MongoDB Optimistic Locking Retry

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MongoDB Optimistic Locking Retry



Suppose I have multiple clients and all are hooked to the same Mongo Database. Each document has a version number to ensure that data isn't lost.



Document #1


{
__v: 0,
_id: 1,
quantity: 5
}



An update hits the server to update the document to quantity 10, using __v as 0.


Collection.update({_id: 1, __v: 0}, {$set: {quantity: 10}, $inc: {__v: 1}});



Now just say that another updates comes in right after as the following:


Collection.update({_id: 1, __v: 0}, {$set: {quantity: 15}, $inc: {__v: 1}});



Since the __v of the update is not 1, this update does not happen, so the client receives an error. This is what I am attempting to resolve.



Issue #1



Merging the 2nd update with the old __v: 0 document could work if the properties don't conflict, but if they do like in this example, not sure how to solve this issue.



I am thinking maybe I could keep the history of the documents, somehow compare the update with the version number (the second so quantity of 15 minus initial document quantity of 5, get an $inc of 10), and then take the latest document version of quantity 10 and $inc 10 to get a total of 20? Not sure if this is a bad idea. What about nested arrays getting deleted/pushed?



Issue #2



Say I am updating 3 different collections and need all 3 to either work or all 3 to fail (they need to be in sync). So what happens if two updates to collections work and then the third fails, I need to roll back the other two collection updates so they remain in sync.



I am using the oplog to attempt to get "live" data, but there are situations where two updates may happen before the live data responds to the clients and I am trying to prevent the user from getting error messages and rather deal with these updates on the server.









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