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

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


1
Conversation SpecificationA New Approach to
Design and Specification of E-Service Composition
  • T. Bultan X. Fu R. Hull J. Su
  • University of California at Santa Barbara
  • Bell Labs, Lucent Technologies

2
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

3
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

4
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

5
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

6
Outline
  • A Model for E-services Compositions
  • Conversations
  • Mealy Peers/Implementations
  • Conversation Specifications (Top-Down)
  • Related work
  • Conclusions

7
E-Composition Schema
  • An E-C schema is a triple (M, P, C )
  • Specifies the infrastructure of composition

conservative aggressive
8
Composition Infrastructure
  • Possible models
  • Peer-to-peer (distributed control)
  • Hub-and-spoke (centralized control)

9
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
10
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

11
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)

12
Outline
  • A Model for E-services Compositions
  • Conversations
  • Mealy Peers/Implementations
  • Conversation Specifications (Top-Down)
  • Related work
  • Conclusions

13
Global Behaviors of Composition
  • Center around composition (collaboration)
  • Rather than individual E-services
  • Behavioral type checking composability is an
    important issue
  • Our focusIs 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

14
Conversations
  • Watcher records the messages (classes) as they
    are sent

authorize
ok
order2
payment1
payment2
receipt2
order1
receipt1
bill2
bill1
a
k
o1
b1
o2
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

15
Outline
  • A Model for E-services Compositions
  • Conversations
  • Mealy Peers/Implementations
  • Conversation Specifications (Top-Down)
  • Related work
  • Conclusions

16
Peers Revisited
Do until halt nondeterministic choice
read an input send an output to some
other peer halt end choice
inputmessages
to othere-services
  • Again, ports and storages are ignored
  • Internal logic of peers finite state control

17
Mealy Peers
  • Mealy machines Finite state machines with input
    (incoming messages) output (outgoing messages)

warehouse2
18
Executing a Mealy Composition
?o2
!r2
?a
!a
!b2
!b2
?o1
!k
?k
!r2
!o1
?p2
?p2
!o2
a
k
o2
!r2



null

warehouse2
bank
store
w1
  • Execution halts if
  • All mealy peers are in final states
  • All queues are empty

19
Outline
  • A Model for E-services Compositions
  • Conversations
  • Mealy Peers/Implementations
  • Conversation Specifications (Top-Down)
  • Related work
  • Conclusions

20
E-Composition Language ? Regular
  • E-C languages are not always regular
  • Example ECL ? ab 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

21
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 wont always work well

22
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

23
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
  • If abc is a part of a conversation, so are bac
    and bca
  • ppi(abc) ppi (bac) ppi (bca) a for i
    1, 2
  • ppi(abc) ppi (bac) ppi (bca) bc for i 3,
    4

b
a
p2
p1
p4
p3
c
24
Join
  • Given languages Li over Si, ? ? i ? n
  • Conversations (ECLs) L are closed under
    projection-join

25
Local Prepone
!a

a
b

!b
c

local view at p
a peer p
  • ppeer(w) should also allow

a
b


26
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

27
The Converse (General Case)
  • There is an Mealy compositions whose ECL is
    notfor every regular languages L

ECL aibci i ?0
28
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)
    isnt a bad idea!

29
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!

30
Summary of Technical Results
  1. ECLs of some Mealy compositions are not
    regular,some others not context free
  2. The prepone and join closure of every regular
    language ECL of some composite Mealy E-services
  3. 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, )

31
Outline
  • A Model for E-services Compositions
  • Conversations
  • Mealy Peers/Implementations
  • Conversation Specifications (Top-Down)
  • Related work
  • Conclusions

32
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

33
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?
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