Service Task Formula

[zzl123]

Hello everyone!
I am really new with BPMN standard and coding, so my question may be very stupid, please be patient

I wish to create a process that gathers information from two operators (team A and team B) and then execute some operations (“Evaluation Process”).

So I created two “user tasks” with Form where the operators can put variables, let say “Input_Variable_TeamA” and “Input_Variable_TeamB”.

Fig 1

Then the output from the two user tasks will be the input in the evaluation process, that performs the operation like “output_EvaluationProcess” = “Input_Variable_TeamA” + “Input_Variable_TeamB”. So, I defined the Service Task “Evaluation Process” with the following expression under Output section as :
value.InputVariable_TeamA + value.InputVariable_TeamB

Fig 2.

I successed to deploy and create a BP instance and the data have been correctly send to the “Evaluation Process” task. I tested this process with value 3 and 5 for the InputVariable_TeamA and InputVariable_TeamB.

Fig 3

As you can see from Fig 3, the expression that I set didn’t work.
I would like that the output from the “Evaluation Process” task gives me as result 8.

Can anyone help me out this this exercise ? or indicate me a tutorial where I can find the solution.
Thank you anticipatedly!

[zzl123]

Fig 2

[zzl123]

Fig 3

[StephanHaarmann]

Hi @zzl123 and welcome to the world of BPMN,

Simply put, service tasks in Camunda 8 work as follows:

1. When a process instance reaches the service task, a corresponding job is put on a queue
2. External Task Workers can pick and complete these jobs
3. Only then is the output expression evaluated

So, if you want to implement a service task, have a look at the available Clients:

Java client | Camunda Platform 8 Docs

Your example would work if you have a simple job worker that takes the job and completes it (without any other step).

However, in your example, you can avoid job worker completely:
Business Rule Tasks can be implemented via DMN models, which can evaluate FEEL expressions and Decisions tables. Have a look at the following:

DMN in Modeler | Camunda Platform 8 Docs

What is FEEL? | Camunda Platform 8 Docs

The final DMN may look like this:
Example.dmn (2.3 KB)

[zzl123]

Thank you !