node.js - How to prevent race conditions in a distributed nodejs web servers architecture backed by mongodb -

here' s description of problem:

1. i have x web workers written in nodejs (express.js) behind load ballancer. these workers write data mongodb (mongoose.js)
2. i've setup endpoint y such @ point in middleware chain of handler i'm doing following logic: if requesting user exists in database, fetch it, update fields store back. if doesn't exist, insert in mongo.

note! i'm unable use mongoose's findandupdateone() - atomic - due domain specific logic, ie. if user exists update value, else insert value.

1. the problem is, quite often, 2 requests same user (who doesn't yet exist in db) hit 2 different workers. when user processing middleware kicks in, both workers determine user doesn't exist , attempt insert update it. naturally causes errors, eg. validation errors: i've setup unique email per user validation , others.

how can prevent this?

you can use $set , $setoninsert update operators achieve that.

from mongodb $setoninsert docs:

if update operation upsert: true results in insert of document, $setoninsert assigns specified values fields in document. if update operation not result in insert, $setoninsert nothing.

and

if db.collection.update() upsert: true had found matching document, mongodb performs update, applying $set operation ignoring $setoninsert operation.

so, should trick:

var = date.now();  users.update({ _id: 1 }, {   $set:         { updatedat: },   $setoninsert: { createdat: } }, {upsert: true}, function(err, doc) {    // resultant doc available here  })