A special virtual collection that exposes MongoDB’s database commands. To use database commands, see Issue Commands.
A field required in every MongoDB document. The _id field must have a unique value. You can think of the _id field as the document’s primary key. If you create a new document without an _id field, MongoDB automatically creates the field and assigns a unique BSON ObjectId.
An expression in the aggregation framework that maintains state between documents in the aggregation pipeline. For a list of accumulator operations, see $group.
An operation the user can perform on a resource. Actions and resources combine to create privileges. See action.
admin database
A privileged database. Users must have access to the admin database to run certain administrative commands. For a list of administrative commands, see Administration Commands.
Any of a variety of operations that reduces and summarizes large sets of data. MongoDB’s aggregate() and mapReduce() methods are two examples of aggregation operations. For more information, see Aggregation.
aggregation framework
The set of MongoDB operators that let you calculate aggregate values without having to use map-reduce. For a list of operators, see Aggregation Reference.
A member of a replica set that exists solely to vote in elections. Arbiters do not replicate data. See Replica Set Arbiter.
MongoDB Atlas is a cloud-hosted database-as-a-service.
Verification of the user identity. See Authentication.
Provisioning of access to databases and operations. See Role-Based Access Control.
A data structure commonly used by database management systems to store indexes. MongoDB uses B-trees for its indexes.
An internal MongoDB process that runs in the context of a sharded cluster and manages the migration of chunks. Administrators must disable the balancer for all maintenance operations on a sharded cluster. See Sharded Cluster Balancer.
A serialization format used to store documents and make remote procedure calls in MongoDB. “BSON” is a portmanteau of the words “binary” and “JSON”. Think of BSON as a binary representation of JSON (JavaScript Object Notation) documents. See BSON Types and MongoDB Extended JSON.
BSON types
The set of types supported by the BSON serialization format. For a list of BSON types, see BSON Types.
CAP Theorem
Given three properties of computing systems, consistency, availability, and partition tolerance, a distributed computing system can provide any two of these features, but never all three.
capped collection
A fixed-sized collection that automatically overwrites its oldest entries when it reaches its maximum size. The MongoDB oplog that is used in replication is a capped collection. See Capped Collections.
The measure of the number of elements within a set of values. For example, the set A = { 2, 4, 6 } contains 3 elements, and has a cardinality of 3. See Shard Key Cardinality.
A calculated value used to ensure data integrity. The md5 algorithm is sometimes used as a checksum.
A contiguous range of shard key values within a particular shard. Chunk ranges are inclusive of the lower boundary and exclusive of the upper boundary. MongoDB splits chunks when they grow beyond the configured chunk size, which by default is 64 megabytes. MongoDB migrates chunks when a shard contains too many chunks of a collection relative to other shards. See Data Partitioning with Chunks and Sharded Cluster Balancer.

The application layer that uses a database for data persistence and storage. Drivers provide the interface level between the application layer and the database server.

Client can also refer to a single thread or process.

