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Ok, here is what I think...

I totally support the first option for its easiness in terms of understanding how it all should work (no need to figure out if some additional objects must be deleted if a workflow has been removed etc. etc.). We actually have two BPS [0] and [1] where the idea was similar to your option #2. But I admit that they’ve been around for a while and I think are obsolete (even though having eventually the same goal of notifying the outside world about executions/tasks events).

The only thing I would like to suggest is how we define a callback (keeping in mind it should be a valid JSON in reality):

POST /executions
    workflow_name=flow
    callbacks=[{
        events: [[on-task-complete, on-execution-complete]
	action: std.http url=‘http://foo.bar.com' method=POST headers=‘{}' ##
    },
   {# another callback}
   ]

and/or

POST /executions
    workflow_name=flow
    callbacks=[{
        events: [[on-task-complete, on-execution-complete]
	action: std.http
	parameters: {
		url: http://foo.bar.com,
		method: POST
		headers: {
			# Whatever headers we need.
		} 
	}
    },
   {# another callback} 
   ]

In other word we can trivially generalise this so that:
we can use not only webhooks but any action accessible in Mistral (e.g. it may be other transport)
it is consistent with our DSL

We might even want to allow “workflow” as well as “action” but not sure if we need to get that far for now.

Thoughts?

[0] https://blueprints.launchpad.net/mistral/+spec/mistral-event-listeners-amqp
[1] https://blueprints.launchpad.net/mistral/+spec/mistral-event-listeners-http

Renat Akhmerov
@ Mirantis Inc.



On 17 Sep 2014, at 10:36, Dmitri Zimine <dzimine at stackstorm.com> wrote:

> Use case: 
> The client software fires the workflow execution and needs to be know when the workflow is complete. There is no good pool strategy as workflow can take arbitrary time from ms to days. Callback notification is needed. 
> 
> Solution is a webhook
> 
> Option 1: pass callback URL as part of starting workflow execution:
> POST /executions
>     workflow_name=flow
>     callback= {
>         events: [[on-task-complete, on-execution-complete]
>         url: http://bla.com
>         method:POST
>         headers: {}
>         … other stuff to form proper HTTP call, like API tokens, etc ...
>     }
>     …..
> 
> 
> Option 2: webhook endpoint
> Mistral exposes /webhook controller 
> Client creates a webhook and receives events for all executions for selected workflows. 
> {  
>   "name": "web",
>   "active": true,
>   "events": [  ]
>   “workflows”: [wf1, wf2] 
>   "url": "http://example.com/webhook",  
> }
> 
> Opinions: 
> 
> DZ: my opinion: 
> Option 1 for it is simple and convenient for a client. 
> It seems like an optimal solution for “tracking executions and tasks” use case.
> 
> Option 2 is an overkill: makes it harder for a client (post workflow, post webhook, post execution, delete workflow, delete webhook) introduces lifecycle management problems (e.g., workflow deleted -> webhook orphaned).
> 
> I vaguely recall someone from Heat compared these options and regretted one of them for security reasons, but can’t remember details.
> 
> 
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