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Apache ODE – BPEL Extensions

BPEL Extensions

Overview

ODE extends the WS-BPEL in areas that aren't covered by the spec. This page details these extensions.

Implicit Correlations

BPEL process instances are stateful — therefore, a client interacting with the BPEL engine must identify the particular instance with which it intends to interact in all of its communications. The BPEL specification defines a mechanism — correlation — which allows the process designer to specify which parts of an incoming message (i.e. a message going from a client to the BPEL server) should be used to identify the target process instance. Correlation is a powerful mechanism — however it is a bit complicated and relies on "in-band" message data to associate a messages with a process instance.

To keep simple cases simple, ODE provides an alternative correlation mechanism — implicit correlation — that automatically handles correlation through "out-of-band" session identifiers. The mechanism is simple: a unique session identifier is associated with every every partner link instance. When a message is sent on a partner link, the session identifier is sent along with the message. The recipient is then able to use the received session identifier in subsequent communications with the process instance. Messages received by the BPEL engine that have a session identifier are routed to the correct instance (and partner link) by that session identifier.

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Activity Failure and Recovery

There are several types of error conditions. In this document we introduce a class of error condition called failures, distinct from faults, and describe how failures are caught and handled by the process engine.

For example, when the process is unable to perform DNS resolution to determine the service endpoint, it generates a failure. An administrator can fix the DNS server and tell the process engine to retry the activity. Had the DNS error been reported as a fault, the process would either terminate or require complex fault handling and recovery logic to proceed past this point of failure.

In short, failures shields the process from common, non-terminal error conditions while retaining simple and straightforward process definitions that do not need to account for these error conditions.

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Extension Activities & Extensible Assign Operations

Apache ODE provides a plug-in architecture for custom activity implementations and custom variable assignment logic. Such plug-ins can be registered to ODE and can be used according to the WS-BPEL 2.0 extensibility mechnisms (<extensionActivity> & <extensionAssignOperation>).

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XPath Extensions

Apache ODE extends the default XPath coverage provided by the WS-BPEL specification mostly by adding support for XPath 2.0 and by offering a few utility extension functions to make some assignments easier.

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XQuery Extensions

Apache ODE extends the default XPath coverage provided by the WS-BPEL specification mostly by adding support for XQuery 1.0 and by offering a few utility extension functions to make some assignments easier.

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External Variables

External variables are an easy way to share data between the process and external systems, by treating those independent entities as BPEL variables that can be used in expressions and assignments. External variables may be records stored in the database, REST resources, etc.

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Headers Handling

There are several situations where one would want to access headers form the wire format in their BPEL process. It could be to assign them to another message and pass them around or to execute some business logic that's header-dependent (often a bad idea but sometimes there's no choice). ODE supports the manipulation of wire format headers in two different ways.

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RESTful BPEL, Part I

Extends the invoke activity to handle RESTful Web services. This extension uses BPEL variables of type xsd:uri and xsd:string instead of partner links, and does away with the WSDL indirection, making it straightforward to develop processes that use RESTful Web services.

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RESTful BPEL, Part II

Extends receive and onEvent to expose RESTful resources that. This extension adds the ability to declare and instantiate resources, and specific handling for the HTTP methods GET, POST, PUT and DELETE.

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Iterable ForEach

Extends the activity so that the counter variable iterates over the items contained in a given sequence. This extension provides an alternative and more dynamic way of generating M branches, where M is the size of the binding sequence. The semantics of the stays the same, in that the loop is deemed to be complete if B out of the M branches complete (successfully), where B is its actual value.

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Flexible Assigns

Extends the activity so that it can be made to either ignore or insert data that is missing in the to-spec of a copy operation. This shortcut allows users to handle what would normally be fault scenarios, in a more graceful and intuitive way.

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Process Contexts

Extends BPEL and ODE to allow the circulation of transport metadata (like security tokens, correlation keys, or tracing informations) from messages to processes and then from processes to messages.

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