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

Analyzing Interactions of Asynchronously Communicating

Software Components

Tevfik BultanDepartment of Computer Science

University of California, Santa [email protected]

http://www.cs.ucsb.edu/~bultan

Acknowledgements

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

Big Picture Motivation

• All software is moving to the network– Mobile or Browser-based thin clients combined with servers hosted

on the cloud are replacing desktop applications• More things are becoming programmable

– Smart-phones and smart-TVs are already common– Smart-glasses, smart watches, programmable cars are soon to

follow• More things are moving to the network

– Internet of things is becoming a reality• you can control your lights with your smart-phone

– Nowadays programmable things come with network connection

• It seems like a good time to focus on specification and analysis of interactions of software systems that communicate over a network!

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)

AtomicService

AtomicService

OrchestratedService

SOAP

WSDL

WSDL

Choreography Web Services Choreography Description Language (WS-CDL)

WS-BPEL

OrchestratedService

WS-BPEL

SOAP

SOAP

SOAP

SOAPWS-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 (SC)

• in messages are sent from the Client endpoint to the Server endpoint (CS)

– 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 as State Machines

Start

Ready$0ReadyWaiting

SC:Success

SC:AckKey

SC:AckKey

CS:GetKey

CS:PollKey

SC:NakKey

SC:AckKey

Implicit State

Motivation 3: Erlang and UBF(B)

• 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...

UBF(B) Specifications as State Machines

+NAME(“IRC SERVER”)... +STATE start logon() => ok() & active

| error() & stop

+STATE active ls() => files() & active getFile() => fileSent() & active

| noFileErr() & stop...

start

stopactive

SC:files

SC:noFileErr

CS:ls

CS:logon

SC:fileSent

SC:ok SC:error

CS:getfile

1:reserve

:TravelAgency

:Airline

:Customer

:Hotel

A2,B2/2:reply1/A1:fligtInquiry

A2:flightAvailability

1/B1:roomInquiry

B2:roomAvailability

Motivation 4: UML Collaboration Diagrams

messagesequencelabel

mustprecede

1:reserve

1/A1:flightInquiry

A2:flightAvailability

1/B1:roomInquiry

B2:roomAvailability

A2,B2/2:reply

Motivation 4: Collaboration Diagrams

• Messages 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 message

Final message

TA A: flightInquiry TAH: roomInquiry

CTA: reserve

ATA: flightAvailability HTA: roomAvailability

TAC: reply

Collaboration Diagrams as State Machines

TAH: roomInquiry

ATA: flightAvailability

TAH: roomInquiry

HTA: roomAvailability

TA A: flightInquiry

ATA: flightAvailability

HTA: roomAvailability

TA A: flightInquiry

Common: Specification of 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 sent messages (aka, conversations):

Conversation: A sequence of messages sent during an execution

Conversation Protocol (aka Choreography): A state machine that 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

SC:Success

SC:AckKey

SC:AckKey

CS:GetKey

CS:PollKey

SC:NakKey

SC: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 voicemail 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 decisionTevfik 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

? receiveS->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 Voicemail Problems

• After the economy started to recover, the university installed a voicemail 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 voicemail!

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 MeriemTevfik

?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 voicemail!

A Model for Composition

S->M(P)

M->S(P)

T->M(D)T->A(D)

Peer S

Peer T

Peer M

• A composition of services consists of– a finite set of peers

• Lunch example with three peers: T, S, M– and a finite set of messages

• Lunch example (TD protocol) with four messages T->S(D), T->M(D), S->M(P), M->S(P)

Communication Model

• We assume that the messages among the peers are exchanged using reliable and asynchronous messaging– FIFO and unbounded message queues

• There are existing messaging platforms that support this type of messaging • Java Messaging Service (JMS)• Java API for XML messaging (JAXM)• MSMQ (Microsoft Message Queuing Service)

T->S(D)Peer T Peer ST->S(D)

Conversations

• Record the messages in the order they are sent

Generated conversation:

• A conversation is a sequence of messages generated during an execution

S->M(P)

T->S(D)

Peer S

Peer T

Peer M

S->M(P)T->S(D)

Properties of Conversations

• The notion of conversation enables us to reason about temporal properties of the composite services

• LTL framework extends naturally to conversations– LTL temporal operators

X (neXt), U (Until), G (Globally), F (Future)– Atomic properties

Predicates on message classes (or contents)

Example: G ( payment F receipt )

• Model checking problem: Given an LTL property, does the conversation set satisfy the property?

