“Dynamic fault handling mechanisms for service-oriented applications” Fabrizio Montesi, Claudio Guidi, Ivan Lanese and Gianluigi Zavattaro Department of Computer Science, University of Bologna

Outline Introduction Key concepts Static approach versus dynamic approach Fault, termination and compensation handlers Faults in the Request-Response pattern Demo Conclusions

Introduction Service oriented applications are usually distributed Orchestrators rely upon a workflow programming approach (e.g. WS-BPEL) Faults can be propagated within a system by involving several services Basic mechanisms: fault, termination and compensation handling. Static approach used so far, but some difficulties exists in some scenarios

Key Concepts (1) A scope is a process container able to manage faults A fault is a signal raised by a process towards the enclosing scope Termination is automatically triggered when a running scope must be smootlhy stopped because of a fault thrown by a parallel process Compensation is invoked to undo the effects of a scope whose execution has already successfully completed. Recovering mechanisms are implemented by exploiting handlers

Key concepts (2) Scope A Scope BScope C FH TH CH P A process P is in parallel with scope B and CFault handlersTermination handlersCompensation handlers f Process P raises fault f QR Scope C finishes with success C Compensation handler of scope C is promoted to the parent scope Scope B, which is still running, terminates its activities When all the child scopes have been termintated, the fault handler of scope A is executed Fault f Compensation handlers can be invoked within a fault handler

The static approach Usually error recovery is managed by statically associating handlers to scopes scope q ( P, FH, TH, CH ) Example: parallel composition between R and scope q where the compensation policy for q requires to compensate the activities executed so far in a reversed order. scope q ( i=0; while( i<100 ){ i=i+1; if (i%2=0) then P else Q }, FH, TH, CH ) R | i=50i=51 f

In this paper… We present a new approach for dealing with fault mechanisms: the dynamic approach The formal model has been presented in a previous paper. It is an extension of SOCK calculus We have implemented such a formal model within JOLIE. JOLIE is the actual implementation of SOCK

The formal model: SOCK SOCK is formal calculus for modelling Service Oriented Computing systems developed at the University of Bologna Three layers: service behaviour layer service engine layer services system layer It supports the RequestResponse communication pattern In a previous work, we have modelled fault handling mechanisms by exploiting a dynamic approach. "On the Interplay Between Fault Handling and Request-Response“ Claudio Guidi, Ivan Lanese, Fabrizio Montesi and Gianluigi Zavattaro ACSD pages , IEEE Computer Society, 2008.

Jolie is the implementation of SOCK It is an open-source project It is developed in Java It is a complete programming language It supplies a new programming paradigm which is the service oriented one In Jolie everything is a service It supports programming primitives for managing XML data Service embedding and redirection

The dynamic approach The basic idea of the dynamic approach is that handlers can be updated while computation progresses scope q ( P, H ) where H is a function which associates fault names to fault handlers and scope names to termination and compensation handlers Dynamic handling installation is addressed by two specific primitives: inst(H’) :updates the current handler function with H’ cH : refers to the previous installed handlers

Updating handlers The install primitive updates the current handler for the specified fault or scope name main { scope( myScope ) { install( f => Handler for f")); line 1"); install( f => Handler updated for f")); line 2"); install( f => cH; handler")); line 3"); throw( f ) } f => Handler for f")f =>f => Handler updated for f")f => Handler updated for f"); handler")

Execution priority for install primitive The install primitive is executed with priority w.r.t. fault processing main { install( myFault => HANDLER FOR myFault ); scope( myScope ) { install( this => Handler for f")); line 1"); install( this => Handler updated for f")); line 2"); install( this => cH; handler")); line 3") } | scope( faultScope ) { throw( myFault ) } f

Termination Termination is always propagated to sibling and child scopes, and it is always completed before the enclosing scope processes the fault handler main { install( f => FAULT HANDLER FOR f ); scope( myScope ) { install( this => HANDLER FOR myScope ); scope( childScope ) { install( this => HANDLER FOR childScope );... } } | throw( f ) } f

Compensation A compensation handler is a termination handler promoted to the parent scope when the child scope finishes successfully. A compensation handler can be activated by means of a specific primitive which can only be used within a handler. main { install( f => FAULT HANDLER FOR f; comp( myScope ) ); scope( myScope ) { install( this => HANDLER FOR myScope; comp( chScope ); scope( chScope ) { install( this => HANDLER FOR childScope );... } } ; throw( f ) } chScope myScope

From static to dynamic Static approach can be trivially simulated with the dynamic one: scope q ( P, FH, TH, CH ) scope q (inst(f=>FH);inst(this=>TH);P;inst(this=>CH),H) We can rewrite the previous example as it follows: scope q ( i=0; while( i P’;cH)} else {Q;inst(this=>Q’;cH)} } ) R | scope q ( i=0; while( i<100 ){ i=i+1; if (i%2=0) then P else Q }, FH, TH, CH )

Request-Response communication pattern Jolie supports the RequestResponse communication pattern which is inspired by the Request-Response and Solicit- Response operations of WSDL 1.1 In a RequestResponse operation a message is received and, after a body process is executed a response is sent to the invoker. Jolie supplies two specific primitives for receiving and sending messages on a RR op_name( request )( response ){ body_process } request )( response )

Faults in the Request-Response pattern Fault management within a RequestResponse pattern is an interesting issue: What happen when a fault is raised within the RR? What happen when a fault is raised by a process on the SR side? SRRR P Behaviour without faults

Faults in the Request-Response pattern (2) What happen when a fault is raised within the RR? In this case, a fault message is automatically sent to the sending primitive of the invoker SRRR P Behaviour without faults

Faults in the Request-Response pattern (3) What happen when a fault is raised by a process on the SR side? In this case, the sending primitive (SR) always waits for the incoming response before the fault is handled by the service SRRR P Behaviour without faults

Demo: the automotive example It describes the scenario where a car failure occurs. The car requests for an orchestrator to the assistance service in order to search for garage, car rental and truck services. In case the failure is about a tyre replacement, the orchestrator will look for a garage service which can send someone to the car for replacing the tyre. In case the failure is about the engine, the orchestrator will look for a garage and for a truck for carrying the car to the garage. Concurrently, it will look for a rental car service in order to find another car to use. The rent car will be sent to the garage coordinates if found, to the faulted car coordinates otherwise.

Demo: the automotive example (2) The engine failure: Assistance Car Register Garages 21 3 Rental 21 3 Truck 21 3 Banks AEVisa mySQL 9

Demo: the automotive example (3) The engine failure involves also the truck services and the car rental ones if no garage services are available the orchestrator will not search for a truck service and, if a rental car service is found the new car is redirected to the faulted one instead of the garage if no truck services are available the garage reservation will be revoked and the car rental one, if found, will be redirected to the faulted car coordinates.

Something to know about Jolie… In Jolie everything is a service Services can be embedded, both statically and dynamically, within another service Services can be passed from a service to another through message exchange and then embedded at run-time: service mobility Jolie exploits the programming power of Java. It is possible to create services developed in Java which can be easily embedded within a Jolie service: JavaServices

Conclusions In Jolie we have implemented the dynamic fault mechanisms formally developed in our previous work the dynamic approach is based upon the install primitive for updating handlers the install primitive must be executed with priority w.r.t. fault mamagement RequestResponse communication pattern has been enhanced with automatic fault communication As a future work we intend to formalize fault mechanisms in our choreography calculus and then implement it

Thank you! Questions?