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Ivan LaneseComputer Science Department

University of BolognaItaly

On the Interplay between Fault Handlingand Request-response Service Invocations

Joint work with Claudio Guidi, Fabrizio Montesi and Gianluigi Zavattaro

Roadmap

Idea of the work

Our approach to error handling

– Extending SOCK

– Dynamic installation of handlers

– Interplay with request-response

The automotive case study

Conclusive remarks

Roadmap

Idea of the work

Our approach to error handling

– Extending SOCK

– Dynamic installation of handlers

– Interplay with request-response

The automotive case study

Conclusive remarks

Service Oriented Computing (SOC)

SOC is a paradigm to program distributed applications– Based on the composition of dynamically discovered, loosely-

coupled services– Services interact using the notification and request-response

patterns Allows integration of services from different companies

– Great code reusability Has to deal with interoperability, dynamicity, security,

Quality of Service, reconfiguration… Based on standards for data (XML), communication

(SOAP), discovery (WSDL and UDDI) and orchestration (BPEL)

Error handling

Safe composition of services requires todeal with faults– Scarce guarentees on components’ behaviour

because of loose coupling

– Unexpected events can happen

A fault is an abnormal situation that forbids the continuation of an activity– An activity that generates a fault is terminated

Faults should be managed so that the whole system can reach a consistent state

BPEL offers various mechanisms for error handling

Fault handling mechanisms

Based on the idea of long running transactions– ACID transactions impossible to obtain

– Compensations are used to reach a consistent state

Faults terminate the current activity and trigger recovery activities specified by suitable handlers

Fault handler: executed as answer to a fault Termination handler: executed to smoothly terminate a

parallel activity Compensation handler: executed to undo the effect of an

already completed activity

What we propose?

A formal model for faults and compensations in SOC– Based on the process calculus SOCK– Rigorous LTS semantics

1. Expressive high-level primitives– Mimicking BPEL ones– Allowing for easy management of different kinds of faults

2. Dynamic installation of handlers– The fault handling code is always up-to-date

3. Faults do not spoil request-response communications– Automatic notification in case of server fault– The client always waits the reply from the server– Possible to recover from remote errors

Why process calculi?

Formal methods are necessary to master the complexity of SOC– Different implementations of BPEL have different behaviours

– Standards are only informally defined

– Difficult to understand the interplay between different features

Process calculi allow to unequivocally specify the behaviour of the different mechanisms…– Clarify their semantics and their interactions

– Drive the implementations

… and prove properties of the resulting system– Good behaviour guarenteed

The underlying language

SOCK (Service Oriented Computing Kernel) is a process calculus for SOC

Explores service interactions– Based on notification and request-response primitives

– Composed using standard operators from imperative languages (while, …) and process calculi (parallel composition, …)

Strongly related to current technologies– WSDL, BPEL

But with full formal semantics Driving the implementation of the language JOLIE

Roadmap

Idea of the work

Our approach to error handling

– Extending SOCK

– Dynamic installation of handlers

– Interplay with request-response

The automotive case study

Conclusive remarks

Extending SOCK with faults and compensations

Code boxed into scopes {P}q

– Provide a hierarchical structure

– Define the boundaries of error handling activities

Primitives for:– Throwing faults: throw(f)

– Installing handlers: inst(u,P)

– Invoking compensations: comp(q)

A few other things

The scope hierarchy

P H

q

P H

q

P H

q

P H

q

P H

q

At runtime the scopes

contain handlers H

Throwing a fault

q1

q2

(f,Q)

Throw

(f)

(q2,T2)

(q1,T1)

Throwing a fault

q1

q2

(f,Q)

(q2,T2)

(q1,T1)

f

Faults kill activities

while propagating

Throwing a fault

T1

q1

T2

q2

(f,Q)

f

Termination handlers

of parallel activities

are executed

Throwing a fault

T1

q1

T2

q2

Q

f

When found, the fault

handler is executed

More on fault propagation

Recovery activities cannot be killed by other faults– Error recovery activities are always completed

But termination overrides fault handling– Global errors more important than internal ones

After having been killed a scope smoothly terminates– Ongoing communications are terminated

– No more faults can be thrown

Compensation handlers

Allow to undo the effect of an already completed activity– The fault handler of a purchase activity could ask to annul a

previously done payment

Has to be explicitly programmed and invoked– Primitive comp(q)

– Available only inside handlers

– Only child activities can be compensated

Roadmap

Idea of the work

Our approach to error handling

– Extending SOCK

– Dynamic installation of handlers

– Interplay with request-response

The automotive case study

Conclusive remarks

Dynamic fault handling

In all the languages we are aware of handlers are statically installed while programming– Java throw … catch …