!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

?M->S:P?T->S:D

!S->M:P

Samik MeriemTevfik

?S->M:P?T->M:D

!M->S:P

!T->S:D !T->M:D

State machines for the TD Protocol

T->S:D

T->M:D

M->S:P

M->T:D M->S:DS->T:D

S->M:D

S->M:PT->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 }

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

Observations & Questions

• 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?

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

ConversationProtocol(ChoreographySpecification)

F(S->M:P M->S:P)? LTL property

InputQueue

...Conversation?

LTL property

Peer T Peer XPeer 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



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

AB: m1

CD: m2

AB: m1BA: m2

AC: m3

BA: m2

AB: m1

• Three unrealizable conversation protocols:

AB: m1

CA: m2

Unrealizable Examples

• This protocol is unrealizable both for synchronous and asynchronous communication!

AB: m1

CD: m2

Conversationprotocol

Conversation “m2 m1m2 m1” will be generated by all implementations which follow the protocol

!m1 ?m1 !m2 ?m2

Peer A Peer B Peer C Peer D

Projections of the protocol to the processes


• This protocol is realizable with synchronous communication but unrealizable with asynchronous communication!

AB: m1

CA: m2


Conversation “m2 m1m2 m1” will be generated by all implementations which follow the protocol

!m1?m1 !m2

?m2

Peer A

Peer B Peer C

Projections of the protocol to the processes


m2 m1 m3

m1

m2

m3

AB: m1BA: m2

AC: m3

BA: m2

AB: m1

A

B

C

m1m2

m3

Conversation:

Generated conversation:

B A, 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 conditions• Lossless join• Synchronous compatible• Autonomous

Together they are sufficient conditions for realizability


• Lossless join– Conversation set should be equivalent to the join of its

projections to each peer

AB: m1

CD: m2

This protocol is not lossless join

Conversation set: {m1m2}

Projection to A: {!m1}Projection to B: {?m1}Projection to C: {!m2}Projection to D: {?m2}

Join of the projections: {m1m2, m2m1}

Not equal to the conversation set!


• 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

AB: m1

CA: m2


?m1 !m2

Peer A

Peer B Peer C

This protocol is not synchronous compatiable

?m2

!m1

C is ready to sendbut A isnot readyto 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)

AB: m1BA: m2

AC: m3

BA: m2

AB: m1

This protocol is not autonomous

A has both a send and a receive transition from this state(B also has both send and receive transitions)

Outline



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

requester server

!r2

?a1 ?a2

!e

!r1

• Conversation set is regular: (Conversation set is regular: (rr11aa11 | | rr22aa22)* )* ee

• During all executions the message queues are bounded

r1, r2

a1, a2

e ?r1

!a1 !a2

?r2

?e

Example 2

• Conversation set is not regularConversation set is not regular• Queues are not bounded

requester server

!r2

?a1 ?a2

!e

!r1

r1, r2

a1, a2

e ?r1

!a1 !a2

?r2

?e

Example 3

• Conversation set is regular: (Conversation set is regular: (rr11 | | rr22 | | rara)* )* ee

• Queues are not bounded

requester server

!r2

?a !r

!e!r1

r1, r2

a1, a2

e

?r1 ?r2

?e

?r !a

State Spaces of the Three Examples

queue length

# o

f st

ates

in

th

ou

san

ds

• 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 Set Size Pass?Source Name #msg #states #trans.

ISSTA’04 SAS 9 12 15 yes

IBM

Conv.

