Deciding Choreography Reliazability Samik Basu Iowa State University Tevfik Bultan University of California at Santa Barbara Meriem Ouederni University of Malaga

Motivation 1: Web Services Web services support basic client/server style interactions Example: Amazon E-Commerce Web Service (AWS-ECS) Service Requester Service Provider Request Response SOAP WSDL Client Server

Service Composition via Choreography 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 Choreography: A specification of how the individual services that participate to a composite service should interact with each other A choreography is a global specification of interactions among services Web Services Choreography Description Language (WS-CDL)

Motivation 2: Singularity OS Experimental OS developed by Microsoft Research to explore new ideas for operating system design focusing on dependability 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 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

Singularity 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; } }

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...

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

Common: Conversations Specifications of message-based asynchronous 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 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

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 Singularity channel contract corresponds to a finite state machine Each message causes a deterministic transition from one state to another state KeyboardDeviceContract Example Singularity Channel Contract Start Ready$0ReadyWaiting S  C:Success S  C:AckKey C  S:GetKey C  S:PollKey S  C:NakKey Implicit State

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:

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 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 does not obey the FCD protocol if asynchronous communication is used Implementation of the TD protocol obeys the TD 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?

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  ?

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 Verification

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

Challenge & Contribution Finite state processes that communicate with FIFO message queues can simulate Turing Machines –Checking conformance to a conversation protocol is undecidable We show that conversation protocol realizability problem is decidable We implemented the realizability check and applied it to many specifications –Demonstrated that realizability can be checked efficiently in practice

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

Asynchronous Communication !a ?b?c?b ?a !b!c!b Process P1 Process P2 Queue: a P1->P2 b P2->P1 c P2->P1 b P2->P1 Conversations sets are equal but some sent messages may never be consumed

Realizability Requirements We refine our realizability condition to eliminate such cases. We have 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

Summary of our contribution Conversation protocol: C Asynchronous System with unbounded buffer: I with k size communication buffer: I k Synchronous System: I 0 C is realizable if and only C is equivalent to determinized I 1 obtained from projections of C

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 [WWW’11: Choreography Conformance via Synchronizability]

Observation 2: Synchronizability ?a !b!c!b Peer P2 This system is synchronizable since the asynchronous and synchronous versions are equivalent in term of sequences of send actions but it is not well-formed !a ?b?c?b Peer P1

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

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

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 –[Stengel and Bultan ISSTA’09]: Application of sufficient realizability conditions to checking Singularity channel contracts –[Halle and Bultan FSE’10]: more relaxed sufficient condition that allows arbitrary initiators [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: [Fu et al. TSE’05]: Sufficient conditions for synchronizability [Basu and Bultan WWW’11]: Necessary and sufficient condition for synchronizability [Basu, Bultan, Ouderni VMCAI’12]: Synchronizability considering send sequences + reachability of synchronized states [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 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:

Future Directions Choreography realizability for other communication models Analyzing failure of realizability –Correcting unrealizable choreographies with minimal changes to the choreography

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