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Release Notes for MongoDB 4.2

MongoDB 4.2 Released Aug 13, 2019

Minor Releases

4.2.1 - Oct 18, 2019

Issues fixed:

Distributed Transactions

Distributed Transactions and Multi-Document Transactions

Starting in MongoDB 4.2, the two terms are synonymous. Distributed transactions refer to multi-document transactions on sharded clusters and replica sets. Multi-document transactions (whether on sharded clusters or replica sets) are also known as distributed transactions starting in MongoDB 4.2.

In version 4.2, MongoDB introduces distributed transactions. Distributed transactions:

  • Adds support for multi-document transactions on sharded clusters.
    • All members of the 4.2 sharded clusters must have featureCompatibilityVersion of 4.2.
    • Clients must use MongoDB drivers updated for MongoDB 4.2
  • Incorporates the existing support for transactions on replica sets.
    • All members of the 4.2 replica set must have featureCompatibilityVersion of 4.2.
    • Clients must use MongoDB drivers updated for MongoDB 4.2
  • Removes the 16MB total size limit for a transaction. In version 4.2, MongoDB creates as many oplog entries (maximum size 16MB each) as necessary to the encapsulate all write operations in a transaction. In MongoDB 4.0, MongoDB creates a single entry for all write operations in a transaction, thereby imposing a 16MB total size limit for a transaction.
  • Extends transaction support to deployments whose secondary members use the in-memory storage engine. That is, transactions are available for deployments that use the WiredTiger storage engine for the primary and either the WiredTiger or the in-memory storage engine for the secondary members. In MongoDB 4.0, transactions are available for deployments that use the WiredTiger storage engine only.

For more information, see Transactions.

Removed MMAPv1 Storage Engine

MongoDB 4.2 removes the deprecated MMAPv1 storage engine.

If your 4.0 deployment uses MMAPv1, you must change the deployment to WiredTiger Storage Engine before upgrading to MongoDB 4.2. For details, see:

MMAPv1 Specific Configuration Options

MongoDB removes the following MMAPv1 specific configuration options:

Removed Configuration File Setting Removed Command-line Option
storage.mmapv1.journal.commitIntervalMs  
storage.mmapv1.journal.debugFlags mongod --journalOptions
storage.mmapv1.nsSize mongod --nssize
storage.mmapv1.preallocDataFiles mongod --noprealloc
storage.mmapv1.quota.enforced mongod --quota
storage.mmapv1.quota.maxFilesPerDB mongod --quotaFiles
storage.mmapv1.smallFiles mongod --smallfiles
storage.repairPath mongod --repairpath
replication.secondaryIndexPrefetch mongod --replIndexPrefetch

Note

Starting in version 4.2, MongoDB processes will not start with these options. Remove any MMAPv1 specific configuration options if using a WiredTiger deployment.

MMAPv1 Specific Parameters

MongoDB removes the following MMAPv1 parameters:

  • newCollectionsUsePowerOf2Sizes
  • replIndexPrefetch

MMAPv1 Specific Command

MongoDB removes the MMAPv1 specific touch command.

MMAPv1 Specific Options for Binaries, Commands and Methods

MongoDB removes the MMAPv1 specific options:

Removed Commands and Methods

Removed Command Removed Method Notes
group db.collection.group() Use db.collection.aggregate() with the $group stage instead.
eval   The MongoDB 4.2 mongo shell methods db.eval() and db.collection.copyTo() can only be run when connected to MongoDB 4.0 or earlier.
copydb  

The corresponding mongo shell helpers db.copyDatabase() can only be run when connected to MongoDB 4.0 or earlier.

As an alternative, users can use mongodump and mongorestore (see Copy/Clone a Database) or write a script using the drivers.

clone  

The corresponding mongo shell helpers db.cloneDatabase() can only be run when connected to MongoDB 4.0 or earlier.

As an alternative, users can use mongodump and mongorestore (see Copy/Clone a Database) or write a script using the drivers.

geoNear  

Use db.collection.aggregate() with the $geoNear stage instead.

For more information, see Remove Support for the geoNear Command.

parallelCollectionScan    
repairDatabase db.repairDatabase() For more information, see Remove Support for the repairDatabase Command.

