Analyzing Interactions of Asynchronously Communicating Software Components Tevfik Bultan Department of Computer Science University of California, Santa Barbara

Acknowledgements Collaborators: –Xiang Fu, Hofstra University –Jianwen Su, University of California, Santa Barbara –Rick Hull, IBM –Aysu Betin Can, Middle East Technical University, Turkey –Zachary Stengel, Microsoft –Chris Ferguson, Active Network –Gwen Salaun, Inria, France –Sylvain Halle, University du Quebec a Chicoutimi, Canada –Samik Basu, Iowa State –Meriem Ouderni, INP-ENSEEIHT, France

Motivation 1: Web Services Web services support basic client/server style interactions Example: Amazon E-Commerce Web Service (AWS-ECS) AWS-ECS WSDL specification lists 40 operations that provide differing ways of browsing Amazon’s product database such as –ItemSearch, CartCreate, CartAdd, CartModify, CartGet, CartClear Based on the AWS-ECS WSDL specification one can implement clients that interact with AWS-ECS Service Requester Service Provider Request Response SOAP WSDL Client Server

Composing Services Can this framework support more than basic client/server style interactions? Can we compose a set of services to construct a new service? For example: –If we are building a bookstore service, we may want to use both Amazon’s service and Barnes & Noble’s service in order to get better prices Another (well-known) example: –A travel agency service that uses other services (such as flight reservation, hotel reservation, and car rental services) to help customers book their trips

Orchestration vs Choreography Orchestration: Define an executable process that interacts with existing services and executes them in a particular order and combines the results to achieve a new goal –From atomic services to stateful services –Web Services Business Process Execution Language (WS-BPEL) Choreography: Specify how the individual services should interact with each other. Find or construct individual services that follow this interaction specification –Global specification of interactions among services –Web Services Choreography Description Language (WS-CDL) A choreography can be realized by writing an orchestration for each peer involved in the choreography –Choreography as global behavior specification –Orchestration as local behavior specification that realizes the global specification

Web Services Standards Stack Data Type Service Orchestration Protocol Web Services Business Process Execution Language (WS-BPEL) Web Services Description Language (WSDL) Simple Object Access Protocol (SOAP) XML Schema (XSD) Extensible Markup Language (XML) Atomic Service Atomic Service Orchestrated Service SOAP WSDL Choreography Web Services Choreography Description Language (WS-CDL) WS-BPEL Orchestrated Service WS-BPEL SOAP WS-CDL

Motivation 2: Singularity OS Experimental OS developed by Microsoft Research to explore new ideas for operating system design Key design principles: –Dependability –Security Key architectural decision: –Implement a sealed process system Software Isolated Processes (SIPs) –Closed code space (no dynamic code loading or code generation) –Closed object space (no shared memory) Inter-process communication occurs via message passing over channels

Singularity Channels Channels allow 2-Party asynchronous communication via FIFO message queues –Sends are non blocking –Receives block until a message is at the head of a receive queue Each channel has exactly two endpoints –Type exposed for each endpoint ( Exp and Imp ) –Each endpoint owned by at most one process at any time Owner of Exp referred to as Server Owner of Imp referred to as Client

Channel Contracts Written in Sing # Contracts specify two things: 1.The messages that may be sent over a channel out message are sent from the Server endpoint to the Client endpoint ( S  C ) in messages are sent from the Client endpoint to the Server endpoint ( C  S ) 2.The set of allowed message sequences out message marked with ! in messages marked with ? public contract KeyboardDeviceContract { out message AckKey( uint key ); out message NakKey(); out message Success(); in message GetKey(); in message PollKey(); state Start { Success! -> Ready; } state Ready { GetKey? -> Waiting; PollKey? -> (AckKey! or NakKey!) -> Ready; } state Waiting { AckKey! -> Ready; NakKey! -> Ready; } }

public contract KeyboardDeviceContract { out message AckKey( uint key ); out message NakKey(); out message Success(); in message GetKey(); in message PollKey(); state Start { Success! -> Ready; } state Ready { GetKey? -> Waiting; PollKey? -> (AckKey! or NakKey!) -> Ready; } state Waiting { AckKey! -> Ready; NakKey! -> Ready; } A contract specifies a finite state machine Each message causes a deterministic transition from one state to another state KeyboardDeviceContract Channel Contracts Start Ready$0ReadyWaiting S  C:Success S  C:AckKey C  S:GetKey C  S:PollKey S  C:NakKey S  C:AckKey Implicit State

