[CAMEL-10910] Revisit if Pipeline should wrap single processor or not

• Type: Task
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 3.0.0.RC1, 3.0.0
• Component/s: camel-core
• Estimated Complexity: Unknown
• Assignee: Claus Ibsen
• Reporter: Tomohisa Igarashi
• Created: 28/Feb/17 15:09
• Updated: 08/Aug/19 06:34
• Resolved: 08/Aug/19 06:34

Description

I noticed that if eventDrivenProcessors contains only one processor, IN message is not copied to OUT even when exchange pattern is InOut while it does if it contains multiple processors.

eventDrivenProcessors are wrapped with Pipeline here when initiating a camel route here:
https://github.com/apache/camel/blob/master/camel-core/src/main/java/org/apache/camel/impl/DefaultRouteContext.java#L161

But if there's only one event driven processor, the processor itself is used instead of wrapping with Pipeline:
https://github.com/apache/camel/blob/master/camel-core/src/main/java/org/apache/camel/processor/Pipeline.java#L57

Pipeline copies the IN message to OUT if it's InOut and OUT message doesn't exist:
https://github.com/apache/camel/blob/master/camel-core/src/main/java/org/apache/camel/processor/Pipeline.java#L107

So the IN->OUT copy happens only if multiple processors exist. It means that whether the response message is returned as OUT or IN depends on if the route results in single processor or not. I think it looks a bit confusing and wondering if we can just wrap it with the Pipleline even if there's single processor.

Having said that as this has been same for 10 years, we can't change this behavior in 2.x as it would break backward compatibility. Let's revisit in 3.0.