Remove maxScan Option

MongoDB removes the deprecated option maxScan for the find command and the mongo shell helper cursor.maxScan(). Use either the maxTimeMS option for the find command or the helper cursor.maxTimeMS() instead.

MongoDB Drivers

The following drivers are feature compatible [1] with MongoDB 4.2:

[1]Field Level Encryption is available as a beta feature. Currently, the 4.2-compatible Java, Node and Python drivers have support for the beta feature.

Sharded Clusters

Mutable Shard Key Values

Starting in MongoDB 4.2, you can update a document’s shard key value unless the shard key field is the immutable _id field. For details on updating the shard key, see Change a Document’s Shard Key Value.

Before MongoDB 4.2, a document’s shard key field value is immutable.

Backups

mongodump and mongorestore cannot be part of a backup strategy for 4.2+ sharded clusters that have sharded transactions in progress as these tools cannot guarantee a atomicity guarantees of data across the shards.

For 4.2+ sharded clusters with in-progress sharded transactions, for coordinated backup and restore processes that maintain the atomicity guarantees of transactions across shards, see:

Balancer State and Autosplit

Starting in MongoDB 4.2:

The mongo methods sh.enableBalancing(namespace) and sh.disableBalancing(namespace) have no affect on the auto-splitting.

mongos Connection Pool

Starting in MongoDB 4.2, MongoDB adds the parameter ShardingTaskExecutorPoolReplicaSetMatching that determines the minimum size (can vary during runtime) of the mongos instance’s connection pools to each member of the sharded cluster.

By default, for each replica set in the sharded cluster (i.e. shard replica set and config servers), mongos maintains connection pools to each replica set secondary that are at least equal to the size of its connection pool to the primary.

To modify, see ShardingTaskExecutorPoolReplicaSetMatching.

Sharded Collections and Replace Documents

Starting in MongoDB 4.2,

  • Replace document operations, such as db.collection.replaceOne() or db.collection.update( query, replacement, … ), attempt to target a single shard, first by using the query filter. If the operation cannot target a single shard by the query filter, it then attempts to target by the replacement document.

    In earlier versions, the operation attempts to target using the replacement document.

  • For a replace document operation that includes upsert: true and is on a sharded collection, the filter must include an equality match on the full shard key.

Security Improvements

Resolved Common Vulnerabilities and Exposures

MongoDB 4.2 includes fixes that resolve the following Common Vulnerabilities and Exposures (CVEs):

New TLS Options

MongoDB 4.2 adds TLS options for the mongod, the mongos, and the mongo shell to replace the corresponding SSL options (deprecated in 4.2). The new TLS options provide identical functionality as the deprecated SSL options as MongoDB has always supported TLS 1.0 and later.

Deprecated SSL Options

MongoDB 4.2 deprecates the SSL options for the mongod, the mongos, and the mongo shell as well as the corresponding net.ssl Options configuration file options.

Use the new TLS options instead.

New tls Parameters

New Parameter Description
tlsWithholdClientCertificate Available for mongod and mongos, the parameter can be set to true to stop the instance from sending its TLS certificate when initiating intra-cluster communications with other mongod or mongos instances. For details, see tlsWithholdClientCertificate.
tlsX509ClusterAuthDNOverride

Available for mongod and mongos, the parameter can be set to an alternative certificate DN to use for x.509 membership authentication. For details, see tlsX509ClusterAuthDNOverride.

You can use this parameter for a rolling update of certificates to new certificates that contain a new DN value. See Rolling Update of x.509 Cluster Certificates that Contain New DN.

New tlsClusterCAFile Option

MongoDB 4.2 adds the --tlsClusterCAFile option/net.tls.clusterCAFile for mongod and mongos, which specifies a .pem file for validating the TLS certificate from a client establishing a connection. This lets you use separate Certificate Authorities to verify the client to server and server to client portions of the TLS handshake.

See also

New TLS Options

Forward Secrecy

Starting in version 4.2 on Linux:

In earlier versions of MongoDB (3.6.14+ and 4.0.3+), MongoDB enables support for Ephemeral Elliptic Curve Diffie-Hellman (ECDHE) if, during compile time, the Linux platform’s OpenSSL supports automatic curve selection of ECDH parameters.

On Windows and macOS, MongoDB’s support for ECDHE and DH remain unchanged from earlier versions; that is, support is implicit through the use of the platform’s respective native TLS/SSL OS libraries.

