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Oracle® Fusion Middleware Concepts and Architecture for Oracle Service Bus
11g Release 1 (11.1.1.6.3)

Part Number E15020-08
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5 Service Composition

This chapter discusses the service composition capabilities of Oracle Service Bus. It highlights operational features that enable service configuration and key concepts associated with message structures and message flow modeling. It is intended for integration-focused IT architects responsible for messaging and service oriented architectures (SOA) and service modelers or designers.

This chapter includes the following sections:

5.1 Dynamic Content-Based Routing

Oracle Service Bus mediates service request and response messages between disparate heterogeneous service endpoints and intelligently routes messages between them. Content-based routing is a mediation capability supported by Oracle Service Bus based on conditional message processing and transformation capabilities. This routing capability allows loose coupling of SOA endpoints and is particularly useful and allows service enrichment and reuse by combining transformation and routing functions.

Oracle Service Bus performs dynamic message routing based on a message content, for cases when services or responses need to be directed to multiple destination service and in scenarios where different versions of a service have to be provisioned based upon business service requests. Dynamic routing is useful when business requirements dictate that certain conditions of a request define where it should be processed. For example, a financial institution's request for a credit report on a customer may use any of several credit services based on where the customer or organization resides.

In dynamic routing, a message is analyzed using conditional checks in conditional branching statements, to retrieve the value of a data element or multiple data elements that determine the routing logic. Different business service destinations are assigned to different value combinations resulting from this conditional check. The message is dynamically routed to one of multiple destination business services based on the data element value. Transformations may be applied to the response message going to one or more of these destinations depending on business-service requirements.

5.1.1 Business Services and Proxy Services

Oracle Service Bus routes message between business services (such as enterprise services and databases) and service clients (such as presentation applications or other business services) through proxy services. The following sections detail the Oracle Service Bus features available for designing and implementing proxy services that support content-based routing.

5.1.1.1 Proxy Services

Proxy services are definitions of generic intermediary Web services, that are hosted locally on Oracle Service Bus. A proxy service communicates with other services in the IT infrastructure through interfaces, which may or may not be identical to that of a service provider or service consumer business service. Proxy services can route messages to multiple business services, using their configured independent interfaces. For more information on proxy services, see "Proxy Services: Creating and Managing" in the Oracle Fusion Middleware Administrator's Guide for Oracle Service Bus.

Proxy services can be defined and configured using the Oracle Service Bus Administration Console. They are configured by specifying its interface, type of transport it uses, and its associated message processing logic. Message handling capabilities of a proxy service are implemented with message flow definitions. For more information on message flow definitions, see "Modeling Message Flow in Oracle Service Bus" in the Oracle Fusion Middleware Administrator's Guide for Oracle Service Bus.

When a proxy service interfaces with multiple business services, a message flow definition is configured to route a message to the appropriate business service and map the message data into the format required by the business service's interface. For more information about the structure of proxy services, see Section 5.2, "Message Flow Modeling."

5.1.1.2 Business Services

Business services are Oracle Service Bus definitions of the enterprise services that exchange messages during business processes. A business service and its interface can be defined and configured using the Oracle Service Bus Administration Console. A business service is configured by specifying its interface, type of transport it uses, its security requirements, and other characteristics. A business service definition is similar to that of a proxy service, but it does not have a pipeline. For information on how to configure a business service using the Oracle Service Bus Administration Console, see "Business Services: Creating and Managing" in the Oracle Fusion Middleware Administrator's Guide for Oracle Service Bus.

A service account can be created to provide authentication when connecting to a business service. It acts as an alias resource for the required username and password pair. Oracle WebLogic Server can be used to directly manage security credentials for a business service requiring credential-level validation. For more information on business service security considerations, see the Oracle Fusion Middleware Developer's Guide for Oracle Service Bus.

5.1.1.3 Proxy Templates

Oracle Service Bus provides the ability to create a generic proxy service that accepts any SOAP or XML messages. This helps masks the underlying complexity of protocol specifications from the service consumer. The proxy service can be configured to analyze SOAP or XML messages that it receives and dynamically route the message using content-based routing logic. Generating proxy services from a generic template, also promotes dynamic protocol switching, which allows end-point protocol selection to be made at run-time.

5.2 Message Flow Modeling

Message flows are definitions used for implementing proxy services within Oracle Service Bus. Message flow modeling involves configuration of message processing logic in proxy service message flow definitions. This logic includes such activities as transformation, publishing, reporting and exception management. Each of these activities are configured as individual actions within the message flow. The graphical modeling tools available in Eclipse and in the Oracle Service Bus Administration Console can be used to perform message modeling.

