使用 Netty 限制 HTTP/2 中的流读取

Question

In the Netty 4 proxy example AUTO_READ is turned off, which according to this answer is so that a channel sending a lot of data to the proxy doesn't use up too many buffers, which makes sense.

However, my understanding is that in HTTP/2 you should never delay reads, and instead deal with the flow control on a stream-by-stream basis. I can't figure out how to do this in Netty though.

It feels like it should be achieved by returning 0 in onDataRead on an Http2ConnectionHandler . The following contrived example is how I expected it to work, where the bytes are not consumed for 100ms:

@Override
public int onDataRead(ChannelHandlerContext ctx, int streamId, ByteBuf data,
                      int padding, boolean endOfStream) throws Http2Exception {
    int processed = data.readableBytes() + padding;

    Http2LocalFlowController flowController = decoder().flowController();
    Http2Stream stream = connection().stream(streamId);
    executorService.schedule(() -> {
        ctx.executor().execute(() -> {
            try {
                // ...do stuff...
                flowController.consumeBytes(stream, processed);
            } catch (Http2Exception e) {
                e.printStackTrace(); // now what?
            }
        });
    }, 100, TimeUnit.MILLISECONDS);

    return 0;
}

When I use this, no exception is thrown, and the client just times out. (Note that if I call the consumeBytes call before returning from onDataRead then it does work.)

With some extra logging, when reading 16k messages I can see immediately that 65535 bytes are unconsumed. After beginning to consume, a window frame is reportedly sent after 32k. After the 4th message, flowController.unconsumedBytes(stream) reports 0 bytes unconsumed, however the next message never comes to onDataRead .

This apparently works for other people so I'm struggling to see where the issue is.

Answer 1

The consumeBytes method returns a boolean that indicates whether or not a threshold was reached meaning a window update can be sent for the stream (ie to say that the stream can send more data).

The method does a ctx.write with the window update frame, but doesn't flush it. So if nothing is flushing the context, you need to do that:

if (flowController.consumeBytes(stream, processed)) {
    ctx.flush();
}