For more information, see Forward Secrecy.

passwordPrompt()

Starting in version 4.2 of the mongo shell, you can use the passwordPrompt() method in conjunction with various user authentication/management methods/commands to prompt for the password instead of specifying the password directly in the method/command call. However, you can still specify the password directly as you would with earlier versions of the mongo shell.

For example:

db.createUser( {
   user:"user123",
   pwd: passwordPrompt(),   // Instead of specifying the password in cleartext
   roles:[ "readWrite" ]
} )

Keyfile Format Change to YAML

Starting in MongoDB 4.2, keyfiles for internal membership authentication use YAML format to allow for multiple keys in a keyfile. The YAML format accepts content of:

  • a single key string (same as in earlier versions),
  • multiple key strings (each string must be enclosed in quotes), or
  • sequence of key strings.

The YAML format is compatible with the existing single-key keyfiles that use the text file format.

The new format allows for rolling upgrade of the keys without downtime. See Rotate Keys for Replica Sets and Rotate Keys for Sharded Clusters.

libldap and libldap_r

For MongoDB 4.2 (and 4.0.9) Enterprise binaries linked against libldap (such as when running on RHEL), access to the libldap is synchronized, incurring some performance/latency costs.

For MongoDB 4.2 (and 4.0.9) Enterprise binaries linked against libldap_r, there is no change in behavior from earlier MongoDB versions.

To avoid the automatic synchronization with libldap, you may wish to link to libldap_r. Contact support for assistance.

Encrypted Storage Engine

For encrypted storage engine configured with AES256-GCM cipher:

  • Restoring from Hot Backup

    Starting in 4.2, if you restore from files taken via “hot” backup (i.e. the mongod is running), MongoDB can detect “dirty” keys on startup and automatically rollover the database key to avoid IV (Initialization Vector) reuse.

  • Restoring from Cold Backup

    However, if you restore from files taken via “cold” backup (i.e. the mongod is not running), MongoDB cannot detect “dirty” keys on startup, and reuse of IV voids confidentiality and integrity guarantees.

    Starting in 4.2, to avoid the reuse of the keys after restoring from a cold filesystem snapshot, MongoDB adds a new command-line option --eseDatabaseKeyRollover. When started with the --eseDatabaseKeyRollover option, the mongod instance rolls over the database keys configured with AES256-GCM cipher and exits.

For more information, see encrypted storage engine and --eseDatabaseKeyRollover.

General Security Enhancements

  • Add serverStatus to the backup built-in role.

  • To connect a client over TLS/SSL connection, MongoDB 4.2 supports matching by IP addresses as well as DNS for Subject Alternative Name (SAN) matching.

    For example, a mongod instance’s x.509 certificate has the following SAN:

    X509v3 Subject Alternative Name:
        DNS:hostname.example.com, DNS:localhost, IP Address:127.0.0.1
    

    Then, to connect a mongo shell to the instance, you can specify the host of 127.0.0.1 or the DNS names:

    • mongo "mongodb:\\127.0.0.1:27017\test" --tls --tlsCAFile /etc/ssl/ca.pem ...
      
    • mongo "mongodb:\\hostname.example.com:27017\test" --tls --tlsCAFile /etc/ssl/ca.pem ...
      
    • mongo "mongodb:\\localhost:27017\test" --tls --tlsCAFile /etc/ssl/ca.pem ...
      

    In previous versions, MongoDB only supported DNS entries for SAN matching.

    • mongo "mongodb:\\hostname.example.com:27017\test" --tls --tlsCAFile /etc/ssl/ca.pem ...
      
    • mongo "mongodb:\\localhost:27017\test" --tls  --tlsCAFile /etc/ssl/ca.pem ...
      

LDAP Query Template {PROVIDED_USER} Token

Starting in version 4.2, MongoDB Enterprise adds a new token {PROVIDED_USER} that can be used in security.ldap.authz.queryTemplate. When used in the template, MongoDB substitutes the supplied username, i.e. before either authentication or LDAP transformation.

Aggregation Improvements

On-Demand Materialized View ($merge Stage)

MongoDB 4.2 adds the $merge aggregation stage.

With the new stage you can:

  • Can output to a collection in the same or different database.
  • Can incorporate results (merge documents, replace documents, keep existing documents, fail the operation, process documents with an custom update pipeline) into an existing collection.
  • Can output to an existing sharded collection.

