Serving a file efficiently using Play 2.3 - file

Question

I need to serve some content from an Action in the form of a file: basically, I am creating CSV content on the fly and sending it to the client.
I cannot do it using sendFile, since the file does not really exist; I tried using the chunked transfer, but I get a really slow response (in localhost I got the file at about 100KB/s, which I think is really strange).
Is there a way for me to set the content type and write the response "line by line", without having to specify the content length "a priori"?

Answer 1

Here's one way using a simple predefined Enumerator that will produce the response from bytes written to an OutputStream:
def csv = Action {
val enumerator = Enumerator.outputStream { out =>
out.write(...)
// Keep writing to the Enumerator
out.close()
}
Ok.chunked(enumerator.andThen(Enumerator.eof)).withHeaders(
"Content-Type" -> "text/csv",
"Content-Disposition" -> s"attachment; filename=test.csv"
)
}
This is simple enough for relatively small files (or if the process of generating the file is slow by nature), however note that from the documentation this has no back-pressure, reading a large file into the OutputStream can quickly fill up memory if the client can't download it quickly enough.
Update:
After testing this some more it seems like the size of the Byte arrays you write to the OutputStream make a huge difference in throughput.
Using this sample stream:
val s = Stream.continually(0.toByte)
Writing in chunks of 1KB to the OutputStream like this resulted in 6MB/s of throughput:
(0 until 1024*1024).foreach{i =>
out.write(s.take(1024).toArray)
}
However if I only write 10 bytes at a time, the throughput slows to less than 100KB/s. So my suggestion for using this method to write CSVs in a chunked form would be to write multiple rows at a time to the OutputStream rather than one row at a time.