Hi, everyone! I am using Ktor and I'm working on ...

# ktor

[Patrik Ziduliak]

Hi, everyone!

I am using Ktor and I'm working on processing streamed data from my DB (Druid). I am trying to process the data and send them to the client in a streamed fashion, however, it seems like the response is not being streamed.

I receive a streamed response, and with Gson's JsonReader and produce a Flow<Row> :

val eventFlow = flow {
                while (jsonReader.hasNext()) {
                    // ... stuff
                    val row = parseRow(jsonReader, ...)
                    emit(row)

            }

Then, in PipelineContext.handleResultFlow, I want to send them in a streamed response. I am using com.google.gson.stream.JsonWriter, but I also tried just using strings. I tried both respondOutputStream and respondBytesWriter, but neither seems to work to allow streaming the response to the client:

private suspend fun PipelineContext<Unit, ApplicationCall>.handleResultFlow(
    flow: Flow<QSResult.Row>,
    ...
) {

    //...
        call.respondBytesWriter(contentType = ContentType.Application.Json) {
            val outputStream = this.toOutputStream()
            val jsonWriter = JsonWriter((OutputStreamWriter(outputStream)))
            jsonWriter.setIndent("") // Disable pretty printing for efficiency

            coroutineScope {
                launch(<http://Dispatchers.IO|Dispatchers.IO>) {
                    jsonWriter.beginObject()

                    // ...
                    flow.collect { row ->
                        jsonWriter.beginObject()

                        // ... write the actual object

                        jsonWriter.endObject()
                        jsonWriter.flush()
                    }

                    jsonWriter.endArray()
                    jsonWriter.endObject()
                    jsonWriter.flush()
                }
            }.join()
}

I tried also installing the PartialContent plugin.

How can I make sure that this response is sent to the client streamed? Which part of the code could be causing a potential issue?

[Aleksei Tirman [JB]]

Can you try writing data to the channel without converting it to the OutputStream ?

[Patrik Ziduliak]

Still the same issue if I use writeStringUtf8 with ByteWriteChannel directly

[Aleksei Tirman [JB]]

How do you test streaming?

[Patrik Ziduliak]

I just use curl / postman for sending a request for a large query.

I used VirtualVM for memory profiling and it shows me the heap getting used up by byte[] objects

[Aleksei Tirman [JB]]

Have you used the --no-buffer option with curl to disable buffering of the response body?