Glossary

$cmd

A virtual collection that exposes MongoDB's database commands. To use database commands, see Issue Commands.

_id

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 to the field.

accumulator

An expression in an aggregation pipeline that maintains state between documents in the aggregation pipeline. For a list of accumulator operations, see $group.

action

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.

aggregation

An operation 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 Operations.

aggregation pipeline

Consists of one or more stages that process documents. Aggregation operators calculate aggregate values without having to use map-reduce. For a list of operators, see Aggregation Reference.

arbiter

A replica set member that exists just to vote in elections. Arbiters do not replicate data. An arbiter participates in elections for a primary but cannot become a primary. For more details, see Replica Set Arbiter.

Atlas

MongoDB Atlas is a cloud-hosted database-as-a-service.

atomic operation

An atomic operation is a write operation that either completes entirely or doesn't complete at all. For distributed transactions, which involve writes to multiple documents, all writes to each document must succeed for the transaction to succeed. Atomic operations cannot partially complete. See Atomicity and Transactions.

authentication

Verification of the user identity. See Authentication on Self-Managed Deployments.

authorization

Provisioning of access to databases and operations. See Role-Based Access Control in Self-Managed Deployments.

automatic encryption

When using In-Use Encryption, automatically performing encryption and decryption based on your preconfigured encryption schema. The Automatic Encryption Shared Library translates MongoDB Query Language into the correct call, meaning you don't need to rewrite your application for specific encrypt and decrypt calls.

B-tree

A data structure commonly used by database management systems to store indexes. MongoDB uses B-tree indexes.

backup cursor

A tailable cursor that points to a list of backup files. Backup cursors are for internal use only.

balancer

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.

big-endian

A byte order in which the most significant byte (big end) of a multibyte data value is stored at the lowest memory address.

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blocking sort

A sort that must be performed in memory before the output is returned. Blocking sorts may impact performance for large data sets. Use an indexed sort to avoid a blocking sort.

See Sort and Index Use for more information on blocking sort operations.

bounded collection scan

A plan used by the query optimizer that excludes documents with specific field value ranges. For example, if a range of date field values is outside of a specified date range, the documents in that range are excluded from the query plan. See Collection Scan.

BSON

A serialization format used to store documents and make remote procedure calls in MongoDB. "BSON" is a combination 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 (v2).

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 the collection reaches its maximum size. The MongoDB oplog that is used in replication is a capped collection. See Capped Collections.

cardinality

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.

cartesian product

The result of combining two data sets where the combined set contains every possible combination of values.

cfq

Complete Fairness Queueing (cfq) is an I/O operation scheduler that allocates bandwidth for incoming request processes.

checksum

A calculated value used to ensure data integrity. The md5 algorithm is sometimes used as a checksum.

chunk

A contiguous range of shard key values within a shard. Chunk ranges are inclusive of the lower boundary and exclusive of the upper boundary. MongoDB splits chunks when they grow bigger than the configured chunk size. The default chunk size is 128 megabytes. MongoDB migrates chunks when a shard contains too many chunks of a collection relative to other shards. For more details, see Data Partitioning with Chunks, Sharded Cluster Balancer, and Manage Sharded Cluster Balancer.

client

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.

A client can also be a single thread or process.

client affinity

A consistent client connection to a specified data source.

cluster

See sharded cluster.

cluster-to-cluster sync

Synchronizes data between sharded clusters. Also known as C2C sync.

clustered collection

A collection that stores documents ordered by a clustered index key. See Clustered Collections.

CMK

Abbreviation of Customer Master Key, see Customer Master Key.

collection

A grouping of MongoDB documents. A collection is the equivalent of an RDBMS table. A collection is in a single database. Collections do not enforce a schema. Documents in a collection can have different fields. Typically, 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 don't use indexes. See Query Optimization for details about query execution strategies.

commit

Saves data changes made after the start of the startSession command. Operations within a transaction are not permanent until they are committed with the commitTransaction command.

commit quorum

During an index build the commit quorum specifies how many secondaries must be ready to commit their local index build before the primary node performs the commit.

