Glossary

  • $cmd
  • A special virtual collection that exposes MongoDB’sdatabase commands.To use database commands, see Issue Commands.
  • _id
  • A field required in every MongoDB document. The_id field must have a unique value. You canthink of the _id field as the document’s primary key.If you create a new document without an _id field, MongoDBautomatically creates the field and assigns a uniqueBSON ObjectId.
  • accumulator
  • An expression in the aggregation framework thatmaintains state between documents in the aggregationpipeline. For a list of accumulator operations, see$group.
  • action
  • An operation the user can perform on a resource. Actions andresources combine to create privileges. See action.
  • admin database
  • A privileged database. Usersmust have access to the admin database to run certainadministrative commands. For a list of administrative commands,see Administration Commands.
  • aggregation
  • Any of a variety of operations that reduces and summarizes largesets of data. MongoDB’s aggregate() andmapReduce() methods are twoexamples of aggregation operations. For more information, seeAggregation.
  • aggregation framework
  • The set of MongoDB operators that let you calculate aggregatevalues without having to use map-reduce. For a list ofoperators, see Aggregation Reference.
  • arbiter
  • A member of a replica set that exists solely to vote inelections. Arbiters do not replicate data. SeeReplica Set Arbiter.
  • Atlas
  • MongoDB Atlasis a cloud-hosted database-as-a-service.
  • authentication
  • Verification of the user identity. SeeAuthentication.
  • authorization
  • Provisioning of access to databases and operations. SeeRole-Based Access Control.
  • B-tree
  • A data structure commonly used by database management systems tostore indexes. MongoDB uses B-trees for its indexes.
  • balancer
  • An internal MongoDB process that runs in the context of asharded cluster and manages the migration of chunks. Administrators must disable the balancer for allmaintenance operations on a sharded cluster. SeeSharded Cluster Balancer.
  • BSON
  • A serialization format used to store documents and makeremote procedure calls in MongoDB. “BSON” is a portmanteau of the words“binary” and “JSON”. Think of BSON as a binary representationof JSON (JavaScript Object Notation) documents. SeeBSON Types andMongoDB Extended JSON (v2).
  • BSON types
  • The set of types supported by the BSON serializationformat. For a list of BSON types, see BSON Types.
  • CAP Theorem
  • Given three properties of computing systems, consistency,availability, and partition tolerance, a distributed computingsystem can provide any two of these features, but never allthree.
  • capped collection
  • A fixed-sized collection that automaticallyoverwrites its oldest entries when it reaches its maximum size.The MongoDB oplog that is used in replication is acapped 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.
  • 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 particularshard. Chunk ranges are inclusive of the lower boundaryand exclusive of the upper boundary. MongoDB splits chunks whenthey grow beyond the configured chunk size, which by default is64 megabytes. MongoDB migrates chunks when a shard contains toomany chunks of a collection relative to other shards. SeeData Partitioning with Chunks and Sharded Cluster Balancer.
  • client
  • The application layer that uses a database for data persistenceand storage. Drivers provide the interfacelevel between the application layer and the database server.

Client can also refer to a single thread or process.

