Open Stack

More responses inline :)

From: Dmitri Zimine <dz at stackstorm.com<mailto:dz at stackstorm.com>>
Date: Monday, March 31, 2014 at 5:59 PM
To: Joshua Harlow <harlowja at yahoo-inc.com<mailto:harlowja at yahoo-inc.com>>
Cc: "OpenStack Development Mailing List (not for usage questions)" <openstack-dev at lists.openstack.org<mailto:openstack-dev at lists.openstack.org>>
Subject: Re: [openstack-dev] [Mistral] How Mistral handling long running delegate tasks

Inline...
On Mar 27, 2014, at 5:10 PM, Joshua Harlow <harlowja at yahoo-inc.com<mailto:harlowja at yahoo-inc.com>> wrote:

Thanks for the description!

The steps here seem very much like what a taskflow engine does (which is good).

To connect this to how I think could work in taskflow.

  1.  Someone creates tasks/flows describing the work to-be-done (converting a DSL -> taskflow tasks/flows/retry[1] objects…)
  2.  On execute(workflow) engine creates a new workflow execution, computes the first batch of tasks, creates executor for those tasks (remote, local…) and executes those tasks.
  3.  Waits for response back from futures<http://docs.python.org/dev/library/concurrent.futures.html> returned from executor.
  4.  Receives futures responses (or receives new response DELAY, for example), or exceptions…
  5.  Continues sending out batches of tasks that can be still be executing (aka tasks that don't have dependency on output of delayed tasks).
  6.  If any delayed tasks after repeating #2-5 as many times as it can, the engine will shut itself down (see http://tinyurl.com/l3x3rrb).

Why would engine treat long running tasks differently? The model Mistral tried out is the engine sends the batch of tasks and goes asleep; the 'worker/executor' is calling engine back when the task(s) complete. Can it be applied

Not all ways of using taskflow I think want to go 'asleep', some active conductors I would think actually stay 'awake', that’s why I was thinking this would be a comprimise, the usage of 'DELAY' (via exception or return) would allow a task to say it is going to be finished sometime in the future (btw started this @ https://review.openstack.org/#/c/84277/ --- WIP, likely won't go into taskflow 0.2 but seems like a reasonable start on something like this proposal). To me offering just the going asleep way means that all users of taskflow need to have some type of entrypoint (rest, mq, other…) that restarts the engine on result finishing. I'd rather not force that model onto all users (since that seems inappropriate).



  1.  On delay task finishing some API/webhook/other (the mechanism imho shouldn't be tied to webhooks, at least not in taskflow, but should be left up to the user of taskflow to decide how to accomplish this) will be/must be responsible for resuming the engine and setting the result for the previous delayed task.

Oh no, webhook is the way to expose it to 3rd party system. From the library standpoint it's just an API call.

One can do it even now by getting the appropriate Flow_details, instantiating and engine (flow, flow_details) and running it to continue from where it left out. Is it how you mean it? But I keep on dreaming of a passive version of TaskFlow engine which treats all tasks the same and exposes one extra method - handle_tasks.

I was thinking that, although I'm unsure on the one extra method idea, can u explain more :) What would handle_tasks do? Restart/resume the engine (basically the work u stated 'getting the appropriate Flow_details, instantiating and engine (flow, flow_details) and running it to continue')? Seems like a tiny helper function if its really wanted, but maybe I'm misunderstanding.  It might be connected into a recent taskflow BP @ https://blueprints.launchpad.net/taskflow/+spec/generic-flow-conductor?

  1.  Repeat 2 -> 7 until all tasks have executed/failed.
  2.  Profit!

This seems like it could be accomplished, although there are race conditions in the #6 (what if multiple delayed requests are received at the same time)? What locking is done to ensure that this doesn't cause conflicts?
Engine must handle concurrent calls of mutation methods - start, stop, handle_action. How -  differs depending on engine running in multiple threads or in event loop on queues of calls.

Agreed, to me this requires some level of locking, likely something that the tooz<https://review.openstack.org/#/c/71167/> library can provide (or in concept is similar to what the jobboard<https://wiki.openstack.org/wiki/TaskFlow/Paradigm_shifts#Workflow_ownership_transfer> concept provides, single 'atomic' engine access to a given workflow, ensuring this with a distributing locking scheme, such as zookeeper).


Does the POC solve that part (no simultaneous step #5 from below)?
Yes although we may want to revisit the current solution.