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 that used in systems with locks, blocks any potentially conflicting operations even if they may not conflict. Optimistic concurrency control, the approach used by WiredTiger, delays checking until after a conflict may have occurred, ending and retrying one of the operations in any write conflict.

config database

An internal database with metadata for a sharded cluster. Typically, you don't modify the config database. For more information about the config database, see Config Database.

config server

A mongod instance that stores all the metadata associated with a sharded cluster. See Config Servers.

connection pool

A cache of database connections maintained by the driver. The cached connections are re-used when connections to the database are required, instead of opening new connections.

connection storm

A scenario where a driver attempts to open more connections to a deployment than that deployment can handle. When requests for new connections fail, the driver requests to establish even more connections in response to the deployment slowing down or failing to open new connections. These continuous requests can overload the deployment and lead to outages.

container

A collected set of software and its dependent libraries that are packaged together to make transferring between computing environments easier. Containers run as compartmentalized processes on your operating system, and can be given their own resource constraints. Common container technologies are Docker and Kubernetes.

contention factor

Multiple operations attempting to modify the same resource, such as a document field, cause conflicts that delay operations. The contention factor is a setting used with Queryable Encryption to internally partition encrypted field/value pairs and optimize operations. See contention.

CRUD

An acronym for the fundamental operations of a database: Create, Read, Update, and Delete. See MongoDB CRUD Operations.

CSV

A text data format with comma-separated values. CSV files can be used to exchange data between relational databases because CSV files have tabular data. You can import CSV files using mongoimport.

cursor

A pointer to the result set of a query. Clients can iterate through a cursor to retrieve results. By default, cursors not opened within a session automatically timeout after 10 minutes of inactivity. Cursors opened in a session close with the end or timeout of the session. See Iterate a Cursor in mongosh.

Customer Master Key

A key that encrypts your Data Encryption Key. The customer master key must be hosted in a remote key provider.

daemon

A background, non-interactive process.

data directory

The file system location where mongod stores data files. dbPath specifies the data directory.

Data Encryption Key

A key you use to encrypt the fields in your MongoDB documents. The encrypted Data Encryption Key is stored in your Key Vault collection. The Data Encryption Key is encrypted by the Customer Master Key.

data files

Store document data and indexes. The dbPath option specifies the file system location for the data files.

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.

database

A container for collections. Each database has a set of files in the file system. One 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 Profiler.

dbpath

The location of MongoDB's data file storage. See dbPath.

DDL (Data Definition Language)

DDL includes commands that create and modify collections and indexes.

DEK

Data Encryption Key. For more details, see Data Encryption Key.

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 (unintentionally deleted databases) or updates that have unforeseen effects on the production database. See Delayed Replica Set Members.

document

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.

draining

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.

driver

A client library for interacting with MongoDB in a particular computer language. See driver.

durable

A write operation is durable when it persists after 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 after the write operation achieves durability on a majority of voting nodes and written to a majority of voting nodes' journals.

election

The process where members of a replica set select a primary on startup and in the event of a failure. See Replica Set Elections.

endianness

In computing, endianness refers to the order in which bytes are arranged. This ordering can refer to transmission over a communication medium or more commonly how the bytes are ordered in computer memory, based on their significance and position. For details, see big-endian and little-endian.

envelope encryption

An encryption procedure where data is encrypted using a Data Encryption Key and the data encryption key is encrypted by another key called the Customer Master Key. The encrypted keys are stored as BSON documents in a MongoDB collection called the KeyVault.

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 aren't required to have the latest updates.

explicit encryption

When using In-Use Encryption, explicitly specifying the encryption or decryption operation, keyID, and query type (for Queryable Encryption) or algorithm (for Client-Side Field Level Encryption) when working with encrypted data. Compare to automatic encryption.

expression

A component of a query that resolves to a value. Expressions are stateless, meaning they return a value without mutating any of the values used to build the expression.

In the MongoDB Query Language, you can build expressions from the following components:

Component	Example
Constants	3
Operators	$add
Field path expressions	"$<path.to.field>"

For example, { $add: [ 3, "$inventory.total" ] } is an expression consisting of the $add operator and two input expressions:

• The constant 3

• The field path expression "$inventory.total"

The expression returns the result of adding 3 to the value at path inventory.total of the input document.

failover

The process that allows a secondary member of a replica set to become primary in the event of a failure. See Automatic Failover.

field

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 a document. To specify a field path, use a string that prefixes the field name with a dollar sign ($).

firewall

A system level network filter that restricts access based on IP addresses and other parameters. Firewalls are part of a secure network. See Firewalls.

fsync

A system call that flushes all dirty, in-memory pages to storage. As applications write data, MongoDB records the data in the storage layer.