The new stage allows users to create on-demand materialized views, where the content of the output collection can be incrementally updated each time the pipeline is run.

Aggregation Trigonometry Expressions

MongoDB 4.2 adds new trigonometry expressions for use in aggregation pipelines.

Trigonometry expressions perform trigonometric operations on numbers. Values that represent angles are always input or output in radians. Use $degreesToRadians and $radiansToDegrees to convert between degree and radian measurements.

Name Description
$sin Returns the sine of a value that is measured in radians.
$cos Returns the cosine of a value that is measured in radians.
$tan Returns the tangent of a value that is measured in radians.
$asin Returns the inverse sin (arc sine) of a value in radians.
$acos Returns the inverse cosine (arc cosine) of a value in radians.
$atan Returns the inverse tangent (arc tangent) of a value in radians.
$atan2 Returns the inverse tangent (arc tangent) of y / x in radians, where y and x are the first and second values passed to the expression respectively.
$asinh Returns the inverse hyperbolic sine (hyperbolic arc sine) of a value in radians.
$acosh Returns the inverse hyperbolic cosine (hyperbolic arc cosine) of a value in radians.
$atanh Returns the inverse hyperbolic tangent (hyperbolic arc tangent) of a value in radians.
$degreesToRadians Converts a value from degrees to radians.
$radiansToDegrees Converts a value from radians to degrees.

Aggregation Arithmetic Expressions

MongoDB 4.2 adds the $round aggregation expression. Use $round to round numerical values to a specific digit or decimal place.

MongoDB 4.2 adds expanded functionality and new syntax to $trunc. Use $trunc with the new syntax to truncate numerical values to a specific digit or decimal place.

Aggregation Regular Expressions (regex) Operators

MongoDB 4.2 adds the following regular expression (regex) pattern matching operators for use in the aggregation pipeline:

Operator Description
$regexFind Applies a regular expression (regex) to a string and returns information on the first matched substring.
$regexFindAll Applies a regular expression (regex) to a string and returns information on all matched substrings.
$regexMatch Applies a regular expression (regex) to a string and returns true if a match is found and false if a match is not found.

Prior to MongoDB 4.2, aggregation pipeline can only use the query operator $regex in the $match stage.

New Stages

MongoDB 4.2 adds the following new aggregation pipeline stages:

New Stage Description
$merge Writes the aggregation results to a collection. The $merge stage can incorporate results (merge documents, replace documents, keep existing documents, fail the operation, process documents with an custom update pipeline) into an existing collection.
$planCacheStats

Provides plan cache information for a collection.

The $planCacheStats aggregation stage is preferred over the following methods and commands, which have been deprecated in 4.2:

$replaceWith Replaces the input document with the specified document. The operation replaces all existing fields in the input document, including the _id field. The new $replaceWith stage is an alias to the $replaceRoot stage.
$set Adds new fields to documents. The stage outputs documents that contains all existing fields from the input documents as well as the newly added fields. The new $set stage is an alias to the $addFields stage.
$unset Excludes fields from documents. The new $unset stage is an alias to the $project stage that excludes fields.

New Variables

MongoDB 4.2 adds the following new aggregation pipeline variables:

Variable Description
NOW Returns the current datetime value.
CLUSTER_TIME

Returns the current timestamp value.

Only available on replica sets and sharded clusters.

Availability

Starting in MongoDB 4.2, you can use the aggregation pipeline for updates in:

Command mongo Shell Methods
findAndModify

db.collection.findAndModify()

db.collection.findOneAndUpdate()

update

db.collection.updateOne()

db.collection.updateMany()

db.collection.update()

For the updates, the pipeline can consist of the following stages:

Using the aggregation pipeline allows for a more expressive update statement, such as expressing conditional updates based on current field values or updating one field using the value of another field(s).

See the individual reference pages for details and examples.

See also

Aggregation

Change Stream

startAfter Option for Change Streams

MongoDB 4.2 adds startAfter as an option for Change Streams, which starts a new change stream after the event indicated by a resume token. With this option, you can start a change stream from an invalidate event, thereby guaranteeing no missed notifications after the previous stream was invalidated.

