OSB - Publish, Routing and Service Callout

1. Service Callout
Used in real time request-response scenarios. Call a service in synchronous way. Being a synchronus call, the thread gets blocked until a response is returned from target service.


2. Publish
Used for Request only scenarios where you don't expect a response back. The nature of Publish action (sync or async) will depend upon the target service you are invoking.

• Invoking an external service through a business service,  then Publish action with Quality of Service(QoS) as "Best Effort" (default) will work like fire and forget and thread won't get blocked (async call).
• Invoking a local proxy service (proxy with transport protocol as "local") from another proxy using publish action then it would be a blocking call (synchronus call) and thread will get blocked untill the processing of local proxy finishes.

3. Routing
Only can be created inside a Route Node and Route Node is the last node in a request processing and  not in the pipeline, it passes all processing to another service (business or proxy). A Route Node indicates that Request Processing will end here and Response Processing will begin. You can not have any node after Route Node in the message flow.

Route Node can be seen as an action which defines where a Request thread stops and Response thread begins, in addition to what ever it does. It is know that by-design, request and response pipelines of a OSB proxy will be in different threads unless other wise configured.

Good Practice
Use service callouts for message enrichment or for doing message validation. Use route node to invoke the actual service for which the proxy service is a 'proxy'.

Try to design OSB proxy services in a standard VETO pattern - Validate, Enrich, Transform, rOute. Use Service callout for Validate and Enrich steps and use route node for rOute step. This approach looks more appealing design wise.

Reference:
Thread: compare Routing action versus Service Callout action versus Publish action?
Thread: Route node and Service Call out in OSB