To provide durable data, WiredTiger uses checkpoints. For more details, see Journaling and the WiredTiger Storage Engine.

geohash

A geohash value is a binary representation of the location on a coordinate grid. See Geohash Values.

GeoJSON

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 https://tools.ietf.org/html/rfc7946#section-3.1.

geospatial

Relating to geographical location. See Geospatial Queries.

GridFS

A convention for storing large files in a MongoDB database. All of the official MongoDB drivers support the GridFS convention, as does the mongofiles program. See GridFS for Self-Managed Deployments.

hashed shard key

A 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.

health manager

A health manager runs health checks on a health manager facet at a specified intensity level. The health manager checks are run at specified time intervals. A health manager can be configured to move a failing mongos out of a cluster automatically.

health manager facet

A set of features that a health manager can be configured to run health checks for. For example, you can configure a health manager to monitor and manage DNS or LDAP cluster health issues automatically. See Health Manager Facets for details.

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. Applications supported by the system can operate without downtime for a long time period. MongoDB replica sets support high availability when deployed according to the best practices.

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

idempotent

An operation produces the same result with the same input when run multiple times.

In-Use Encryption

Encryption that secures data when transmitted, stored, and processed, and enables supported queries on that encrypted data. MongoDB provides two approaches to In-Use Encryption: Queryable Encryption and Client-Side Field Level Encryption.

index

A data structure that optimizes queries. See Indexes.

index bounds

The range of index values that MongoDB searches when using an index to run a query. To learn more, see Multikey Index Bounds.

indexed sort

A sort where an index provides the sorted result. Sort operations that use an index often have better performance than a blocking sort. See Use Indexed to Sort Query Results for more information.

init script

A shell script used by a Linux platform's init system to start, restart, or stop a daemon process. If you installed MongoDB using a package manager, an init script is provided for your system as part of the installation. See the respective Installation Guide for your operating system.

init system

The init system is the first process started on a Linux platform after the kernel starts, and manages all other processes on the system. The init system uses an init script to start, restart, or stop a daemon process, such as mongod or mongos. Recent Linux versions typically use the systemd init system and the systemctl command. Older Linux versions typically use the System V init system and the service command. See the Installation Guide for your operating system.

initial sync

The replica set operation that replicates data from an existing replica set member to a new replica set member. See Initial Sync.

intent lock

A lock on a resource that indicates the lock holder will read from (intent shared) or write to (intent exclusive) the resource using concurrency control at a finer granularity than that of the resource with the intent lock. Intent locks allow concurrent readers and writers of a resource. See What type of locking does MongoDB use?.

interrupt point

A point in an operation when it can safely end. MongoDB only ends an operation at designated interrupt points. See Terminate Running Operations.

IPv6

A revision to the IP (Internet Protocol) standard with a large address space to support Internet hosts.

ISODate

The international date format used by mongosh to display dates. The format is YYYY-MM-DD HH:MM.SS.millis.

JavaScript

A scripting language. mongosh, the legacy mongo shell, and certain server functions use a JavaScript interpreter. See Server-side JavaScript for more information.

journal

A sequential, binary transaction log used to bring the database into a valid state in the event of a hard shutdown. Journaling writes data first to the journal and then to the core data files. MongoDB enables journaling by default for 64-bit builds of MongoDB version 2.0 and newer. Journal files are pre-allocated and exist as files in the data directory. See Journaling.

JSON

JavaScript Object Notation. A plain text format for expressing structured data with support in many programming languages. For more information, see http://www.json.org. Certain MongoDB tools render an approximation of MongoDB BSON documents in JSON format. See MongoDB Extended JSON (v2).

JSON document

A JSON document is a collection of fields and values in a structured format. For sample JSON documents, see http://json.org/example.html.

JSONP

JSON with padding. Refers to a method of injecting JSON into applications. Presents potential security concerns.

jumbo chunk

A chunk that grows beyond the specified chunk size and cannot split into smaller chunks. For more details, see Indivisible/Jumbo Chunks.

key material

The random string of bits used by an encryption algorithm to encrypt and decrypt data.

