The document proposes a decentralized approach for executing composite web services using a chemical programming paradigm. Key points: 1) Current workflow execution is centralized, with problems of scalability and bottlenecks. The chemical model allows for decentralized, dynamic, and autonomic execution. 2) In the chemical model, a program is like a chemical solution with data as molecules and computations as chemical reactions. The multiset data structure contains all molecules. 3) The architecture uses "chemical web services" that encapsulate actual web services with a chemical engine for local execution. Services communicate through a shared distributed multiset acting as the coordination mechanism. 4) A high-order chemical language is used to define workflows by

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Decentralized Approach for Execution
of Composite Web Services using the
Chemical Paradigm
Hector Fernandez, Thierry Priol and Cedric Tedeschi
AUTOCHEM (ANR-07-BLAN-0323)

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Context
• Service oriented computing
• Service composition expressed as workflow
– Workflow execution
Introduction
• 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
Introduction
• 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:
Chemical Programming Paradigm
• 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].

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High-Order Chemical Language (HOCL)
• The High-Order Chemical Language, HOCL [Banâtre et al., 2006], extension of Gamma.
Chemical Programming Paradigm
• 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
Decentralized Composite Web Services Execution
• 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.
using a Chemical Paradigm
• Loose coupling among web services.
• Each node is co-responsible of the execution.

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Building Block: Chemical Web Service
Decentralized Compositeblock Services Execution
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
Building Web
– HOCL interpreter (Chemical workflow engine)
using a Chemical Paradigm

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Architecture
Decentralized Composite Web Services Execution
• 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.
Architecture
using a Chemical Paradigm
•
• Multiset mirrors DSM's (Distributed Shared Memory) behavior.

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Chemical Workflow Definition
Decentralized Compositeblock Services Execution
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.
Building Web
In a chemical workflow definition
using a Chemical Paradigm
• Workflow definition using chemical expressions.
• Distributes data and control dependencies to each ChWS.
• Executes in parts a workflow in each ChWS.
Multiset

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Chemical Representations for Workflow
Decentralized Compositeblock Services Execution
• 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.
Building Web
using a Chemical Paradigm

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Chemical rules for distributed execution
Decentralized Composite Web Services 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).
using a Chemical Paradigm
• Execution of the workflow step by step (MR_invoke).

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Example

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Parallel Synchronization
Service2
Service1 Service4
Service3

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• Workflow will be executed in parts.

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More complex examples

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Multi choice Synchronization merge
Despatch police
Emergency request Transfer patient
Despatch
ambulance

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Multi-choice
control dependencies
Synchronization merge
control dependencies
Data dependence

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Conclusion
• Demand for more decentralized and dynamic interaction schemas.
• Chemical model is well featured for decentralized workflow execution.
Summary
• 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).
Summary
• Implementation of a prototype (on-going work).
• Autonomic behaviour in decentralized workflow execution.

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Questions ?