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Glossary

$cmd

A special 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.

accumulator

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

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 Instance Administration Commands.

aggregation

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 Concepts.

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.

arbiter

A member of a replica set that exists solely to vote in elections. Arbiters do not replicate data. See Replica Set Arbiter.

B-tree

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

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 Collection Balancing.

BSON

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 also

Documents, BSON Types and Data Type Fidelity

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.

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 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 and Sharding Mechanics.

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.

cluster

See sharded cluster.

collection

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 What is a namespace in MongoDB?.

compound index

An index consisting of two or more keys. See Compound Indexes.

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. A production sharded cluster requires three config servers, each on a separate machine. See Config Servers.

control script

A simple shell script, typically located in the /etc/rc.d or /etc/init.d directory, and used by the system’s initialization process to start, restart or stop a daemon process.

CRUD

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

CSV

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.

cursor

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 Cursors.

daemon

The conventional name for a background, non-interactive process.

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 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.

datum

A set of values used to define measurements on the earth. MongoDB uses the WGS84 datum in certain geospatial calculations. See Geospatial Indexes and Queries.

dbpath

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.

diagnostic log

A verbose log of operations stored in the dbpath. See diaglog.

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 a subdocument. 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 language. See MongoDB Drivers and Client Libraries.

election

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 MongoDB, reads to a primary have strict consistency; reads to secondaries have eventual consistency.

expression

In the context of aggregation framework, expressions are the stateless transformations that operate on the data that passes through a pipeline. See Aggregation Concepts.

failover

The process that allows a secondary member of a replica set to become primary in the event of a failure. See Replica Set High Availability.

field

A name-value pair in a document. A document has zero or more fields. Fields are analogous to columns in relational databases.

firewall

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.

fsync

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.

geohash

A geohash value is a binary representation of the location on a coordinate grid. See Calculation of Geohash Values for 2d Indexes.

GeoJSON

A geospatial data interchange format based on JavaScript Object Notation (JSON). GeoJSON is used in geospatial queries. For supported GeoJSON objects, see Location Data. For the GeoJSON format specification, see http://geojson.org/geojson-spec.html.

geospatial

Data that relates to geographical location. In MongoDB, you may store, index, and query data according to geographical parameters. See Geospatial Indexes and Queries.

GridFS

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 Index.

haystack index

A geospatial index that enhances searches by creating “buckets” of objects grouped by a second criterion. See Haystack Indexes.

hidden member

A replica set member that cannot become primary and are invisible to client applications. See Hidden Replica Set Members.

idempotent

The quality of an operation to produce the same result given the same input, whether run once or run multiple times.

index

A data structure that optimizes queries. See Index Concepts.

initial sync

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

IPv6

A revision to the IP (Internet Protocol) standard that provides a significantly larger address space to more effectively support the number of hosts on the contemporary Internet.

ISODate

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

JavaScript

A popular scripting language originally designed for web browsers. The MongoDB shell and certain server-side 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 consistent 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 Mechanics.

JSON

JavaScript Object Notation. A human-readable, 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.

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.

legacy coordinate pairs

The format used for geospatial data prior to MongoDB version 2.4. This format stores geospatial data as points on a planar coordinate system (e.g. [ x, y ]). See Geospatial Indexes and Queries.

LineString

A LineString is defined by 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: http://geojson.org/geojson-spec.html#linestring. To use a LineString in MongoDB, see Store GeoJSON Objects.

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 Backup and Restore Using LVM on a Linux System.

map-reduce

A data processing and aggregation paradigm consisting of a “map” phase that selects data and a “reduce” phase that transforms the data. In MongoDB, you can run arbitrary aggregations over data using map-reduce. For map-reduce implementation, see Map-Reduce. For all approaches to aggregation, see Aggregation Concepts.

mapping type

A Structure in programming languages that associate keys with values, where keys may nest other pairs of keys and values (e.g. dictionaries, hashes, maps, and associative arrays). The properties of these structures depend on the language specification and implementation. Generally the order of keys in mapping types is arbitrary and not guaranteed.

master

The database that receives all writes in a conventional master-slave replication. In MongoDB, replica sets replace master-slave replication for most use cases. For more information on master-slave replication, see Master Slave Replication.

md5

A hashing algorithm used to efficiently provide reproducible unique strings to identify and checksum data. MongoDB uses md5 to identify chunks of data for GridFS. See filemd5.