  • cluster
  • See sharded cluster.
  • collection
  • A grouping of MongoDB documents. A collectionis the equivalent of an RDBMS table. A collection existswithin a single database. Collections do not enforce aschema. Documents within a collection can have different fields.Typically, all documents in a collection have a similar or relatedpurpose. See Namespaces.
  • collection scan
  • Collection scans are a query execution strategy where MongoDB mustinspect every document in a collection to see if it matches thequery criteria. These queries are very inefficient and do not useindexes. See Query Optimization for details aboutquery execution strategies.
  • compound index
  • An index consisting of two or more keys. SeeCompound Indexes.
  • concurrency control
  • Concurrency control ensures that database operations can beexecuted concurrently without compromising correctness.Pessimistic concurrency control, such as used in systemswith locks, will block any potentiallyconflicting operations even if they may not turn out toactually conflict. Optimistic concurrency control, the approachused by WiredTiger, will delaychecking until after a conflict may have occurred, aborting andretrying one of the operations involved in any writeconflict that arises.
  • config database
  • An internal database that holds the metadata associated with asharded cluster. Applications and administrators shouldnot modify the config database in the course of normaloperation. See Config Database.
  • config server
  • A mongod instance that stores all the metadataassociated with a sharded cluster.See Config Servers.
  • 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 relationaldatabases since the format is well-suited to tabular data. You canimport CSV files using mongoimport.
  • cursor
  • A pointer to the result set of a query. Clients caniterate through a cursor to retrieve results. By default, cursorstimeout after 10 minutes of inactivity. SeeIterate a Cursor in the mongo Shell.
  • daemon
  • The conventional name for a background, non-interactiveprocess.
  • data directory
  • The file-system location where the mongod stores datafiles. The dbPath option specifies the data directory.
  • data partition
  • A distributed system architecture that splits data into ranges.Sharding uses partitioning. SeeData Partitioning with Chunks.
  • data-center awareness
  • A property that allows clients to address members in a systembased on their locations. Replica setsimplement data-center awareness using tagging. SeeData Center Awareness.
  • database
  • A physical container for collections.Each database gets its own set of files on the filesystem. A single MongoDB server typically has multipledatabases.
  • database command
  • A MongoDB operation, other than an insert, update, remove, orquery. For a list of database commands, seeDatabase Commands. To use database commands, seeIssue Commands.
  • database profiler
  • A tool that, when enabled, keeps a record on all long-runningoperations in a database’s system.profile collection. Theprofiler is most often used to diagnose slow queries. SeeDatabase Profiling.
  • dbpath
  • The location of MongoDB’s data file storage. SeedbPath.
  • delayed member
  • A replica set member that cannot become primary andapplies operations at a specified delay. The delay is useful forprotecting data from human error (i.e. unintentionally deleteddatabases) or updates that have unforeseen effects on theproduction database. See Delayed Replica Set Members.
  • document
  • A record in a MongoDB collection and the basic unit ofdata in MongoDB. Documents are analogous to JSON objectsbut exist in the database in a more type-rich format known asBSON. See Documents.
