Large response - how to handle asynchronously

[Łukasz Osipiuk]

Hi guys,

We are implementing server application using Netty protocol framework
and we have problem implementing handling of large responses.
We will be handling many simultaneously connected clients.
In protocol we are implementing, responses to client requests can be
huge (lets assume that larger than server memory).
While handling request we have to read part of response data from
external storage - send it to client - wait until he receives the data
and continue with next part.

We approached problem with following code in handler (simplified
version, no error handling and so on):

   @Override
   public void messageReceived(ChannelHandlerContext ctx, MessageEvent
e) throws Exception {
       XXXRequest req = (XXXRequest) e.getMessage();
       writePartResponse(ctx, req);
   }

    private void writePartResponse(final ChannelHandlerContext ctx,
final XXXRequest req) {
        if (moreDataToBeRead(req)) {// anything more to be done?
            XXXPartResponse resPart = readSomeData(req); // read some
data from storage
            ChannelFuture f = ctx.getChannel().write(resPart);
            f.addListener(new ChannelFutureListener() {
                public void operationComplete(ChannelFuture f_) throws
Exception {
                    writePartResponse(ctx, req);
                }
            });
        }
    }

Above approach does not work.

Problem 1.

While handling request we have to block other client's requests  so
they are not handled simultaneously.
In above code when messageReceived() method terminates any pending
client requests are delivered to handler and handled even though
asynchronous handling of previous request might still be in progress.
We do not know how to block handling of any pending requests.

Problem 2.

I client is quick enough to read data from socket f.addListener
immediatelly triggers calling of futer.operationComplete in the same
thread.
This leads to StackOverflowError.
(btw. It appears that StackOverflowError it is not handled correctly
by netty in this place and application is not notified about it being
thrown - handling of request is just silently terminated).

Any way to avoid above problems?
I guess the approach is broken - can anybody lead us how to implement
such (trivial one might think) behaviour?

Best regards, Łukasz Osipiuk


-- 
-- 
Łukasz Osipiuk
mailto:lukasz at osipiuk.net