mongodb impossible (?) E11000 duplicate key error dup key when upserting

Question

My understanding is that update with upsert:true on a single document is an atomic operation so this should never result in a duplicate key error, especially not on the primary _id key, when the collection has no unique-ly indexed fields:

Order.update({ _id: order._id }, query, { upsert: true }, cb) // with mongoose 

But this appears in the mongod.log:

    2015-03-27T09:39:10.349-0400 I WRITE    [conn258236] update xyz.orders  query: { _id: "6353f880-c6a7-4260-809f-98e0af27b9a2" } update: { $set: { ...  } keyUpdates:0 writeConflicts:0 **exception: E11000 duplicate key error dup  key: { : "6353f880-c6a7-4260-809f-98e0af27b9a2" } code:11000** numYields:1  locks:{} 138ms       2015-03-27T09:39:10.349-0400 I COMMAND  [conn258236] command xyz.$cmd  command: update { update: "orders", writeConcern: { w: 1 }, ordered: true,  updates: [ { q: { _id: "6353f880-c6a7-4260-809f-98e0af27b9a2" }, u: { $set: {  ... } }, multi: false, upsert: true } ] } keyUpdates:0 writeConflicts:0  numYields:0 reslen:235 locks:{} 139ms 

Here is the output from db.orders.getIndexes():

{     "v" : 1,     "key" : {         "_id" : 1     },     "name" : "_id_",     "ns" : "xyz.orders" }, 

We are using MongoDB version 3.0.0 with WiredTiger.

Answer 1

I am afraid that this is an ongoing problem. I had the same problem and I found a jira ticket about this:

https://jira.mongodb.org/browse/SERVER-14322

It is possible that two updates come in with upsert:true, resulting in neither finding a document and both inserting new documents which conflict on unique index violations of the query predicate.

The "solution" here is to add a retry code into the client.