Motivation 3: Erlang Erlang is a general purpose programming language developed initially at Ericsson for improving dependability of telephony applications In Erlang distributed processes do not share memory and only interact with each other via exchanging messages asynchronously UBF(B) is a language for specifying communication contracts in distributed Erlang programs. UBF(B) specifications list transitions between states where each transition is identified with a request (the message received) and response (the message sent) +NAME(“IRC SERVER”)... +STATE start logon() => ok() & active | error() & stop +STATE active ls() => files() & active getFile() => fileSent() & active | noFileErr() & stop...

1:order :Store :CDSupplier :Customer :BookSupplier A2,B2/2:orderReply 1/A1:cdInquiry A2:cdAvailability 1/B1:bookInquiry B2:bookAvailability Motivation 4: UML Collaboration Diagrams message sequence label must precede

1:order 1/A1:cdInquiry A2:cdAvailability 1/B1:bookInquiry B2:bookAvailability A2,B2/2:orderReply Motivation 4: Collaboration Diagrams Message send events are ordered based on two rules –Implicit: The sequence labels that have the same prefix must be ordered based on their sequence number –Explicit: The events listed before “/” must precede the current event initial event final event

Common: Conversations Specifications of message-based communication –Web Service Choreography Specifications: Global specification of interactions for composition of services –Singularity Channel Contracts: Coordinating inter-process communication in Singularity OS –Erlang Communication Contracts: Coordinating interactions among distributed processes –UML Collaboration diagrams: Specifying interactions among components All these specifications can be modeled as state machines and they all specify sequences of send actions (aka, conversations): Conversation: A sequence of send actions Conversation Protocol (aka Choreography): Specifies a set of conversations

Common: Asynchronous Messaging Sender does not have to wait for the receiver –Message is inserted to a message queue –Messaging platform guarantees the delivery of the message Why support asynchronous messaging? –Otherwise the sender has to block and wait for the receiver –Sender may not need any data to be returned –If the sender needs some data to be returned, it should only wait when it needs to use that data –Asynchronous messaging can alleviate the latency of message transmission through the Internet –Asynchronous messaging can prevent sender from blocking if the receiver service is temporarily unavailable Rather then creating a thread to handle the send, use asynchronous messaging

Each contract state machine specifies a set of conversations, i.e., it is a conversation protocol: KeyboardDeviceContract Example Singularity Channel Contract Start Ready$0ReadyWaiting S  C:Success S  C:AckKey C  S:GetKey C  S:PollKey S  C:NakKey Success(GetKey(AckKey|NakKey)|PollKey(AckKey|NakKey))* Conversation set:

Outline Motivation –Composition of Web Services –Singularity Channel Contracts Conversations Realizability Synchronizability Applications Conclusions

Going to Lunch at UCSB At UCSB Samik, Meriem and I were using the following protocol for going to lunch: –Sometime around noon one of us would call another one by phone and tell him/her where and when we would meet for lunch. –The receiver of this first call would call the remaining peer and pass the information. Let’s call this protocol the First Caller Decides (FCD) protocol. At the time we did not have answering machines or voic due to budget cuts at UC!

FCD Protocol Scenarios Possible scenario 1.Tevfik calls Samik with the decision of where and when to eat 2.Samik calls Meriem and passes the information Another scenario 1.Samik calls Tevfik with the decision of where and when to eat 2.Tevfik calls Meriem and passes the information Yet another scenario 1.Tevfik calls Meriem with the decision of where and when to eat Maybe Samik also calls Meriem at the same time with a different decision. But the phone is busy. Samik keeps calling. But Meriem is not going to answer because according to the protocol the next thing Meriem has to do is to call Samik. 2.Meriem calls Samik and passes the information

FCD Protocol: Tevfik’s Behavior Tevfik calls Samik with the lunch decision Let’s look at all possible behaviors of Tevfik based on the FCD protocol Tevfik is hungry Tevfik calls Meriem with the lunch decision Tevfik receives a call from Samik passing him the lunch decision Tevfik receives a call from Meriem passing him the lunch decision Tevfik receives a call from Meriem telling him the lunch decision that Tevfik has to pass to Samik

