[concurrency-interest] Concurrency Architecture

David Holmes davidcholmes at aapt.net.au
Sun Jun 14 17:37:08 EDT 2009


Norman,

There are many many ways to set this up. It could be as simple as checking
if the current server has changed after a timeout - if so then someone did
the change; if not then search for a new server yourself. It all depends on
the exact semantics you want - eg whether the new server has to be known to
be good before anyone else starts using it; and how long a "down" server is
excluded from being used etc. A CountDownLatch used as a simple gate can be
used to make other threads wait while a search is in progress, if you don't
want to use the lock directly for this purposes. (Using a Semaphore seems
tricky because you don't know how many threads may be waiting.)

David Holmes

> -----Original Message-----
> From: concurrency-interest-bounces at cs.oswego.edu
> [mailto:concurrency-interest-bounces at cs.oswego.edu]On Behalf Of Norman
> Elton
> Sent: Monday, 15 June 2009 2:37 AM
> To: concurrency-interest at cs.oswego.edu
> Subject: [concurrency-interest] Concurrency Architecture
>
>
>
> I've been using the java.util.concurrent classes over the past few
> years, they've worked well for every circumstance I could throw at
> them. Today, I've stumbled upon a case that I just can't figure out
> how best to tackle.
>
> I've got a connection generator which builds and returns connections
> to a server. In case of failover, I'm maintaining a list of possible
> servers to which it can generate connections. My wrapper
> getConnection() method takes this into account by first attempting a
> connection to the "current" server. If that times out, it scans the
> list for a new "current" server.
>
> Here's some pseudocode:
>
> public Connection getConnection() {
> 	try {
> 		return this.generator.getConnection();
> 	} catch (TimeoutException e) {
> 	}
>
> 	synchronized(this.generator) {
> 		foreach (this.servers as curr_server) {
> 			try {
> 				this.generator.server = curr_server;
> 				return this.generator.getConnection();
> 			} catch (TimeoutException e) {
> 			}
> 		}
> 	}
>
> 	throw new Exception("Sorry, couldn't find a server");
> }
>
> In this case, if three threads simultaneously call getConnection(),
> they will all timeout and hit the synchronized block together. The
> first one will identify a new server, but the other two have no way to
> know that a new server has been identified. They each will loop
> through the possible servers.
>
> I'm looking for some way for all threads to be able to call the first
> part of the procedure (attempt a connection to the current server). If
> that fails, then the first thread will attempt to find a new server,
> while others wait for the result.
>
> I've dug through all the possible types of locks, nothing quite meets
> the need. Of course, I could be wrong! Can anyone think of a good way
> to tackle these requirements?
>
> Thanks!
>
> Norman
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