  • dot notation
  • MongoDB uses the dot notation to access the elements of an arrayand to access the fields of an embedded document. SeeDot Notation.
  • draining
  • The process of removing or “shedding” chunks fromone shard to another. Administrators must drain shardsbefore removing them from the cluster. SeeRemove Shards from an Existing Sharded Cluster.
  • driver
  • A client library for interacting with MongoDB in a particularlanguage. See /drivers.
  • durable
  • A write operation is durable when it will persist across ashutdown (or crash) and restart of one or more server processes.For a single mongod server, a write operation isconsidered durable when it has been written to the server’sjournal file. For a replica set, a write operation isconsidered durable once the write operation is durable on amajority of voting nodes; i.e. written to a majority of votingnodes’ journals.
  • election
  • The process by which members of a replica set select aprimary on startup and in the event of a failure. SeeReplica Set Elections.
  • eventual consistency
  • A property of a distributed system that allows changes to thesystem to propagate gradually. In a database system, this meansthat readable members are not required to reflect the latestwrites at all times.
  • expression
  • In the context of aggregation framework, expressions arethe stateless transformations that operate on the data that passesthrough a pipeline. See Aggregation Pipeline.
  • failover
  • The process that allows a secondary member of areplica set to become primary in the event of afailure. See Automatic Failover.
  • field
  • A name-value pair in a document. A document haszero or more fields. Fields are analogous to columns in relationaldatabases. See Document Structure.
  • field path
  • Path to a field in the document. To specify a field path, use astring that prefixes the field name with a dollar sign ($).
  • firewall
  • A system level networking filter that restricts access based on,among other things, IP address. Firewalls form a part of aneffective network security strategy. SeeFirewalls.
  • fsync
  • A system call that flushes all dirty, in-memory pages todisk. MongoDB calls fsync() on its database files at leastevery 60 seconds. See fsync.
  • geohash
  • A geohash value is a binary representation of the location on acoordinate grid. See Calculation of Geohash Values for 2d Indexes.
  • GeoJSON
  • A geospatial data interchange format based on JavaScriptObject Notation (JSON). GeoJSON is used ingeospatial queries. Forsupported GeoJSON objects, see Geospatial Data.For the GeoJSON format specification, seehttps://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 ofthe official MongoDB drivers support this convention, as does themongofiles program. See GridFS.
  • hashed shard key
  • A special type of shard key that uses a hash of the valuein the shard key field to distribute documents among members ofthe sharded cluster. See Hashed Indexes.
  • haystack index
  • A geospatial index that enhances searches by creating“buckets” of objects grouped by a second criterion. SeegeoHaystack Indexes.
  • hidden member
  • A replica set member that cannot become primaryand are invisible to client applications. SeeHidden Replica Set Members.
  • high availability
  • High availability indicates a system designed for durability,redundancy, and automatic failover such that the applicationssupported by the system can operate continuously and withoutdowntime for a long period of time. MongoDBreplica sets supporthigh availability when deployed according to our documentedbest practices.

