We can also identify our replica set members by different tags (that is, reporting, east-coast servers, HQ servers) and specify a custom write concern per operation.

Using the usual procedure of connecting to the primary via mongo shell as follows:

> conf = rs.conf()
> conf.members[0].tags = { "location": "UK", "use": "production", "location_uk":"true"  }
> conf.members[1].tags = { "location": "UK", "use": "reporting", "location_uk":"true"  }
> conf.members[2].tags = { "location": "Ireland", "use": "production"  }

We can now set a custom write concern as follows:

> conf.settings = { getLastErrorModes: { UKWrites : { "location_uk": 2} } }

And after applying it as follows:

> rs.reconfig(conf)

We can now start using it by setting writeConcern in our writes as follows:

> db.mongo_books.insert({<our insert object>}, { writeConcern: { w: "UKWrites" } })

This means that our write will only be acknowledged if the UKWrites write concern is satisfied, which in turn will be satisfied by at least two servers with tag location_uk verifying it. Since we only have two servers located in the UK, we can make sure that with this custom write concern we have written our data to all of our UK-based servers.