– BPEL handlers

Not always easy to write the desired compensation

Example

{ throw(f) | while (i <100) if i%2=0 then P else Q , H}q

We want to compensate each completed execution of P and Q in the reverse order of execution

We need auxiliary variables to track the executions of P and Q– Complex and error-prone

Atomicity problem– Suppose P has been executed but the auxiliary variables have

not been updated yet– If a fault occurs then the last execution of P is not

compensated

Our solution

{ throw(f) | while (i <100) if i%2=0 then P ; inst(f,P’;cH)

else Q ; inst(f,Q’;cH) , H0}q

P’ compensates P, Q’ compensates Q The handlers are dynamically installed cH (for current handler) allows to recover the previous

handler for updating inst is a special primitive that has higher priority than

fault execution– No atomicity problem

Installing an handler

Inst

(f,Q)

As soon as inst is

enabled it is executed

Installing an handler

(f,Q)

Compensation handlers

When an activity terminates the last defined termination handler becomes its compensation handler

Same handling for faults immediatly before activity termination and immediatly after activity termination

Installing compensation handlers

q

q’

Inst

(q,Q)

Installing compensation handlers

q

(q,Q)

Q terminates

q’

Installing compensation handlers

(q,Q)

Handlers in q’

can compensate

q using comp(q)

q’

Roadmap

Idea of the work

Our approach to error handling

– Extending SOCK

– Dynamic installation of handlers

– Interplay with request-response

The automotive case study

Conclusive remarks

Fault handling and request-response

Request-response is a long lasting interaction Faults on one side influence the other side Two possibilities:

– Faults on server side during the interaction

– Faults on client side while waiting for the answer

Faults on server side

A client asks a payment to the bank, the bank fails In ActiveBPEL the client receives a generic “missing-

reply” exception Our approach

– The exact fault is notified to the client

– The notification acts as a fault for the client

– Suitable actions can be taken to manage the remote fault

Faults on client side

A client asks a payment to the bank, then fails before the answer

In BPEL the return message is discarded Our approach

– The return message is waited for

– The handlers can be updated according to whether or not a non-faulty message is received

– The remote activity can be compensated if necessary

Roadmap

Idea of the work

Our approach to error handling

– Extending SOCK

– Dynamic installation of handlers

– Interplay with request-response

The automotive case study

Conclusive remarks

Automotive case study

A car failure forces the car to stop The car service system looks for

– A garage to repair the car

– A tow truck to take the car to the garage

– A car rental to take the driver home

The suitability of the services is checked The services are booked and paid via a bank

(Part of) the automotive case study in SOCK

CARP ::= RP j (GP ;TP )

RP ::= book@R(Gcoords;hRacc;R idi );pay@B(hRpr ices;R id;Racci ;Rpayid)

GP ::= book@G(f ailure;hGacc;Gidi );pay@B(hGpr ice;Gid;Gacci ;Gpayid)

TP ::= book@T(hCARcoords;Gi ;hTacc;Tidi );pay@B(hTpr ice;Tid;Tacci ;Tpayid)

Adding tow truck faults

CARP ::= finst(f T;comp(g) j comp(r));( RP j (GP ;TP ) )

gmainRP ::= f : : :grGP ::= f : : :ggTP ::= f

inst(f B ;comp(tb);throw(f T)); fbook@T(hCARcoords;Gi ;hTacc;Tidi );inst(tb;revbook@T(Tid))

gtb;pay@B(hTpr ice;Tid;Tacci ;Tpayid)gt

Screenshot from JOLIE

Feedback from the case study

Easy to write the desired error handling policies All the mechanisms are used

– Useful to allow for frozen variables in handlers code

All unexpected behaviours catched

Roadmap

Idea of the work

Our approach to error handling

– Extending SOCK

– Dynamic installation of handlers

– Interplay with request-response

The automotive case study

Conclusive remarks

Conclusions

Formal framework for error handling in SOC– Near to current technologies (BPEL)…

– … but with formal semantics

Dynamic installation of handlers– Allows to update the termination handler as the activity

progresses

Errors do not spoil the request-response protocol– Either the fault or the normal answer is sent back

– The answer can be used during error recovery

Future work

Check whether the approach can be applied to other languages– Other languages for SOC (COWS, SSCC, CASPIS)

– π-calculus

Study the relationships between different kinds of primitives for fault handling– Static vs dynamic– Hierarchical vs flat

End of talk