Friday, May 16, 2008

OpenLDAP 2.4.9 Multi-Master Replication Stability Improved

I pulled in OpenLDAP 2.4.9 to re-visit some multi-master testing I had done on the previous version. 2.4.9 was a better experience for me than 2.4.8. I re-ran some tests of running two replicated servers on the same machine. I would continuously load in several hundred or thousand user records on one server, then delete them.

The stability is improved from 2.4.8. I have yet to have a server core on me--even after abruptly shutting done one or both servers during a batch load or delete.

My only issue now is that when performing a bulk load, if you interrupt the client during the load ( Control-c ldapadd ) the servers get out of sync. The server you directly attach to in order to load the records contains more records than the "replicated" server. Under most circumstances, this would not be a huge issue, but it does show an easy way for a client to get the servers out of sync.

With the 2.4.9 release of OpenLDAP, I would trust the multi-master synchronization enough now for use on production systems.

Thursday, May 8, 2008

Simplified BlazeDS and JMS

I've been using Flex for some time now, and finally got around to trying out BlazeDS. What really peaked my interest was it's claim to publish and consume JMS messages from both topics and queues.

The one area in web development that I always struggle with is asynchronous messaging from the server to clients over HTTP. I'm pleased to announce that I got a simple application up and running.

The goal of this post is to share the example and lessons learned in order to save others some of the problems I ran into.

My project had a few main goals:
  • Get a simple pub/sub example up and running
  • Avoid using the "pre-configured" download. I wanted to get a project up and running "from scratch"
  • Use my existing ActiveMQ broker running in it's own JVM
  • Use my existing Tomcat install
  • Use mxml as much as possible and avoid writing Actionscript
  • Keep it simple to get a working example

Before I get into the example code, a few things I ran into:
  • The BlazeDS documentation is good, but far from perfect. The messaging examples switch from mxml to actionscript, so it makes it a little difficult to piece things together. Also, it's not clear in the examples what many of the values point to--i.e. is that the JMS Topic name or the destination name???? I also found a few syntax problems in the examples which caused some "cut and paste" problems for me.
  • A key piece I discovered after nothing was working at all, was the use of the "-services" flag to the mxmlc compiler. You have to compile your mxml file with the services file you plan to call.
  • Don't rely on client side faults. Prior to finding out about the "-services" flag, I would call the send method in the client, but nothing happened--the faultHandler was never called on the client, and nothing appeared in the server logs. Once I compiled with the "-services" flag, things began to click.
  • Make sure the JNDI names are correct. BlazeDS looks up the JMS ConnectionFactory and Topics via JNDI calls to Tomcat's JNDI. Make sure you specify the full JNDI name of the resource. i.e java:comp/env/jms/messageTopic

The example contains 5 main files:
  1. A standard web.xml file with a BlazeDS servlet to handle the remote calls
  2. A Tomcat context.xml file to add the JNDI values for the required JMS resources
  3. A BlazeDS services-context.xml file to set up the services and channels
  4. A BlazeDS messaging-config.xml that stores the messaging service(s).
  5. A test.mxml file which is compiled into the Flex application.

The image to the right shows the file locations in the final WAR layout.

The web.xml file is pretty basic. The main 2 things to point out are the HttpFlexSession listener and the MessageBrokerServlet. The servlet loads up you services, so it is passed the location of the services-config.xml file inside the WAR file.

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE web-app PUBLIC "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN" "">
<description>BlazeDS Application</description>

<!-- Http Flex Session attribute and binding listener support -->

<!-- MessageBroker Servlet -->

The context.xml file is for Tomcat. You can read more about it in the Tomcat documentation. The key take away is the "name" attribute is the JNDI name that has to match the jndi name used in the messaging-config.xml file.

<?xml version="1.0" encoding="UTF-8"?>
<Context path="/BlazeTest">
<Resource name="jms/flex/TopicConnectionFactory"
description="JMS Connection Factory"
<Resource name="jms/messageTopic"
description="a simple topic"

The services-config.xml file is used by BlazeDS. It is first used when you go to compile the mxml file. When you deploy the war file to Tomcat, this file is also loaded up by the MessageBrokerServlet.

