ResponseEntity created from Source<ByteString,M> - how to react to (exceptional) completion of this stream?

Akka
1

Hi,

I am completing one of my routes with an entity created from a Source<ByteString,M> as follows:

HttpResponse.create().withEntity(HttpEntities.create(TEXT_CSV_UTF8, mysource));

Since this source does I/O, it may fail both during materialization and while running. I haven’t observed the latter yet, but a failure during materialization:

  • makes the server close the connection after the headers have been sent to the client - this is the behaviour I would expect
  • does not log anything as far as I can see - this seems slightly unexpected.

Most importantly, all result transformation directives and exception handlers have already been applied at this point, so I have no natural point where I could catch this problem and do some server-side reporting/recovery. (It’s clear to me that I cannot do anything to help the client at this point, the headers are already sent so the damage is done.)

Is there a way to get something, for example a CompletableFuture, from the API, that would allow me to check whether mysource and the entire “response body builder graph” have completed normally or exceptionally?

2

You can use watchTermination operator on mysource to get a CompletableFuture that will be completed when the stream completes with success or with failure. You can get access to that future in the callback to mapMaterializeValue.

3

Thank you very much. I was aware of watchTermination but forgot that I could access it via mapMaterializedValue even if the materialization of the graph happens outside of my code’s control.