Key Vault Collection

A MongoDB collection that stores the encrypted Data Encryption Keys as BSON documents.

least privilege

An authorization policy that grants a user only the access that is essential to that user's work.

legacy coordinate pairs

The format used for geospatial data before MongoDB version 2.4. This format stores geospatial data as points on a planar coordinate system (for example, [ x, y ]). See Geospatial Queries.

LineString

A LineString is an array of two or more positions. A closed LineString with four or more positions is called a LinearRing, as described in the GeoJSON LineString specification: https://tools.ietf.org/html/rfc7946#section-3.1.4. To use a LineString in MongoDB, see GeoJSON Objects.

little-endian

A byte order in which the least significant byte (little end) of a multibyte data value is stored at the lowest memory address.

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lock

MongoDB uses locks to ensure that concurrency does not affect correctness. MongoDB uses read locks, write locks and intent locks. For more information, see What type of locking does MongoDB use?.

log files

Contain server events, such as incoming connections, commands run, and issues encountered. For more details, see Log Messages.

LVM

Logical volume manager. LVM is a program that abstracts disk images from physical devices and provides a number of raw disk manipulation and snapshot capabilities useful for system management. For information on LVM and MongoDB, see Back Up and Restore Using LVM on Linux.

map-reduce

An aggregation process that has a "map" phase that selects the data and a "reduce" phase that transforms the data. In MongoDB, you can run arbitrary aggregations over data using map-reduce. For the map-reduce implementation, see Map-Reduce. For all approaches to aggregation, see Aggregation Operations.

mapping type

A structure in programming languages that associate keys with values. Keys may contain embedded pairs of keys and values (for example, dictionaries, hashes, maps, and associative arrays). The properties of these structures depend on the language specification and implementation. Typically, the order of keys in mapping types is arbitrary and not guaranteed.

md5

A hashing algorithm that calculates a checksum for the supplied data. The algorithm returns a unique value to identify the data. MongoDB uses md5 to identify chunks of data for GridFS. See filemd5.

mean

Average of a set of numbers.

median

In a dataset, the median is the percentile value where 50% of the data falls at or below that value.

member

An individual mongod process. A replica set has multiple members. A member is also known as a node.

metadata collection

In Queryable Encryption, the internal collections MongoDB uses to enable querying on encrypted fields. See Metadata Collections.

MIME

Multipurpose Internet Mail Extensions. A standard set of type and encoding definitions used to declare the encoding and type of data in multiple data storage, transmission, and email contexts. The mongofiles tool provides an option to specify a MIME type to describe a file inserted into GridFS storage.

mode

Number that occurs most frequently in a set of numbers.

mongo

The legacy MongoDB shell. The mongo process starts the legacy shell as a daemon connected to either a mongod or mongos instance. The shell has a JavaScript interface.

Starting in MongoDB v5.0, mongo is deprecated and mongosh replaces mongo as the client shell. See mongosh.

mongod

The MongoDB database server. The mongod process starts the MongoDB server as a daemon. The MongoDB server manages data requests and background operations. See mongod.

mongos

The MongoDB sharded cluster query router. The mongos process starts the MongoDB router as a daemon. The MongoDB router acts as an interface between an application and a MongoDB sharded cluster and handles all routing and load balancing across the cluster. See mongos.

mongosh

MongoDB Shell. mongosh provides a shell interface to either a mongod or a mongos instance.

Starting in MongoDB v5.0, mongosh replaces mongo as the preferred shell.

namespace

A namespace is a combination of the database name and the name of the collection or index: <database-name>.<collection-or-index-name>. All documents belong to a namespace. See Namespaces.

natural order

The order recordIds are created and stored in the WiredTiger index. The default sort order for a collection scan run on a single instance is natural order.

In replica sets, natural order is not guaranteed to be consistent and can differ between members.

In sharded collections, natural order is not defined. However, using $natural still forces each shard to perform a collection scan.

For details, see $natural and Return in Natural Order.

network partition

A network failure that separates a distributed system into partitions such that nodes in one partition cannot communicate with the nodes in the other partition.

