Conversation Specification: A New Approach to Design and Specification of E-Service Composition

Conversation Specification:A New Approach to Design and Specification of E-Service Composition T. Bultan X. FuR. HullJ. Su University of California at Santa Barbara Bell Labs, Lucent Technologies

The E-Services Paradigm • E-services : network-resident software services accessible via standardized protocols • In e-commerce, telecom, science • Possibility of automatic discovery, composition, invocation, monitoring • Primary roots : • Process description formalisms, including automata and workflow • Data management (including models, transforms, mediation, transactions) • Distributed computing middleware WWW 2003

E-Services Composition • Web  very flexible forms of distributed computing (SOAP, WSDL) • Composition: distributed, flexible, and complex • More flexible, less structured than CORBA • Data management plays a large role • Increased structure helps understanding fundamental issues • “Glue” languages: WSFL, XLANG, BPEL4WS, BPML • “Behavioral” signatures: automata-based, WSCL, “session types” • Formalisms to describe e-services: DAML-S pre- and post-conditions WWW 2003

Fundamental Issues in Composition • How to build composite e-services from atomic ones? • Various standards proposed; different disciplines addressed • Most pursue a procedural approach • Approaches to “synthesize” (automatic composition) e-compositions from desired global properties • How to analyze composite e-services? • “Correctness”, behaviors, composable? compatibility? • Tools for analysis of compositions • Formal foundations not yet clear WWW 2003

Summary of Contributions • Propose a model of global behaviors for composite e-services • E-service interactions via messaging (e.g. in the spirit of JMS, BizTalk): asynchronous + FIFO queue • Use formal language techniques • Technical results concerning Mealy machines as participating e-services: • Global behaviors are not always regular languages • The “prepone” and “join” closure of every regular language = global behavior of some composite e-service • The converse of 2. is not true • Implications to composition design: • Top-down is better than bottom-up • Bounded queues vs unbounded WWW 2003

Outline • A Model for E-services & Compositions • Conversations • Mealy Peers/Implementations • Conversation Specifications (Top-Down) • Related work • Conclusions WWW 2003

authorize store bank • M : finite set ofmessage classes ok receipt2 payment1 • P : finite set of peers(e-services) order2 bill1 payment2 bill2 receipt1 order1 • C : finite set of peer to peer channels ware- house1 ware- house2 E-Composition Schema • An E-C schema is a triple (M, P, C ) Specifies the infrastructure of composition “conservative” “aggressive” WWW 2003

store bank s b k’ a’ a k r b m p o mediator • Our technical results do not rely on special rolesof peers (in the spirit of P2P) p2 o1 w2 w1 b2 r1 authorize r2 b1 store bank ok o2 p1 ware- house2 ware- house1 order2 s b receipt2 receipt1 payment2 order1 bill2 payment1 w1 w2 bill1 ware- house1 ware- house2 Composition Infrastructure • Possible models: • Peer-to-peer (distributed control) • Hub-and-spoke (centralized control) WWW 2003

ware- house1 store order1 o1 Communication Channels • Channels are assumed to be reliable • Asynchronous, for example, the following channel: send Order1 … • Queues are FIFO, unbounded length • Can simulate synchronousand also bounded queues send Order1 receive Receipt1 … WWW 2003

ware- house1 store order1 Messages • Messages are classified into classes • Each class is associated with one channel • Each message class may have additional attributes which can carry the contents of messages • For this paper, analysis involves no contents • Results immediately apply to “finite domain” contents WWW 2003

inputmessages inputmessages to othere-services to othere-services Do until halt nondeterministic choice: read an input; send an output to some other peer; halt; end choice Do until halt nondeterministic choice: read an input; send an output to some other peer; halt; end choice message log message log local store local store Peers (E-services) • In the most general case, a peer can be a Turing machine • Impossible to analyze • Essence of BPEL4WS, BPML, etc. standards: • Finite control + data structures • Infinite state system and thus difficult to analyze • Our approach: • Finite control+ (finite number of) message classes(+ finite domain contents) • Open to extend to allow data structures (not in this paper) WWW 2003

Global Behaviors of Composition • Center around composition (collaboration) • Rather than individual E-services • “Behavioral type” checking: composability is an important issue • Our focus:Is the specification of a composite E-service “correct”? • How, when, and what do peers communicate? • Correctness: properties of communication during possible executions • Ignore port-level details WWW 2003

store bank Watcher ware- house1 ware- house2 Conversations • Watcher: “records” the messages (classes) as they are sent authorize ok payment1 order2 payment2 receipt2 receipt1 order1 bill2 bill1 a k o1 o2 b1 p1 r1 r2 b2 p2 • A conversation is a sequence of messages the watcher sees in a successful run (or session) • E-composition (ec) language: the set of all possible conversations WWW 2003

message log local store Peers Revisited • Again, ports and storages are ignored • Internal logic of peers : finitestate control Do until halt nondeterministic choice: read an input; send an output to some other peer; halt; end choice inputmessages to othere-services WWW 2003

