What are Capped Collections?

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Capped collections are fixed-size collections that act like a circular buffer — once full, new inserts overwrite the oldest documents. Insertion order is preserved. Fixed size — defined at creation; size in bytes, optional max document count. High-throughput inserts — very fast; documents stored in insertion order on disk. Auto-aging — oldest documents removed when full; no explicit deletes needed. Common uses — logs, recent events, in-memory queue replacement. Limitations — can't shard, can't shrink, can't delete individual documents. Modern alternative — TTL indexes for time-based expiry; capped collections fit special use cases. Capped = fixed-size FIFO (oldest auto-removed). Use for: logs, caching, event streaming. Tailable cursors for real-time reading. Cannot delete individual docs. Know size and max options. This is a very commonly asked MongoDB feature.