Is Journal + Majority 100% safe?

Question

I have been reading a few articles on MongoDB fault tolerance, and some people complain it can never really be achieved with MongoDB (like in this article: http://hackingdistributed.com/2013/01/29/mongo-ft/), and this got be confused.

Can someone confirm (and if possible show me the appropriate docs) that using the Write Concern "Journal + Majority" is enough to make sure that 100% of the writes that were reported as success by my driver are durably written and won't be lost even if any replica fails just after the write?

I'm talking about a 3 replica setup. I'm ok with the system no longer accepting writes in case of failure, but when a write is reported as successful by the driver, I need it to be durably committed (regardless of the number of replica failing after that).

Answer 1

Right so if you choose a journal write you are basically ensuring the write has made it to disk of a single node. If you choose to do a majority write, you are ensuring that the write has made it to memory of at least x number of nodes in your replica set.

By default, mongodb will flush from memory to journal every 100ms. By having your replica nodes on different machines (physical or virtual), ideally in different data centres, you are very unlikely to ever see ALL nodes in a geographically ditributed replica set go down within the same 100ms before one gets to disk.

Alternatively to guarantee that write made it to disk of a single node - use journal write.