[appengine-java] Re: is it possible that repeated task executions happen in parallel ?

[Prashant]

anyone from google ?

On 23 March 2011 15:45, Prashant  wrote:

> hi,
>
> appengine docs says -
>
> *"*When implementing the code for tasks (as worker URLs within your app),
> it is important to consider whether the task is 
> idempotent.
> App Engine's Task Queue API is designed to only invoke a given task once,
> however *it is possible in exceptional circumstances that a task may
> execute multiple times* (e.g. in the unlikely case of major system
> failure). Thus, your code must ensure that there are no harmful side-effects
> of repeated execution.*"*
>
>
> my doubt is -
>
> in exceptional circumstances, is it possible that repeated task executions
> happen in parallel (i.e. multiple invocations of same task on same or
> different systems at the same time) ?
>
> --
> Prashant
>

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[appengine-java] Re: is it possible that repeated task executions happen in parallel ?

[Colin Hawkett]

I asked this question a while back and didn't really get a definitive 
response. 

https://groups.google.com/d/topic/google-appengine/YRAK6lHV1XQ/discussion 
(that's the link groups is giving me, but I'm having trouble with it - title 
of the thread is 'Idempotence & multiple task execution' if you want to 
search for it).

Ikai's response sounded definitive - '*The same task should not be executed 
multiple times concurrently*'. But the subsequent discussion seemed to cast 
doubt on that statement and it was never cleared up. I have never seen it 
happen.

The answer to this question makes a difference in how the idempotent 
contract is implemented - especially whether transactional semantics are 
required or not. That might sound trivial, but implementing a transaction 
has significant performance implications, and is not something you would do 
if you don't need to.

I disagree that 'make it idempotent' is a sufficient answer to this 
question. It is reasonable to want to understand the constraints of the task 
queue and make implementation decisions on that basis. Cheers,

Colin

*
*
*
*

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Re: [appengine-java] Re: is it possible that repeated task executions happen in parallel ?

[JT]

I am not from google, but yes.
On Mar 23, 2011 3:07 PM, "Prashant"  wrote:
> anyone from google ?
>
> On 23 March 2011 15:45, Prashant  wrote:
>
>> hi,
>>
>> appengine docs says -
>>
>> *"*When implementing the code for tasks (as worker URLs within your app),
>> it is important to consider whether the task is idempotent<
http://en.wikipedia.org/wiki/idempotent>.
>> App Engine's Task Queue API is designed to only invoke a given task once,
>> however *it is possible in exceptional circumstances that a task may
>> execute multiple times* (e.g. in the unlikely case of major system
>> failure). Thus, your code must ensure that there are no harmful
side-effects
>> of repeated execution.*"*
>>
>>
>> my doubt is -
>>
>> in exceptional circumstances, is it possible that repeated task
executions
>> happen in parallel (i.e. multiple invocations of same task on same or
>> different systems at the same time) ?
>>
>> --
>> Prashant
>>
>
> --
> You received this message because you are subscribed to the Google Groups
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> To post to this group, send email to
google-appengine-java@googlegroups.com.
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> For more options, visit this group at
http://groups.google.com/group/google-appengine-java?hl=en.
>

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