candidates(r)=[{ak}. If candidates(r)=∅, the
a ready-to-select. We devised several strategies to k
protocolr belongstoisnomoreconsideredintheselection
perform this selection but here we only describe one, the 
process. Moreover,as there exists several participantroles
largestset’sstrategy,whichlooksfortherole mostagents
in1-Nprotocols,someofthemmayreceivetheirfirstmes-
selected.Ifthereexistsaroler thatallthe agentspointed
i
sagefromotherparticipantroles.Thus,weintroduceanew
out,thenthisoneisselectedforalltheagentswhichreplied
aready-to-select.Otherwise,welookforarolethat relation,father:giventworolesr1andr2ofa1-Nprotocol,
will involve the largest set of agents. Therefore, we con- r1 =father(r2)|=r1isthesenderof ther2′sfirstmessage.
For each protocol retained after the candidates sets con-
structan array where each indexı containsa collectionof
struction, the initiator agent constructs a tree t wherein
all the roles which exactly ı agents candidated for. As we
nodes are the roles of the protocol. A node r is child
didn’tsucceedinfindingoutaroleforallthenagentswhich m
of another node r if r = father(r ). t is traversed in a
sentaready-to-select,thehighestindexinthisarray n n m
breadth-firstwayandforeachnoder oftanagenta isas-
pointstoacollectionofrolesthatexactlyn−1agentscan-  
signed to r from candidates(r ). Assigning a role to an
didated for. We traverse the array from the highest index  
agent can be performedby any well known resource allo-
downtothelowest.Whileexploringindexk,ifthecollec-
cationalgorithm(exelection).Thisassignmentisachieved
tionofrolesisnotempty,werepresentforeachroler the
ı
forallthetreesandtheinitiatoragentusesastrategytose-
sete ofagentswhichcandidatedforit.
ı
lectoneofthetotallyassignedtrees.Animprovementdur-
1. Ifallthee areequal,thenwerandomlyselectar and ing the roles assignment is to avoid situations where the
i p
same agent plays several roles in a protocol. Thus, when each participant agent a constructs the collection of can-

candidates is a singleton, its only one agent is re- didate roles r which we refer to in the remainder of this

moved from all other candidates sets it appears in section as collection(a ,t ). The roles are then selected
 k
when these are not singletons. As well, while explor- from collection(a ,t ) and instantiated so that the inter-
 k
ingt,oncearolehasbeenassignedtoanagentwe should actioncantakeplace.Theindividualprotocolselection,al-
remove this agent from all the candidates sets it ap- thoughmoresophisticatedandpowerful,can leadtointer-
pears in provided these are not singletons. The singleton actioninconsistencies.Indeed,asindividuallyselectedroles
criterionmayguideatreeselectionstrategy. may mismatch, the exchangedmessages’ contentor struc-
ture(performative,ontology,language,etc.)maybewrong.
Thus,weprovideagentswithtechniquestoanticipatesuch
3.2 Beyondthe jointprotocol selection
errors by checking incoming messages over structure and
contentcompliance.Whencollection(a ,t )isasingleton,
Thejointprotocolselectionmechanismdoesnotapplytoall  k
the only onerole is instantiated in orderto interact. If any
interactioncontexts.One evidentissue is that genericpro-
erroroccursduringtheinteractionnorecoverywouldhave
tocols are thought to be invariably specified in the MAS.
beenpossible.Dynamicallyselectingprotocolsismoreap-
However, the only one aspect that remains invariable in
pealingwhenthe collection(a ,t ) containsseveralroles.
generic protocols’ specification is the description of ex-  k
In this case we explore collection(a ,t ) either sequen-
changed messages which is imposed by communication  k
tiallyorinparallel.Inthispaper,weonlydescribeindivid-
languages(KQML,FIPAACL)andembeddedintheproto-
ual1-1protocolsselectionsincetheselectionmechanismis
cols’specification.Hence,thereisnoguaranteeforgeneric
quite similar for the other two typesof protocolsand only
protocolstobespecifiedinauniqueformalismandagents
some extensions are required to fit the specificity of these
mightfailtointerpretsomeoftheformalismsusedtospec-
protocols.