See sharded cluster.
A grouping of MongoDB documents. A collection is the equivalent of an RDBMS table. A collection exists within a single database. Collections do not enforce a schema. Documents within a collection can have different fields. Typically, all documents in a collection have a similar or related purpose. See Namespaces.
collection scan
Collection scans are a query execution strategy where MongoDB must inspect every document in a collection to see if it matches the query criteria. These queries are very inefficient and do not use indexes. See Query Optimization for details about query execution strategies.
compound index
An index consisting of two or more keys. See Compound Indexes.
concurrency control
Concurrency control ensures that database operations can be executed concurrently without compromising correctness. Pessimistic concurrency control, such as used in systems with locks, will block any potentially conflicting operations even if they may not turn out to actually conflict. Optimistic concurrency control, the approach used by WiredTiger, will delay checking until after a conflict may have occurred, aborting and retrying one of the operations involved in any write conflict that arises.
config database
An internal database that holds the metadata associated with a sharded cluster. Applications and administrators should not modify the config database in the course of normal operation. See Config Database.
config server
A mongod instance that stores all the metadata associated with a sharded cluster. See Config Servers.
An acronym for the fundamental operations of a database: Create, Read, Update, and Delete. See MongoDB CRUD Operations.
A text-based data format consisting of comma-separated values. This format is commonly used to exchange data between relational databases since the format is well-suited to tabular data. You can import CSV files using mongoimport.
A pointer to the result set of a query. Clients can iterate through a cursor to retrieve results. By default, cursors timeout after 10 minutes of inactivity. See Iterate a Cursor in the mongo Shell.
The conventional name for a background, non-interactive process.
data directory
The file-system location where the mongod stores data files. The dbPath option specifies the data directory.
data partition
A distributed system architecture that splits data into ranges. Sharding uses partitioning. See Data Partitioning with Chunks.
data-center awareness
A property that allows clients to address members in a system based on their locations. Replica sets implement data-center awareness using tagging. See Data Center Awareness.
A physical container for collections. Each database gets its own set of files on the file system. A single MongoDB server typically has multiple databases.
database command
A MongoDB operation, other than an insert, update, remove, or query. For a list of database commands, see Database Commands. To use database commands, see Issue Commands.
database profiler
A tool that, when enabled, keeps a record on all long-running operations in a database’s system.profile collection. The profiler is most often used to diagnose slow queries. See Database Profiling.
The location of MongoDB’s data file storage. See dbPath.
delayed member
A replica set member that cannot become primary and applies operations at a specified delay. The delay is useful for protecting data from human error (i.e. unintentionally deleted databases) or updates that have unforeseen effects on the production database. See Delayed Replica Set Members.
A record in a MongoDB collection and the basic unit of data in MongoDB. Documents are analogous to JSON objects but exist in the database in a more type-rich format known as BSON. See Documents.
dot notation
MongoDB uses the dot notation to access the elements of an array and to access the fields of an embedded document. See Dot Notation.
The process of removing or “shedding” chunks from one shard to another. Administrators must drain shards before removing them from the cluster. See Remove Shards from an Existing Sharded Cluster.
A client library for interacting with MongoDB in a particular language. See /drivers .
A write operation is durable when it will persist across a shutdown (or crash) and restart of one or more server processes. For a single mongod server, a write operation is considered durable when it has been written to the server’s journal file. For a replica set, a write operation is considered durable once the write operation is durable on a majority of voting nodes; i.e. written to a majority of voting nodes’ journals.
The process by which members of a replica set select a primary on startup and in the event of a failure. See Replica Set Elections.
eventual consistency
A property of a distributed system that allows changes to the system to propagate gradually. In a database system, this means that readable members are not required to reflect the latest writes at all times.
In the context of aggregation framework, expressions are the stateless transformations that operate on the data that passes through a pipeline. See Aggregation Pipeline.
The process that allows a secondary member of a replica set to become primary in the event of a failure. See Automatic Failover.
A name-value pair in a document. A document has zero or more fields. Fields are analogous to columns in relational databases. See Document Structure.
field path
Path to a field in the document. To specify a field path, use a string that prefixes the field name with a dollar sign ( $).
A system level networking filter that restricts access based on, among other things, IP address. Firewalls form a part of an effective network security strategy. See Firewalls.
A system call that flushes all dirty, in-memory pages to disk. MongoDB calls fsync() on its database files at least every 60 seconds. See fsync.
A geohash value is a binary representation of the location on a coordinate grid. See Calculation of Geohash Values for 2d Indexes.
A geospatial data interchange format based on JavaScript Object Notation ( JSON). GeoJSON is used in geospatial queries. For supported GeoJSON objects, see Geospatial Data. For the GeoJSON format specification, see .
Relating to geographical location. See Geospatial Queries.
A convention for storing large files in a MongoDB database. All of the official MongoDB drivers support this convention, as does the mongofiles program. See GridFS.
hashed shard key
A special type of shard key that uses a hash of the value in the shard key field to distribute documents among members of the sharded cluster. See Hashed Indexes.
haystack index
A geospatial index that enhances searches by creating “buckets” of objects grouped by a second criterion. See geoHaystack Indexes.
hidden member
A replica set member that cannot become primary and are invisible to client applications. See Hidden Replica Set Members.
high availability

High availability indicates a system designed for durability, redundancy, and automatic failover such that the applications supported by the system can operate continuously and without downtime for a long period of time. MongoDB replica sets support high availability when deployed according to our documented best practices.

For guidance on replica set deployment architecture, see Replica Set Deployment Architectures.

The quality of an operation to produce the same result given the same input, whether run once or run multiple times.
A data structure that optimizes queries. See Indexes.
init script
A simple shell script used by a Linux platform’s init system to start, restart, or stop a