Oracle Service Bus proxy service implementations are defined in message flow definitions using components such as pipelines, branch nodes, and route nodes. The following figure shows a high-level view of the message flow definition components.

For more information on message flow modeling, see "Modeling Message Flow in Oracle Service Bus" in the Oracle Fusion Middleware Administrator's Guide for Oracle Service Bus.

Figure 5-1 Message Flow Components

Description of Figure 5-1 follows
Description of "Figure 5-1 Message Flow Components "

5.2.1 Message Pipelines

A pipeline is a named sequence of stages, representing a non-branching one-way processing path. It is used to specify the message flow for service requests and responses. Pipelines fall into one of the following three categories:

  • Request Pipelines: used for processing the request path of the message flow

  • Response Pipelines: used for processing the response path of the message flow

  • Error Pipelines: used as error handlers

5.2.1.1 Operational Pipelines

A single service level request pipeline in a stage might optionally branch out into operational pipelines (at most one per operation, and optionally a default operational pipeline). The operation is determined by user-selected criteria. The response processing starts with the relevant operation pipeline which then merges into a single service-level response pipeline. The following figure shows an example of operation pipelines in a proxy service.

Figure 5-2 Sample Operational Pipeline

Description of Figure 5-2 follows
Description of "Figure 5-2 Sample Operational Pipeline"

For one-way operations, the response pipeline is executed with an empty message. This permits a response to be constructed for the proxy service, enabling bridging between request/response and one-way operations. The bridging mechanism means that proxy service input can be one-way while its output is request/response or vice versa. The proxy service either absorbs the response from the invoked service or generates one for the client. Actions in the response flow may also be used to do post processing on the message after it has been routed to the business service or the proxy service.

5.2.1.2 Branch Nodes

A branch node allows processing to proceed down exactly one of several possible paths. Branching is driven by a simple lookup table with each branch tagged with a simple but unique string value.A variable in the message context is designated as the lookup variable for that node, and its value is used to determine which branch to follow. If no branch matches the value of the lookup variable, then a default branch is followed. The value of the lookup variable must be set before reaching the branch node. This approach ensures that exceptions do not occur within the branch node itself. A branch node may have several descendants in the message flow tree: one for each branch including the default branch.

Figure 5-3 Branch Nodes in a Message Flow

Description of Figure 5-3 follows
Description of "Figure 5-3 Branch Nodes in a Message Flow"

5.2.1.3 Route Nodes

The route node is used to perform request and response communication with another service. It represents the boundary between request and response processing for the proxy service, and therefore, cannot have any descendants in the message flow tree. When the route node dispatches a request message, request processing is considered finished. When the route node receives a response message, response processing begins.

The route node is very flexible in its specification and supports conditional routing as well as outbound and response transformations. It allows if structures and case structures to be combined (and nested) to define a single endpoint and operation to route the message. For information about how to configure route nodes, see "Adding Route Nodes to Message Flows" in the Oracle Fusion Middleware Administrator's Guide for Oracle Service Bus.

Figure 5-4 Proxy Service Route Node Communicates With Services

Description of Figure 5-4 follows
Description of "Figure 5-4 Proxy Service Route Node Communicates With Services"

Echo node is a route node that routes (or echoes) a message from the end of the request pipeline to the start of the response pipeline. The message is not routed from the proxy service to another service, but remains within the proxy service.

5.2.1.4 Pipeline Pairs

Pipeline logic occurs in pairs of definitions consisting of a request pipeline definition and a response pipeline definition. The request pipeline definition specifies the actions that Oracle Service Bus performs on request messages to the proxy service before invoking a business service or another proxy service. The response pipeline definition specifies the processing that Oracle Service Bus performs on responses from the service invoked by the proxy service before the proxy service returns a response. Routing is performed by a route node at the end of the message flow.

To create the request and response paths, request and response pipelines are paired together and organized into a single-rooted tree structure. A branch node allows these pipeline pairs to be executed conditionally, and route nodes at the ends of the branches perform the request and response dispatching. A pipeline tree chains together instances of the following top-level components - pipeline pair node, branch node, route node or echo node.

A pipeline pair node ties together a single request and a single response pipeline into one top-level element. Only the request pipeline is executed during request processing, and only the response pipeline is executed when reversing the path for response processing.

5.2.1.5 Pipeline Execution Stages and Actions

Each pipeline is a sequence of stages that contain actions. An action is a user-configured processing step such as transformation or publishing. Messages fed into the message flow are accompanied by a set of message context variables that contain the message contents and can be accessed or modified by actions in the pipeline stages.

