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Re: implementing long poll in grails

by redheat :: Rate this Message:

Dave,

I'm also implementing long polling. Have you been able to solve your original problem?

I'm using plain while with return, not a closure with break. When I tried my code with break like you did I didn't have any success.

For your checking of changes, how do you keep track of the last change? I used the session context together with beforeInsert in all classes so that a Date session.objectcreated gets set when an instance of the class is created.

Also I'm still having a couple of things left that I couldn't yet solve.

I have an alarm button on all of my views, so all of them re-open a request to the server script.
When you now switch between views in the browser, firefox aborts running requests while IE6 doesn't. So I used req.transport.abort to be safe(using prototype) when req is the handler to my xmlhttprequest. This solved the problem partially.
On the server side however the controller with the polling loop continues to run until its timeout.

Do you know a good way to recognize that the request was aborted?

A related probem occurs when the user logs out (using acegi). This creates an unknown state exception on the server, because the session context is no longer there in which I stored my time stamps for the updates. However, I can't use a check whether the session context is null because the controller is still running with its local instance of the session context so the result is not nul, strangely.

I would greatly appreciate if you don't mind sharing some of your experience.

Thanks, Martin

Dave Crane-3 wrote:

Hi,

I'm trying to implement a simple long poll in a Grails controller, so that
the controller will go to sleep until something interesting happens, and
maybe take 10-20 seconds before rendering the response. (Yeah, I know this
is a naive, inefficient implementation, and that DWR/Jetty continuations
could do it nicely!) To see whether anything interesting has happened, I'm
sending the latest known version numbers of each domain entity with the
request, and then looking at the domain classes for changes periodically.
So, something like this:

def longpoll={
//read known version numbers
def counter = 0
localPoll: while (counter < 10){
def changed=getChanges()
if (changed){
//render a response to say something has changed
break localPoll
}else{
Thread.currentThread.sleep(5000)
counter+=1
}
}
if (counter>10){
//render a null response
}
}

def getChanges(){
def things=Thing.findAll()
for (t in things){
//is version number different? If so, return true
}
return false
}

So in the longpoll closure, I enter a loop, and check every 5 seconds for
changes, based on incremented version numbers, by iterating through every
object in the domain (like I said, I'm not looking for efficiency here!) If I
find a change, I report back immediately, and only return a "no change"
response after a set timeout, here 50 seconds. My 5 second interval is long,
but better for debugging - because I'm just polling on the server, not across
the network. I could go for a much smaller interval.

Anyway, it's looking like a good pedagogical example of how to do things the
hard way, but I find that Thing.findAll() doesn't return the changes in the
version number, even if I go in and manually update an entity through a
scaffolding/admin screen.

I'm guessing that this is because the entire controller closure is wrapped in
a single transaction or hibernate session, which is an entirely sensible
thing to do when the response returns almost-instantly, as in the majority of
web apps. So, my question is, is there a way in which I can manually create a
new transaction within the controller closure, so that I can see updates that
occur between the request first arriving and the response being rendered?

Thanks in advance,

Dave

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