FCD Protocol: Tevfik’s Behavior !T->S:D !T->M:D ?S->T:P ?M->T:P ?M->T:D ?S->T:D !T->S:P !T->M:P T->S:D Tevfik calls Samik with the lunch decision Message Labels: ! send ? receive S->M:P Samik calls Meriem to pass the decision

!T->S:D ?M->T:D !T->M:D ?S->T:D !T->M:P Tevfik !T->S:P ?S->T:P ?M->T:P !M->S:D ?T->M:D !M->T:D ?S->M:D !M->T:P Meriem !M->S:P ?S->M:P ?T->M:P !S->T:D ?M->S:D !S->M:D ?T->S:D !S->M:P Samik !S->T:P ?T->S:P ?M->S:P State machines for the FCD Protocol Three state machines characterizing the behaviors of Tevfik, Meriem and Samik according to the FCD protocol

FCD Protocol Has Voic Problems After the economy started to recover, the university installed a voic system FCD protocol started causing problems –We were showing up at different restaurants at different times! Example scenario: –Tevfik calls Meriem with the lunch decision –Samik also calls Meriem with the lunch decision The phone is busy (Meriem is talking to Tevfik) so Samik leaves a message – Meriem calls Samik passing the lunch decision Samik does not answer (he already left for lunch) so Meriem leaves a message –Samik shows up at a different restaurant! Message sequence is: T->M:D S->M:D M->S:P –The messages S->M:D and M->S:P are never consumed This scenario is not possible without voic !

A Different Lunch Protocol To fix this problem, I suggested that we change our lunch protocol as follows: –As the most senior researcher among us I would make the first call to either Meriem or Samik and tell when and where we would meet for lunch. –Then, the receiver of this call would pass the information to the other peer. Let’s call this protocol the Tevfik Decides (TD) protocol

?M->S:P ?T->S:D !S->M:P Samik Meriem Tevfik ?S->M:P ?T->M:D !M->S:P !T->S:D !T->M:D State machines for the TD Protocol TD protocol works fine with voic !

T->S:D T->M:D M->S:P M->T:D M->S:D S->T:D S->M:D S->M:P T->S:P S->T:P T->M:P M->T:P FCD Protocol T->S:D T->M:D S->M:P M->S:P TD Protocol FCD and TD Conversation Protocols Conversation set: { T->M:D M->S:P, T->S:D S->M:P, M->T:D T->S:P, M->S:D S->T:P, S->T:D T->M:P, S->M:D M->T:P } Conversation set: { T->S:D S->M:P, T->M:D M->S:P }

Observation & Question The implementation of the FCD protocol behaves differently with synchronous and asynchronous communication whereas the implementation of the TD protocol behaves the same. –Can we find a way to identify such implementations? The implementation of the FCD protocol does not obey the FCD protocol if asynchronous communication is used whereas the implementation of the JD protocol obeys the JD protocol even if asynchronous communication used. –Given a conversation protocol can we figure out if there is an implementation which generates the same conversation set?

Conversations, Choreography, Orchestration Peer state machines are orchestrations –A peer state machine can be specified using an orchestration language such as WS-BPEL –One can translate WS-BPEL specifications to peer state machines A conversation protocol is a choreography specification –A conversation set corresponds to a choreography –A conversation set can be specified using a choreography language such as WS-CDL –One can translate WS-CDL specifications to conversation protocols

Bottom-Up vs. Top-Down Bottom-up approach Specify the behavior of each peer –For example using an orchestration language such as WS-BPEL The global communication behavior (conversation set) is implicitly defined based on the composed behavior of the peers Global communication behavior is hard to understand and analyze Top-down approach Specify the global communication behavior (conversation set) explicitly as a protocol –For example using a choreography language such as WS-CDL Ensure that the conversations generated by the peers obey the protocol

Conversation Protocol (Choreography Specification) F( S->M:P  M->S:P ) ? LTL property Input Queue... Conversation ? LTL property Peer TPeer X Peer J T->S:D T->M:D S->M:P M->S:P F( S->M:P  M->S:P ) !T->S:D !T->M:D ?M->S:P ?T->S:D !S->M:P ?S->M:P ?T->M:D !M->S:P Top-Down vs. Bottom-Up Verification

Outline Motivation –Composition of Web Services –Singularity Channel Contracts Conversations Realizability Synchronizability Applications Conclusions

Realizability Conversation protocols identify the global communication behavior –How do we implement processes that conform to the conversation protocol? Realizability question: –Given a conversation protocol, are there processes whose communication behavior in terms of conversations (i.e., send sequences) is equal to the set of conversations (i.e., send sequences) specified by the conversation protocol? The FCD protocol is unrealizable The TD protocol is realizable Conversations generated by some processes Conversations specified by the conversation protocol  ?