Change Streams Resume Tokens

MongoDB 4.2 uses the version 1 (i.e. v1) change streams resume tokens, introduced in version 4.0.7.

Starting in MongoDB 4.2, change streams will throw an exception if the change stream aggregation pipeline modifies an event’s _id field.

Availability

Starting in MongoDB 4.2, change streams are available regardless of the "majority" read concern support; that is, read concern majority support can be either enabled (default) or disabled to use change streams.

In MongoDB 4.0 and earlier, change streams are available only if "majority" read concern support is enabled (default).

Change Stream Pipeline

Starting in MongoDB 4.2, you can use additional stages in the change stream aggregation pipeline to modify the change stream output (i.e. the event documents):

Starting in MongoDB 4.2, change streams will throw an exception if the change stream aggregation pipeline modifies an event’s _id field.

Update Enhancements

Update and Aggregation

Starting in MongoDB 4.2, you can use the aggregation pipeline for updates in:

Command mongo Shell Methods
findAndModify

db.collection.findAndModify()

db.collection.findOneAndUpdate()

update

db.collection.updateOne()

db.collection.updateMany()

db.collection.update()

For the updates, the pipeline can consist of the following stages:

Using the aggregation pipeline allows for a more expressive update statement, such as expressing conditional updates based on current field values or updating one field using the value of another field(s).

See the individual reference pages for details and examples.

Update and Hint

Starting in MongoDB 4.2, the update command and the associated mongo shell method db.collection.update() can accept a hint argument to specify the index to use. See:

Sharded Collections and Replace Documents

Starting in MongoDB 4.2,

  • Replace document operations, such as db.collection.replaceOne() or db.collection.update( query, replacement, … ), attempt to target a single shard, first by using the query filter. If the operation cannot target a single shard by the query filter, it then attempts to target by the replacement document.

    In earlier versions, the operation attempts to target using the replacement document.

  • For a replace document operation that includes upsert: true and is on a sharded collection, the filter must include an equality match on the full shard key.

Wildcard Indexes

MongoDB 4.2 introduces wildcard indexes for supporting queries against fields whose names are unknown or arbitrary.

Consider an application that captures user-defined data under the userMetadata field and supports querying against that data:

{ "userMetadata" : { "likes" : [ "dogs", "cats" ] } }
{ "userMetadata" : { "dislikes" : "pickles" } }
{ "userMetadata" : { "age" : 45 } }
{ "userMetadata" : "inactive" }

Administrators want to create indexes to support queries on any subfield of userMetadata.

A wildcard index on userMetadata can support single-field queries on userMetadata, userMetadata.likes, userMetadata.dislikes, and userMetadata.age:

db.userData.createIndex( { "userMetadata.$**" : 1 } )

The index can support the following queries:

db.userData.find({ "userMetadata.likes" : "dogs" })
db.userData.find({ "userMetadata.dislikes" : "pickles" })
db.userData.find({ "userMetadata.age" : { $gt : 30 } })
db.userData.find({ "userMetadata" : "inactive" })

A non-wildcard index on userMetadata can only support queries on values of userMetadata.

Important

Wildcard indexes are not designed to replace workload-based index planning. For more information on creating indexes to support queries, see Create Indexes to Support Your Queries. For complete documentation on wildcard index limitations, see Wildcard Index Restrictions.

The mongod featureCompatibilityVersion must be 4.2 to create wildcard indexes. For instructions on setting the fCV, see Set Feature Compatibility Version on MongoDB 4.2 Deployments.

You can create a wildcard index using the createIndexes database command or its shell helpers db.collection.createIndex() and db.collection.createIndexes(). For examples of creating a wildcard index, see Create a Wildcard Index.

See Wildcard Indexes for complete documentation.

Platform Support

  • MongoDB 4.2 adds support for:
    • Ubuntu 18.04 on ARM64
  • MongoDB 4.2 removes support for:
    • Debian 8
    • Ubuntu 14.04
    • Ubuntu 16.04 ARM64 for MongoDB Community Edition
    • Ubuntu 16.04 POWER/PPC64LE (Also removed in version 3.6.13 and 3.4.21)
    • macOS 10.11

See Supported Platforms.

MongoDB Tools

FIPS Mode

Starting in version 4.2, MongoDB removes the --sslFIPSMode option for the following programs:

The programs will use FIPS compliant connections to mongod/mongos if the mongod/mongos instances are configured to use FIPS mode.

