Howto avoid concurrent call to conversation problem in Seam

It’s very common that in your Seam web-application you will get “HTTP Status 503 – Concurrent call to conversation” error and an exception stacktrace if you in debug-mode.

That can be caused by several reasons, one of them is double click on the ajax-link like <a4j:commandLink action=”#{myAction.action} value=”Simple link”/>
A4J/RichFaces allow the solultion for that problem – you have just to specify eventsQueue attrbute
<a4j:commandLink action=”#{myAction.action} value=”Simple link” eventsQueue=”myQueue” />
In that way even if user will click on that link twice – the events (requests) will be sent only one-by-one

As a side-note it worth also to specify “requestDelay” (specify the time the request will wait in queue before send) and ignoreDupResponses=”true” (A4J will ignore ajax response if another “similar” request is in queue). It’s worth for everyone to read that in RichFaces docs

But, it not always solve your problem – users are unpredicted creatures and they can click on your h:commandLink (which is not ajax and can’t be set to queue) or even on some link which actually triggers an action.  So,  you are still not safe since user have a lot of ways to break your application by click on another element which will trigger action inside current conversation.

Besides the fact that using a4j-queues is a good practice (it improve the overall perfomance of application also) – to be absolutely safe you have to disable ALL THE LINKS in the document or make them inaccessible for the user. And the solution exists, it rather simple but very powerful. So what you have to do – is to show <rich:modalPanel> once user is click on the link (or start ajax request with <a4j:support>) and hide it after ajax-response is complete. For h:commandLink or h:commandButton – you don’t have to worry about hiding it, since once new page will be loaded rich:modalPanel will be disappeared automatically (since it belongs to previous page/DOM-document). Once rich:modalPanel is shown – it makes impossible to use any navigation elements on the page except the elements on that form itself.

You can even automatically enable it for all your ajax-request’s with help of <a4j:status> and “onstart” “onstop” handlers.  In the example below the onRequestStart() function is called by onstart handler of aj4:status component. In the example below

//javascript
var waitDialogShown = false;
var useTimerBeforeShowWaitDialog = true;
var waitDialogTimeout = 50;
var waitDialogTimer;

function showWaitDialog() {
    //avoid attempt to show it if it is already shown
    if (!waitDialogShown) {
        Richfaces.showModalPanel('wait-dialog');
        waitDialogShown = true;
    }
}

function onRequestStart() {
    if (useTimerBeforeShowWaitDialog) {
        waitDialogTimer = setTimeout("showWaitDialog();", waitDialogTimeout);
    } else {
        showWaitDialog();
    }
}
function onRequestEnd() {
    if (waitDialogShown) {
        Richfaces.hideModalPanel('wait-dialog');
        waitDialogShown = false;
    } else if (useTimerBeforeShowWaitDialog && waitDialogTimer) {
        clearTimeout(waitDialogTimer);
    }
}

<a4j:status onstart="onRequestStart()" onstop="onRequestEnd()"/>

<rich:modalPanel id="wait-dialog" resizeable="false" autosized="false" width="200" height="65" moveable="false" shadowOpacity="0">
  <f:facet name="header">One moment please..</f:facet>
  <h:outputText value="Your request is being processed."/>
</rich:modalPanel>

<!-- Your wait dialog can be invisible for the users if you don't want your screen blink with that dialog on each request -->
<rich:modalPanel id="wait-dialog-invisible" resizeable="false" autosized="false"
width="10" height="10" moveable="false" left="-100" top="-100" shadowOpacity="0"/>

And if you don’t want to apply that for every component or want to modify prevention behavior you can do one of the:

1. Explicitly define onclick function of your links/buttons
   <h:commandButton onclick=”onRequestStart();” value=”Button”/>
   for ajax you also have to define oncomplete to hide the modalPanel, otherwise your browser-page will be blocked
   <a4j:commandButton onclick=”onRequestStart();” oncomplete=”onRequestEnd();” value=”Button”/>
2. Define a4j:status with id and forceId=”true” and use status attribute of your ajax components to specify the a4j:status component they will use.
   

   <a4j:status id="waitStatus" forceId="true" onstart="onRequestStart()" onstop="onRequestEnd()"/>
   <a4j:commandLink status="waitStatus"/>

In reality things are more complex than we think about them – you may also take a look at the related article
How to avoid primary page conversation timeout on background requests in Seam. Hacking around Seam request lifecycle – it describe another problem and solution to it. When on the page you have background process (periodically executed ajax requests) – they can also lead to conversation timeout and kill your primary conversation.

About Andrey Chorniy

https://www.facebook.com/achorniy

‹ Howto improve Seam application security

JBoss SSL configuration ›

Tagged with: ajax, howto, java, jsf, richfaces, seam, web-application
Posted in Software Development, Tips and Tricks