On Aug 26, 2011, at 6:41 PM, John Dickinson wrote:
>
> On Aug 26, 2011, at 6:10 PM, Monsyne Dragon wrote:
>>
>> The proper way, IMHO, for this to work is that a request generates a
>> workorder with a set of tasks.
>> This gets sent to something (scheduler, probably) which looks at the first
>> uncompleted task on the workorder, makes the decision on where to send it,
>> and routes the whole workorder there.
>> The service that gets it performs the task (i.e. executes the method),
>> possibly attaching info (like id of newly created instance) to the
>> workorder, and possibly pushing an 'undo' task to the top of a list of tasks
>> to perform if things fail somewhere.
>> Then the whole workorder gets sent back to the origin (again, scheduler?)
>> This looks at the next uncompleted task, and starts the cycle again.
>> Repeat until done.
>>
>> If there is a failure, the scheduler works through the 'undo' list on the
>> workorder, and then makes whatever decisions are needed to redo the tasks
>> elsewhere. The workorder contains the record of the failed attempt, so it
>> doesn't, for example, try to send the server build back to the same hosts
>> that just failed.
>>
>> The workorder acts as an environment for the tasks, and could be passed to
>> tasks (rpc methods) as an attribute of the context object.
>
>
> This sounds very similar to Google's app engine pipeline project
> (http://code.google.com/p/appengine-pipeline/).
Somewhat. It's a pretty common, simplified 'workflow' type design. Basically
a glorified visitor pattern. I just avoided the term, because I think we
only need a fairly simple, specific, implementation, not the be-all, end-all
all-singing-all-dancing dessert topping and floor wax that most things labeled
'workflow' tend to try to be. :>
--
Monsyne M. Dragon
OpenStack/Nova
cell 210-441-0965
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