Version 3.4 mongos instances cannot connect to earlier versions of mongod instances.

Default port for mongod instances with the shardsvr role is 27018 . To use a different port, specify net.port setting or --port option.

For a 3.4 sharded cluster, mongod instances for the shards must explicitly specify its role as a shardsvr , either via the configuration file setting sharding.clusterRole or via the command line option --shardsvr .

Starting in 3.4, sharded cluster components (shards, config servers, mongos instances) recognize their membership in a sharded cluster, including the name of the sharded cluster, the location of the config servers.

The balancer process has moved from the mongos to the primary member of the config server replica set. Associated with this change:

Removal of Support for SCCC Config Servers¶

3.4 sharded clusters no longer support the use of mirrored (SCCC) mongod instances as config servers. The use of SCCC config servers, deprecated in the 3.2 release, is no longer valid. Instead, deploy your config servers as a replica set (CSRS).

To upgrade your sharded cluster to version 3.4, the config servers must be running as a replica set.

To convert your existing config servers from SCCC to CSRS, see the Upgrade Config Servers to Replica Set.