MongoDB is a NoSQL data store. NoSQL offers a range of features, namely, performance, cost, scale, and elasticity, that are at best difficult and expensive, if not impossible, to achieve with traditional databases. So MongoDB could be cool. In fact, it used to be cool, because it was one of the earliest NoSQL systems out there, but there are second-generation data stores now and we can afford to apply normal standards. And by any standard, a data repository, especially one whose main selling point is high scale, needs to deal with failures. It lies
So, let's imagine that you're building an Instagram clone that you'll eventually sell to Facebook for $1B, and see if we can accomplish that with MongoDB. Your site probably has a front-end web server where users upload data, like putting up new pictures or sending messages or whatever else. You can't really drop this data on the ground -- if you could lose data without consequences, you would just do precisely that all the time, skip having to configure MongoDB, and read all those dev blogs. And if you possess the least bit of engineering pride, you'll want to do an honest stab at storing this data properly. So you need to store the data, shown in yellow, in the database in a fault-tolerant fashion, or else face a lot of angry users. Luckily for you, MongoDB can be configured to perform replication. Let's assume that you indeed configured MongoDB correctly to perform such replication. So, here's the $1 billion dollar question: what does it mean when MongoDB says that a write (aka insert) is complete?
The answer is none of the above. MongoDB v2.0 will consider a write to be complete, done, finito as soon as it has been buffered in the outgoing socket buffer of the client host. Read that sentence over again.