Unrealizable Conversation Protocols A  B: m1 C  D: m2 A  B: m1 B  A: m2 A  C: m3 B  A: m2 A  B: m1 There are unrealizable conversation protocols: A  B: m1 C  A: m2

Unrealizable Examples Some conversation protocols are unrealizable! A  B: m1 C  D: m2 Conversation protocol m2 m1 Conversation “ m2 m1 ” will be generated by all implementations which follow the protocol !m1 ?m1 !m2 ?m2 Peer APeer BPeer CPeer D Projections of the protocol to the processes

Unrealizable Examples Some conversation protocols are unrealizable! A  B: m1 C  A: m2 Conversation protocol m2 m1 Conversation “ m2 m1 ” will be generated by all implementations which follow the protocol !m1 ?m1 !m2 ?m2 Peer A Peer BPeer C Projections of the protocol to the processes

Unrealizable Examples m2 m1 m3 m1 m2 m3 A  B: m1 B  A: m2 A  C: m3 B  A: m2 A  B: m1 A B C m1 m2 m3 Conversation: Generated conversation: BA, C

Challenges Finite state processes that communicate with FIFO message queues can simulate Turing Machines –In general analyzing properties of asynchronously communicating finite state machines is undecidable –For example, checking conformance to a conversation protocol is undecidable

Sufficient Conditions for Realizability Three sufficient conditions for realizability Lossless join –Conversation set should be equivalent to the join of its projections to each peer Synchronous compatible –When the projections are composed synchronously, there should not be a state where a peer is ready to send a message while the corresponding receiver is not ready to receive Autonomous –At any state, each peer should be able to do only one of the following: send, receive or terminate (a peer can still choose among multiple messages)

Outline Motivation –Composition of Web Services –Singularity Channel Contracts Conversations Realizability Synchronizability Applications Conclusions

Bottom-Up Approach We know that analyzing conversations of composite web services is difficult due to asynchronous communication –Model checking for conversation properties is undecidable even for finite state peers The question is: –Can we identify the composite web services where asynchronous communication does not create a problem? We call such compositions synchronizable The implementation of the JD protocol is synchronizable The implementation of the FCD protocol is not synchronizable

Three Examples, Example 1 requesterserver !r 2 ?a 1 ?a 2 !e !r 1 Conversation set is regular: ( r 1 a 1 | r 2 a 2 )* eConversation set is regular: ( r 1 a 1 | r 2 a 2 )* e During all executions the message queues are bounded r 1, r 2 a 1, a 2 e ?r 1 !a 1 !a 2 ?r 2 ?e

Example 2 Conversation set is not regularConversation set is not regular Queues are not bounded requesterserver !r 2 ?a 1 ?a 2 !e !r 1 r 1, r 2 a 1, a 2 e ?r 1 !a 1 !a 2 ?r 2 ?e

Example 3 Conversation set is regular: ( r 1 | r 2 | ra )* eConversation set is regular: ( r 1 | r 2 | ra )* e Queues are not bounded requesterserver !r 2 ?a!r !e !r 1 r 1, r 2 a 1, a 2 e ?r 1 ?r 2 ?e ?r !a

State Spaces of the Three Examples queue length # of states in thousands Verification of Examples 2 and 3 are difficult even if we bound the queue length How can we distinguish Examples 1 and 3 (with regular conversation sets) from 2? –Synchronizability Analysis

Synchronizability Analysis A composite web service is synchronizable if its conversation set does not change –when asynchronous communication is replaced with synchronous communication If a composite web service is synchronizable we can check the properties about its conversations using synchronous communication semantics –For finite state peers this is a finite state model checking problem

Synchronizability Analysis Sufficient conditions for synchronizability: A composite web service is synchronizable, if it satisfies the synchronous compatible and autonomous conditions Connection between realizability and synchronizability: –A conversation protocol is realizable if its projections to peers are synchronizable and the protocol itself satisfies the lossless join condition

Are These Conditions Too Restrictive? Problem SetSizePass? SourceName#msg#states#trans. ISSTA’04SAS91215yes IBM Conv. Support Project CvSetup444yes MetaConv446no Chat245yes Buy556yes Haggle858no AMAB81015yes BPEL spec shipping233yes Loan666yes Auction9910yes Collaxa. com StarLoan677yes Cauction576yes

Necessary and Sufficient Conditions More recently we identified necessary and sufficient conditions for realizability and synchronizability

Refining Realizability Just looking at equivalence of the conversation sets is not enough Conversations generated by some processes Conversations specified by the conversation protocol  ?

