decentralized workflow execution using a chemical metaphor

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AUTOCHEM (ANR-07-BLAN-0323)

Decentralized Approach for Execution of Composite Web Services using the

Chemical Paradigm

Hector Fernandez, Thierry Priol and Cedric Tedeschi

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Context

• Service oriented computing

• Service composition expressed as workflow

– Workflow execution

• Centralized Workflow execution:

• A single coordinator node.

• The composite web service will be executed in this node.

• Responsible for coordination of all data and control flow between the web services.

➔ Problems [Alonso et al., 1997][Chafle et al., 2004]:

• Scientific issues: poor scalability, communication and performance bottlenecks.

• Societal issues: privacy

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Objectives

• Ensure an execution of composite services:

• Decentralized

• Dynamic

• Autonomic

• Loose coupling (coordination mechanism)

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“Nature-inspired metaphors have been shown to be of high interest for service coordination.”

[Viroli et al., 2009]

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Chemical Programming Model• A program can be seen as a chemical solution:

• Data: “floating” molecules in the solution.

• Computation: chemical reactions between the molecules.

• Implicit parallelism and autonomy of reactions until inertia.

• Data structure: Multiset.

• Containing all data molecules.

• Reaction rules re-writing the multiset.

• Pioneered by Gamma (General Abstract Model for Multiset Manipulation) [Banâtre et al., 1990].C

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High-Order Chemical Language (HOCL)• The High-Order Chemical Language, HOCL [Banâtre et al., 2006], extension of Gamma.

• Reaction rules apply on reaction rules.

• Every entity is a molecule in the multiset.

• Autonomic behaviors.

• Dynamic (runtime).

• Example:

• A reaction rules is written

replace-one P by M if C

where P is a pattern which matches the required molecule, C is the reaction condition and M the result of the reaction.

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Decentralized Composite Web Services Execution using a Chemical Paradigm

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Architecture overview

• Nodes communicating through a persistent and fault-tolerant shared address space.

• Composite web service is executed in a fully decentralized manner.

• Workflow is executed in parts corresponding with each web service

• Data and control transfer through the shared address space.

• Loose coupling among web services.

• Each node is co-responsible of the execution.

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Building Block: Chemical Web Service

The architecture is composed of:

• Chemical web services (ChWSes), a chemical encapsulation of a web service.

• Service invocation

• Storage space (Contains part of multiset)

• Chemical engine– HOCL interpreter (Chemical workflow engine)

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Architecture• Multiset, dynamic and decentralized coordination mechanism.

– Acts as a shared address space containing both control and data flows.– ChWSes communicate through the multiset. (reading and writing)– Physically distributed over ChWSes storage spaces.

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• Multiset mirrors DSM's (Distributed Shared Memory) behavior.

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Chemical Workflow Definition

In a classical workflow definition

• Executable languages like BPEL [OASIS, 2007], XPDL [WfMC, 2008] or YAWL [Van der Aalst et al., 2005].

• All data and control dependencies in the same file.

• Made to be executed in a centralized way.

In a chemical workflow definition

• Workflow definition using chemical expressions.

• Distributes data and control dependencies to each ChWS.

• Executes in parts a workflow in each ChWS.

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Multiset

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Chemical Representations for Workflow

• Express all data and control dependencies (reaction rules and molecules).

• We use the High-Order Chemical Language (HOCL)

• The whole solution represents the multiset containing all information.

• The solution itself is composed of as many sub-solutions as ChWSes.

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Chemical rules for distributed execution

• Independent from any chemical workflow representation.

• Local chemical engines use these chemical rules.

• Two sets of chemical rules:

• Control and data transfer among ChWSes (MR_transfer).

• Execution of the workflow step by step (MR_invoke).

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Example

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Service2

Service3

Service4Service1

SynchronizationParallel

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• Workflow will be executed in parts.

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More complex examples

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Despatch police

Despatch ambulance

Transfer patientEmergency request

Synchronization mergeMulti choice

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Synchronization mergecontrol dependencies

Multi-choicecontrol dependencies

Data dependenceData dependence

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Conclusion

• Demand for more decentralized and dynamic interaction schemas.

• Chemical model is well featured for decentralized workflow execution.

• Our proposal: High-level decentralized coordination mechanism.

• Decentralized Architecture:– Chemical web services working as local engines.– Distributed Multiset

– Data and control dependencies– Temporal composition

• A High-order chemical language for composite web services:– Concepts for decentralized coordination.– Autonomic behavior

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Future Work

• Decentralized workflow coordination for composite web services (Chemical engines).

• Implementation of a prototype (on-going work).

• Autonomic behaviour in decentralized workflow execution.

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Questions ?

decentralized workflow execution using a chemical metaphor

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