--uri Option

Starting in version 4.2,

  • For the following command-line tools, if the write concern is specified in both the --uri connection string and the --writeConcern option, the --writeConcern option overrides the one in the connection string:
  • For the following command-line tools, if the read preference is specified in both the --uri connection string and the --readPreference option, the --readPreference option overrides the one in the connection string:

Extended JSON v2

Starting in version 4.2:

bsondump Uses Extended JSON v2.0 (Canonical mode) format.
mongodump

Use Extended JSON v2.0 (Canonical mode) format for the metadata. Requires mongorestore version 4.2+ that supports Extended JSON v2.0 (Canonical mode or Relaxed) format.

Tip

In general, use corresponding versions of mongodump and mongorestore. That is, to restore data files created with a specific version of mongodump, use the corresponding version of mongorestore.

mongoexport
Creates output data in Extended JSON v2.0 (Relaxed mode) by default.
Creates output data in Extended JSON v2.0 (Canonical mode) if used with --jsonFormat canonical.
mongoimport
Expects import data to be in Extended JSON v2.0 (either Relaxed or Canonical mode) by default.
Can recognize data that is in Extended JSON v1.0 format if the option --legacy is specified.

Tip

In general, the versions of mongoexport and mongoimport should match. That is, to import data created from mongoexport, you should use the corresponding version of mongoimport.

For details on MongoDB extended JSON v2, see MongoDB Extended JSON (v2).

See also

--query Options

mongofiles

The mongofiles command get_id and delete_id can accept both ObjectId or non-ObjectId values for the _id.

Bulk Operations for mongoimport and mongorestore

mongoimport

Starting in version 4.2:

  • mongoimport uses maximum batch size of 100,000 to perform bulk insert/upsert operations.
  • mongoimport by default, continues when it encounters duplicate key and document validation errors. To ensure that the program stops on these errors, specify --stopOnError.
  • Specifying --maintainInsertionOrder for mongoimport:
    • Maintains document insertion order using ordered bulk write operations; i.e. both the batch order and document order within the batches are maintained. In earlier versions, only the batch order is maintained; document order within batches are not maintained.
    • Enables --stopOnError and sets numInsertionWorkers to 1.
mongorestore

Starting in version 4.2:

  • mongorestore by default, continues when it encounters duplicate key and document validation errors. To ensure that the program stops on these errors, specify --stopOnError.
  • Specifying --maintainInsertionOrder for mongorestore:
    • Maintains document insertion order using ordered bulk write operations; i.e. both the batch order and document order within the batches are maintained. In earlier versions, only the batch order is maintained; document order within batches are not maintained.
    • Enables --stopOnError and sets --numInsertionWorkersPerCollection to 1.

Lock Optimization for Specific DDL Operations

Starting with MongoDB 4.2, the following operations take an exclusive collection lock instead of an exclusive database lock:

Commands Methods
create

db.createCollection()

db.createView()

createIndexes

db.collection.createIndex()

db.collection.createIndexes()

drop db.collection.drop()
dropIndexes

db.collection.dropIndex()

db.collection.dropIndexes()

renameCollection db.collection.renameCollection()

Prior to MongoDB 4.2, these operations took an exclusive lock on the database, blocking all operations on the database and its collections until the operation completed.

In earlier versions, get_id and delete_id can only accept ObjectId values for the _id.

Monitoring

Starting in version 4.2, the Storage Node Watchdog is available in both the MongoDB Community edition and the MongoDB Enterprise edition.

In earlier versions, the feature is available in the MongoDB Enterprise edition only.

Flow Control

MongoDB 4.2 introduces a flow control mechanism to control the rate at which the primary applies its writes in order to keep the majority committed lag under a specified maximum value.

Flow control is enabled by default.

Note

For flow control to engage, the replica set/sharded cluster must have: featureCompatibilityVersion (FCV) of 4.2 and read concern majority enabled. That is, enabled flow control has no effect if FCV is not 4.2 or if read concern majority is disabled.

For more information, see Replication Lag and Flow Control.

Logging and Diagnostics

Logging

  • Added INITSYNC component to log messages.

  • Added ELECTION component to log messages.