Sometimes, partitions are partial or asymmetric. An example partial partition is the a division of the nodes of a network into three sets, where members of the first set cannot communicate with members of the second set, and the reverse, but all nodes can communicate with members of the third set.

In an asymmetric partition, communication may be possible only when it originates with certain nodes. For example, nodes on one side of the partition can communicate with the other side only if they originate the communications channel.

node

An individual mongod process. A replica set has multiple nodes. A node is also known as a member.

noop

No Operation (noop), is an I/O operation scheduler that allocates I/O bandwidth for incoming processes based on a first in, first out queue.

NVMe

NVMe (Non-Volatile Memory Express) is a protocol for accessing high-speed storage media.

object identifier

See ObjectId.

ObjectId

A 12-byte BSON type that is unique within a collection. The ObjectId is generated using the timestamp, computer ID, process ID, and a local process incremental counter. MongoDB uses ObjectId values as the default values for _id fields.

operation log

See oplog.

operation time

See optime.

operator

A keyword beginning with a $ used to express an update, complex query, or data transformation. For example, $gt is the query language's "greater than" operator. For available operators, see Operators.

oplog

A capped collection that stores an ordered history of logical writes to a MongoDB database. The oplog is the basic mechanism enabling replication in MongoDB. See Replica Set Oplog.

oplog hole

A temporary gap in the oplog because the oplog writes aren't in sequence. Replica set primaries apply oplog entries in parallel as a batch operation. As a result, temporary gaps in the oplog can occur from entries that aren't yet written from a batch.

oplog window

oplog entries are time-stamped. The oplog window is the time difference between the newest and the oldest timestamps in the oplog. If a secondary node loses connection with the primary, it can only use replication to sync up again if the connection is restored within the oplog window.

optime

A reference to a position in the replication oplog. The optime value is a document that contains:

• ts, the Timestamp of the operation.

• t, the term in which the operation was originally generated on the primary.

ordered query plan

A query plan that returns results in the order consistent with the sort() order. See Query Plans.

orphaned cursor

A cursor that is not correctly closed or iterated over in your application code. Orphaned cursors can cause performance issues in your MongoDB deployment.

orphaned document

In a sharded cluster, orphaned documents are those documents on a shard that also exist in chunks on other shards. This is caused by a failed migration or an incomplete migration cleanup because of an atypical shutdown.

Orphaned documents are cleaned up automatically after a chunk migration completes. You no longer need to run cleanupOrphaned to delete orphaned documents.

passive member

A member of a replica set that cannot become primary because its members[n].priority is 0. See Priority 0 Replica Set Members.

percentile

In a dataset, a percentile is a value where that percentage of the data is at or below the specified value. For details, see Calculation Considerations.

PID

A process identifier. UNIX-like systems assign a unique-integer PID to each running process. You can use a PID to inspect a running process and send signals to it. See /proc File System.

pipe

A communication channel in UNIX-like systems allowing independent processes to send and receive data. In the UNIX shell, piped operations allow users to direct the output of one command into the input of another.

pipeline

A series of operations in an aggregation. See Aggregation Pipeline.

Point

A single coordinate pair as described in the GeoJSON Point specification: https://tools.ietf.org/html/rfc7946#section-3.1.2. To use a Point in MongoDB, see GeoJSON Objects.

Polygon

An array of LinearRing coordinate arrays, as described in the GeoJSON Polygon specification: https://tools.ietf.org/html/rfc7946#section-3.1.6. For Polygons with multiple rings, the first must be the exterior ring and any others must be interior rings or holes.

MongoDB does not permit the exterior ring to self-intersect. Interior rings must be fully contained within the outer loop and cannot intersect or overlap with each other. See GeoJSON Objects.

post-image document

A document after it was inserted, replaced, or updated. See Change Streams with Document Pre- and Post-Images.

powerOf2Sizes

A setting for each collection that allocates space for each document to maximize storage reuse and reduce fragmentation. powerOf2Sizes is the default for TTL Collections. To change collection settings, see collMod.

pre-image document

A document before it was replaced, updated, or deleted. See Change Streams with Document Pre- and Post-Images.

pre-splitting

An operation performed before inserting data that divides the range of possible shard key values into chunks to facilitate easy insertion and high write throughput. In some cases pre-splitting expedites the initial distribution of documents in sharded cluster by manually dividing the collection rather than waiting for the MongoDB balancer to do so. See Create Ranges in a Sharded Cluster.

