Newbie questions

["이희승 (Trustin Lee)"]

On 08/15/2009 03:49 PM, Bozo Juretic wrote:
> Ernesto A. wrote:
>> Hello,
>>
>>   
>>> 1. What is the best way to implement reconnecting mechanism for TCP
>>> client in which the connection with the server should be up at all
>>> times. I need to send periodic "ping status" messages to the server as
>>> well. At the start of app I need to connect to host (and retry if host
>>> is not available), and if the connection goes down I need to reconnect
>>> up with the server in some smart way (perhaps you have reconnecting
>>> strategies implemented somewhere in netty code?).
>>>     
>>
>> About this first question, I think this part belongs to what in netty
>> is called the business logic, or the object in your network framework
>> that will take care of handling connections/disconnections, maintain
>> session state etc. You should check the javadocs for
>> SimpleChannelHandler, this class provides exactly what you need
>> through callbacks. There are plenty of examples about how to use it on
>> netty's website, I recommend reading the guide first and foremost, so
>> you could get in touch with netty principles and get an idea of how
>> things fit together.
>>
>>   
> I did reconnects using the IdleStateHandler and bootstrap.connect() in
> the main thread. This seems to be a problem, so it seems that the
> bootstrap.connect should be run in a separate thread. Thus now I have
> this in the main thread:
> 
>         Channel channel;
>         do {
>             logger.info("Connecting to server " + host + ":" + port);
>             ChannelFuture f = bootstrap.connect(new
> InetSocketAddress(host, port));
> 
>             // Wait until the connection attempt succeeds or fails
>             f.addListener(new ChannelFutureListener() {
> 
>                 public void operationComplete(ChannelFuture future) {
>                     if (future.isSuccess()) {
>                         logger.info("Connected to server");
>                         connected = true;
>                     } else {
>                         logger.info("Connection to server failed: " +
> future.getCause().getMessage());
>                     }
>                 }
>             });
> 
>             try {
>                 Thread.sleep(15000);
>             } catch (InterruptedException ex) {
>                 logger.warn("interrupted", ex);
>             }
> 
>         } while (!connected);
> 
> This is a problem since there is some other code that needs to be run
> afterwards which takes over this thread (JMS consumer.receive()).
> 
> I did reconnects of a failed existing connection with IdleStateHandler's
> help. Is there a way to put this "first time connect" code (above) in
> the handler's channelOpen(), so that it is run by netty, and my main
> thread is left to me for my tasks, since I would like to avoid forking
> threads on my own (maybe the code will be ported to application server
> at some point)?
> 
> Also, it the Thread.sleep() a good way to pause execution in netty (i.e.
> if I want to wait before I attempt another (re) connect)?

I would add a listener to the closeFuture of a channel to get notified
on disconnection:

  channel.getCloseFuture().addListener(... /* reconnect */ ...);

If you need some delay between disconnection and reconnection, you could
use a java.util.Timer or something similar to schedule reconnection in
your closeFuture listener.  Netty provides
org.jboss.netty.util.HashedWheelTimer, so you might want to give it a try.

Please let me know if I missed something.

HTH,
Trustin