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

  • idempotent
  • The quality of an operation to produce the same result given thesame input, whether run once or run multiple times.
  • index
  • A data structure that optimizes queries. See Indexes.
  • init script
  • A simple shell script, typically located in the /etc/rc.d or/etc/init.d directory, and used by the system’s initializationprocess to start, restart or stop a daemon process.
  • initial sync
  • The replica set operation that replicates data from anexisting replica set member to a new replica set member. SeeInitial Sync.
  • intent lock
  • A lock on a resource that indicates that the holderof the lock will read (intent shared) or write (intentexclusive) the resource using concurrency control ata finer granularity than that of the resource with the intentlock. Intent locks allow concurrent readers and writers of aresource. See What type of locking does MongoDB use?.
  • interrupt point
  • A point in an operation’s lifecycle when it cansafely abort. MongoDB only terminates an operationat designated interrupt points. SeeTerminate Running Operations.
  • IPv6
  • A revision to the IP (Internet Protocol) standard thatprovides a significantly larger address space to more effectivelysupport the number of hosts on the contemporary Internet.
  • ISODate
  • The international date format used by mongoto display dates. The format is: YYYY-MM-DD HH:MM.SS.millis.
  • JavaScript
  • A popular scripting language originally designed for webbrowsers. The MongoDB shell and certain server-side functions usea JavaScript interpreter. SeeServer-side JavaScript for more information.
  • journal
  • A sequential, binary transaction log used to bring the databaseinto a valid state in the event of a hard shutdown.Journaling writes data first to the journal and then to the coredata files. MongoDB enables journaling by default for 64-bitbuilds of MongoDB version 2.0 and newer. Journal files arepre-allocated and exist as files in the data directory.See Journaling.
  • JSON
  • JavaScript Object Notation. A human-readable, plain text formatfor expressing structured data with support in many programminglanguages. For more information, see http://www.json.org.Certain MongoDB tools render an approximation of MongoDBBSON documents in JSON format. SeeMongoDB Extended JSON (v2).
  • JSON document
  • A JSON document is a collection of fields and values in astructured format. For sample JSON documents, seehttp://json.org/example.html.
  • JSONP
  • JSON with Padding. Refers to a method of injecting JSONinto applications. Presents potential security concerns.
  • least privilege
  • An authorization policy that gives a user only the amount of accessthat is essential to that user’s work and no more.
  • legacy coordinate pairs
  • The format used for geospatial data prior to MongoDBversion 2.4. This format stores geospatial data as points on aplanar coordinate system (e.g. [ x, y ]). SeeGeospatial Queries.
  • LineString
  • A LineString is defined by an array of two or more positions. Aclosed LineString with four or more positions is called aLinearRing, as described in the GeoJSON LineString specification:https://tools.ietf.org/html/rfc7946#section-3.1.4. To use aLineString in MongoDB, seeGeoJSON Objects.
  • lock
  • MongoDB uses locks to ensure that concurrencydoes not affect correctness. MongoDB uses read locks, write locks andintent locks. For more information, seeWhat type of locking does MongoDB use?.
  • LVM
  • Logical volume manager. LVM is a program that abstracts diskimages from physical devices and provides a number of raw diskmanipulation and snapshot capabilities useful for systemmanagement. For information on LVM and MongoDB, seeBack Up and Restore Using LVM on Linux.
  • map-reduce
  • A data processing and aggregation paradigm consisting of a “map”phase that selects data and a “reduce” phase that transforms thedata. In MongoDB, you can run arbitrary aggregations over datausing map-reduce. For map-reduce implementation, seeMap-Reduce. For all approaches to aggregation,see Aggregation.
  • mapping type
  • A Structure in programming languages that associate keys withvalues, 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 languagespecification and implementation. Generally the order of keys inmapping types is arbitrary and not guaranteed.
  • md5
  • A hashing algorithm used to efficiently providereproducible unique strings to identify and checksumdata. MongoDB uses md5 to identify chunks of data forGridFS. See filemd5.
  • MIB
  • Management Information Base. MongoDB uses MIB files to define the type ofdata tracked by SNMP in the MongoDB Enterprise edition.
  • MIME
  • Multipurpose Internet Mail Extensions. A standard set of type andencoding definitions used to declare the encoding and type of datain multiple data storage, transmission, and email contexts. Themongofiles tool provides an option to specify a MIMEtype to describe a file inserted into GridFS storage.
  • mongo
  • The MongoDB shell. The mongo process starts the MongoDBshell as a daemon connected to either a mongod ormongos instance. The shell has a JavaScript interface.See mongo and mongo Shell Methods.
  • mongod
  • The MongoDB database server. The mongod process startsthe MongoDB server as a daemon. The MongoDB server manages datarequests and formats and manages background operations. Seemongod.
  • mongos
  • The routing and load balancing process that acts an interfacebetween an application and a MongoDB sharded cluster. Seemongos.
  • namespace
  • The canonical name for a collection or index in MongoDB.The namespace is a combination of the database name andthe name of the collection or index, like so:[database-name].[collection-or-index-name]. All documentsbelong to a namespace. See Namespaces.
  • natural order
  • The order in which the database refers to documents on disk. This is thedefault sort order. See $natural andReturn in Natural Order.
  • network partition
  • A network failure that separates a distributed system intopartitions such that nodes in one partition cannot communicatewith the nodes in the other partition.

