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POImplementing keep-alive messages in Netty using WriteTimeoutHandler
 

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    <p>I am using Netty 3.2.7. I am trying to write functionality in my client such that if no messages are written after a certain amount of time (say, 30 seconds), a "keep-alive" message is sent to the server.</p> <p>After some digging, I found that WriteTimeoutHandler should enable me to do this. I found this explanation here: <a href="https://issues.jboss.org/browse/NETTY-79" rel="noreferrer">https://issues.jboss.org/browse/NETTY-79</a>.</p> <p>The example given in the Netty documentation is:</p> <pre><code>public ChannelPipeline getPipeline() { // An example configuration that implements 30-second write timeout: return Channels.pipeline( new WriteTimeoutHandler(timer, 30), // timer must be shared. new MyHandler()); } </code></pre> <p>In my test client, I have done just this. In MyHandler, I also overrided the exceptionCaught() method:</p> <pre><code>public void exceptionCaught(ChannelHandlerContext ctx, ExceptionEvent e) { if (e.getCause() instanceof WriteTimeoutException) { log.info("Client sending keep alive!"); ChannelBuffer keepAlive = ChannelBuffers.buffer(KEEP_ALIVE_MSG_STR.length()); keepAlive.writeBytes(KEEP_ALIVE_MSG_STR.getBytes()); Channels.write(ctx, Channels.future(e.getChannel()), keepAlive); } } </code></pre> <p>No matter what duration the client does not write anything to the channel, the exceptionCaught() method I have overridden is never called.</p> <p>Looking at the source of WriteTimeoutHandler, its writeRequested() implementation is:</p> <pre><code>public void writeRequested(ChannelHandlerContext ctx, MessageEvent e) throws Exception { long timeoutMillis = getTimeoutMillis(e); if (timeoutMillis &gt; 0) { // Set timeout only when getTimeoutMillis() returns a positive value. ChannelFuture future = e.getFuture(); final Timeout timeout = timer.newTimeout( new WriteTimeoutTask(ctx, future), timeoutMillis, TimeUnit.MILLISECONDS); future.addListener(new TimeoutCanceller(timeout)); } super.writeRequested(ctx, e); } </code></pre> <p>Here, it seems that this implementation says, "When a write is requested, make a new timeout. When the write succeeds, cancel the timeout."</p> <p>Using a debugger, it does seem that this is what is happening. As soon as the write completes, the timeout is cancelled. This is not the behavior I want. The behavior I want is: "If the client has not written any information to the channel for 30 seconds, throw a WriteTimeoutException."</p> <p>So, is this not what WriteTimeoutHandler is for? This is how I interpreted it from what I've read online, but the implementation does not seem to work this way. Am I using it wrong? Should I use something else? In our Mina version of the same client I am trying to rewrite, I see that the sessionIdle() method is overridden to achieve the behavior I want, but this method is not available in Netty.</p>
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