Another Conversation Protocol a P1->P2 b P2->P1 c P2->P1 b P2->P1 A conversation protocol for 2 processes: P1 and P2

Projections on P1 and P2 !a ?b?c?b ?a !b!c!b Process P1 Process P2

Synchronous Communication !a ?b?c?b ?a !b!c!b Process P1 Process P2 a P1->P2 b P2->P1 c P2->P1

Synchronous Communication !a ?b?c?b ?a !b!c!b Process P1 Process P2 a P1->P2 b P2->P1 c P2->P1 b P2->P1

Synchronous Communication !a ?b?c?b ?a !b!c!b Process P1 Process P2 a P1->P2 b P2->P1 c P2->P1 b P2->P1

Synchronous Communication !a ?b?c?b ?a !b!c!b Process P1 Process P2 a P1->P2 b P2->P1 c P2->P1 b P2->P1

Synchronous Communication !a ?b?c?b ?a !b!c!b Process P1 Process P2 a P1->P2 b P2->P1 c P2->P1 b P2->P1 BLOCKED

Synchronous Communication !a ?b?c?b ?a !b!c!b Process P1 Process P2 a P1->P2 b P2->P1 c P2->P1 b P2->P1 Conversations sets are equal but processes may get stuck

Asynchronous Communication !a ?b?c?b ?a !b!c!b Process P1 Process P2 Queue:

Asynchronous Communication !a ?b?c?b ?a !b!c!b Process P1 Process P2 Queue: Queue: a a P1->P2

Asynchronous Communication !a ?b?c?b ?a !b!c!b Process P1 Process P2 Queue: a P1->P2

Asynchronous Communication !a ?b?c?b ?a !b!c!b Process P1 Process P2 Queue: a P1->P2 b P2->P1

Asynchronous Communication !a ?b?c?b ?a !b!c!b Process P1 Process P2 Queue: Queue: a a P1->P2 b P2->P1

Asynchronous Communication !a ?b?c?b ?a !b!c!b Process P1 Process P2 Queue: a P1->P2 b P2->P1

Asynchronous Communication !a ?b?c?b ?a !b!c!b Process P1 Process P2 Queue: c Queue: a P1->P2 c P2->P1 b P2->P1

Asynchronous Communication !a ?b?c?b ?a !b!c!b Process P1 Process P2 Queue: c Queue: a P1->P2 c P2->P1 b P2->P1 Cannot consume c

Asynchronous Communication !a ?b?c?b ?a !b!c!b Process P1 Process P2 Queue: c Queue: a P1->P2 c P2->P1 b P2->P1 Cannot consume c

Realizability Requirements So we need two requirements for realizability: 1.Conversations specified by the conversation protocol = Conversations generated by the asynchronous system 2.Asynchronous system is well-formed: All sent messages can be eventually consumed Conversation protocol is realizable if and only if there exists such an asynchronous system

Determinizing Projections ?a !b!c!b Process P2 !a ?b?c?b Process P1

Determinizing Projections !a ?b?c?b ?a !b!c!b Peer P1 Peer P2 !a ?c ?b ?a !c !b

Observation 1: Behavioral Order Behavior exhibited by projections when communicating synchronously is larger than the conversation Behavior exhibited by projections when communicating asynchronously is larger than that exhibited by projections when communicating synchronously C ≤ I 0 ≤ I 1 ≤ I 2 ≤ … ≤ I

Observation 1: Behavioral Ordering a P1->P2 c P2->P1 b P3->P4 !a?c ?a!c P2 !b P3 ?b P4 P1

Observation 1: Behavioral Ordering a P1->P2 c P2->P1 b P3->P4 !a?c Synchronous System ?a!c P2 !b P3 ?b P4 a P1->P2 b P3->P4 c P2->P1 b P3->P4 c P2->P1 a P1->P2 P1

Observation 1: Behavioral Ordering a P1->P2 c P2->P1 b P3->P4 !a?c Synchronous System ?a!c P2 !b P3 ?b P4 a P1->P2 b P3->P4 c P2->P1 b P3->P4 c P2->P1 a P1->P2 c P2->P1 Asynchronous System P1

Observation 2: Synchronizability A system is synchronizable if and only if its behaviors are identical for asynchronous and synchronous communication For synchronizable systems: Forall k ≥ 0: I k is equivalent to I I is synchronizable iff I 0 is equivalent to I 1 This is the key result!

