Adding a new shard to our cluster is as easy as connecting to mongos, connecting to the admin database, and invoking runCommand with:

> db.runCommand( {
addShard: "mongo_books_replica_set/rs01.packtdb.com:27017", maxSize: 18000, name: "packt_mongo_shard_UK"
} )

This adds a new shard from the replica set named mongo_books_replica_set from host rs01.packtdb.com running on port 27017. We also define the maxSize of data for this shard as 18,000 MB (or we can set it to 0 for no limit) and the name of the new shard as packt_mongo_shard_UK.

Removing a shard, on the other hand, requires more involvement since we have to make sure that we won't lose any data on the way.

First we need to make sure that the balancer is enabled using sh.getBalancerState().

Then, after identifying the shard we want to remove using any one of the sh.status(), db.printShardingStatus(), or listShards admin commands, we connect to the admin database and invoke removeShard:

> use admin
> db.runCommand( { removeShard: "packt_mongo_shard_UK" } )

The output should contain this part:

...
  "msg" : "draining started successfully",
  "state" : "started",
...

Then if we invoke the same command again, we get:

> db.runCommand( { removeShard: "packt_mongo_shard_UK" } )
…
"msg" : "draining ongoing",
  "state" : "ongoing",
  "remaining" : {
     "chunks" : NumberLong(2),
     "dbs" : NumberLong(3)
  },
…

The remaining document in the result contains the number of chunks and DBs that are still being transferred. In our case it's two and three respectively.

An extra complication in removing a shard can arise if the shard we want to remove serves as the primary shard for one or more of the databases that it contains.

The primary shard is allocated by MongoDB when we initiate sharding, so when we remove the shard, we need to manually move these databases to a new shard.

We will know whether we need to perform this operation by a section of the result from removeShard() like this:

...
"note" : "you need to drop or movePrimary these databases",
   "dbsToMove" : [
      "mongo_books"
   ],
...

So we need movePrimary for our mongo_books database. The way to do it is again from the admin database:

The result should contain the following before proceeding:

 ..."remaining" : {
     "chunks" : NumberLong(0) }...
> db.runCommand( { movePrimary: "mongo_books", to: "packt_mongo_shard_EU" })

This command will invoke a blocking operation, and when it returns, it should have a result like this:

{ "primary" : "packt_mongo_shard_EU", "ok" : 1 }

Invoking the same removeShard() command after we are all done should return a result like:

> db.runCommand( { removeShard: "packt_mongo_shard_UK" } )

... "msg" : "removeshard completed successfully",
   "state" : "completed",
   "shard" : "packt_mongo_shard_UK"
   "ok" : 1
...

After we get to state: completed and ok: 1, it is safe to remove our packt_mongo_shard_UK shard.

Removing a shard is naturally more involving than adding one. We need to allow some time, hope for the best, and plan for the worst when performing potentially destructive operations on our live cluster.