Remove Shards from an Existing Sharded Cluster

Considerations

• A shard removal may cause an open change stream cursor to close, and the closed change stream cursor may not be fully resumable.

• You can safely restart a cluster during a shard removal process. If you restart a cluster during an ongoing draining process, draining continues automatically after the cluster components restart. MongoDB records the shard draining status in the config.shards collection.

Ensure the Balancer Process is Enabled

To successfully migrate data from a shard, the balancer process must be enabled. Check the balancer state using the sh.getBalancerState() helper in mongosh. For more information, see the section on balancer operations.

Determine the Name of the Shard to Remove

To determine the name of the shard, connect to a mongos instance with mongosh and either:

• Use the listShards command, as in the following:

  db.adminCommand( { listShards: 1 } )

• Run either the sh.status() or the db.printShardingStatus() method.

The shards._id field lists the name of each shard.

Remove Chunks from the Shard

From the admin database, run the removeShard command. This begins "draining" chunks from the shard you are removing to other shards in the cluster. For example, for a shard named mongodb0, run:

db.adminCommand( { removeShard: "mongodb0" } )

mongos converts the write concern of the removeShard command to "majority".

This operation returns with the following response:

{
   "msg" : "draining started successfully",
   "state" : "started",
   "shard" : "mongodb0",
   "note" : "you need to drop or movePrimary these databases",
   "dbsToMove" : [
      "fiz",
      "buzz"
   ],
   "ok" : 1,
   "operationTime" : Timestamp(1575398919, 2),
   "$clusterTime" : {
      "clusterTime" : Timestamp(1575398919, 2),
      "signature" : {
         "hash" : BinData(0,"Oi68poWCFCA7b9kyhIcg+TzaGiA="),
         "keyId" : NumberLong("6766255701040824328")
      }
   }
}

The balancer begins migrating chunks from the shard named mongodb0 to other shards in the cluster. These migrations happens slowly to avoid placing undue load on the overall cluster. Depending on your network capacity and the amount of data, this operation can take from a few minutes to several days to complete.

Check the Status of the Migration

To check the progress of the migration at any stage in the process, run removeShard from the admin database again. For example, for a shard named mongodb0, run:

db.adminCommand( { removeShard: "mongodb0" } )

mongos converts the write concern of the removeShard command to "majority".

The command returns output similar to the following:

{
   "msg" : "draining ongoing",
   "state" : "ongoing",
   "remaining" : {
      "chunks" : NumberLong(2),
      "dbs" : NumberLong(2),
      "jumboChunks" : NumberLong(0)         // Available starting in 4.2.2 (and 4.0.14)
   },
   "note" : "you need to drop or movePrimary these databases",
   "dbsToMove" : [
      "fizz",
      "buzz"
   ],
   "ok" : 1,
   "operationTime" : Timestamp(1575399086, 1655),
   "$clusterTime" : {
      "clusterTime" : Timestamp(1575399086, 1655),
      "signature" : {
         "hash" : BinData(0,"XBrTmjMMe82fUtVLRm13GBVtRE8="),
         "keyId" : NumberLong("6766255701040824328")
      }
   }
}

In the output, the remaining field includes the following fields:

Continue checking the status of the removeShard command until the number of chunks remaining is 0.

db.adminCommand( { removeShard: "mongodb0" } )

Move Databases to Another Primary Shard

If the shard is the primary shard for one or more databases in the cluster, then you must make that database use a different shard as its primary shard. removeShard lists any databases that you need to move in the dbsToMove field in the command output. If the shard is not the primary shard for any databases, skip to the next task, Finalize the Migration.

To move a database to another shard, use the movePrimary command.

To migrate the fizz database from mongodb0 to mongodb1, issue the following command:

db.adminCommand( { movePrimary: "fizz", to: "mongodb1" })

mongos uses "majority" write concern for movePrimary.

This command does not return until MongoDB completes moving all data. The response from this command will resemble the following:

{
   "ok" : 1,
   "operationTime" : Timestamp(1575400369, 9),
   "$clusterTime" : {
   "clusterTime" : Timestamp(1575400369, 9),
   "signature" : {
      "hash" : BinData(0,"2Nz8QCcVXB0LJLm1hsXfpTCaM0M="),
      "keyId" : NumberLong("6766255701040824328")
   }
}
}

Finalize the Migration

To clean up all metadata information and finalize the removal, run removeShard again. For example, for a shard named mongodb0, run:

db.adminCommand( { removeShard: "mongodb0" } )

mongos converts the write concern of the removeShard command to "majority".

A success message appears at completion:

{
    "msg" : "removeshard completed successfully",
    "state" : "completed",
    "shard" : "mongodb0",
    "ok" : 1,
    "operationTime" : Timestamp(1575400370, 2),
    "$clusterTime" : {
       "clusterTime" : Timestamp(1575400370, 2),
       "signature" : {
          "hash" : BinData(0,"JjSRciHECXDBXo0e5nJv9mdRG8M="),
          "keyId" : NumberLong("6766255701040824328")
       }
    }
}

Once the value of the state field is "completed", you may safely stop the instances comprising the mongodb0 shard.