Observation 3: Synchronizability & Determinism A synchronizable system that consists of deterministic processes is well- formed (all sent messages are eventually consumed)

Observation 3 ?a !b!c!b Peer P2 Synchronizable but not well-formed Synchronizable and well-formed !a ?c ?b ?a !c !b !a ?b?c?b Peer P1

Outline of the Realizability Check Project conversations to processes Determinize peers Check equivalence between conversation C and I 1 –C = I 1 if and only if I is synchronizable [Obs 1, 2] and C = I –C = I 1 implies I is well-formed [Obs 3] C = I 1 if and only if C is realizable

Outline Motivation –Composition of Web Services –Singularity Channel Contracts Conversations Realizability Synchronizability Applications Conclusions

Implementation Implemented using CADP toolbox –Automatically generate a LOTOS specification for the conversation protocol –Generate determinized projections (in LOTOS) –Check equivalence of the 1-bounded asynchronous system and the conversation protocol Checked realizability of –9 web service choreography specifications 8 are realizable –9 collaboration diagrams 8 are realizable –86 Singularity channel contracts 84 are realizable Realizability check takes about 14 seconds on average

Singularity Channel Contract Verification State machine construction allows for automated verification and analysis of channel communication Singularity compiler automatically checks compliance of client and server processes to the specified contract Claim from Singularity documentation: –"clients and servers that have been verified separately against the same contract C are guaranteed not to deadlock when allowed to communicate according to C.“ This claim is wrong!

IO_RUNNING$0ReadyState$1 ReadyState$0 IO_RUNNING Deadlock Example: The TpmContract IO_RUNNING$0 IO_RUNNINGReadyState ReadyState$1 ReadyState$0 ReadyState Server Projection Client Projection C  S:SendS  C:AckStartSend S  C:SendComplete C  S:GetTpmStatus S  C:TpmStatus Send? Send! AckStartSend! AckStartSend? SendComplete! GetTpmStatus! GetTpmStatus? TpmStatus! GetTpmStatus! TpmStatus? TpmStatus! GetTpmStatus? Server Receive Queue Client Receive Queue Conversation SendComplete?

Deadlock Example: The TpmContract IO_RUNNING$0 IO_RUNNINGReadyState ReadyState$1 ReadyState$0 Send? IO_RUNNING$0 IO_RUNNINGReadyState ReadyState$1 ReadyState$0 Server Projection Send! AckStartSend! AckStartSend? SendComplete! GetTpmStatus! GetTpmStatus? TpmStatus! GetTpmStatus! TpmStatus? SendComplete? TpmStatus! GetTpmStatus? Server Receive Queue Client Receive Queue Send AckStartSendSendComplete GetTpmStatus TpmStatus Conversation C  S: Send S  C: AckStartSend S  C: SendComplete C  S: GetTpmStatus S  C: TpmStatus Client Projection

Realizability Problem KeyboardDeviceContract is not realizable –It violates the autonomous condition As I mentioned earlier, autonomous condition is sufficient (but not necessary) for realizability of two-party protocols (Singularity channel contracts are two-party protocols) –If a contract is autonomous, it is guaranteed to be realizable –However, it can be realizable but not autonomous i.e., false positives are possible when we use autonomous condition as our realizability check

Autonomous condition and false positives Example: TpmContract Since autonomous condition is not a necessary condition, it can cause false positives when used for checking realizability Using our recent results we can show that this modified protocol is realizable using the necessary and sufficient condition for realizability IO_RUNNING$0 IO_RUNNINGReadyState ReadyState$1 ReadyState$0 C  S:SendS  C:AckStartSend S  C:SendComplete C  S:GetTpmStatus S  C:TpmStatus IO_RUNNING$1 S  C:SendCompleteS  C:TpmStatus Fixed Violates Autonomous condition

