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POHow to prevent Sidekiq from parallel modifications to database
 

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  1. POHow to prevent Sidekiq from parallel modifications to database
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    <p>I have a worker that tries to find a match for a photo based on some conditions. Each photo as to have a unique match. I coded something like this:</p> <pre><code>class PhotoDeliveryWorker include Sidekiq::Worker def perform(photo_id) photo = Photo.find(photo_id) unless photo.match matches = Photo.where(some_condition: "some_value") match = matches.first if match # Do something to photo photo.match = match if photo.save match.some_condition = "another_value" else schedule photo_id end else # Couldn't find a match schedule photo_id end end end private def schedule(photo_id) PhotoDeliveryWorker.perform_in 1.hours, photo_id end end </code></pre> <p>As you can see, worker gets the first model object that passes the conditions and then changes the <code>match</code> to exclude it from future workers matches list. </p> <p>The issue is when several workers perform at a time, they all get the same <code>matches</code> list and as a result modify the same entity. But I need a unique match for each photo.</p> <p>How can I work it around?</p> <p>Additional info:</p> <p>Q: Why do I use workers?</p> <p>A: If I can't find a match, I need to retry later.</p> <p>Q: Why do I use Sidekiq with multiple threads?</p> <p>A: I need do process photos as fast as I could.</p> <p>Probably I could get count of currently active workers in the beginning of each one and take nth match instead of the first. But that solution smells a bit, doesn't it?</p> <p><strong>UPDATE:</strong></p> <p>Additional question: Could I use something about ActiveRecord locking with this issue? I'm not very familiar with <code>with_lock</code> and all this stuff.</p>
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    1. PTQuestion
    1. USBardt
    1. USBardt
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    1. COYou could drop down to raw SQL for this: `UPDATE photos SET match_id = ? where id = ? and match_id IS NULL`. This way, only the first update will find and update record.
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      1. POHow to prevent Sidekiq from parallel modifications to database
      1. USSergio Tulentsev
    2. COAssuming a simple condition, you could keep a redis list/set per condition with the Photo IDs then use `BRPOP` or `RPOP` to fetch a matching ID and ensuring this ID is never reused. Edit: You probably could also handle a list/set with all IDs to ensure it's not reused within redis
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      1. POHow to prevent Sidekiq from parallel modifications to database
      1. USlwe
 

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