Hi,
This is an additional email to continue discussion about new Mistral DSL
capabilities. This time about conditional transitions and task dependencies.
Here’s the link to the corresponding etherpad:
https://etherpad.openstack.org/p/mistral-poc.
For convenience, I’m providing the snippet about conditional transitions right
in this message.
"
Conditional Transitions
Option #1 ('requires' with condition)
In this case we add additional optional condition in dependencies declaration.
task1 # task1 doesn't have dependencies
action: MyRest:action1
task2:
requires: task1 # task2 is allowed to run if task1 has
finished with success
action: MyRest:action2
task3:
requires: # task3 is allowed to run if task1
and task2 have finished and corresponding conditions are true
task1: $.value1 = 34
task2: $.value2 = 245
action: MyRest:action3
task4:
requires: [task2, task3] # task4 is allowed to run if task2 and
task3 has finished with success
action: MyRest:action4
Option #2 (direct transition)
In this case after task completion we decided what to do next depending on task
result.
task1:
action: MyRest:action1
SUCCESS:
- task2 # If task1 has finished with success then start task2
- task3: $.value1 == 123 # If after completion of task1 variable
"value1" in the context equals to "123" then start task3
ERROR:
- task10 # If task1 has finished with error then start task10
Option #3 (hybrid of 'requires' and direct transitions)
In this case we can specify both "requires" and direct transitions. In the
example below task3 starts only when task1 and task2 have finished. Therefore
task2 is an implicit dependency of task3 since task1 initiated execution of
task2. It means that a task can start only if the entire sub-workflow initiated
by "requires" clause has finished.
task1:
action: MyRest:action1
SUCCESS
- task2
task2:
action: MyRest:action2
task3:
action: MyRest:action3
requires:
task1: $.value1 = 34
SUCCESS:
task4: '$.value1 == 123'
"
Note that we’re now considering 3 options to proceed with and our current
preference is option #3 (mix of ‘requires’ and direct transitions) since it
would give a lot of flexibility for designing real life workflows. In some
cases it would be more convenient to describe a business process using task
dependencies and in other cases use direct conditional transitions if
“straightforward” flow definition is preferred.
Regardless of the option described here a user would start new workflow
execution with specifying a particular task in a graph.
As always, we would like to get your feedback on this.
Thanks!
Renat Akhmerov
@ Mirantis Inc.
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