Java – Asynchronous Java

At a previous job, we used what I jokingly called "redundant array of inexpensive computers" {1} to distribute a number of jobs. Some took seconds to run, others took more than a day (quarterly reports). All were designed to be capable of being restarted if they crashed. Some jobs ran on a schedule, some were run only on demand. The schedule was sophisticated enough to include things as simple as "run daily" or "run monthly" or as complicated as "run monthly, except for the last week of the quarter run daily". Any of the regularly scheduled tasks could be run "now" (on demand).

Because the array {2} lacked displays, the "scoreboard" of who was running what job was read from a database table. In order to determine if a job crashed or hung, I took a concept from embedded computers called a "watch dog". In this case, a running job would periodically update a status column with a 0. The monitoring software would periodically increment that status column, and if the number got above a threshold in a certain period of time, would send a message to a person to basically reboot machine X. The idea was to put this "heart beat" at the end of a loop, but not the innermost loop. I planned to have the "pulse" go off about every minute with the checking circuit checking every 5 or 10 minutes.

Notes:
1 - Folks might now call this a "grid" or "cluster", but they were a pile of Pentium 75 and 100 that hadn't been depreciated, so I could use them.
2 - Well, a 1 dimensional array, as they stacked reasonably well and were worthless enough that if they fell over and broke, no one would miss them. They hadn't been fully depreciated yet, so technically could not be discarded, but were slow enough that they were useless as desktop computers.

using XSLT to transform messages is a very common technique that you'll find supported in most ESB frameworks such as Apache Camel, Mule, Spring Integration, Ikasan etc.

The trade off is that XSLT can be difficult to maintain and there can be a lack of flexibility in the programming model that it offers. I've found that I'd often have to call out to a datasource to do a lookup or a POJO to perform some other complex transform. In the end up we only used XSLT for fairly simple transforms and had other components in the ESB flow do the rest.

There is also a performance cost in terms of performing the transform and the validation afterwards. There's a large array of libraries to choose from which perform the transform and validation with varying performance characteristics (think memory and CPU impacts).

There's another serialisation cost when you eventually transform back to a Java object.