To ensure high availability and failover in a MongoDB sharded cluster, there are several best practices that you can follow:
1. Set up replica sets: In a sharded environment, each shard should have a minimum of three (3) nodes configured as a replica set for fault tolerance. This helps to ensure that data is continuously available in the event of a primary replica set node failure. You can set up replica sets by using the rs.initiate() method.
2. Use a configuration replica set: A dedicated replica set should be used to store the cluster’s metadata, such as the configuration data and shard key ranges. This replica set should have a minimum of three (3) nodes to ensure high availability. You can set up a configuration replica set by using the sh.addShard() method.
3. Monitor the health of the cluster: To prevent failures, you need to continually monitor the health of the cluster. MongoDB offers monitoring tools like Ops Manager and Cloud Manager. You can also use serverStatus and replSetGetStatus methods to get cluster health information programmatically. If any node or replica set goes down, you will get an alert and take necessary actions to recover the failed node.
4. Use shard key optimization: Proper shard key selection is critical to the performance and scalability of the sharded cluster. The chosen shard key should be uniformly distributed across all shards to prevent hotspots. MongoDB 4.4 introduces intelligent global clustering to help auto-select the right shard key based on popular queries and traffic.
5. Use graceful shutdowns: If you need to take a node or a shard offline, use a graceful shutdown. This ensures that the node is removed from the cluster without causing data loss or significant performance degradation. You can use the sh.removeShard() method to remove a shard.
6. Test failovers and recoveries: It’s essential to test failovers and recoveries regularly to ensure that the system can withstand failures. You should run simulated failovers and recoveries during off-peak times to evaluate the effectiveness of the failover procedures.
Overall, following these best practices will ensure that your MongoDB sharded cluster is highly available and fault-tolerant.