Unique Indexes

To create unique indexes using the following procedure, you must stop all writes to the collection during this procedure.

If you cannot stop all writes to the collection during this procedure, do not use the procedure on this page. Instead, build your unique index on the collection by issuing db.collection.createIndex() on the mongos for a sharded cluster.

Oplog Size

Ensure that your oplog is large enough to permit the indexing or re-indexing operation to complete without falling too far behind to catch up. See the oplog sizing documentation for additional information.

A. Stop the Balancer

Connect a mongo shell to a mongos instance in the sharded cluster, and run sh.stopBalancer() to disable the balancer:

sh.stopBalancer()

To verify that the balancer is disabled, run sh.getBalancerState(), which returns false if the balancer is disabled:

sh.getBalancerState()

B. Determine the Distribution of the Collection

From the mongo shell connected to the mongos, refresh the cached routing table for that mongos to avoid returning stale distribution information for the collection. Once refreshed, run db.collection.getShardDistribution() for the collection you wish to build the index.

For example, if you want to an ascending index on the records collection in the test database:

db.adminCommand( { flushRouterConfig: "test.records" } );
db.records.getShardDistribution();

The method outputs the shard distribution. For example, consider a sharded cluster with 3 shards shardA, shardB, and shardC and the db.collection.getShardDistribution() returns the following:

Shard shardA at shardA/s1-mongo1.example.net:27018,s1-mongo2.example.net:27018,s1-mongo3.example.net:27018
 data : 1KiB docs : 50 chunks : 1
 estimated data per chunk : 1KiB
 estimated docs per chunk : 50

Shard shardC at shardC/s3-mongo1.example.net:27018,s3-mongo2.example.net:27018,s3-mongo3.example.net:27018
 data : 1KiB docs : 50 chunks : 1
 estimated data per chunk : 1KiB
 estimated docs per chunk : 50

Totals
 data : 3KiB docs : 100 chunks : 2
 Shard shardA contains 50% data, 50% docs in cluster, avg obj size on shard : 40B
 Shard shardC contains 50% data, 50% docs in cluster, avg obj size on shard : 40B

From the output, you only build the indexes for test.records on shardA and shardC.

C. Build Indexes on the Shards That Contain Collection Chunks

For each shard that contains chunks for the collection, follow the procedure to build the index on the shard.

net:
   bindIp: localhost,<hostname(s)|ip address(es)>
   port: 27218
#   port: 27018
#replication:
#   replSetName: shardA
#sharding:
#   clusterRole: shardsvr
setParameter:
   skipShardingConfigurationChecks: true
   disableLogicalSessionCacheRefresh: true

mongod --config <path/To/ConfigFile>

mongod --port 27218 --setParameter skipShardingConfigurationChecks=true --setParameter disableLogicalSessionCacheRefresh=true

db.records.createIndex( { username: 1 } )