ifygenericprotocolsintheMAS.Particularly,plugginghet-
erogeneous sub-systems together in a MAS increases the
riskformultiplegenericprotocolsspecificationformalisms. 4.1 Sequential Roles Instantiation
ThejointprotocolselectionthenfallsshortinaMASwhere
Inthepurposeofstartinganinteractionorreplacingafail-
generic protocols are specified in several formalisms and
ingroleduringaninteraction,aparticipantagentrandomly
agents are unable to interpret all those formalisms. In ad-
(or using another strategy we’ll define later) selects roles
dition,therearealsosituationswhereagentsdonotalways
fromthecollectiononeaftertheotheruntilthereisnoavail-
trustoneanother.Then,basingprotocolsselectiononspec-
able roleto select orthe interactioneventuallysafelyends
ificationsexchangebecomesunsafe.
up. Once selected, roles are removed from the collection
Toaddressthesedrawbacks,wedevelopedanindividual
in order to avoid selecting them anew during the same in-
protocolselectionmethod.
teraction. When a message is wrong the participant agent
must recover from this error by replacing the failing role.
Therecoveryprocessliesontheinteraction’sjournalwhere
4 Individual Protocol Selection
agentslogtheexecutedmethodsandtherelatedevents(in-
put:eventswhichfiredthemethod,andoutput:eventsgen-
This selection form is carried out concomitantly to the
erated by the method’s execution). Each method and its
targeted interaction. Likewise the joint protocol selection
events form a record. The following four steps define the
form, the initiator agent is in charge of starting the selec-
errorrecoveryprocess:
tion processwhen it locates a collaborativetask’s descrip-
tions. It finds out some protocols which comply with the 1. Ifanagentdetectsanerror,itnotifiesitsinterlocutor;
task’sdescriptionsandwhereinitcanplayaninitiatorrole. 2. a then purges collection(a ,t ) and selects another
  k
Theinitiatoragentmayadoptastaticbehaviourduringthis role;
selection by choosing a protocolamong the candidates. in
3. a computes the point where the interaction should

thiscase,thestrategyitadoptsisrequiredtobefair.Itmay
continueatin the newlyselected roleandnotifiesthe
alsobegiventhepossibilitytoexhibitdynamicbehaviours
initiator;
bychangingprotocolsinordertoaddressoccurringincon-
4. Bothagentsupdatetheirjournalsbyerasingthewrong
sistencies.Inthispaperweconsiderthefirstcase.
recordsandtheinteractionproceeds.
The initiator agent sends the initial message m0 of the
selected protocol p to one or several potential partici- Theparticipantrolereplacementduringerrorrecoverycan
ı
pant agents. m0 actually denotes a need for a new in- requiretheinitiatoragenttorollsomeactionsbackinorder
teraction and any agent which receives it selects a par- to synchronise with the newly instantiated role. To purge
ticipant role r which starts with m0’s reception. Hence, collection(a,tk),a:
1. Removes from collection(a ,t ) all the roles whose ror occurred at. However, doing so can bring inconsisten-
 k
description,fromthebeginningoftheroletothepoint cies in the interaction execution because the roles, though
theerroroccursat,doesnotmatchthejournal; enactedbythesameagent,donotnecessarilyusethesame
methods.These inconsistenciescould be avoided by look-
2. If the message structure is wrong and the error has
ingformethodsofthenewrolewhichfollowthesamese-
been detected by the initiator agent, removes from
quenceorderasinthejournalfromthestartingpoint.This
collection(a ,t ) the roles that generated the wrong
 k
setofmethodswon’tbere-executed.Werepresentthemeth-
message.If theerrorhasbeendetectedbya itself, it

odsofthenewroleasnodesofadirectedgraphwhereinan
removesfromcollection(a ,t )therolesthatcan’tre-
 k
edgem m meansthatmethodm can be executedimme-
ceivetheclaimederroneousmessage; i j j
diately after m completes and the conditions for its exe-
i
3. If the message content is wrong and if the er-
