passing custom messages downstream in spring cloud function's MessageRoutingCallback

Question

Hey I m using MessageRoutingCallback to route to a function in spring cloud functions. It needs FunctionRoutingResult for routing. I also wanted to edit the message. The spring cloud docs says. "Additionally, the FunctionRoutingResult provides another constructor allowing you to provide an instance of Message as second argument to be used down stream".

But the problem is the constructor with Message type in FunctionRoutingResult is internal and cannot be accessed outside.

Am I doing something wrong here. Any insight would be helpful

Answer 1

Couple of things. As the documentation explains it is made to assist with routing decisions. For example if routing decision should be made based on payload which may need to be temporarily converted. The reality is that it is a very bad practice to let framework make such decisions based on the payload, since payload is a privileged information. Similar to the letter in the envelope where mailman does not read the actual letter to make proper routing decisions. .. those all come from the envelope itself. So I will actually update the documentation to remove that paragraph.

And it is definitely not there to modify the message. That would be improper use of MessageRoutingCallback. To modify message you can use function composition. For example MessageRoutingCallback you check some header in the incoming message, determined that the function name should be foo but then actually output modifier|foo as function definition.