Enterprise Service Bus - Mule

I love ESB. No, thats not a drug, it stands for Enterprise Service Bus and its a Java Enterprise Component. A lot of development work is taking 2 or more existing things and making them talk together. Moving into the server side arena, this usualy entails spanning multiple machines, importing data, transforming data, and ensuring that data that needs to be imported is never lost.

Now, anyone can write a program that watches a folder for new files and parses them when they arrive. But, add in the following and the problem becomes a little harder:

1. The server that needs these files may be on a different machine than the folder being watched.
2. If the server goes down half way through parsing a file, I want to recover when I come back up. Loosing a file is unacceptable.
   
3. At any point in time I want to be able to monitor files that are being processed.
4. Depending on how powerful machines are, I want to control how many files are parsed at a time and when.
5. More than one component may be interested in the data that these files contain.
6. Maybe I not only want to be able to watch folders, but maybe also FTP site, email accounts, etc.
   

Now things are much harder. Just adding in common error handling and recovery can be a pretty difficult task.

Enter the Enterprise Service Bus (ESB). This is a java architecture where you configure endpoints (Hot folders, email accounts, ftp accounts, anything), message flows, and connect them all together. It handles error handling, splicing and duplicating messages, delivery and since it runs in a container threading and resource management is handled for you.

In ESB talk, the example above would be configure a File endpoint to watch a hot folder, a transform to parse the file into relevant data, and another endpoint that talks to the server using whatever transport mechanism fits best. If the message is stopped at any point throughout this process (say, because a server is down) it simply waits until it can move on. Plus, since you are using a well-known API, the leaning curve is greatly redulced and it can be expanded easily.

So go learn about the Enterprise Service Bus architecture. Some good ones are:

1. Mule
2. ServiceMix
3. JBoss ESB

To see some code here is an programmatic example using Mule. I will configure an "In" folder and an "Out" folder. When files are dropped in the "In" folder, they are successfully moved through a dummy component (called a pass through component) and to the out folder.

QuickConfigurationBuilder builder = new QuickConfigurationBuilder();
     // this will create an admin agent
builder.createStartedManager(true, "tcp://localhost:60504");

// in endpoint
UMOEndpoint inboundEndpoint = new MuleEndpoint("file://c:/MuleTest/In",true);
// out endpoint
UMOEndpoint outboundEndpoint = new MuleEndpoint(file://c:/MuleTest/Out",false);
 
     // creating a flow using 2 endpoints and a passthrough component.
builder.registerComponent(PassThroughComponent.class.getName(), "My Test Component",
inboundEndpoint, outboundEndpoint, new HashMap());

And thats it. By using these few lines we get a file copier with full error handling and recovery. Plus, you can actually wire multiple Mule ESBs together to route these files from machine to machine to allow scalability and interpolation.