  • For debug messages, include the verbosity level (i.e. D[1-5]). For example, if verbosity level is 2, MongoDB logs D2. In previous versions, MongoDB log messages only specified D for Debug level.

  • When logging to syslog, the format of the message text includes the component. For example:

    ...  ACCESS   [repl writer worker 5] Unsupported modification to roles collection ...
    

    Previously, the syslog message text did not include the component. For example:

    ... [repl writer worker 1] Unsupported modification to roles collection ...
    
  • MongoDB 4.2 adds a usedDisk indicator to the profiler log messages and diagnostic log messages for the aggregate operation. The usedDisk indicates whether any stages of an aggregate operation wrote data to temporary files due to memory restrictions. For more information on aggregation memory restrictions, see Memory Restrictions.

  • Starting in version 4.2 (also available starting in 4.0.6), secondary members of a replica set now log oplog entries that take longer than the slow operation threshold to apply. These messages are logged for the secondaries under the REPL component with the text applied op: <oplog entry> took <num>ms.

    2018-11-16T12:31:35.886-0500 I REPL   [repl writer worker 13] applied op: command { ... }, took 112ms
    

    The slow oplog application logging on secondaries are:

    For more information on setting the slow operation threshold, see

  • Starting in MongoDB 4.2, the getLog command truncates any event that contains more than 1024 characters. In earlier versions, getLog truncates after 512 characters.

  • Starting in MongoDB 4.2 (and in 4.0.9), for slow operations, the profiler entries and diagnostic log messages include storage information.

  • Starting in MongoDB 4.2, the profiler entries and the diagnostic log messages (i.e. mongod/mongos log messages) for read/write operations include:

    • queryHash to help identify slow queries with the same query shape.
    • planCacheKey to provide more insight into the query plan cache for slow queries.

    See Query Plan Improvements.

currentOp

MongoDB 4.2 adds a new option idleCursors to the $currentOp aggregation stage in order to return information on idle cursors.

In addition, MongoDB 4.2 adds the following new fields to the documents returned from the $currentOp aggregation stage, currentOp command, and db.currentOp() helper:

$currentOp currentOp/db.currentOp() Description
$currentOp.type currentOp.type Specifies whether the reported operation is an op, idleSession, or idleCursor.
$currentOp.cursor currentOp.cursor Specifies cursor details. Available when returning getmore operations or idleCursor information.
$currentOp.effectiveUsers currentOp.effectiveUsers Specifies users associated with the operation.
$currentOp.prepareReadConflicts currentOp.prepareReadConflicts Specifies the number of times the current operation had to wait for a prepared transaction with a write to commit or abort.
$currentOp.runBy currentOp.runBy Specifies users that are impersonating the effective users for the operation.
$currentOp.writeConflicts currentOp.writeConflicts Specifies the number of times the current operation conflicted with another write operation.

See also 4.2 current op compatibility changes

Replica Set Status Metrics

Starting in version MongoDB 4.2, replSetGetStatus and its mongo shell helper rs.status() return:

MongoDB 4.2 deprecates the field lastStableCheckpointTimestamp.

Lock Diagnostics Reporting

Starting in version 4.2, MongoDB reports on ReplicationStateTransition lock information.

In addition, MongoDB 4.2 separates ParallelBatchWriterMode lock information from Global lock information. Earlier MongoDB versions report ParallelBatchWriterMode lock information as part of Global locks.

For operations that report on lock information, see:

collStats Improvements

Starting in MongoDB 4.2, the $collStats aggregation, the collStats command, and the mongo shell helper db.collection.stats return information on indexes that are currently being built.

For details, see:

Starting in MongoDB 4.2, the $collStats aggregation, the collStats command, and the mongo shell helper db.collection.stats return the scaleFactor used to scale the various size data.

dbStats Improvements

Starting in MongoDB 4.2, the dbStats command, and the mongo shell helper db.stats() return the scaleFactor used to scale the various size data.

General Improvements

Externally Sourced Values for Configuration Files

MongoDB supports using expansion directives in configuration files to load externally sourced values. Expansion directives can load values for specific configuration file options or load the entire configuration file.

The following expansion directives are available:

Expansion Directive Description
__rest Allows users to specify a REST endpoint as the external source for configuration file options or the full configuration file.
__exec Allows users to specify a shell or terminal command as the external source for configuration file options or the full configuration file.