The following tables describe the actions supported in Oracle Service Bus pipeline stage, branch, and route nodes. For information about actions, including how to configure them, see "Proxy Services: Actions" in the Oracle Fusion Middleware Administrator's Guide for Oracle Service Bus.

Table 5-1 Oracle Service Bus Communication Actions

Action Summary Description

Dynamic Publish

Publish a message to a service identified by an Xquery expression

Publish

Publish a message to a statically specified service.

Publish Table

Publish a message to zero or more statically specified services. Switch-style condition logic is used to determine at runtime which services will be used for the publish.

Routing Options

Modify any or all of the following properties in the outbound request: URI, Quality of Service, Mode, Retry parameters.

Service Callout

Configure a synchronous (blocking) callout to an Oracle Service Bus-registered proxy or business service.

Transport Headers

Set the transport header values in messages.


Table 5-2 Oracle Service Bus Flow Control Actions

Action Summary Description

For Each

Iterate over a sequence of values and execute a block of actions.

If... Then...

Perform an action or set of actions conditionally, based on the Boolean result of an XQuery expression

Raise Error

Raise an exception with a specified error code and description

Reply

Specify that an immediate reply is sent to the invoker; can be a reply with success or failure

Resume

Resume message flow after an error is handled by an error handler.

Skip

Specify that at run time, the execution of the current stage is skipped and the processing proceeds to the next stage in the message flow.


Table 5-3 Oracle Service Bus Message Processing Actions

Action Summary Description

Assign

Assign the result of an XQuery expression to a context variable

Delete

Delete a context variable or a set of nodes specified by an XPath expression

Insert

Insert the result of an XQuery expression at an identified place relative to nodes selected by an XPath expression

Java Callout

Invoke a Java method from within the pipeline.

MFL Transform

Convert non-XML to XML or XML to non-XML in the pipeline.

Rename

Rename elements selected by an XPath expression without modifying the contents of the element

Replace

Replace a node or the contents of a node specified by an XPath expression

Validate

Validate elements selected by an XPath expression against an XML schema element or a WSDL resource


Table 5-4 Oracle Service Bus Reporting Actions

Action Summary Description

Alert

Send an alert notification based on pipeline message context.

Log

Construct a message to be logged

Report

Enable message reporting for a proxy service


5.2.1.6 Operational Branching

Since message flow is typically used with WSDL-based services, operation-specific processing must frequently be performed. Rather than requiring manual configurations of a branching node based on operation, the Oracle Service Bus provides a zero-configuration branching node that branches automatically. In other words, if no operational branching is configured for a service end point, message processing will automatically branch to the appropriate operation based on the operation specified in the message context.

5.2.1.7 Service Callouts

Oracle Service Bus provides a service callout action that offers greater flexibility for more sophisticated message flows. Service Callouts are message processing request actions from one message flow, that invoke other services registered within Oracle Service Bus. This action is generally used in response to decisions made in complex dynamic-routing processing, or to perform message enrichment. The service callout action is used inside a message flow routing stage, to call on the destination service to perform some action on the message. The destination service returns a response to the message flow, which gets assigned to a local variable. The variable may be used within the current message flow for conditional branching.

For information about the Service Callout functionality, see "Constructing Service Callout Messages" in "Modeling Message Flow in Oracle Service Bus" in the Oracle Fusion Middleware Administrator's Guide for Oracle Service Bus.

Service callouts allow custom Java code to be invoked from within proxy services. Oracle Service Bus supports a Java exit mechanism via a Java Callout action that allows call out to a Plain Old Java Object (POJO). Static methods can be accessed from any POJO. The POJO and its parameters are visible in the Oracle Service Bus Administration Console at design time; the parameters can be mapped to message context variables. For information about configuring a Java Callout to a POJO, see "Adding Java Callout Actions" in the Oracle Fusion Middleware Administrator's Guide for Oracle Service Bus.

5.3 Transformations

Transformations are used when disparate message data types exist between source and destination services, requiring data mapping to ensure service compatibility. Oracle Service Bus supports data mapping using XQuery and the eXtensible Stylesheet Language Transformation (XSLT) standard. Messages can be transformed in two ways:

Transformations can be created by a developer and imported into Oracle Service Bus, or scripted using XQuery in the Oracle Service Bus Administration Console. Transformations can occur at different locations depending on the message-flow configuration of the proxy service.

In Oracle Service Bus, the Message Flow defines the implementation of a proxy service. You configure Oracle Service Bus proxy services in the Oracle Service Bus Administration Console, which is described in the Oracle Fusion Middleware Administrator's Guide for Oracle Service Bus.

