Question about listeners in Service tasks of type external

[jaxx]

Hello
When using a Service task implemented as external, is there a listener which is executed at a moment after the pending topic has been recorded in the engine DB?
I would like to send a message to the external worker to start polling.

Thanks in advance

[aravindhrs]

@jaxx

When using a Service task implemented as external, is there a listener which is executed at a moment after the pending topic has been recorded in the engine DB?

Service Task (External) has execution listeners which will get triggered at two event states (start & end).

@jaxx

I would like to send a message to the external worker to start polling.

You can’t push work items into Camunda External Task client. It works on pull based approach where external worker threads query Camunda for “external service tasks”.

Refer this post about external task pattern:

Quick question about External Task and their usage

Hi @francesco_giberti, Camunda External Task Client allows to set up remote Service Tasks for your workflow. The process engine supports two ways of executing service tasks: Internal Service tasks: Synchronous invocation of code deployed along with a process application. External tasks: Providing a unit of work in a list that can be polled by workers. Process engine publishes a unit of work to a worker to fetch and complete, and this process is called as external task pattern. Pul…

[jaxx]

Hello @aravindhrs
Yes, I understand that the external task is based on a pull approach. However my external worker is not running all the time, so I need to start it and I am trying to find the appropriate place.
The “start” listener of the external task will be invoked before the pending topic becomes available in the engine DB?

[Ingo_Richtsmeier]

Hi @jaxx,

@jaxx

However my external worker is not running all the time, so I need to start it and I am trying to find the appropriate place.

I have seen other customers solving this with a single, generic external task worker. The worker polls and pushes the task data to an AWS lambda function.

Hope this helps, Ingo

[aravindhrs]

@Ingo_Richtsmeier

I have seen other customers solving this with a single, generic external task worker.

@Ingo_Richtsmeier Generic task workers means it pulls all the items irrespective of topic? (pulls all tasks from all topics and let the lambda function to choose work on specific topic?)

[Ingo_Richtsmeier]

Hi @aravindhrs,

different options are possible. Somewhere you need a distribution. It could be done in the worker with different handlers, when listening to all topics or with a single topic using variables to differentiate.

Could also be done in a fixed Lambda. It depends on your Lambdas and what would be the easiest way for you to add another function…

Hope this helps, Ingo