[LRUG] Queue-related war stories

Mike Kelly mikekelly321 at gmail.com
Mon Mar 16 05:54:43 PDT 2015


Hey Ali,

One I've seen a few times is workers that perform "transactional"
activities but their behaviour is non-idempotent and/or suffers race
conditions ie. systems that assume all queued messages are guaranteed to
only be processed once, combined with queue that is unable to actually
deliver on that guarantee.


On Mon, Mar 16, 2015 at 9:12 AM, Najaf Ali <ali at happybearsoftware.com>
wrote:

> Hi all,
>
> I'm trying to identify some general good practices (based on real-life
> problems) when it comes to working with async job queues (think DJ, Resque
> and Sidekiq).
>
> So far I've been doing this by collecting stories of how they've failed
> catastrophically (e.g. sending thousands of spurious SMS's to your
> customers) and seeing if I can identify any common themes based on those.
>
> Here are some examples of what I mean (anonymised to protect the innocent):
>
> * Having a (e.g. hourly) cron job that checks if a job has been done and
> then enqueues the job if it hasn't. It knows this because the successfully
> completed job would leave some sort of evidence of completion in e.g. the
> database. If your workers go down for a day, this means the same job would
> be enqueued over and over again superfluously.
>
> * Sending multiple emails (hundreds) in a single job lead to a problem
> where if just one of those emails (say the 24th) fails to be delivered, the
> entire job fails and emails 1-23 get sent again when your worker retries it
> again and again and again.
>
> * With the workers/app running the same codebase but on different virtual
> servers, deploying only to the application server (and not the server
> running the workers) resulted in the app servers queueing jobs that the
> workers didn't know how to process.
>
> It would be great to hear what sort of issues/incidents you've come across
> while using async job queues like the above. I don't think I have enough
> examples to make any generalisations about the "right way" to use them yet,
> so more interested in just things that went wrong and how you fixed them at
> the moment.
>
> Feel free to reply off-list if you'd rather not share with everyone, I
> intend to put the findings together in a blog post with a few guesses as to
> how to avoid these sorts of problems.
>
> All the best,
>
> -Ali
>
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-- 
Mike

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