Friday, 2 October 2015

Task type differentiation and looking a bit more SoA

At the moment all tasks are considered the same. If a client wanted to process only “widget painter” tasks it wouldn’t be able to, unless it pulled tasks from the queue then decided whether to process them or not. If not – there’s not much it could do, as the tasks are off the queue. It could throw them away, which isn’t very constructive.

More practically, the client could hand tasks off to components suited to dealing with them. This complicates the client architecture, as it introduces the need for a broker component that can do a fine-grained distribution of work, along with internal queuing based on task type. That can quickly become a full-blown project in itself.

To avoid the need for a broker component (but also not to obviate them entirely) it makes sense to handle this queueing of tasks by type in the workflow management domain; to advertise submission of tasks of each type using a distinct channel, and allow clients to subscribe to channels that advertise submission of tasks that they can handle. In my workflow system that means having a dedicated submitted queue for each arbitrary task type, and in turn, submission messages would need to be published on dedicated channels.

The supporting functionality would all need to become task-type aware – popping a task would involve knowing what task type to pop; pause and release would need to know which queues to place tasks back in; and so on.

Not only do we need dedicated submitted:type queues, but if we’re going to ask questions about all the submitted tasks then I’ll need a submittedTypes set just so I can locate all the submitted:type queues that are in use.

Finally; what should the task type specified be? The most straightforward immediate answer is for it to be an arbitrary string. That means we’ll be creating an arbitrary subschema of lookups that map a set of client-defined types with tasks. That looks like the beginning of a general system I have in mind that allows the client to associate any number of arbitrary properties with tasks, not just a type; but to meet this use case – allowing the client to differentiate tasks by a given type – this should be enough.

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