cutionhold.Algorithm1achievesthiscomputationandre-
ror was detected by the initiator agent, removes
turnstherecoverypointsforboththe initiatorandthepar-
from collection(a ,t ) the roles that cannot gener-
 k ticipant.Inthisalgorithm,ıisthenumberofthelatestmes-
ate the same message structure at the point the er-
ror occurs at and removes from collection(a ,t )
 k Algorithm1recoverypointscomputation
the roles that use the same method as the one caus-
Input:Journal≡theparticipant’sjournal
ing the error. If the error was detected by the par- Input:Graph≡thenewrole’smethodsgraph
ticipant agent, removes from collection(a ,t ) the Result:Initiatorandparticipantrecoverypoints
 k stop←false;
roles that do not receive the same message struc- ı←1;
ture at the point the error occurred at and also re- ←1;
mthjournal←readinit(journal);
moves from collection(a,tk) the roles that do not mthgraph←getinitnode(graph);
receive the same message content at the point the er- ifmthjournal6=mthgraphthen
stop←true;
roroccurredat endif
whilestop=falsedo
Sinceit’snousecheckingthecontentwhenthemessage mthjournal←Succ(mthjournal);
ifmthjournal=nullthen
structureiswrong,structurecomplianceischeckedpriorto
stop←true;
content’s. When a role does not comply with the current elseifmthjournal∈follow(mthgraph)then
mthgraph←mthjournal;
execution,itis removedfromcollection(a,tk). Rolesre- ←+1;
movalactuallyconsistsinmarkingthemsothattheycanno ifInputisMessagethen
ı←ı+1;
morebe instantiated in the currentinteraction.Then,from
endif
theupdatedcollection(a ,t ),a selectsanewrolefollow- else
 k 
stop←true;
ingthestrategydescribedhereby:
endif
endwhile
1. If the message content is wrong: (a) for each role of initiatorrecoverypoint:ı;
participantrecoverypoint:;
collection(a ,t ),constructthesetofmessages(gen-
 k
erated or received) at the point the error occurred at;
(b)withdrawfromthese sets the messagethatcaused sagetheinitiatorshouldbeconsideringithassenttothepar-
the error; (c) compare these sub sets and select the ticipantandthepointwheretheparticipantistostartexe-
weakest one; Then the role the selected sub set orig- cutingitsnewrolefrom.Thethirdeventtypeconsideredin
inatesfromisinstantiated.Theweakestmessagessub thejournal(datavaluechangeinadditiontomessageemis-
set is the one containing the higher number of weak sion and reception) accounts for the difference between ı
messages.Weakmessagesarethosewhichleadtoin- and.Oncetheinitiatorreceivesı,itlooksfortherecordin
teraction termination; these messages are potentially itsjournalrepresentingtheıthmessageitsenttothepartic-
weakerthanthosewhichcontinuetheinteraction.The ipant.Alltherecordsfollowingthisonewillbeerasedfrom
reason why we prefer the weakest messages sub set thejournal.Theparticipantalsoupdatesitsjournalinquite
is that we wish to avoid producing another message thesamewaybasingon.
than the “structurally” correct one we generated pri- Suppose d1 wants to identify the language its docu-
orly.Whenthereareseveralsubsetscandidateforse- mentiswrittenin;thistask(t2)requiresaninteractionwith
lectionorwhentherearenone,aroleisrandomlyse- a rule agent. We assume d1 finds out a protocol which
lected. startswith anask-oneemission andexpectsa tell. Con-
siderd1contactedaruleagentc1whoseinteractionmodel
2. If the message structure is wrong: randomly select a
is partially depicted in figure 2. In this figure, for exam-
roleinthecollection(a ,t ).
 k ple the given portion of r1 can be interpreted as: r1 re-
Onceanewrolehasbeenselected,theparticipantagent ceivesanask-oneandcanreplyaninsertorasorry.
mightexpecttocontinueitsexecutionfromthepointtheer- collection(c1,t2)={r1,r2,r3,r4}.
1. Alltheactivatedrolesarestopped;
2. Ifthemessagestructureiswrong:allthem aswellas
k
the messages having the same structure are removed
from C and their roles are stopped. The participant
z
selectsanothermessagemk′ followingthesameprin-
cipleasm ’sselection.