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.

mongo

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

mongod

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

MongoDB

An open-source document-based database system. “MongoDB” derives from the word “humongous” because of the database’s ability to scale up with ease and hold very large amounts of data. MongoDB stores documents in collections within databases.

mongos

The routing and load balancing process that acts an interface between an application and a MongoDB sharded cluster. See mongos.

namespace

The canonical name for a collection or index in MongoDB. The namespace is a combination of the database name and the name of the collection or index, like so: [database-name].[collection-or-index-name]. All documents belong to a namespace. See What is a namespace in MongoDB?.

natural order

The order that a database stores documents on disk. Typically, the order of documents on disks reflects insertion order, except when a document moves internally because an update operation increases its size. In capped collections, documents do not move internally, and therefore insertion order and natural order are identical in capped collections. MongoDB returns documents in forward natural order for a find() query with no parameters. MongoDB returns documents in reverse natural order for a find() query sorted with a parameter of $natural:-1. See $natural.

ObjectId

A special 12-byte BSON type that guarantees uniqueness within the collection. The ObjectID is generated based on timestamp, machine ID, process ID, and a process-local incremental counter. MongoDB uses ObjectId values as the default values for _id fields.

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.

ordered query plan

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

padding

The extra space allocated to document on the disk to prevent moving a document when it grows as the result of update() operations. See Padding Factor.

padding factor

An automatically-calibrated constant used to determine how much extra space MongoDB should allocate per document container on disk. A padding factor of 1 means that MongoDB will allocate only the amount of space needed for the document. A padding factor of 2 means that MongoDB will allocate twice the amount of space required by the document. See Padding Factor.

page fault

The event that occurs when a process requests stored data (i.e. a page) from memory that the operating system has moved to disk. See What are page faults?.

partition

A distributed system architecture that splits data into ranges. Sharding uses partitioning. See Data Partitioning.

passive member

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

pcap

A packet-capture format used by mongosniff to record packets captured from network interfaces and display them as human-readable MongoDB operations. See Options.

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 process. See Aggregation Concepts.

Point

A single coordinate pair as described in the GeoJSON Point specification: http://geojson.org/geojson-spec.html#point. To use a Point in MongoDB, see Store GeoJSON Objects.

Polygon

An array of LinearRing coordinate arrays, as described in the GeoJSON Polygon specification: http://geojson.org/geojson-spec.html#polygon. 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 Store GeoJSON Objects.

powerOf2Sizes

A per-collection setting that changes and normalizes the way MongoDB allocates space for each document, in an effort to maximize storage reuse and to reduce fragmentation. This is the default for TTL Collections. See collMod and usePowerOf2Sizes.

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 Chunks (Pre-Splitting).

primary

In a replica set, the primary member is the current master instance, which receives all write operations. See Primary.

primary key

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

primary shard

The shard that holds all the un-sharded collections. See Primary Shard.

priority

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

projection

A document given to a query that specifies which fields MongoDB returns in the result set. See Limit Fields to Return from a Query. For a list of projection operators, see Projection Operators.

query

A read request. MongoDB uses a JSON-like query language that includes a variety of query operators with names that begin with a $ character. In the mongo shell, you can issue queries using the find() and findOne() methods. See Read Operations.

query optimizer

A process that generates query plans. For each query, the optimizer generates a plan that matches the query to the index that will return 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.

RDBMS

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

read lock

In the context of a reader-writer lock, a lock 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 shards. By default, MongoDB directs reads to primaries for strict consistency. However, you may also direct reads to secondaries for eventually consistent reads. See Read Preference.

record size

The space allocated for a document including the padding. For more information on padding, see Padding Factor and compact.

recovering

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

replica pairs

The precursor to the MongoDB replica sets.

