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I'm currently trying my hand at developing a simple web based game using rails and Mongoid. I've ran into some concurrency issues that i'm not sure how to solve.

The issue is i'm not sure how to atomically do a check and take an action based upon it in Mongoid.

Here is a sample of the relevant parts of the controller code to give you an idea of what i'm trying to do:

battle = current_user.battle
battle.submitted = true
battle.save

if Battle.where(opponent: current_user._id, submitted: true, resolving: false).any?
  battle.update_attribute(:resolving, true)
  #Resolve turn

A battle is between two users, but i only want one of the threads to run the #Resolve turn. Now unless i'm completely off both threads could check the condition one after another, but before setting resolving to true, therefore both end up running the '#Resolve turn' code.

I would much appreciate any ideas on how to solve this issue.

I am however getting an increasing feeling that doing user synchronization in this way is fairly impractical and that there's a better way altogether. So suggestions for other techniques that could accomplish the same thing would be greatly appreciated!

EasyPush
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1 Answers1

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Sounds like you want the mongo findAndModify command which allows you to atomically retrieve and update a row.

Unfortunately mongoid doesn't appear to expose this part of the mongo api, so it looks like you'll have to drop down to the driver level for this one bit:

battle = Battle.collection.find_and_modify(query: {oppenent: current_user._id, ...},
                                           update: {'$set' => {resolving: true})

By default the returned object does not include the modification made, but you can turn this on if you want (pass {:new => true})

The value returned is a raw hash, if my memory is correct you can do Battle.instantiate(doc) to get a Battle object back.

Frederick Cheung
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  • Hi! Thanks for your reply. I could however not get that command to work. The output i get is: Mongo::OperationFailure (Database command 'findandmodify' failed: (errmsg: 'no such cmd'; bad cmd: '{"findandmodify"=>"battles", "query"=>{"opponent"=>BSON::ObjectId('4f970eb047911507fe000205'), "submitted"=>true}, "update"=>{"$set"=>{"resolving"=>true}}, "new"=>true}'; ok: '0.0').): I tried to get it working for an hour or so but could not figure out why it didn't. Need i mention i am quite new to mongo? :) – EasyPush Apr 29 '12 at 20:19
  • Oh, and the exact command i'm running is the following: battle = Battle.collection.find_and_modify(query: {opponent: current_user._id, submitted: true}, update: {'$set' => {resolving: true}}, :new => true) – EasyPush Apr 29 '12 at 20:21
  • What version of mongo are you running? – Frederick Cheung Apr 29 '12 at 20:29
  • Ah, i'm running 1.2.2. Assumed it was up to date for some reason! Saw find and modify is only in 1.3 and after, i'll update and try again! – EasyPush Apr 29 '12 at 20:34