Support

Project

CvSetup 4 4 4 yesMetaConv 4 4 6 no

Chat 2 4 5 yesBuy 5 5 6 yes

Haggle 8 5 8 noAMAB 8 10 15 yes

BPEL

spec

shipping 2 3 3 yes

Loan 6 6 6 yes

Auction 9 9 10 yesCollaxa.

com

StarLoan 6 7 7 yesCauction 5 7 6 yes

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

• In addition to the above equivalence we may also want that the peers do not get stuck or some messages may never be consumed

Conversations generated by some processes

Conversations specified by the conversation protocol

?

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

Observation 1: Behavioral Order

• Behavior exhibited by projections when communicating synchronously can be same or larger than the conversation set

• Behavior exhibited by projections when communicating asynchronously can be same or larger than that exhibited by projections when communicating synchronously– For bounded channels, increasing the channel size leads to same

or larger conversation set

C ≤ I0 ≤ I1 ≤ I2 ≤ … ≤ I

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: Ik is equivalent to I

I is synchronizable iff I0 is equivalent to I1

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)

Outline of the Realizability Check

1. Project conversations to processes

2. Determinize peers

3. Check equivalence between conversation C and I1

– C = I1 if and only if I is synchronizable [Obs 1, 2] and C = I

– C = I1 implies I is well-formed [Obs 3]

C = I1 if and only if C is realizable

Outline



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

ClientProjection

CS:Send SC:AckStartSend

SC:SendComplete

CS:GetTpmStatusCS:GetTpmStatusSC:TpmStatus SC:TpmStatus

Send?

Send!

AckStartSend!

AckStartSend?

SendComplete!

GetTpmStatus!

GetTpmStatus?TpmStatus!

GetTpmStatus!TpmStatus? TpmStatus?

TpmStatus!GetTpmStatus?

ServerReceive Queue

ClientReceive Queue

Conversation

SendComplete?

Deadlock Example: The TpmContract

IO_RUNNING$0


ReadyState$1

ReadyState$0

Send?

IO_RUNNING$0


ReadyState$1

ReadyState$0

Server Projection

Send!

AckStartSend!

AckStartSend?

SendComplete!

GetTpmStatus!

GetTpmStatus?TpmStatus!

GetTpmStatus!TpmStatus? TpmStatus?

SendComplete?

TpmStatus!GetTpmStatus?

ServerReceive Queue

ClientReceive Queue

Send

AckStartSendSendComplete

GetTpmStatus

TpmStatus

ConversationCS: SendSC: AckStartSendSC: SendCompleteCS: GetTpmStatusSC: TpmStatus

ClientProjection

Realizability Problem

• TpmContract 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: TpmContractSince 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


ReadyState$1

ReadyState$0

CS:Send SC:AckStartSend

SC:SendComplete

CS:GetTpmStatusCS:GetTpmStatusSC:TpmStatus SC:TpmStatus

IO_RUNNING$1

SC:SendCompleteSC: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

…

Sk

SC:m1

SC:m1

SC:m1

SC:m2

SC:m2

SC:m2

CS: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

…

Sk

SC:m1

SC:m1

SC:m1

SC:m2

SC:m2

SC:m2

CS: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: Focuses on two-party communication– [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: http://codeplex.com/singularity• More recent work on Session Types on multi-party communication

– [Honda, Yoshida, Carbone POPL’08] Multiparty Asynchronous Session Types

– [Danielou, Yoshida, ESOP’12] Multiparty Session Types Meet Communicating Automata

– Correspond to sufficient conditions for realizability

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

Earlier results related to 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

http://www.cse.yorku.ca/~franck/research/drafts/

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

• Extensions– 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• Adoption of choreography specification and analysis in practice

– Web service choreography specification languages have been out there for years, are they being used in practice?

– Message-based interactions appear in many domains, can the general analysis techniques for message-based interactions be adopted in practice in multiple domains?

• All software is moving to the network/cloud and all devices are becoming programmable– How can we integrate conversation specification and analysis to modern

applications in practice?

THE END

Documents

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