Monotonic increasing sequences in MongoDB

mongo_sequence.js

// vim: set ts=4 sw=4 noet:

// create a collection to hold the named sequences
db.createCollection('counters');

// shell helper. initialize, if necessary, and return the next sequence
function getCounterFor(name) {
	return db.counters.findAndModify({
		query: { _id: name },
		update: { $inc: { sequence: 1 } },
		upsert: true,
		new: true
	}).sequence;
}

N.B. for a non-sharded collection, you can just use a Timestamp field - it's in monotonic order even in the face of replicaset failover and clock skew (at least with MongoDB 3.2+)...

db.collection.insert({ts: new Timestamp(), data: "whatever"});

The server fills in the timestamp for you. If a server's clock is behind and takes over as primary, it keeps making timestamps in the same (future) second as the last TS from the old primary until its clock catches up (it has 32-bits worth of timestamps before it will run out). So you still get monotonic ordering.