Explicit state verification is expensive using asynchronous communication –Exponential state space explosion in the worst case Example: BlowupKContract Model checking efficiency S1 S2 … SkSk S  C:m1 S  C:m2 C  S:m3

Model checking efficiency If contract is realizable, conversations generated using asynchronous communication and synchronous communication are the same –Therefore, synchronous communication model can be used for verification S1 S2 … SkSk S  C:m1 S  C:m2 C  S:m3

Analysis Efficiency Performed realizability check and exhaustive deadlock search for ~95% of contracts to compare analysis time Results show clear advantage to performing the realizability check

LTL Property Validation Selected 10 contracts for LTL property validation Both synchronous and asynchronous models were used to compare performance Verification using the synchronous model is more efficient as expected, demonstrating the usefulness of realizability/synchronizability checks

Some of our papers [Fu et al. TCS’04]: Sufficient conditions for realizability [Fu et al. TSE’05]: Sufficient conditions for synchronizability [Bultan and Fu SOCA’08]: Application of realizability analysis to collaboration diagrams [Stengel and Bultan ISSTA’09]: Application of realizability analysis to Singularity channel contracts [Halle and Bultan FSE’10]: more relaxed sufficient condition that allows arbitrary initiators [Basu and Bultan WWW’11]: Necessary and sufficient condition for synchronizability [Basu, Bultan, Ouderni POPL’12]: Necessary and sufficient condition for realizability [Basu, Bultan, Ouderni VMCAI’12]: Synchronizability for send sequences + reachability of synchronized states

Related Work Singularity: –[Hunt, Larus SIGOPS ‘07] Singularity: rethinking the software stack –[Fähndrich, Aiken, Hawblitzel, et. al SIGOPS/Eurosys ‘07] Language support for fast and reliable message-based communication in singularity os. –Influenced by work on Session Types [Honda, Vasconcelos, Kubo ESOP ’98] Language primitives and type discipline for structured communication-based programming –Source code and RDK:

Related Work Sufficient conditions for realizability: –[Fu et al. TCS’04] Conversation Protocols [Honda et al. POPL’08] has similar conditions for session types –Arbitrary Initiators are not allowed: Conversation protocol cannot have two different peers initiating send actions from the same state [Kazhamiakin, Pistore FORTE’06]: Realizability for restricted communication models [Lohmann, Wolf ICSOC’11]: Shows decidability of realizability with unbounded asynchronous communication when messages are not ordered (i.e., FIFO requirement is dropped)!

Related Work Message Sequence Charts (MSC) –[Alur, Etassami, Yannakakis ICSE’00, ICALP’01] Realizability of MSCs and MSC Graphs Defines similar notion of realizability –[Uchitel, Kramer, Magee ACM TOSEM 04] Implied Scenarios in MSCs –Different conversation model

Related Work Results on synchronizability: [Manohar, Martin MPC 98] Slack elasticity –Presents conditions under which changing the size of communication queues does not effect the behavior of the system –Behavior definition also takes the decision points into account in addition to message sequences –It gives sufficient conditions for slack elasticity and discusses how to construct systems to ensure slack elasticity

Related Work Verification of web services –Petri Nets [Narayanan, McIlraith WWW’02] Simulation, verification, composition of web services using a Petri net model –Process Algebras [Foster, Uchitel, Magee, Kramer ASE’03] Using MSC to model BPEL web services which are translated to labeled transition systems and verified using model checking –Model Checking Tools [Nakajima ICWE’04] Model checking Web Service Flow Language specifications using SPIN See the survey on BPEL verification –[Van Breugel, Koshkina 06] Models and Verification of BPEL

Related Work Specification approaches that are similar to conversation protocols –[Parunak ICMAS 96] Visualizing agent conversations: Using enhanced Dooley graphs for agent design and analysis. –[Hanson, Nandi, Kumaran EDOCC’02] Conversation support for business process integration

Related Work Modeling Choreography & Orchestration – Process algebras, synchronous communication [Busi, Gorrieri, Guidi, Lucchi, Zavattaro ICSOC’05] [Qiu, Zhao, Chao, Yang WWW’07] –Activity based (rather than message based) approaches [Berardi, Calvanese, DeGiacomo, Hull, Mecella VLDB’05]

Future Directions Choreography realizability, synchronizability for other communication models Branching time realizability Analyzing failure of realizability/synchronizability –Automatically repairing unrealizable choreographies with minimal changes to the choreography

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