prefix compression

Reduces memory and disk consumption by storing any identical index key prefixes only once, per page of memory. See: Compression for more about WiredTiger's compression behavior.

primary

In a replica set, the primary is the member that receives all write operations. See Primary.

primary key

A record's unique immutable identifier. In RDBMS software, the primary key is typically an integer stored in each row's id field. In MongoDB, the _id field stores a document's primary key, which is typically a BSON ObjectId.

primary shard

The shard that stores all the unsharded collections. See Primary Shard.

priority

A configurable value that helps determine which members in a replica set are most likely to become primary. See members[n].priority.

privilege

A combination of specified resource and actions permitted on the resource. See privilege.

projection

A document supplied to a query that specifies the fields MongoDB returns in the result set. For more information about projections, see Project Fields to Return from Query and Projection Operators.

query

A read request. MongoDB uses a JSON form of query language that includes query operators with names that begin with a $ character. In mongosh, you can run queries using the db.collection.find() and db.collection.findOne() methods. See Query Documents.

query framework

A combination of the query optimizer and query execution engine that processes an operation.

query operator

A keyword beginning with $ in a query. For example, $gt is the "greater than" operator. For a list of query operators, see query operators.

query optimizer

A process that generates query plans. For each query, the optimizer generates a plan that matches the query to the index that returns the results as efficiently as possible. The optimizer reuses the query plan each time the query runs. If a collection changes significantly, the optimizer creates a new query plan. See Query Plans.

query plan

Most efficient execution plan chosen by the query planner. For more details, see Query Plans.

query predicate

An expression that returns a boolean indicating whether a document matches the specified query. For example, { name: { $eq: "Alice" } }, which returns documents that have a field "name" whose value is the string "Alice".

Query predicates can contain child expressions and operators for more complex matching. To see available query operators, see Query and Projection Operators.

query shape

A combination of query predicate, sort, projection, and collation. The query shape allows MongoDB to identify logically equivalent queries and analyze their performance.

For the query predicate, only the structure of the predicate, including the field names, are significant. The values in the query predicate are insignificant. Therefore, a query predicate { type: 'food' } is equivalent to the query predicate { type: 'utensil' } for a query shape.

To help identify slow queries with the same query shape, each query shape is associated with a queryHash. The queryHash is a hexadecimal string that represents a hash of the query shape and is dependent only on the query shape.

Note

As with any hash function, two different query shapes may result in the same hash value. However, the occurrence of hash collisions between different query shapes is unlikely.

range

A contiguous range of shard key values within a chunk. Data ranges include the lower boundary and exclude the upper boundary. MongoDB migrates data when a shard contains too much data of a collection relative to other shards. See Data Partitioning with Chunks and Sharded Cluster Balancer.

RDBMS

Relational Database Management System. A database management system based on the relational model, typically using SQL as the query language.

read concern

Specifies a level of isolation for read operations. For example, you can use read concern to only read data that has propagated to a majority of nodes in a replica set. See Read Concern.

read lock

A shared lock on a resource such as a collection or database that, while held, allows concurrent readers but no writers. See What type of locking does MongoDB use?.

read preference

A setting that determines how clients direct read operations. Read preference affects all replica sets, including shard replica sets. By default, MongoDB directs reads to primaries. However, you may also direct reads to secondaries for eventually consistent reads. See Read Preference.

recovering

A replica set member status indicating that a member is not ready to begin activities of a secondary or primary. Recovering members are unavailable for reads.

replica set

A cluster of MongoDB servers that implements replication and automated failover. MongoDB's recommended replication strategy. See Replication.

replication

A feature allowing multiple database servers to share the same data. Replication ensures data redundancy and enables load balancing. See Replication.

replication lag

The time period between the last operation in the primary's oplog and the last operation applied to a particular secondary. You typically want replication lag as short as possible. See Replication Lag.

resident memory

The subset of an application's memory currently stored in physical RAM. Resident memory is a subset of virtual memory, which includes memory mapped to physical RAM and to storage.

resource

A database, collection, set of collections, or cluster. A privilege permits actions on a specified resource. See resource.

role

A set of privileges that permit actions on specified resources. Roles assigned to a user determine the user's access to resources and operations. See Security.

