Problem Statement
What steps and considerations are involved in migrating a large production database from a single MongoDB instance to a sharded cluster with minimal downtime?
Explanation
Migrating a production database to a sharded cluster is a complex operation requiring careful planning, testing, and execution to minimize downtime and ensure data integrity. The process involves multiple phases and careful coordination.
Phase one is planning and preparation. First, analyze your current workload, data size, growth rate, and query patterns. Identify the optimal shard key based on query patterns, data distribution, and cardinality. A poor shard key choice cannot be changed without rebuilding, so this is critical. Second, design your target sharded cluster architecture including the number of shards, shard replica sets, config servers, and mongos routers.
Third, prepare the infrastructure by provisioning servers for config servers, shards, and mongos routers. Configure networking, security, and monitoring. Fourth, test the entire migration process in a staging environment with a copy of production data. Verify application compatibility with mongos, test failover scenarios, and measure performance.
Phase two is deploying the sharded infrastructure. Deploy config servers as a replica set, deploy each shard as a replica set, deploy mongos routers, and configure authentication and TLS. At this point, the sharded cluster is empty and ready to receive data.
Phase three is the actual migration with minimal downtime. The recommended approach uses MongoDB's live migration capabilities. First, add your existing replica set as the first shard in the cluster. Second, enable sharding on the database using sh.enableSharding. Third, shard the collections using sh.shardCollection with your chosen shard key.
At this point, all data is on one shard. Fourth, add additional empty shards to the cluster. Fifth, the balancer automatically begins migrating chunks to new shards, distributing data. Sixth, update application connection strings to use mongos instead of direct replica set connections. Use connection pooling and retry logic for resilience.
During migration, monitor chunk migration progress, watch for any application errors or timeouts, verify data distribution across shards is balanced, and monitor performance metrics on all components.
Phase four is post-migration validation. Verify all data migrated successfully by comparing document counts, test application functionality thoroughly including read and write operations, monitor performance and optimize indexes if needed, and establish backup and disaster recovery procedures for the sharded cluster.
Key considerations include choosing an optimal shard key, extensive testing in staging before production, having a rollback plan, communicating with stakeholders about migration timeline, and planning for increased operational complexity of managing a sharded cluster.