Is it using some form of database locking? :(


There was a mention of a watch-dog (ideally to ensure that delayed tasks can't just sit around forever), was that implemented?
If _delayed_ tasks and 'normal' tasks are treat alike, this is just a matter of timeout as a generic property on a task. So Mistral didn't have to have it. For the proposal above, a separate treatment is necessary for _delayed_ tasks.

Who responds to the timeout though? Isn't that process the watchdog then? Likely the triggering of a timeout causes something to react (in both cases).

[1] https://wiki.openstack.org/wiki/TaskFlow#Retries (new feature!)
This is nice. I would call it a 'repeater': running a sub flow several times with various data for various reasons is reacher then 'retry'.
What about the 'retry policy' on individual task?

We went back and forth on names a few times (controller, retry, and a few others I think), haha. Retry is what currently stuck :-P

Repeater does sound nice also though, damn naming of 'things' ;)

For retries on individual tasks since taskflow supports flow nesting its not that hard to do this (although it might not be the most elegant).

>>> flow = LinearFlow("blah blah")
>>> flow.add(LinearFlow("subblahblah", retry=XYZ).add(OneTask()) <<< where the one task is literally one task, haha.

A running example @ http://tinyurl.com/kv8q2t8

Room for improvement obviously (isn't there always), but that would/should work.

-Josh



From: Dmitri Zimine <dz at stackstorm.com<mailto:dz at stackstorm.com>>
Reply-To: "OpenStack Development Mailing List (not for usage questions)" <openstack-dev at lists.openstack.org<mailto:openstack-dev at lists.openstack.org>>
Date: Thursday, March 27, 2014 at 4:43 PM
To: "OpenStack Development Mailing List (not for usage questions)" <openstack-dev at lists.openstack.org<mailto:openstack-dev at lists.openstack.org>>
Subject: [openstack-dev] [Mistral] How Mistral handling long running delegate tasks

Following up on http://tinyurl.com/l8gtmsw and http://tinyurl.com/n3v9lt8: this explains how Mistral handles long running delegate tasks. Note that a 'passive' workflow engine can handle both normal tasks and delegates the same way. I'll also put that on ActionDesign wiki, after discussion.

Diagram:
https://docs.google.com/a/stackstorm.com/drawings/d/147_EpdatpN_sOLQ0LS07SWhaC3N85c95TkKMAeQ_a4c/edit?usp=sharing

1. On start(workflow), engine creates a new workflow execution, computes the first batch of tasks, sends them to ActionRunner [1].
2. ActionRunner creates an action and calls action.run(input)
3. Action does the work (compute (10!)), produce the results,  and return the results to executor. If it returns, status=SUCCESS. If it fails it throws exception, status=ERROR.
4. ActionRunner notifies Engine that the task is complete task_done(execution, task, status, results)[2]
5. Engine computes the next task(s) ready to trigger, according to control flow and data flow, and sends them to ActionRunner.
6. Like step 2: ActionRunner calls the action's run(input)
7. A delegate action doesn't produce results: it calls out the 3rd party system, which is expected to make a callback to a workflow service with the results. It returns to ActionRunner without results, "immediately".
8. ActionRunner marks status=RUNNING [?]
9. 3rd party system takes 'long time' == longer then any system component can be assumed to stay alive.
10. 3rd party component calls Mistral WebHook which resolves to engine.task_complete(workbook, id, status, results)

Comments:
* One Engine handles multiple executions of multiple workflows. It exposes two main operations: start(workflow) and task_complete(execution, task, status, results), and responsible for defining the next batch of tasks based on control flow and data flow. Engine is passive - it runs in a hosts' thread. Engine and ActionRunner communicate via task queues asynchronously, for details, see  https://wiki.openstack.org/wiki/Mistral/POC

* Engine doesn't distinct sync and async actions, it doesn't deal with Actions at all. It only reacts to task completions, handling the results, updating the state, and queuing next set of tasks.

* Only Action can know and define if it is a delegate or not. Some protocol required to let ActionRunner know that the action is not returning the results immediately. A convention of returning None may be sufficient.

* Mistral exposes  engine.task_done in the REST API so 3rd party systems can call a web hook.

DZ.

[1]  I use ActionRunner instead of Executor (current name) to avoid confusion: it is Engine which is responsible for executions, and ActionRunner only runs actions. We should rename it in the code.

[2] I use task_done for briefly and out of pure spite, in the code it is conveny_task_results.

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.openstack.org/pipermail/openstack-dev/attachments/20140401/45126169/attachment.html>