rollback

A process that reverts write operations to ensure the consistency of all replica set members. See Rollbacks During Replica Set Failover.

secondary

A replica set member that replicates the contents of the master database. Secondary members may run read requests, but only the primary members can run write operations. See Secondaries.

secondary index

A database index that improves query performance by minimizing the amount of work that the query engine must perform to run a query. See Indexes.

secondary member

See secondary. Also known as a secondary node.

seed list

A seed list is used by drivers and clients (like mongosh) for initial discovery of the replica set configuration. Seed lists can be provided as a list of host:port pairs (see Standard Connection String Format or through DNS entries.) For more information, see SRV Connection Format.

self-managed

A MongoDB instance that is set up and maintained by an individual or organization, and not an external management or third-party services (such as MongoDB Atlas).

set name

The arbitrary name given to a replica set. All members of a replica set must have the same name specified with the replSetName setting or the --replSet option.

shard

A single mongod instance or replica set that stores part of a sharded cluster's total data set. Typically, in a production deployment, ensure all shards are part of replica sets. See Shards.

shard key

The field MongoDB uses to distribute documents among members of a sharded cluster. See Shard Keys.

sharded cluster

The set of nodes comprising a sharded MongoDB deployment. A sharded cluster consists of config servers, shards, and one or more mongos routing processes. See Sharded Cluster Components.

sharding

A database architecture that partitions data by key ranges and distributes the data among two or more database instances. Sharding enables horizontal scaling. See Sharding.

shell helper

A method in mongosh that has a concise syntax for a database command. Shell helpers improve the interactive experience. See mongosh Methods.

single-master replication

A replication topology where only a single database instance accepts writes. Single-master replication ensures consistency and is the replication topology used by MongoDB. See Replica Set Primary.

snappy

A compression/decompression library to balance efficient computation requirements with reasonable compression rates. Snappy is the default compression library for MongoDB's use of WiredTiger. See Snappy and the WiredTiger compression documentation for more information.

snapshot

A snapshot is a copy of the data in a mongod instance at a specific point in time. You can retrieve snapshot metadata for the whole cluster or replica set, or for a single config server in a cluster.

sort key

The value compared against when sorting fields. To learn how MongoDB determines the sort key for non-numeric fields, see Comparison/Sort Order.

split

The division between chunks in a sharded cluster. See Data Partitioning with Chunks.

SQL

Structured Query Language (SQL) is used for interaction with relational databases.

SSD

Solid State Disk. High-performance storage that uses solid state electronics for persistence instead of rotating platters and movable read/write heads used by mechanical hard drives.

stale read

A stale read refers to when a transaction reads old (stale) data that has been modified by another transaction but not yet committed to the database.

standalone

An instance of mongod that runs as a single server and not as part of a replica set. To convert it to a replica set, see Convert a Standalone Self-Managed mongod to a Replica Set.

stash collection

A temporary collection that may be created during resharding. When resharding completes successfully, any stash collections that were created are removed during the operation.

step down

The primary member of the replica set removes itself as primary and becomes a secondary member.

• If a replica set loses contact with the primary, the secondaries elect a new primary. When the old primary learns of the election, it steps down and rejoins the replica set as a secondary.

• If the user runs the replSetStepDown command, the primary steps down, forcing the replica set to elect a new primary.

storage engine

The part of a database that is responsible for managing how data is stored and accessed, both in memory and on disk. Different storage engines perform better for specific workloads. See Storage Engines for Self-Managed Deployments for specific details on the built-in storage engines in MongoDB.

storage order

See natural order.

strict consistency

A property of a distributed system requiring that all members contain the latest changes to the system. In a database system, this means that any system that can provide data must contain the latest writes.

Subject Alternative Name

Subject Alternative Name (SAN) is an extension of the X.509 certificate which allows an array of values such as IP addresses and domain names that specify the resources a single security certificate may secure.

sync

The replica set operation where members replicate data from the primary. Sync first occurs when MongoDB creates or restores a member, which is called initial sync. Sync then occurs continually to keep the member updated with changes to the replica set's data. See Replica Set Data Synchronization.

syslog

On UNIX-like systems, a logging process that provides a uniform standard for servers and processes to submit logging information. MongoDB provides an option to send output to the host’s syslog system. See syslogFacility.

tag

A label applied to a replica set member and used by clients to issue data-center-aware operations. For more information on using tags with replica sets, see Read Preference Tag Set Lists.