Oracle Service Bus supports data mapping that uses XQuery and the eXtensible Stylesheet Language Transformation (XSLT) standards. XSLT maps describe XML-to-XML mappings, whereas XQuery maps can describe XML-to-XML, XML to non-XML, and non-XML to XML mappings. For more information on transformations, see "Performing Transformations in Message Flows" in the Oracle Fusion Middleware Administrator's Guide for Oracle Service Bus.

For more information, see "XQuery Transformations" and "XSL Transformations" in the Oracle Fusion Middleware Administrator's Guide for Oracle Service Bus. For information on using the Oracle XQuery Mapper to create XQueries, see the Oracle Fusion Middleware Developer's Guide for Oracle Service Bus.

5.3.1 Transformation Maps

Transformation maps describe the mapping between two incompatible data types. Oracle Service Bus supports data mapping using either XQuery or the eXtensible Stylesheet Language Transformation (XSLT) standard. XSLT maps describe XML-to-XML mappings, whereas XQuery maps can describe XML-to-XML, XML to non-XML, and non-XML to XML mappings. In addition, MFL described data is automatically converted to the equivalent XML for transformation with XQuery or XSLT. The resulting XML is automatically converted to MFL if the target service requires it. For more information, see "XQuery Transformations" and "XSL Transformations" in the Oracle Fusion Middleware Administrator's Guide for Oracle Service Bus. For information on using the Oracle XQuery Mapper to create XQueries, see the Oracle Fusion Middleware Developer's Guide for Oracle Service Bus.

5.3.2 Message Manipulation

Message manipulation is a kind of transformation in which the contents of a message, rather than its whole structure, are manipulated to make the message compatible with the destination service. This is performed by adding, replacing or removing actions to the request or response pipelines of the message flow. The different actions available to transform a message through content manipulation are described in the following table:

Action Description

Insert

Inserts a data element into the message. This insertion may occur anywhere within the message context as specified during configuration.

Delete

Deletes a data element in the message. This is used when the target destination does not expect a specific data element in the message.

Replace

Replaces a series of text within the message with a different series. This can be used, for example, to replace the namespace of a message.


5.4 Error Handling

Oracle Service Bus provides robust and flexible error handling for configured services. It can handle errors in the following ways:

5.4.1 Message Validation

Oracle Service Bus provides the capability for incoming or outgoing messages to be validated against a WSDL or XML schema with a validation action. This action can occur at any time within the message flow and ensures that the incoming or outgoing message is in the format expected by the destination service's consumer or provider. Messages that fail validation can log the failure or create an error. In the latter case, an error stage can be used to apply alternative actions.

Message validation can be used for service versioning to validate messages against different versions of a schema or WSDL. This is to ensure the message is routed to the proper version of the service end point, or to check whether transformation must be applied prior to sending the message.

5.4.1.1 Error Handling Pipeline

Oracle Service Bus provides a mechanism to handle errors by allowing error handlers to be defined. An error handler is a pipeline that allows various actions such as logging, transformation, and publishing to be performed to handle errors appropriately. If an error occurs within a stage a sequence of steps are executed. This sequence of steps constitutes an error pipeline for that stage.

The error pipeline allows you to handle the error in the following ways:

  • Publish the original message to an alternate endpoint

  • Formulate an error response message to be returned to the invoker of the proxy service

  • Log the message

  • Continue processing the message through the pipeline after modifying the context

  • Raise an exception. Raising an exception transfers control to the next higher scoped error pipeline.

Errors can occur during message flow processing for various reasons. For example, security errors occur if a username is not correctly validated or authorized; transformation errors occur if Oracle Service Bus is unable to successfully transform or validate a message; a routing error is raised if a routing service is unavailable, and so on. Typically, these errors originate from a specific stage, route node or from the proxy service, as this is where most of the message flow logic is implemented.

Each stage can have a sequence of steps to execute if an error occurs in that stage. This sequence of steps constitute an error pipeline for that stage. In addition, an error pipeline can be defined for a pipeline (request or response) or for an entire proxy service. The lowest scoped error pipeline that exists is invoked on an error.

Figure 5-5 Stage, Node, and Service-Level Error Handlers

Description of Figure 5-5 follows
Description of "Figure 5-5 Stage, Node, and Service-Level Error Handlers"

The Oracle Service Bus Administration Console can be used to track messages to obtain an accurate picture of a message flow. This could enable error visibility; for example, the original reported message could be viewed, indicating it was submitted for processing, and then the subsequent reported error could be viewed, indicating that the message was not processed correctly. This would provide a complete view of both the message flow and error flow.

5.4.2 Related Topics