mongodb/mongo

Glossary

A reference for project-specific vocabulary. Most of these terms appear repeatedly across src/mongo/ and the docs in docs/.

Term	Meaning
`mongod`	The data-bearing server process. Hosts collections, indexes, the replication coordinator, and the storage engine.
`mongos`	The sharding router process. Routes commands to shards, runs the cluster transaction protocol, and consults the config server replica set (CSRS).
`jstestshell`	The in-tree JavaScript shell used by `jstests/`. Built on SpiderMonkey. Formerly named the `mongo` shell.
CSRS	Config Server Replica Set. Hosts cluster metadata (the routing table, sharding catalog, and similar collections).
Replica set	A group of `mongod` instances that elect a single primary and replicate the oplog asynchronously. See Replication.
Oplog	The capped collection (`local.oplog.rs`) that records every write on a replica set primary. Secondaries tail it to apply writes.
Primary / Secondary	Roles within a replica set. The primary accepts writes; secondaries replicate them.
WiredTiger	The default storage engine. Lives under `src/third_party/wiredtiger/` and is wrapped by `src/mongo/db/storage/wiredtiger/`.
BSON	Binary JSON. The on-disk and over-the-wire data format. See BSON.
IDL	Interface Definition Language. YAML-driven schema for commands, BSON types, and server parameters. See IDL.
Command	A `Command` subclass registered via `MONGO_REGISTER_COMMAND`. Wire-protocol requests are dispatched to commands; see `src/mongo/db/commands/`.
Service entry point (SEP)	The top-level request dispatcher per binary role. `service_entry_point_shard_role.cpp` for `mongod`, `service_entry_point_router_role.cpp` for `mongos`.
`OperationContext`	The per-operation state container threaded through the codebase. Owns the storage transaction, deadline, kill flag, and decorations.
Decoration	A typed slot attached to `Service`, `Client`, `OperationContext`, or `Collection`. Lets a subsystem store its state without touching the holder.
`ServiceContext`	The process-wide state for a `mongod`/`mongos`. Holds registries, clocks, and the storage engine.
`OpObserver`	The hook that runs on every CRUD operation. Writes oplog entries and notifies subscribers (e.g. change streams). See `src/mongo/db/op_observer/`.
Shard role / Router role	Two `Service` instances that can co-exist in the same process. The shard role accepts shard-targeted operations; the router role does fan-out.
Catalog cache	The `mongos`/`mongod` cache of the cluster's routing table. Lives under `src/mongo/s/catalog_cache.h`.
Chunk	A range of the shard key space. The unit of data placement and balancing in a sharded collection.
Resharding	The online redistribution of an already-sharded collection to a new shard key. See `src/mongo/s/resharding/` and `src/mongo/db/s/resharding/`.
DDL coordinator	A primary-only service that drives sharded DDL (create, drop, rename, shard, reshard). See `src/mongo/db/global_catalog/ddl/`.
Primary-only service (POS)	A framework for stateful services that only run on a replica-set primary. Documented in `docs/primary_only_service.md`.
Fail point	A named runtime hook that can be enabled by tests to inject delays, errors, or branches. See Fail points.
`Status` / `StatusWith<T>`	The error-handling primitives. `Status` is a result code + message; `StatusWith<T>` carries a value or status. See `docs/exception_architecture.md`.
`uassert` / `tassert` / `iassert`	Assertion macros that throw a typed exception with a stable error code. See `docs/exception_architecture.md`.
`ErrorCode`	A stable numeric error identifier defined by IDL (`src/mongo/base/error_codes.yml`). The wire-visible code attached to errors.
Future / Promise	The asynchronous primitives in `src/mongo/util/future.h`. Documented in `docs/futures_and_promises.md`.
Baton	A light-weight task queue bound to a thread. Used to run continuations on the originating thread. See `docs/baton.md`.
logv2	The structured logging system. JSON-first. See logv2 and `docs/logging.md`.
Server parameter	A runtime-tunable knob declared in IDL. Read or written via `getParameter`/`setParameter`. See `docs/server_parameters.md`.
Cluster parameter	A server parameter that's persisted in the cluster's config and propagated to all members.
Feature flag	An IDL-declared flag gating a feature on a binary version (`featureFlagXxx`). See `src/mongo/db/feature_flag.h`.
FCV	Feature Compatibility Version. The cluster's effective protocol version. Controls which feature flags are visible.
Resmoke	The Python test runner. Lives in `buildscripts/resmoke.py` plus `src/mongo/resmoke/`. Knows about suite definitions and JS hooks.
Evergreen	MongoDB's CI system. Configuration is under `evergreen/`.
OP_MSG	The wire-protocol message type used for nearly all commands since 3.6.
SBE	Slot-Based Execution. The newer query execution engine alongside the classic engine. Lives under `src/mongo/db/exec/sbe/`.
Classic engine	The original tree-shaped query execution engine. Under `src/mongo/db/exec/classic/`.
Pipeline / aggregation	The aggregation framework. Pipeline stages live under `src/mongo/db/pipeline/`.
Change stream	A long-running cursor over a logical change feed sourced from the oplog. See Change streams.
DBDirectClient	An in-process "client" that runs commands without going over the network. Used by background services.

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