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POMongoDB Database Semaphores and Node.js Process.NextTick()
 

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    <p>This may be a vary bad idea, or a possible solution that we have to a database concurrency problem.</p> <p>We have a method that is called to do an update of a mongo record. We are seeing some concurrency problems - process A reads the record, process B reads the record, process A makes mods and saves the record, process makes B mods and saves the record. Because B reads after A, before A writes, it doesn't know about the changes A made, and we lose the data from A.</p> <p>I'm wondering if we could not use a database semaphore, basically a field on the collection, that is a boolean. If we read the record at the start of the method, and the field is true, it's being edited. At that point, re-call the method using process.nexttick(), with the same data. Otherwise, set the semaphore, and carry on.</p> <p>There would still be a bit of time between the read and the save, but it should be/could be faster than what we are doing now.</p> <p>Be something like this. Any thoughts, anyone done anything like this? Will it even work?</p> <pre><code>function remove_source(service_id,session, next) { var User = Mongoose.model("User"); /* get the user, based on the session user id */ User.findById(session.me,function(err,user_info) { if (user_info.semaphore === true) { process.nextTick(remove_source(service_id,session,next)); } else { user_info.semaphore = true; user_info.save(function(err,user_new) { if (err) next(err,user_new); else continue_on(null,user_new); }); } function continue_on(user_new) { etc....... } </code></pre> <p>Edit: New Code:</p> <p>The function now looks as follows. I'm doing individual updates to the arrays. This of course means that I now have the possibility, if the transaction fails between the first and second transactions, of having data out of sync. I'm thinking that I could simply resave the user object that I retrieved on entry into the function, overwriting my changes. I don't know if Mongoose/Mongo will not do the save if I have not changed that object, will have to try and see. Any more thoughts?</p> <pre><code> var User = Mongoose.model("User"); /* get the user, based on the session user id */ User.findById(session.me,function(err,user_info) { if (err) { next(err,user_info,null); return; } if (!user_info || user_info.length === 0) { next(_e("ACCOUNT_NOT_FOUND"),"user_id: " + session.me); return; } var source_service_info = _.where(user_info.credentials, {"source_service_id": service_id}); var source_service = source_service_info.source_service; User.findByIdAndUpdate(session.me,{$pull: {"credentials": {"source_service_id": service_id}}},{},function(err,user_credential_removed) { if (err) { next(err,user_info,null); return; } User.findByIdAndUpdate(session.me,{$pull: {"criteria": {"source_service": source_service}}},{},function(err,user_criteria_removed) { if (err) { next(err,user_info,null); return; } else { next(null,user_criteria_removed); } }); }); }); </code></pre> <p>};</p>
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    1. COYou might just want to add a version to your documents and conditionally update based on the version being unchanged since it was read. If it fails to update, you'll know that there was a modification since it last read.
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