8.12.2. Dealing with errors

However carefully your application is designed to queue concurrent requests to your conversational component, there is a risk that the server will overload. When overload occurs, not all requests will be processed before the concurrent-request-timeout period expires. In this case, Seam throws a ConcurrentRequestTimeoutException, which is handled in pages.xml. We recommend sending a HTTP 503 error: 
<exception class="org.jboss.seam.ConcurrentRequestTimeoutException" 
           log-level="trace"> 
  <http-error error-code="503" /> 
</exception>

Note

The server is currently unable to handle the request due to a temporary overloading or maintenance of the server. The implication is that this is a temporary condition which will be alleviated after some delay.
Alternatively you could redirect to an error page: 
<exception class="org.jboss.seam.ConcurrentRequestTimeoutException" 
           log-level="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>
ICEfaces, RichFaces AJAX and Seam Remoting can all handle HTTP error codes. Seam Remoting will pop up a dialog box showing the HTTP error. ICEfaces will indicate the error in its connection status component. RichFaces provides the most complete support for handling HTTP errors by providing a user definable callback. For example, to show the error message to the user: 
<script type="text/javascript"> 
  A4J.AJAX.onError = function(req,status,message) { 
    alert("An error occurred"); 
  }; 
</script>
If, rather than an error code, the server reports that the view has expired, perhaps because a session timed out, use a separate callback function in RichFaces: 
<script type="text/javascript"> 
  A4J.AJAX.onExpired = function(loc,message) { 
    alert("View expired"); 
  }; 
</script>
Alternatively, you can allow RichFaces to handle the error. In this case, the user will receive a prompt reading, "View state could not be restored — reload page?" This message can be globally customized by setting the following message key in an application resource bundle: 
AJAX_VIEW_EXPIRED=View expired. Please reload the page.