Sealyu

A general discussion of concurrent calls to Seam components can be found in Section 4.1.10, “Concurrency model”. Here we will discuss the most common situation in which you will encounter concurrency — accessing conversational components from AJAX requests. We're going to discuss the options that a Ajax client library should provide to control events originating at the client — and we'll look at the options RichFaces gives you.

Conversational components don't allow real concurrent access therefore Seam queues each request to process them serially. This allows each request to be executed in a deterministic fashion. However, a simple queue isn't that great — firstly, if a method is, for some reason, taking a very long time to complete, running it over and over again whenever the client generates a request is bad idea (potential for Denial of Service attacks), and, secondly, AJAX is often to used to provide a quick status update to the user, so continuing to run the action after a long time isn't useful.

Therefore, when you are working inside a long running conversation, Seam queues the action event for a period of time (the concurrent request timeout); if it can't process the event in time, it creates a temporary conversation and prints out a message to the user to let them know what's going on. It's therefore very important not to flood the server with AJAX events!

We can set a sensible default for the concurrent request timeout (in ms) in components.xml:

<core:manager concurrent-request-timeout="500" />

We can also fine tune the concurrent request timeout on a page-by-page basis:

<page view-id="/book.xhtml" 



         conversation-required="true" 



         login-required="true"



         concurrent-request-timeout="2000" />

So far we've discussed AJAX requests which appear serial to the user - the client tells the server that an event has occur, and then rerenders part of the page based on the result. This approach is great when the AJAX request is lightweight (the methods called are simple e.g. calculating the sum of a column of numbers). But what if we need to do a complex computation thats going to take a minute?

For heavy computation we should use a poll based approach — the client sends an AJAX request to the server, which causes action to be executed asynchronously on the server (the response to the client is immediate) and the client then polls the server for updates. This is good approach when you have a long-running action for which it is important that every action executes (you don't want some to timeout).

}

   <exception class="org.jboss.seam.ConcurrentRequestTimeoutException" logLevel="trace">



      <http-error error-code="503" />



   </exception>

<exception class="org.jboss.seam.ConcurrentRequestTimeoutException" logLevel="trace">



   <end-conversation/>



   <redirect view-id="/error.xhtml">



      <message>The server is too busy to process your request, please try again later</message>



   </redirect>



</exception>

<script type="text/javascript">

A4J.AJAX.onError = function(req,status,message) { 

alert("message");    

};

</script>