MongoDB Glossary

A quick-reference glossary of key MongoDB terms and concepts. Click any letter to jump to that section, or scroll through the full list.

A

Aggregation Pipeline

A framework for transforming and analyzing documents by passing them through a sequence of stages. Each stage performs an operation such as filtering, grouping, or reshaping the data. Explore Aggregation Pipeline in Mongon →

Atlas

MongoDB Atlas is the fully managed cloud database service provided by MongoDB Inc. It handles provisioning, scaling, backups, and security so you can focus on building your application. Connect to MongoDB Atlas with Mongon →

B

BSON

Binary JSON (BSON) is the binary-encoded serialization format used by MongoDB to store documents internally. It extends JSON with additional data types such as Date, ObjectId, and binary data.

C

Capped Collection

A fixed-size collection that automatically overwrites its oldest entries when it reaches its maximum size. Capped collections maintain insertion order and are useful for logs or cache-like workloads.

Change Stream

A feature that lets applications subscribe to real-time data changes on a collection, database, or deployment. Change streams use the oplog internally and provide a resumable, ordered stream of change events. Learn about Change Streams →

Collection

A grouping of MongoDB documents, analogous to a table in relational databases. Collections do not enforce a fixed schema, so documents within the same collection can have different fields.

Compound Index

An index that references multiple fields in a document. Compound indexes can support queries that match on a prefix of the indexed fields, improving performance for complex query patterns. Read the MongoDB Index guide →

Cursor

A pointer to the result set of a query. Instead of returning all matching documents at once, MongoDB returns a cursor that the client iterates over in batches, reducing memory usage.

D

Document

The basic unit of data in MongoDB, stored in BSON format. A document is similar to a JSON object and consists of field-value pairs. Each document in a collection has a unique _id field.

E

Embedded Document

A document nested inside another document as the value of a field. Embedding related data in a single document can reduce the need for joins and improve read performance.

G

GridFS

A specification for storing and retrieving files that exceed the 16 MB BSON document size limit. GridFS splits files into chunks and stores them across two collections: fs.files and fs.chunks.

I

Index

A data structure that improves the speed of read operations on a collection. Without indexes, MongoDB must scan every document in a collection to find matching results. Read the MongoDB Index guide →

ISODate

The date wrapper used in MongoDB queries and the mongo shell. ISODate() parses a date string in ISO 8601 format and stores it as a BSON Date object internally. Use Date Macros instead of manual ISODate →

M

mongosh

The modern MongoDB Shell, a fully functional JavaScript and Node.js REPL for interacting with MongoDB deployments from the command line. It replaces the legacy mongo shell.

O

ObjectId

A 12-byte unique identifier that MongoDB automatically assigns to the _id field of every document. It encodes a timestamp, machine identifier, process ID, and an incrementing counter. Navigate ObjectId relations in Mongon →

Oplog

Short for operations log, the oplog is a special capped collection that records all write operations in a replica set. Secondary members replicate data by tailing the primary's oplog.

P

Pipeline Stage

A single operation within an aggregation pipeline. Common stages include $match (filter documents), $group (aggregate values), $project (reshape documents), and $sort (order results).

Projection

A query option that specifies which fields to include or exclude in the returned documents. Projections reduce the amount of data transferred over the network and can improve query performance.

R

Replica Set

A group of MongoDB instances that maintain the same data set for high availability. A replica set consists of a primary node that receives writes and one or more secondary nodes that replicate the data.

S

Schema

The structure and organization of documents within a collection. MongoDB uses a flexible schema model, meaning documents in the same collection are not required to share an identical set of fields.

Shard

A horizontal partition of data distributed across multiple servers. Sharding allows MongoDB to scale out by splitting large data sets and distributing read and write load across a cluster.

T

TTL Index

A special single-field index that automatically removes documents from a collection after a specified number of seconds. TTL indexes are commonly used for session data, logs, and temporary records.

U

Update Operator

Special operators used to modify specific fields in a document without replacing the entire document. Common update operators include $set, $inc, $push, $pull, and $unset.

Upsert

An operation that updates an existing document if one matches the query filter, or inserts a new document if no match is found. Upserts combine update and insert logic into a single atomic operation.

W

WiredTiger

The default storage engine for MongoDB since version 3.2. WiredTiger provides document-level concurrency control, compression, and support for both in-memory and on-disk workloads.

Write Concern

The level of acknowledgment requested from MongoDB for write operations. Write concern lets you trade off between write speed and durability — higher levels wait for more replicas to confirm the write.