Implement exclusive jobs

Basic hooks already available, most notably the property in the jobs table. No schema changes will be necessary.

Initial version committed in svn branch ACT-989

Merged into trunk in rev 3078

From the mail:

>> I just committed a fix which makes jobs exclusive by default.
>> I think this is a good approach also in the interest of the-least-WTF
>> from a user perspective.
>>
>> Jobs can be made non-exclusive by specifying
>> activiti:exclusive="false"
>> on a timer definition or an async task.
>>
>> So an async, non-exclusive service would look like this:
>> <serviceTask .. activiti:async="true" activiti:exclusive="false" />

is this resolved now?

So this are correct examples, right?

<serviceTask ... activiti:async="true" activiti:exclusive="false" />

<timerEventDefinition activiti:exclusive="false">
<timeDuration>PT5M</timeDuration>
</timerEventDefinition>

The examples are correct but proper documentation in the user guide is still TODO.

Currently, there is one Job Acquisition Thread per cluster node, which puts the acquired Jobs into a queue, which is then processed by the worker threads. This queue is of type List<List<String>> with the inner list being a list of exclusive job ids that belong to the same process instance.

New exclusive Jobs that are created, while another exclusive Job is executed, go currently not into the existing exclusive jobs set, but are free to be aquired by any Job Acquisition Thread of the cluster and processed by any worker thread.

Our idea is that a thread immediately locks new exclusive jobs for its cluster node and adds it to the list of exclusive jobs it is currently processing.

This way ActivitiOptimisticLockingExceptions at parallel joins can be completely avoided even in clustered environments.

Another observation:
The aquisition of exclusive jobs uses (too?) many SQL queries and may result in idle threads.

Improving support for exclusive jobs (see Falko's comment above):

http://fisheye.codehaus.org/changelog/activiti/?cs=3144
http://fisheye.codehaus.org/changelog/activiti/?cs=3145
http://fisheye.codehaus.org/changelog/activiti/?cs=3146

Quick question: does it mean that having async=true and (by default) exclusive=true, it's really not asynchronously/parallelly run in multiple threads. Aren't we negating what async provides with setting exclusive default option - could you pls clear this confusion, Thanks.

implemented and documented