Proccessing data collections in Data Flow

Data Flow mechanism should be able to deal with collections, not only with fixed parameters. A typical scenario is when we want to run the same task multiple times with different sets of parameters. For instance, we want to create 100 VMs and it would be ugly to have 100 separate tasks in a workflow definition. So in this case we task could be smart enough to understand how to iterate through input collection containing parameter sets.

The current idea is as follows:

attach-vm:
    for-each:
        vminfo: $.vms
    action: create_vm name=$.vminfo.name flavor=$.vminfo.flavor
    action: create_vm name={$.server_name} image_id={$.vminfo.image_id} image_id={$.vminfo.flavor_id}

So 'for-each' just makes the task run over an array of elements where each element may be any structure (e.g. dictionary).

See https://etherpad.openstack.org/p/mistral-workflow-dataflow-collections for more details.