Sometimes, partitions are partial or asymmetric. An example of apartial partition would be a division of the nodes of a networkinto three sets, where members of the first set cannotcommunicate with members of the second set, and vice versa, butall nodes can communicate with members of the third set. In anasymmetric partition, communication may be possible only when itoriginates with certain nodes. For example, nodes on one side ofthe partition can communicate to the other side only if theyoriginate the communications channel.

  • ObjectId
  • A special 12-byte BSON type that guarantees uniquenesswithin the collection. The ObjectId is generated based ontimestamp, machine ID, process ID, and a process-local incrementalcounter. 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 thequery language’s “greater than” operator. For available operators,see Operators.
  • oplog
  • A capped collection that stores an ordered history oflogical writes to a MongoDB database. The oplog is thebasic mechanism enabling replication in MongoDB.See Replica Set Oplog.
  • optime

Changed in version 3.2: The following describes the optime format used byprotocolVersion: 1, introduced inMongoDB 3.2.

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

  • ts, the Timestamp ofthe operation.
  • t, the term in which theoperation was originally generated on the primary.
    • ordered query plan
    • A query plan that returns results in the order consistent with thesort() order. SeeQuery Plans.
    • orphaned document
    • In a sharded cluster, orphaned documents are those documents on ashard that also exist in chunks on other shards as a result offailed migrations or incomplete migration cleanup due to abnormalshutdown. Delete orphaned documents usingcleanupOrphaned to reclaim disk space and reduceconfusion.
    • passive member
    • A member of a replica set that cannot become primarybecause its members[n].priority is0. See Priority 0 Replica Set Members.
    • pcap
    • A packet-capture format that mongoreplay can use toproduce a BSON-formatted playback file to play back to anotherMongoDB instance. See: mongoreplay recordfor more information.
    • PID
    • A process identifier. UNIX-like systems assign a unique-integerPID to each running process. You can use a PID to inspect arunning process and send signals to it. See/proc File System.
    • pipe
    • A communication channel in UNIX-like systems allowing independentprocesses to send and receive data. In the UNIX shell, pipedoperations allow users to direct the output of one command intothe input of another.
    • pipeline
    • A series of operations in an aggregation process.See Aggregation Pipeline.
    • Point
    • A single coordinate pair as described in the GeoJSON Pointspecification: https://tools.ietf.org/html/rfc7946#section-3.1.2. Touse a Point in MongoDB, seeGeoJSON Objects.
    • Polygon
    • An array of LinearRing coordinate arrays, asdescribed in the GeoJSON Polygon specification:https://tools.ietf.org/html/rfc7946#section-3.1.6. For Polygonswith multiple rings, the first must be the exterior ring andany 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 andcannot intersect or overlap with each other. SeeGeoJSON Objects.

  • powerOf2Sizes
  • A per-collection setting that changes and normalizes the wayMongoDB allocates space for each document, in an effort tomaximize storage reuse and to reduce fragmentation. This is thedefault for TTL Collections. SeecollMod andusePowerOf2Sizes.
  • pre-splitting
  • An operation performed before inserting data that divides therange of possible shard key values into chunks to facilitate easyinsertion and high write throughput. In some cases pre-splittingexpedites the initial distribution of documents in shardedcluster by manually dividing the collection rather than waitingfor the MongoDB balancer to do so. SeeCreate Chunks in a Sharded Cluster.
  • prefix compression
  • Reduces memory and disk consumption by storing any identical indexkey prefixes only once, per page of memory. See:Compression for more about WiredTiger’scompression behavior.
  • primary
  • In a replica set, the primary is the member thatreceives all write operations. SeePrimary.
  • primary key
  • A record’s unique immutable identifier. In an RDBMS, the primarykey is typically an integer stored in each row’s id field.In MongoDB, the _id field holds a document’s primarykey which is usually a BSON ObjectId.
  • primary shard
  • The shard that holds all the un-sharded collections. SeePrimary Shard.
  • priority
  • A configurable value that helps determine which members ina replica set are most likely to become primary.See members[n].priority.
  • privilege
  • A combination of specified resource andactions permitted on the resource. Seeprivilege.
  • projection
  • A document given to a query that specifies which fieldsMongoDB returns in the result set. See Project Fields to Return from Query. For alist of projection operators, seeProjection Operators.
  • query
  • A read request. MongoDB uses a JSON-like query languagethat includes a variety of query operators withnames that begin with a $ character. In the mongoshell, you can issue queries using thedb.collection.find() anddb.collection.findOne() methods. SeeQuery Documents.
  • query optimizer
  • A process that generates query plans. For each query, theoptimizer generates a plan that matches the query to the indexthat will return results as efficiently as possible. Theoptimizer reuses the query plan each time the query runs. If acollection changes significantly, the optimizer creates a newquery plan. See Query Plans.
  • query shape
  • A combination of query predicate, sort, and projection.

For the query predicate, only the structure of the predicate,including the field names, are significant; the values in thequery predicate are insignificant. As such, 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,starting in MongoDB 4.2, each query shape is associated witha queryHash. The queryHash is ahexadecimal string that represents a hash of the query shape andis dependent only on the query shape.

