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How does mongod work in a sharded cluster?
In a sharded cluster, mongod and mongos instances monitor the replica sets in the sharded cluster (e.g. shard replica sets, config server replica set). If all members of a replica set shard are unavailable, all data held in that shard is unavailable.
How can I troubleshoot a MongoDB shard key?
To troubleshoot a shard key, see Troubleshoot Shard Keys. Each shard should be a replica set, if a specific mongod instance fails, the replica set members will elect another to be primary and continue operation. However, if an entire shard is unreachable or fails for some reason, that data will be unavailable.
Can a MongoDB cluster be deployed as a replica?
Starting in MongoDB 3.2, config servers can be deployed as replica sets. The mongos instances for the sharded cluster must specify the same config server replica set name but can specify hostname and port of different members of the replica set.
How is a rolling index built in MongoDB?
Rolling index builds take at most one shard replica set member out at a time, starting with the secondary members, and builds the index on that member as a standalone. Rolling index builds require at least one replica set election per shard.
What should be included in a sharded cluster?
Each member of a sharded cluster must be able to connect to all other members in the cluster. This includes all shards and config servers. Ensure that network and security systems, including all interface and firewalls, allow these connections.
How to convert a replica set to a sharded cluster?
From mongosh, run rs.initiate () on replica set member 0. Run rs.initiate () on just one and only one mongod instance for the replica set. When possible, use a logical DNS hostname instead of an ip address, particularly when configuring replica set members or sharded cluster members.
How to convert MongoDB config servers to CSRS?
To convert your config servers from SCCC to CSRS, see the MongoDB 3.4 manual Upgrade Config Servers to Replica Set. With earlier versions of MongoDB sharded clusters that use the topology of three mirrored mongod instances for config servers, mongos instances in a sharded cluster must specify identical configDB string.