Figure2.agentc’sinteractionmodel k
3. Ifthemessagecontentiswrong:allthem messages
k
as well as those having the same content pattern are
• Whateverrolec1 selects,ifitrepliesasorrytheinter- removedfromC andtheirrolesarestopped.Thepar-
z
actionwillendup,maybeprematurely-thefirstmes- ticipantselectsanothermessagemk′ havingthesame
sage has not been validated yet. To get sure it’s not
structurebutadifferentcontentpattern.
so d1 issues a warning: “May be premature interac-
Whensomerolesstayedactivated,theyallgeneratetheir
tion termination!”.c1 tries to select anotherrole and
messages.Ifthemessageshavethesamestructureandcon-
theinteractionproceedsordefinitelystops.
tent, all these roles remain activated. Otherwise, only one
• Ifc1selectedr4andsentatellwhosecontentiswrong message is selected and all the roles whose message have
d1 notifies c1a wrong message contenterror. c1 then not been selected are deactivated. If all the previously ac-
stopsr4,purgesitscollection(a,tk)byremovingr1 tivatedroleshavebeenstopped,theparticipantagentreac-
(sinceitcannotgenerateatellattheerrorlocation), tivates the most recently deactivated role but cares about
selectsr3becauseitcorrespondstotheweakestsubset earlyinteractiontermination.Whentherearemorethanone
andcomputestherecoverypoints:ı =  = Rec♯1.c1 suchroles,theyallarereactivated.Theparticipantrolereac-
updatesitsjournalandrepliesanewtotheask-one. tivation,mightrequiretheinitiatortorollsomeactionsback
toarecoverypoint.Analgorithmsimilartoalgorithm1per-
In order to avoid inconsistencies at the end of interac-
formstherecoverypointscomputationforboththeinitiator
tions,werequirebothagentstoexplicitlynotifyeachother
andparticipant.Foreachrolethealgorithmisapplied,con-
theprotocol’stermination.Insteadofselectingrolesonthe
sideringtheroles’currentexecution,andthefinalrecovery
basisofmessagestheycangenerate,itsoundstoselectthem
pointistheearliest.
onthebasisofmessagestheyreallygenerated.Thisispos-
sibleonlyifallcandidatesrolesareinstantiatedatthesame
time. This parallel roles instantiation is only known from 5 Related Work
theparticipantagent;theinitiatoragentstillhasthepercep-
tionofasequentialrolesinstantiation.
Protocols selection in agents interactions design is some-
thing generally done at design time. Indeed, most of
4.2 MixedRolesInstantiation the agent-oriented design methodologies (Gaia [7] and
MaSE[3]toquoteafew)allmakedesignersdecidewhich
Alltherolesa identifiedincollection(a ,t )areinstanti- role agents should play for each single interaction. How-
  k
atedatthesametime.Theyhandlethereceivedmessageand ever dynamic behaviours and openness in MAS demand
generatetheirreplymessageswhicharestoredinacontrol greaterflexibility.
zone(C );C alsocontainthemessageswhicharedestined To date, there have been some efforts to overcome this
z z
tocurrentlyactivatedroles.Therolesarethendeactivated. limitation. [4] introduces more flexibility in agents’ coor-
Only onemessage m is selected from C and sent to the dinationbutitonlyappliesto planningmechanismsofthe
k z
initiator.Thisselectioncanbeperformedfollowingseveral individualagents. [2] also proposesa frameworkbasedon
strategies.Forexample,theparticipantagentcanrandomly multi-agentMarkovdecisionprocesses.Ratherthanidenti-
selectamessageamongthosewhichdon’tshortentheinter- fyinga coordinationmechanismwhichsuits best fora sit-
action.Therefore,ifaninsert,asorry,anerrorand uation, this work deals with optimal reasoning within the
a tell are generated in reply to an ask-one, insert contextofagivencoordinationmechanism.[1]proposeda
andtellwillbepreferredtosorryanderror,andthe frameworkthatenablesautonomousagentstodynamically
random selection will be performed between the first two select the mechanism they employ in order to coordinate
messages. After a message m has been selected, all the their inter-related activities. Using this framework, agents
k
roleswhichgeneratedamessageofthesamestructureand select their coordination mechanisms reasoning about the
content are activated. In this instantiation mode, when an rewardstheycanobtainfromcollaborativetasksexecution
erroroccurs,theparticipantagentrecoversfromitbystop- aswellastheprobabilityforthesetaskstosucceed.
pingthewrongrolesandbyreactivatingoneorseveralother Themainrequirementtheselectionprocessfacesinpro-
roles.Thus,ifanerroroccursonm : tocol based coordination mechanisms is whether or not
k
thereexistsintheagent’sinteractionmodelrolescapableof gain,ouraimwastodescribethetheoreticalbasisofady-
supportingthedesiredinteraction.Tofillthisvoid,wepro- namicprotocolsselectionmethod.Our methodintensively
posedamethodtoenableagentstodynamicallyselectpro- benefitsfromtheagents’capacitytointerpret,relateandup-
tocolsbasingontheirinteractioncapacities. datemodelsembeddedinsidethem.