Note

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

  • RDBMS
  • Relational Database Management System. A database managementsystem based on the relational model, typically usingSQL 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 toa majority of nodes in a replica set. SeeRead Concern.
  • read lock
  • A shared lock on a resource such as a collection ordatabase that, while held, allows concurrent readers but nowriters. 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 replicasets. By default, MongoDB directs reads to primaries. However, you may also direct reads to secondaries foreventually consistent reads. SeeRead Preference.
  • recovering
  • A replica set member status indicating that a memberis 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 implementsreplication and automated failover. MongoDB’s recommendedreplication strategy. See Replication.
  • replication
  • A feature allowing multiple database servers to share the samedata, thereby ensuring redundancy and facilitating load balancing.See Replication.
  • replication lag
  • The length of time between the last operation in theprimary’soplog and the last operationapplied to a particular secondary. In general, you want tokeep replication lag as small as possible. See ReplicationLag.
  • resident memory
  • The subset of an application’s memory currently stored inphysical RAM. Resident memory is a subset of virtual memory,which includes memory mapped to physical RAM and to disk.
  • resource
  • A database, collection, set of collections, or cluster. Aprivilege permits actions on a specifiedresource. See resource.
  • role
  • A set of privileges that permit actions onspecified resources. Roles assigned to a userdetermine the user’s access to resources and operations. SeeSecurity.
  • rollback
  • A process that reverts writes operations to ensure the consistencyof all replica set members. See Rollbacks During Replica Set Failover.
  • secondary
  • A replica set member that replicates the contents of themaster database. Secondary members may handle read requests, butonly the primary members can handle write operations. SeeSecondaries.
  • secondary index
  • A database index that improves query performance byminimizing the amount of work that the query engine must performto fulfill a query. See Indexes.
  • set name
  • The arbitrary name given to a replica set. All members of areplica set must have the same name specified with thereplSetName setting or the —replSet option.
  • shard
  • A single mongod instance or replica set thatstores some portion of a sharded cluster’s total data set. In production, all shards should bereplica sets. See Shards.
  • shard key
  • The field MongoDB uses to distribute documents among members of asharded cluster. See Shard Keys.
  • sharded cluster
  • The set of nodes comprising a sharded MongoDBdeployment. A sharded cluster consists of config servers,shards, and one or more mongosrouting processes. See Sharded Cluster Components.
  • sharding
  • A database architecture that partitions data by key ranges anddistributes 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 concisesyntax for a database command. Shell helpersimprove the general interactive experience. Seemongo Shell Methods.
  • single-master replication
  • A replication topology where only a single databaseinstance accepts writes. Single-master replication ensuresconsistency and is the replication topology employed by MongoDB.See Replica Set Primary.
  • snappy
  • A compression/decompression library designed to balanceefficient computation requirements with reasonable compression rates.Snappy is the default compressionlibrary for MongoDB’s use of WiredTiger. See Snappy and the WiredTiger compressiondocumentationfor more information.
  • split
  • The division between chunks in a shardedcluster. See Data Partitioning with Chunks.
  • SQL
  • Structured Query Language (SQL) is a common special-purposeprogramming language used for interaction with a relationaldatabase, including access control, insertions,updates, queries, and deletions. There are some similarelements in the basic SQL syntax supported by different databasevendors, but most implementations have their own dialects, datatypes, and interpretations of proposed SQL standards. ComplexSQL is generally not directly portable between majorRDBMS products. SQL is often used asmetonym for relational databases.
  • SSD
  • Solid State Disk. A high-performance disk drive that uses solidstate electronics for persistence, as opposed to the rotating plattersand movable read/write heads used by traditional mechanical hard drives.
  • standalone
  • An instance of mongod that is running as a singleserver and not as part of a replica set. To convert astandalone into a replica set, seeConvert a Standalone to a Replica Set.
  • storage engine
  • The part of a database that is responsible for managing how datais stored and accessed, both in memory and on disk. Differentstorage engines perform better for specific workloads. SeeStorage Engines for specific details on the built-instorage engines in MongoDB.
  • storage order
  • See natural order.
  • strict consistency
  • A property of a distributed system requiring that all membersalways reflect the latest changes to the system. In a databasesystem, this means that any system that can provide data mustreflect the latest writes at all times.
  • sync
  • The replica set operation where members replicate datafrom the primary. Sync first occurs when MongoDB createsor restores a member, which is called initial sync. Syncthen occurs continually to keep the member updated with changes tothe replica set’s data. See Replica Set Data Synchronization.
  • syslog
  • On UNIX-like systems, a logging process that provides a uniformstandard for servers and processes to submit logging information.MongoDB provides an option to send output to the host’s syslogsystem. See syslogFacility.
  • tag
  • A label applied to a replica set member and used byclients to issue data-center-aware operations. For more informationon using tags with replica sets, see the followingsections of this manual: Tag Set.

