Features

This section covers the cross-cutting capabilities of the MongoDB server. Each one spans several directories under src/mongo/ and integrates with both mongod and mongos.

Pages

• Replication — replica sets, oplog, elections, initial sync, dbcheck.
• Sharding — chunks, balancer, migrations, the catalog cache, routing.
• Query engine — the classic engine, SBE, plan caching, query planning.
• Aggregation pipeline — pipeline stages, optimization, document/value model.
• Transactions — single-shard and cluster ACID transactions.
• Change streams — the streaming change feed.
• Index builds — two-phase index build coordination.
• Time-series — bucket-based collections optimized for telemetry.
• Auth — authentication, authorization, role-based access control.
• Storage — the storage engine API and the WiredTiger integration.

Other capabilities not getting their own page

The following are real features but don't currently warrant a dedicated wiki page; they are documented in line above or in the docs/ tree:

• Queryable Encryption (FLE2) — src/mongo/crypto/, src/mongo/db/query/fle/. Adds typed-encryption support for equality, range, and prefix queries on encrypted fields.
• Field-level encryption (FLE1) — src/mongo/db/auth/, src/mongo/db/commands/. Predates FLE2 and is preserved for compatibility.
• Audit log — src/mongo/db/audit*. Pluggable auditing of authentication, command dispatch, and other events.
• Resharding — covered by Sharding. The dedicated coordinators live under src/mongo/db/s/resharding/ and src/mongo/s/resharding/.
• Free monitoring — small client that ships diagnostic data to a MongoDB-managed endpoint when enabled.
• Traffic recorder / replay — src/mongo/db/traffic_recorder.h, src/mongo/replay/. Captures wire-protocol traffic for replay testing.
• OpenTelemetry export — src/mongo/otel/. Exports tracing and metrics to OTLP collectors.

The Architecture page has the cross-cutting picture.