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

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

Client can also refer to a single thread or process.

See sharded cluster.
A grouping of MongoDB documents. A collection is the equivalent of an RDBMS table. A collection exists within a single database. Collections do not enforce a schema. Documents within a collection can have different fields. Typically, all documents in a collection have a similar or related purpose. See Namespaces.
collection scan
Collection scans are a query execution strategy where MongoDB must inspect every document in a collection to see if it matches the query criteria. These queries are very inefficient and do not use indexes. See Query Optimization for details about query execution strategies.
compound index
An index consisting of two or more keys. See Compound Indexes.
concurrency control
Concurrency control ensures that database operations can be executed concurrently without compromising correctness. Pessimistic concurrency control, such as used in systems with locks, will block any potentially conflicting operations even if they may not turn out to actually conflict. Optimistic concurrency control, the approach used by WiredTiger, will delay checking until after a conflict may have occurred, aborting and retrying one of the operations involved in any write conflict that arises.
config database
An internal database that holds the metadata associated with a sharded cluster. Applications and administrators should not modify the config database in the course of normal operation. See Config Database.
config server
A mongod instance that stores all the metadata associated with a sharded cluster. See Config Servers.
An acronym for the fundamental operations of a database: Create, Read, Update, and Delete. See MongoDB CRUD Operations.
A text-based data format consisting of comma-separated values. This format is commonly used to exchange data between relational databases since the format is well-suited to tabular data. You can import CSV files using mongoimport.
A pointer to the result set of a query. Clients can iterate through a cursor to retrieve results. By default, cursors timeout after 10 minutes of inactivity. See Iterate a Cursor in the mongo Shell.
The conventional name for a background, non-interactive process.
data directory
The file-system location where the mongod stores data files. The dbPath option specifies the data directory.
data partition
A distributed system architecture that splits data into ranges. Sharding uses partitioning. See Data Partitioning with Chunks.
data-center awareness
A property that allows clients to address members in a system based on their locations. Replica sets implement data-center awareness using tagging. See Data Center Awareness.
A physical container for collections. Each database gets its own set of files on the file system. A single MongoDB server typically has multiple databases.
database command
A MongoDB operation, other than an insert, update, remove, or query. For a list of database commands, see Database Commands. To use database commands, see Issue Commands.
database profiler
A tool that, when enabled, keeps a record on all long-running operations in a database’s system.profile collection. The profiler is most often used to diagnose slow queries. See Database Profiling.
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.
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 the --diaglog option.
A record in a MongoDB collection and the basic unit of data in MongoDB. Documents are analogous to JSON objects but exist in the database in a more type-rich format known as BSON. See Documents.
dot notation
MongoDB uses the dot notation to access the elements of an array and to access the fields of an embedded document. See Dot Notation.
The process of removing or “shedding” chunks from one shard to another. Administrators must drain shards before removing them from the cluster. See Remove Shards from an Existing Sharded Cluster.
A client library for interacting with MongoDB in a particular language. See MongoDB Drivers and Client Libraries.
A write operation is durable when it will persist across a shutdown (or crash) and restart of one or more server processes. For a single mongod server, a write operation is considered durable when it has been written to the server’s journal file. For a replica set, a write operation is considerable durable once the write operation is durable on a majority of voting nodes in the replica set; i.e. written to a majority of voting nodes’ journal files.
The process by which members of a replica set select a primary on startup and in the event of a failure. See Replica Set Elections.
eventual consistency
A property of a distributed system that allows changes to the system to propagate gradually. In a database system, this means that readable members are not required to reflect the latest writes at all times.
In the context of aggregation framework, expressions are the stateless transformations that operate on the data that passes through a pipeline. See Aggregation Pipeline.
The process that allows a secondary member of a replica set to become primary in the event of a failure. See Replica Set High Availability.
A name-value pair in a document. A document has zero or more fields. Fields are analogous to columns in relational databases. See Document Structure.
field path
Path to a field in the document. To specify a field path, use a string that prefixes the field name with a dollar sign ($).
A system level networking filter that restricts access based on, among other things, IP address. Firewalls form a part of an effective network security strategy. See Firewalls.
A system call that flushes all dirty, in-memory pages to disk. MongoDB calls fsync() on its database files at least every 60 seconds. See fsync.
A geohash value is a binary representation of the location on a coordinate grid. See Calculation of Geohash Values for 2d Indexes.
A geospatial data interchange format based on JavaScript Object Notation (JSON). GeoJSON is used in geospatial queries. For supported GeoJSON objects, see Location Data. For the GeoJSON format specification, see
Data that relates to geographical location. In MongoDB, you may store, index, and query data according to geographical parameters. See Geospatial Indexes and Queries.
A convention for storing large files in a MongoDB database. All of the official MongoDB drivers support this convention, as does the mongofiles program. See GridFS.
hashed shard key
A special type of shard key that uses a hash of the value in the shard key field to distribute documents among members of the sharded cluster. See Hashed Indexes.
haystack index
A geospatial index that enhances searches by creating “buckets” of objects grouped by a second criterion. See geoHaystack Indexes.
hidden member
A replica set member that cannot become primary and are invisible to client applications. See Hidden Replica Set Members.
The quality of an operation to produce the same result given the same input, whether run once or run multiple times.
A data structure that optimizes queries. See Indexes.
init script
A simple shell script, 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.
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 that the holder of the lock will read (intent shared) or write (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’s lifecycle when it can safely abort. MongoDB only terminates an operation at designated interrupt points. See Terminate Running Operations.
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.
The international date format used by mongo to display dates. The format is: YYYY-MM-DD HH:MM.SS.millis.
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.
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.
JavaScript Object Notation. A human-readable, plain text format for expressing structured data with support in many programming languages. For more information, see 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
JSON with Padding. Refers to a method of injecting JSON into applications. Presents potential security concerns.
least privilege
An authorization policy that gives a user only the amount of access that is essential to that user’s work and no more.
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.
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: To use a LineString in MongoDB, see GeoJSON Objects.
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?.
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.
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.
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.
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.
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.
Management Information Base. MongoDB uses MIB files to define the type of data tracked by SNMP in the MongoDB Enterprise edition.
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.
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.
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.
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.
MongoDB Enterprise
A commercial edition of MongoDB that includes additional features. For more information, see MongoDB Subscriptions.
The routing and load balancing process that acts an interface between an application and a MongoDB sharded cluster. See mongos.
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 Namespaces.
natural order
The order in which the database refers to documents on disk. This is the default sort order. 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 of a partial partition would be 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 vice versa, 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 to the other side only if they originate the communications channel.

