Converting from Enumerator to Sink when using java OutputStream

[Dragisa Krsmanovic]

I have a controller that returns chunked output and needs an OtputStream.
Implementation using Enumerators works fine:

    val enumerator = Enumerator.outputStream(os => report.write(os))
    Ok.chunked(enumerator)

Since chunked with Enumerator is now deprecated, I am trying to convert it to Akka streams. I have something like this:

    val source = StreamConverters.asOutputStream().mapMaterializedValue(os => report.write(os))
    Ok.chunked(source)


Unfortunately this is not working. Requests just time out.
I can see that report.write(os) gets called, but no output gets written to Http request.

What is the correct way to convert Enumerator.outputStream(...) to Akka streams ?

Thanks,
Dragisa

[Nick Howes]

Does report.write(os) write the whole output? Have you tried calling os.close() afterwards (inside mapMaterializedValue)? It may be waiting for the output stream to be closed or flushed.

[Dragisa Krsmanovic]

Yes, report.write() writes whole output.

FYI, it's Apache POI. I am writing an Excel spreadsheet in chunks to avoid rendering it all in memory.

I tried closing the output stream too

    val source = StreamConverters
      .asOutputStream()
      .mapMaterializedValue { os =>
        try {
          log.debug("Writing to os")
          report.write(os)
        } finally {
          log.debug("Closing os")
          os.close()
        }
      }

It never enters the finally block.

The example with Enumerator works fine.

[Christian Schmitt]

Just for reference could you try something like that:

val inputStream = new PipedInputStream()
val outputStream = new PipedOutputStream(ios)
Future{ /** Write to outputStream */ } 
Ok.chunked(StreamConverters.fromInputStream(() => inputStream))

[Dragisa Krsmanovic]

Christian,

That does seem to work. Thank you !

I wonder, what is the proper use of StreamConverters.asOutputStream() ?

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[Dragisa Krsmanovic]

Interestingly,

val source = StreamConverters.asOutputStream().mapMaterializedValue(os => Future{report.write(os)})
Ok.chunked(source)

Doesn't seem to work. The request times out. However, it's different from when you don't have a Future. It starts to upload chunks but never finishes. (Yes, I tried putting try-finally around report.write to close output stream.)

On Fri, Jun 24, 2016 at 3:13 PM, Christian Schmitt <c.sc...@briefdomain.de> wrote:

Actually I guess the following could work, too:

val source = StreamConverters.asOutputStream().mapMaterializedValue(os => Future{report.write(os)})
Ok.chunked(source)

Btw. it's best to close the outputStream after write, so that the source will be closed and the upstream handler knows that data writing is finished.

Actually I guess that works, too:

val (outputStream, publisher) = StreamConverters.asOutputStream().toMat(Sink.asPublisher(false))(Keep.both).run()
Future { /* write to outputstream */ } 
Ok.chunked(Source.fromPublisher(publisher))

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