Changed in version 3.4: In MongoDB 3.4, sharded cluster zones replacetags.

  • tag set
  • A document containing zero or more tags.
  • tailable cursor
  • For a capped collection, a tailable cursor is a cursor thatremains open after the client exhausts the results in the initialcursor. 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 increasingnumber that corresponds to an election attempt.
  • topology
  • The state of a deployment of MongoDB instances, includingthe type of deployment (i.e. standalone, replica set, or shardedcluster) as well as the availability of servers, and the role ofeach server (i.e. primary, secondary,config server, or mongos.)
  • TSV
  • A text-based data format consisting of tab-separated values.This format is commonly used to exchange data between relationaldatabases, since the format is well-suited to tabular data. You canimport TSV files using mongoimport.
  • TTL
  • Stands for “time to live” and represents an expiration time orperiod for a given piece of information to remain in a cache orother temporary storage before the system deletes it or ages itout. MongoDB has a TTL collection feature. SeeExpire Data from Collections by Setting TTL.
  • unique index
  • An index that enforces uniqueness for a particular field acrossa 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 thesort() order.See Query Plans.
  • upsert
  • An option for update operations; e.g.db.collection.update(),db.collection.findAndModify(). If set to true, theupdate operation will either update the document(s) matched bythe specified query or if no documents match, insert a newdocument. The new document will have the fields indicated in theoperation. See Insert a New Document if No Match Exists (Upsert).
  • virtual memory
  • An application’s working memory, typically residing on bothdisk and in physical RAM.
  • WGS84
  • The default reference system and geodetic datum that MongoDB usesto calculate geometry over an Earth-like sphere for geospatialqueries 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.
  • write concern
  • Specifies whether a write operation has succeeded. Write concernallows your application to detect insertion errors or unavailablemongod instances. For replica sets, you can configure write concern to confirm replication to aspecified number of members. See Write Concern.
  • write conflict
  • A situation in which two concurrent operations, at leastone of which is a write, attempt to use a resource in a waythat would violate constraints imposed by a storage engineusing optimistic concurrency control. MongoDB willtransparently abort and retry one of the conflicting operations.
  • write lock
  • An exclusive lock on a resource such as a collectionor database. When a process writes to a resource, it takesan exclusive write lock to prevent other processes from writingto or reading from that resource. For more information onlocks, see FAQ: Concurrency.
  • writeBacks
  • The process within the sharding system that ensures that writesissued to a shard that is not responsible for therelevant chunk get applied to the proper shard. For relatedinformation, see What does writebacklisten in the log mean? andwriteBacksQueued.
  • zlib
  • A data compression library that provides higher compression ratesat the cost of more CPU, compared to MongoDB’s use ofsnappy. You can configure WiredTiger to use zlib as its compression library. Seehttp://www.zlib.net and the WiredTiger compression documentation for moreinformation.
  • zone

New in version 3.4: A grouping of documents based on ranges of shard key valuesfor a given sharded collection. Each shard in the sharded cluster canassociate with one or more zones. In a balanced cluster, MongoDBdirects reads and writes covered by a zone only to those shardsinside the zone. See the Zones manual page for moreinformation.

Zones supersede functionality described by tags inMongoDB 3.2.

  • zstd

New in version 4.2.

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