6 Conclusion References
Designing agents for open and dynamic environments is [1] R. Bourne, C. B. Excelente-Toledo, and N. R. Jen-
still a challenging task, especially in regard to protocol nings. Run-timeselectionofcoordinationmechanisms
based interactions. Two main concerns arise from interac- in multi-agent systems. In Proceedings of the 14th
tionsmodellinganddesignin such systems. First, howin- European Conference on Artificial Intelligence, pages
teractionswhicharebasedongenericprotocolsareconfig- 348–352,Berlin,Germany,August2000.
uredso thatconsistentmessagesexchangecan takeplace?
[2] C.Boutilier. Sequentialoptimalityandcoordinationin
Second, does it sound that designers always decide which
multiagentsystems. InProceedinsoftheSixteenthIn-
protocolsandrolestouseeverytimeaninteractionisasked
ternational Joint Conference on Artificial Intelligence
for?Weaddressbothissuesbydevelopingseveralmethods.
(IJCAI-99),pages478–485,1999.
In this paper we focus on the second concern. We argued
[3] S. DeloachandM. Wood. Anoverviewofthemultia-
that due to openness and dynamic behaviours more flexi-
gentsystemsengineeringmethodology. InP.Ciancar-
bility is needed in protocolsselection. Furthermore,in the
iniandM.Wooldridge,editors,Proceedingsofthe1st
contextofcomplexapplicationsdemandingmulti-protocols
InterationalWorkshoponAgentOrientedSoftwareEn-
agents, moving from static to dynamic protocol selection
gineering,volume1957.SpringerVerlag,June2000.
greatlyincreasessuchsystems’efficiencyandproperlyhan-
dles the situation tightly related to openness where all the [4] E. H. Durfee. Practically coordinating. AI Magazine,
protocolsare notknown at design time. Thus, we enabled 20(1):99–116,1999.
agentsto dynamicallyselect protocolsuponthe prevailing
[5] FIPA. Fipa interaction protocol library specification.
circumstances.
Technical report, Foundation for Intelligent Physical
One outcome of the dynamic protocol selection is that
Agents,2001.
the protocols to use are no more hard-coded in all agents
[6] SmithR.G. Thecontractnetprotocol:High-levelcom-
source code. Rather, programmers mention collaborative
municationandcontrolinadistributedproblemsolver.
tasksdescriptionsin the initiatoragent’ssourcecodeonly
IEEETrans.OnComputers,29(12):1104–1113,1980.
makingthelatterinchargeoffiringtheinteraction.Agents
are given two ways to select protocols. First, the initiator [7] M. Wooldridge, N. Jennings, and D. Kinny. The gaia
agent and all (or a part of them) the potential participant methodology for agent-oriented analysis and design.
agentsitidentifiedcanjointogetherandshareinformation Autonomous Agents and Multi-Agent Systems, 3:285–
andpreferencesabouttheprotocolsathandin ordertose- 312,2000.
lect a protocol and assign a role to each agent. Second,
agents are given the possibility to individually select their
protocols and roles anticipating errors. We focus on two
types of errors: wrong message structure and wrong mes-
sagecontent.Asrolesreplacementareperformedassoonas
ananomalyisdetected,weconstrainactionsexecuteddur-
ing interactions to be reversible and not to render critical
side effect. Furthermore, when there are several candidate
protocolsintheindividualprotocolselection,wedeveloped
twoexplorationmechanismsforthesecandidates:(1)ase-
quentialexplorationand(2)amixedexplorationmodes.
Bothmethodshavebeenproposedandtestedinthecon-
textofa Europeanprojectdedicatedforinformationfilter-
ing.Theyprovedtheirusefulnesstoefficientlymanagethe
multipleinteractionsthattakeplacebetweenagents.Inthis
paper, we don’t provide the results we obtained from the
application of these methods since they need to be inter-
preted and compared to static selection cases. In the bar-