meteor/meteor — pull request #3290 by DanielDent (2014-12-14 15:26:11+00:00)
Title: Avoids a race condition

Doing a remove and then an insert means that there is a period of time when there isn't a configuration entry. MongoDB does not have transactions, atomic upserts are used instead. If there are multiple meteor servers backed by one MongoDB database, this race condition could cause user-visible issues.