By default, the write operations in MongoDB replica sets will be acknowledged once the write has been acknowledged by the primary server. If we want to change this behavior, we can do so in two different ways:

• We can request a different write concern per operation, in cases where we want to make sure that a write has propagated to multiple members of our replica set before marking it as complete, as follows:

> db.mongo_books.insert(
  { name: "Mastering MongoDB", isbn: "1001" },
  { writeConcern: { w: 2, wtimeout: 5000 } }
)

In the preceding example, we are waiting for the write to be confirmed by two servers (the primary, plus any one of the secondaries). We are also setting a timeout of 5000 milliseconds to avoid our write from blocking in cases where the network is slow or we just don't have enough servers to acknowledge the request.

• We can also change the default write concern across the entire replica set, as follows:

> cfg = rs.conf()
> cfg.settings.getLastErrorDefaults = { w: "majority", wtimeout: 5000 }
> rs.reconfig(cfg)

Here, we set the write concern to majority with a timeout of 5 seconds. The write concern majority makes sure that our writes will propagate to at least n/2+1 servers, where n is the number of our replica set members.