Note

Sharded cluster zones replace tags.

tag set

A document containing zero or more tags.

tailable cursor

For a capped collection, a tailable cursor is a cursor that remains open after the client exhausts the results in the initial cursor. As clients insert new documents into the capped collection, the tailable cursor continues to retrieve documents.

term

For the members of a replica set, a monotonically increasing number that corresponds to an election attempt.

time series collection

A collection that efficiently stores sequences of measurements over a period of time. See Time Series.

topology

The state of a deployment of MongoDB instances. Includes:

• Type of deployment (standalone, replica set, or sharded cluster).

• Availability of servers.

• Role of each server (primary, secondary, config server, or mongos).

transaction

Group of read or write operations. For details, see Transactions.

transaction coordinator

A component of MongoDB that manages transactions in a replica set or a sharded cluster. It coordinates the execution and completion of multi-document transactions across nodes and allows a complex operation to be treated as an atomic operation.

TSV

A text-based data format consisting of tab-separated values. This format is commonly used to exchange data between relational databases because the format is suited to tabular data. You can import TSV files using mongoimport.

TTL

Time-to-live (TTL) is an expiration time or period for a given piece of information to remain in a cache or other temporary storage before the system deletes it or ages it out. MongoDB has a TTL collection feature. See Expire Data from Collections by Setting TTL.

unbounded array

An array that consistently grows larger over time. If a document field value is an unbounded array, the array may negatively impact performance. In general, design your schema to avoid unbounded arrays.

unique index

An index that enforces uniqueness for a particular field in a single collection. See Unique Indexes.

unix epoch

January 1st, 1970 at 00:00:00 UTC. Commonly used in expressing time, where the number of seconds or milliseconds since this point is counted.

unordered query plan

A query plan that returns results in an order inconsistent with the sort() order. See Query Plans.

upsert

An option for update operations. For example: db.collection.updateOne(), db.collection.findAndModify(). If upsert is true, the update operation either:

• updates the document(s) matched by the query.

• or if no documents match, inserts a new document. The new document has the field values specified in the update operation.

For more information about upserts, see Insert a New Document if No Match Exists (Upsert).

virtual memory

An application's working memory, typically residing on both disk and in physical RAM.

WGS84

The default reference system and geodetic datum that MongoDB uses to calculate geometry over an Earth-like sphere for geospatial queries on GeoJSON objects. See the "EPSG:4326: WGS 84" specification: http://spatialreference.org/ref/epsg/4326/.

window operator

Returns values from a span of documents from a collection. See window operators.

working set

The data that MongoDB uses most often.

write concern

Specifies whether a write operation has succeeded. Write concern allows your application to detect insertion errors or unavailable mongod instances. For replica sets, you can configure write concern to confirm replication to a specified number of members. See Write Concern.

write conflict

A situation where two concurrent operations, at least one of which is a write, try to use a resource that violates the constraints for a storage engine that uses optimistic concurrency control. MongoDB automatically ends and retries one of the conflicting write operations.

write lock

An exclusive lock on a resource such as a collection or database. When a process writes to a resource, it takes an exclusive write lock to prevent other processes from writing to or reading from that resource. For more information on locks, see FAQ: Concurrency.

writeBacks

The process in the sharding system that ensures writes sent to a shard that is not responsible for the relevant chunk are applied to the correct shard. For more information, see What does writebacklisten in the log mean? and writeBacksQueued.

zlib

A data compression library that provides higher compression rates at the cost of more CPU, compared to MongoDB's use of snappy. You can configure WiredTiger to use zlib as its compression library. See http://www.zlib.net and the WiredTiger compression documentation for more information.

zone

A grouping of documents based on ranges of shard key values for a given sharded collection. Each shard in the sharded cluster can be in one or more zones. In a balanced cluster, MongoDB directs reads and writes for a zone only to those shards inside that zone. See the Zones manual page for more information.

Zones supersede functionality described by tags in MongoDB 3.2.

zstd

A data compression library that provides higher compression rates and lower CPU usage when compared to zlib.