During compilation, you pass it in like this using the relative path to it from your mxml file.

~/java_libraries/flex3/bin/mxmlc -services=WEB-INF/flex/services-config.xml test.mxml

My current file turns on logging which can help during resolve messaging problems like jndi resources not being found. This file loads in the messaging-config.xml file which is described below.

The channels determine how the client will talk to the server. The endpoint has to match the servlet-mapping url-pattern contained in the web.xml. Since the servlet is grabbing /messagebroker/* and my application is deployed under BlazeTest, the endpoint becomes:


The "amf" on the end of the URL can be called anything--it's what distinguishes one channel url from another channel url. Here, I am only using one channel.

<?xml version="1.0" encoding="UTF-8"?>
<service-include file-path="messaging-config.xml" />
<channel-definition id="my-amf" class="mx.messaging.channels.AMFChannel">
<endpoint url="http://localhost:8080/BlazeTest/messagebroker/amf" class="flex.messaging.endpoints.AMFEndpoint"/>
<target class="flex.messaging.log.ConsoleTarget" level="Debug">
<prefix>[BlazeDS] </prefix>

This file stores the JMS services and tells BlazeDS where to get the JMS ConnectionFactory and Topics from the JNDI lookup. The part here that caused be a little problem was making sure the connection-factory and destination-jndi-name values had the correct JNDI name Tomcat expected. Also, note the destination id value. When you set up the consumer or producer in the mxml file, you don't tell it what Topic or Queue to connect to, you tell it what destination to connect to. This makes sense, since the mx:Producer and mx:Consumer attribute is called "destination", but it did throw me for a little loop.

<?xml version="1.0" encoding="UTF-8"?>
<service id="message-service" class="">
<adapter-definition id="actionscript" class="" default="true" />
<adapter-definition id="jms" class=""/>
<channel ref="my-amf"/>
<destination id="message-destination">
<channel ref="my-amf"/>
<adapter ref="jms"/>

The test.mxml is the final file. It's the GUI code. The file gets compiled into a swf file as I showed up above. My version uses mx:Producer and mx:Consumer mxml tags, though you can also instantiate these using Actionscript if you'd like. I prefer to write as little script as possible. The main key in this file was that I had to subscribe the consumer to it's destination before it would consume messages. Everything else is pretty simple, and you can see how easy it is to get a client sending and receiving messages to the server-side JMS broker over HTTP. This is why I like Flex!

<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx='' xmlns:xsi=''
backgroundGradientColors="[0xcfe4ff, 0xffffff]" paddingTop="5" paddingLeft="5"
paddingRight="5" paddingBottom="0" applicationComplete="logon()">
import mx.messaging.*;
import mx.messaging.messages.*;
import mx.controls.Alert;

private function messageHandler(event:MessageEvent):void {
receiveTextArea.text = event.message.body + "\n" + receiveTextArea.text;
private function acknowledgeHandler(event:MessageAckEvent):void{
private function faultHandler(event:MessageFaultEvent):void {'Fault!', 'Alert Box', mx.controls.Alert.OK);

private function logon():void {

private function sendMessage():void {
var message:AsyncMessage = new AsyncMessage();
message.body = sendTextArea.text;

<mx:Producer id="producer" destination="message-destination" acknowledge="acknowledgeHandler(event);" fault=" faultHandler(event)"/>
<mx:Consumer id="consumer" destination="message-destination" message="messageHandler(event)" acknowledge="acknowledgeHandler(event);" fault=" faultHandler(event)"/>
<mx:TextInput id="sendTextArea"/>
<mx:Button label="Send" click="sendMessage();"/>
<mx:TextArea id="receiveTextArea" width="50%" height="200"/>

The client app looks something like this:

In Conclusion, BlazeDS is an extremely simple way to use JMS with remote Flex clients over HTTP connections. The back-end configuation is somewhat complex, but mostly boilerplate and not too bad once you piece it together.

I haven't "stress" tested the messaging yet, or done very complex messaging to be able to say how robust and scalable it is, but so far, It does live up to it's claims. Best of all, I can now do asynchronous messaging to remote clients without writing code.