using JMS for long running processes?

Question

Can someone point me to a tutorial or similar code where JMS is used by a web app to execute a long running background process? (instead of using threads), I'm fairly familiar with the concepts of JMS messaging, but never used any JMS API or brokers (i'm looking at learning Apache ActiveMQ)

I'd like to be able to: submit a message to the queue to run a process check the status (progress) on that process at arbitrary times

Thanks!

Answer 1

The real point of using JMS in your context is to start tasks asynchronously. This is called fire and forget in middleware lingo. JMS has guaranteed delivery semantics, meaning that once the message has been put on the queue it is guaranteed to get there ... eventually.

The idea is you do any tasks you need to do and if you have any tasks in the process that can be done at a later time, then you put a message on a queue and later it will execute. This allows you to cut down processing by a significant amount while somebody is waiting for a response.

Another benefit of JMS is that the different parts of the system do not need to be running at the same time. The part that consumes messages can be down for maintenance while your front end still works.

Answer 2

The previous post is accurate in terms of a model to put orders or requests into a queue asynchronously and then have them be picked up later. However, it doesn't really address the question of long running processes.

In terms of queues and topics, the benefit of persistent queues is that if there are no consumers on the queue then messages will be waiting for consumption until there is a subscriber. In a topic, you need to create a durable subscription in order to make sure a consumer that is not connected will receive messages that are sent in its absence once it reconnects.

So, how are you defining a long-running-process? For a multi-step process you would typically use something like a workflow engine. There are options like a BPM tool or something like "OS Workflow". You can also do a home-grown solution that could look like the example below

1) There would need to be some sort of workflow definition that defines the steps in the process. This could be a properties file or an XML file. 2) Web App puts a message on a queue or topic (pub/sub) with an indication of the process to be executed (or you can have specific destinations for different processes) 3) A Dispatcher MDB picks the 'order' up off the queue with a status of 'NEW' and starts processing the first step. 4) Once the step is complete, the MDB puts a new message on the queue indicating the process being executed and either the next step to be executed, or the last step that was executed (depending on how deterministic you want the process to be) 5) The MDB picks up the message and sees that the process is 'IN_PROGRESS'. It either determines the next step to be executed or reads the step to be executed next from the message (either a JMS header value or within the message, perhaps in an XML format) 6) Steps 4 & 5 are repeated until the process instance is complete

In this case you will need an external representation of the order and process instance information. This will allow you to check the status of a request from your WebApp. Your order would need to be read and persisted with an updated status after each step in the process such that the WebApp could access the status information.

The key component of this architecture is the dispatcher MDB that listens for messages and executes the next step of the process. When I worked with OS Workflow that was one key piece that was missing. In this manner, you can control the number of threads that are executing process steps by controlling the number of MDB's in the pool and consumers on the queue. In this architecture I would recommend a queue over a topic for the workflow steps. However after each process step you could publish a message to a topic for subscribers to get updated status information.

With the Java EE6 technologies including JPA you could easily create an XSD, generate domain data model POJO's with JAXB and use JPA for persistence. We did a webcast earlier this year that covered the JEE6 technologies that are currently supported in WebLogic. Here are the replays: http://www.oracle.com/technetwork/middleware/weblogic/learnmore/weblogic-javaee6-webcasts-358613.html .

I'm also still interested to speak with you about your JBoss migration :) jeffrey.west@oracle.com