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.
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.
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.
orphaned document
In a sharded cluster, orphaned documents are those documents on a shard that also exist in chunks on other shards as a result of failed migrations or incomplete migration cleanup due to abnormal shutdown. Delete orphaned documents using cleanupOrphaned to reclaim disk space and reduce confusion.
The extra space allocated to document on the disk to prevent moving a document when it grows as the result of update() operations. See Record Allocation Strategies.
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 Record Allocation Strategies.
page fault

With the MMAPv1 storage engine, page faults can occur as MongoDB reads from or writes data to parts of its data files that are not currently located in physical memory. In contrast, operating system page faults happen when physical memory is exhausted and pages of physical memory are swapped to disk.

See Page Faults and What are page faults?.

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.
A packet-capture format used by mongosniff to record packets captured from network interfaces and display them as human-readable MongoDB operations. See Options.
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.
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.
A series of operations in an aggregation process. See Aggregation Pipeline.
A single coordinate pair as described in the GeoJSON Point specification: To use a Point in MongoDB, see GeoJSON Objects.

An array of LinearRing coordinate arrays, as described in the GeoJSON Polygon specification: 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.

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.
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 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.
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.
A configurable value that helps determine which members in a replica set are most likely to become primary. See members[n].priority.
A combination of specified resource and actions permitted on the resource. See privilege.
A document given to a query that specifies which fields MongoDB returns in the result set. See Project Fields to Return from Query. For a list of projection operators, see Projection Operators.
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 db.collection.find() and db.collection.findOne() methods. See Query Documents.
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.
query shape

A combination of query predicate, sort, and projection specifications.

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

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.
record size
The space allocated for a document including the padding. For more information on padding, see Record Allocation Strategies and compact.
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.
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.
A database, collection, set of collections, or cluster. A privilege permits actions on a specified resource. See resource.

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 API.

Deprecated since version 3.2: HTTP interface for MongoDB

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.
A process that reverts writes operations to ensure the consistency of all replica set members. See Rollbacks During Replica Set Failover.
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 replSetName setting or the --replSet option.
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 config servers, shards, and one or more mongos routing processes. See Sharded Cluster Components.
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.
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.
A compression/decompression library designed 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.
The division between chunks in a sharded cluster. See Data Partitioning with Chunks.
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.
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.
Refers to the amount of time a secondary member of a replica set trails behind the current state of the primary’soplog. If a secondary becomes too stale, it can no longer use replication to catch up to the current state of the primary. See Replica Set Oplog and Replica Set Data Synchronization for more information.
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.
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 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 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.
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.
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.
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 Tag Aware Sharding.
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.
The state of a deployment of MongoDB instances, including the type of deployment (i.e. standalone, replica set, or sharded cluster) as well as the availability of servers, and the role of each server (i.e. primary, secondary, config server, or mongos.)
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.
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.
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.
An option for update operations; e.g. update(), findAndModify(). If set to true, the update operation will either update the document(s) matched by the specified query or if no documents match, insert a new document. The new document will have the fields indicated in the operation. See Upsert Option.
virtual memory
An application’s working memory, typically residing on both disk and in physical RAM.
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:
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 conflict
A situation in which two concurrent operations, at least one of which is a write, attempt to use a resource in a way that would violate constraints imposed by a storage engine using optimistic concurrency control. MongoDB will transparently abort and retry one of the conflicting 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.
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.
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 and the WiredTiger compression documentation for more information.
[1]In some circumstances, two nodes in a replica set may transiently believe that they are the primary, but at most, one of them will be able to complete writes with { w: "majority" } write concern. The node that can complete { w: "majority" } writes is the current primary, and the other node is a former primary that has not yet recognized its demotion, typically due to a network partition. When this occurs, clients that connect to the former primary may observe stale data despite having requested read preference primary, and new writes to the former primary will eventually roll back.

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