?o2 !r2 !b2 !b2 !r2 ?p2 ?p2 !r2 null Mealy Peers • Mealy machines: Finite state machines with input (incoming messages) & output (outgoing messages) warehouse2 WWW 2003

Executing a Mealy Composition • Execution halts if • All mealy peers are in final states • All queues are empty ?o2 !r2 !a ?a !b2 !b2 ?o1 ?k !k !r2 !o1 ?p2 ?p2 !o2 k o2 a !r2 … … … null … store w1 warehouse2 bank WWW 2003

E-Composition Language Regular • E-C languages are not always regular • Example: ECL a*b*=anbn ?a a !a ?b !b b p1 p2 • Not context free for some Mealy compositions • Causes: asynchronous communication & unbounded queue • Bounded queues or synchronous: ECL always regular WWW 2003

Practical Implications • Simply composing peers without a global sense can make the E-composition behaviors very complicated • Non regular means many model checking tools are out of reach (for correctness) • Bottom up won’t always work well WWW 2003

An Alternative • Given a regular language L as the global behavior, find Mealy peers so that the ECL =L • A quick answer: no • But, wait… WWW 2003

Local Views • Local view of a conversation for a peer: part of the execution that is related to the peer • Defined as projection: pp(w) for a conversation w • Two conversations cannot be distinguished if they have exactly the same set of local views • Ifabcis a part of a conversation, so are bacand bca • ppi(abc) = ppi(bac) = ppi(bca) = a for i = 1, 2 • ppi(abc) = ppi(bac) = ppi(bca) = bcfor i = 3, 4 b a p1 p2 p3 p4 c WWW 2003

Join • Given languages Liover Si,   i  n • Conversations (ECLs) L are closed under “projection-join”: WWW 2003

Local Prepone ppeer(w) should also allow !a … a b … !b … c local view atp a peer p … b a … WWW 2003

A Synthesis Result • Given a regular language L, we can find a Mealy composition such that its ECL is the closure: • Intuitively: given a regular L (e.g., ako1…), we can find Mealy peers whose conversations are not arbitrary • Opportunity for automatic composition • But some Mealy compositions do not relate to any regular languages in this way WWW 2003

!b p2 ?a p1 b p3 !c ?b !a ?c a c The Converse (General Case) • There is an Mealy compositions whose ECL is notfor every regular languages L ECL = { aibci | i 0 } WWW 2003

The Tree Case • When the peer-channel graph is a tree, then the Mealy composition has an ECL equal tofor some regular languages L • Intuitively: the global behavior of bottom-up composition is still predictable if the composition infrastructure is a tree • In particular, adding an mediator (hub-spoke) isn’t a bad idea! WWW 2003

Hub-and-spoke • For every star-shaped E-composition infrastructure, and every regular language L, we can construct an Mealy composition whose ECL =L • Good news for hub-and-spoke! WWW 2003

Summary of Technical Results • ECLs of some Mealy compositions are not regular,some others not context free • The “prepone” and “join” closure of every regular language = ECL of some composite Mealy E-services • The converse of 2. is not true in general, true in special cases • However: if bounded queue or synchronous: ECL of every Mealy composition is regular • Design time decision! Need to be explicit in specifications (BPEL4WS, BPLM, …) WWW 2003

Related Work • Similar E-service models: • BPEL4WS (WSFL, XLANG), BPML, WSCL • Workflow, 1-safe Petri-nets • p-calculus: synchronous but can simulate unbounded buffer effect • Other synchronous models • CSP [Hoare ’85], I/O automata [Lynch-Tuttle ’87], interface automata [Henzinger et al ’01 ] • Other asynchronous models • Communicating FSA [Brand-Zafiropulo ’82], Message Sequence Charts [Alur et al ’00] WWW 2003

Conclusions • Conversations are an interesting model for global behaviors • Only a beginning, more need to be understood (see also [Hull et al PODS ’03]) • Would like ECLs to be regular, some sufficient conditions are given in [Fu-Bultan-S. CIAA ’03] • Infinite domain message contents? • Design tools, e.g., verification tools? • Spawning new processes? • … WWW 2003

Conversation Specification: A New Approach to Design and Specification of E-Service Composition