Synchronous-over-asynchronous invocation

Avoid using synchronous-over-asynchronous invocation, which occurs when a component synchronously calls another component that has an asynchronous implementation.

When synchronous-over-asynchronous invocation is detected at run time, the Service Component Architecture (SCA) automatically changes the synchronous call to an asynchronous call, which creates a new transaction boundary and can cause one or more of the following problems:

• Lack of transaction propagation
• Multiple retries that result in multiple invocations of the same service
• Time outs when the default wait time for invocations is exceeded
• Blocked threads and thread pool depletion, which are caused when a thread is allocated for each asynchronous call and waits for a response

Most synchronous-over-asynchronous invocations occur when you have a component that synchronously calls one of the following components that typically have asynchronous implementations:

• Long-running BPEL and BPMN business processes
• Human tasks
• JMS imports
• MQ imports
• Java POJO components that implement the ServiceAsynchImpl interface

For example, a web service export always performs a synchronous call. If an export synchronously calls a long-running BPEL process, a synchronous-over-asynchronous invocation occurs. Similarly, a microflow always performs a synchronous call for a request-response operation. If a microflow invokes an MQ import for a request-response operation, a synchronous-over-asynchronous invocation occurs.

In general, avoid the following scenarios that might result in a synchronous-over-asynchronous invocation:

• A web services export calling a long-running BPEL or BPMN business process
• A web services export calling a message-binding import, such as an MQ or JMS import
• A BPEL microflow calling a long-running BPEL or BPMN process for a request-response operation
• A BPEL microflow calling a message-binding import (such as an MQ or JMS import) for a request-response operation

When a synchronous-over-asynchronous invocation is detected, the following warning message is displayed in the SystemOut.log file:

000000ac Core          W   CWSCA2011W: Service Component Architecture (SCA) is switching a synchronous call to an asynchronous call.
Request will be sent in a new local transaction. Default timeout is 115000 milliseconds.

Depending on your specific application scenario, there are several approaches to avoiding a synchronous-over-asynchronous invocation. Many of these approaches are described in the article Process implementation types: Patterns-based design for process-based solutions. For example, if you have a microflow that invokes a long-running BPEL process over MQ for a request-response operation, you might want to incorporate a briefly persisted process to avoid a synchronous-over-asynchronous invocation. The use of briefly persisted processes is described in the "Briefly persisted process" section of the article.

For general information about invocation styles, see the Invocation styles topic.