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Sharded Cluster High Availability¶
On this page
- Application Servers or mongos Instances Become Unavailable
- A Single mongod Becomes Unavailable in a Shard
- All Members of a Shard Become Unavailable
- A Config Server Replica Set Member Become Unavailable
- Renaming Mirrored Config Servers and Cluster Availability
- Shard Keys and Cluster Availability
A production cluster has no single point of failure. This section introduces the availability concerns for MongoDB deployments in general and highlights potential failure scenarios and available resolutions.
Renaming Mirrored Config Servers and Cluster Availability¶
If the sharded cluster is using mirrored config servers instead of a replica set and the name or address that a sharded cluster uses to connect to a config server changes, you must restart every mongod and mongos instance in the sharded cluster. Avoid downtime by using CNAMEs to identify config servers within the MongoDB deployment.
To avoid downtime when renaming config servers, use DNS names unrelated to physical or virtual hostnames to refer to your config servers.
Generally, refer to each config server using the DNS alias (e.g. a CNAME record). When specifying the config server connection string to mongos, use these names. These records make it possible to change the IP address or rename config servers without changing the connection string and without having to restart the entire cluster.
Shard Keys and Cluster Availability¶
The most important consideration when choosing a shard key are:
- to ensure that MongoDB will be able to distribute data evenly among shards, and
- to scale writes across the cluster, and
- to ensure that mongos can isolate most queries to a specific mongod.
Furthermore:
- 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.
- If the shard key allows the mongos to isolate most operations to a single shard, then the failure of a single shard will only render some data unavailable.
- If your shard key distributes data required for every operation throughout the cluster, then the failure of the entire shard will render the entire cluster unavailable.
In essence, this concern for reliability simply underscores the importance of choosing a shard key that isolates query operations to a single shard.
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