Deprecated since version 1.6.

replica set

A cluster of MongoDB servers that implements master-slave replication and automated failover. MongoDB’s recommended replication strategy. See Replication.

replication

A feature allowing multiple database servers to share the same data, thereby ensuring redundancy and facilitating load balancing. See Replication.

replication lag

The length of time between the last operation in the primary’s oplog and the last operation applied to a particular secondary. In general, you want to keep replication lag as small 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 disk.

REST

An API design pattern centered around the idea of resources and the CRUD operations that apply to them. Typically REST is implemented over HTTP. MongoDB provides a simple HTTP REST interface that allows HTTP clients to run commands against the server. See REST Interface and REST API.

rollback

A process that reverts writes 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 handle read requests, but only the primary members can handle 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 fulfill a query. See Indexes.

set name

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

shard

A single mongod instance or replica set that stores some portion of a sharded cluster’s total data set. In production, all shards should be 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 three config processes, one or more replica sets, 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 the mongo shell that provides a more concise syntax for a database command. Shell helpers improve the general interactive experience. See mongo Shell 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 employed by MongoDB. See Replica Set Primary.

slave

A read-only database that replicates operations from a master database in conventional master/slave replication. In MongoDB, replica sets replace master/slave replication for most use cases. However, for information on master/slave replication, see Master Slave Replication.

split

The division between chunks in a sharded cluster. See Chunk Splits in a Sharded Cluster.

SQL

Structured Query Language (SQL) is a common special-purpose programming language used for interaction with a relational database, including access control, insertions, updates, queries, and deletions. There are some similar elements in the basic SQL syntax supported by different database vendors, but most implementations have their own dialects, data types, and interpretations of proposed SQL standards. Complex SQL is generally not directly portable between major RDBMS products. SQL is often used as metonym for relational databases.

SSD

Solid State Disk. A high-performance disk drive that uses solid state electronics for persistence, as opposed to the rotating platters and movable read/write heads used by traditional mechanical hard drives.

standalone

An instance of mongod that is running as a single server and not as part of a replica set. To convert a standalone into a replica set, see Convert a Standalone to a Replica Set.

strict consistency

A property of a distributed system requiring that all members always reflect the latest changes to the system. In a database system, this means that any system that can provide data must reflect the latest writes at all times. In MongoDB, reads from a primary have strict consistency; reads from secondary members have eventual consistency.

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 syslog.

tag

A label applied to a replica set member or shard and used by clients to issue data-center-aware operations. For more information on using tags with replica sets and with shards, see the following sections of this manual: Tag Sets and Behavior and Operations.

TSV

A text-based data format consisting of tab-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 TSV files using mongoimport.

TTL

Stands for “time to live” and represents 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.

unique index

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

unordered query plan

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

upsert

An operation that will either update the first document matched by a query or insert a new document if none matches. The new document will have the fields implied by the operation. You perform upserts with the update() operation. See Upsert Parameter.

virtual memory

An application’s working memory, typically residing on both disk an in physical RAM.

WGS84

The default datum MongoDB uses to calculate geometry over an Earth-like sphere. MongoDB uses the WGS84 datum for geospatial queries on GeoJSON objects. See the “EPSG:4326: WGS 84” specification: http://spatialreference.org/ref/epsg/4326/.

working set

The data that MongoDB uses most often. This data is preferably held in RAM, solid-state drive (SSD), or other fast media. See What is the working set?.

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 lock

A lock on the database for a given writer. When a process writes to the database, it takes an exclusive write lock to prevent other processes from writing or reading. For more information on locks, see FAQ: Concurrency.

writeBacks

The process within the sharding system that ensures that writes issued to a shard that is not responsible for the relevant chunk get applied to the proper shard. For related information, see What does writebacklisten in the log mean? and writeBacksQueued.

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