Secondaries fell behind longer than the oplog window and now report RS102 "too stale to catch up", leaving the primary without redundancy. Grow the oplog, resync the stale members by initial sync or from a snapshot, and protect the window from heavy writes.
One shard holds most of the data or takes most of the writes. Read balancer status, find jumbo chunks and monotonic shard keys, then fix with balancer windows, clearing jumbo flags, refining or resharding the key.
Choose between mongodump (logical, small data), filesystem/volume snapshots (large data) and managed backups. Take consistent backups from a replica set, restore them, replay the oplog for point-in-time recovery and test regularly.
The mongosh commands a MongoDB DBA uses every week - server health, running operations, replica sets, sizes, indexes, profiler, users, sharding and mongodump/mongorestore.
The shard key decides whether a sharded cluster spreads load evenly or piles it onto one shard. Evaluate candidates on cardinality, frequency and monotonic growth, make sure common queries include the key, prefer compound or hashed keys where needed, and know your resharding options before you commit.
MongoDB schemas should follow how the application reads and writes data, not a normalised relational model. Embed data read together, reference data that grows without bound or is shared, use patterns such as subset, bucket and computed fields, and enforce structure with schema validation.
MongoDB performs well while the working set and indexes fit in memory. Measure WiredTiger cache usage and eviction pressure, check index sizes and usage, remove unused indexes, design compound indexes with the ESR rule, and right-size instances or shard before the working set outgrows RAM.
Applications get NotWritablePrimary or "no primary found", and rs.status() shows only SECONDARY members. Work out whether a voting majority is reachable, fix network or member problems, and use a forced reconfiguration only when the majority is truly gone.
Move MongoDB replica sets off self-managed VMs. Choose between Atlas (MongoDB's own service) and Cosmos DB for MongoDB vCore (MongoDB-compatible on Azure), check feature compatibility, migrate online or with dump and restore, and switch connection strings.
Design a MongoDB replica set that survives a node or zone failure. Covers member layout, priorities, arbiters (and why to avoid them), majority write concern, read preferences, rolling maintenance and monitoring.
Find slow operations with the profiler or logs, read explain('executionStats') for COLLSCAN and in-memory sorts, and build compound indexes with the Equality-Sort-Range rule.
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