For complete documentation, see Externally Sourced Configuration File Values.

outputConfig Option

MongoDB 4.2 adds the --outputConfig option for mongod and mongos. The option outputs to stdout the mongod/mongos instance’s configuration, in YAML format.

If the configuration uses any Externally Sourced Configuration File Values, the option returns the resolved value for those options.

Warning

This may include any configured passwords or secrets previously obfuscated through the external source.

For usage examples, see:

Remove Index Key Size Limit

Starting in MongoDB 4.2, for featureCompatibilityVersion set to "4.2" or greater, MongoDB removes the Index Key Limit. For fCV set to "4.0", the limit still applies.

Remove Index Name Length Limit

Starting in version 4.2, for featureCompatibilityVersion set to "4.2" or greater, MongoDB removes the Index Name Length limit of 127 byte maximum. In previous versions or MongoDB versions with featureCompatibilityVersion (fCV) set to "4.0", index names must fall within the limit.

Improvements to dropIndexes

Drop multiple indexes

Starting in MongoDB 4.2, you can specify multiple indexes to the dropIndexes command and its mongo shell helper db.collection.dropIndexes(). To specify multiple indexes to drop, pass an array of index names to dropIndexes/db.collection.dropIndexes().

zstd Availability

Starting in version 4.2, MongoDB supports zstd for:

bulkWrite() Error Handling inside Transactions

Starting in MongoDB 4.2, if a db.collection.bulkWrite() operation encounters an error inside a transaction, the method throws a BulkWriteException (same as outside a transaction).

In 4.0, if the bulkWrite operation encounters an error inside a transaction, the error thrown is not wrapped as a BulkWriteException.

Inside a transaction, the first error in a bulk write causes the entire bulk write to fail and aborts the transaction, even if the bulk write is unordered.

Query Plan Improvements

Plan Cache States

Starting in MongoDB 4.2, the cache entry is associated with a state:

Associating states with entries helps reduce the likelihood that sub-optimal cache entries remain in the cache. For more information, see Query Plans.

queryHash and planCacheKey

  • queryHash

    To help identify slow queries with the same query shape, starting in MongoDB 4.2, each query shape is associated with a queryHash. The queryHash is a hexadecimal string that represents a hash of the query shape and is dependent only on the query shape.

    Note

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

  • planCacheKey

    To provide more insight into the query plan cache, MongoDB 4.2 introduces the planCacheKey.

    planCacheKey is a hash of the key for the plan cache entry associated with the query.

    Note

    Unlike the queryHash, the planCacheKey is a function of both the query shape and the currently available indexes for the shape. That is, if indexes that can support the query shape are added/dropped, the planCacheKey value may change whereas the queryHash value would not change.

    See also

    planCacheKey

  • The queryHash and planCacheKey are available in:

The fields are also available in operations that return information about the query plan cache:

$regex and $not

Starting in MongoDB 4.2 (and 4.0.7), $not operator can perform logical NOT operation on $regex operator expressions as well as on regular expression objects (i.e. /pattern/).

In 4.0 and earlier versions, you could use $not operator with regular expression objects (i.e. /pattern/) but not with $regex operator expressions.

Kill Own Cursors

Starting in MongoDB 4.2, users can always kill their own cursors, regardless of whether the users have the privilege to killCursors. As such, the killCursors privilege has no effect starting in MongoDB 4.2.

In MongoDB 4.0, users required the killCursors privilege in order to kill their own cursors.

New Parameters

MongoDB 4.2 adds the parameter replBatchLimitBytes to configure the maximum oplog application batch size. The parameter is also available starting in MongoDB 4.0.10.

Retryable Writes on Certain Single-Document Upserts

MongoDB 4.2 will retry certain single-document upserts (update with upsert: true and multi: false) that encounter a duplicate key exception. See Duplicate Key Errors on Upsert for conditions.

Prior to MongoDB 4.2, MongoDB would not retry upsert operations that encountered a duplicate key error.

db.dropDatabase() and Write Concern

Starting in MongODB 4.2, the mongo shell method db.dropDatabase() can take an optional write concern document.

dropConnections

The dropConnections command drops the mongod/mongos instance’s outgoing connections to the specified hosts. The dropConnections must be run against the admin database.

Client Disconnection

For the following operations, if the issuing client disconnects before the operation completes, MongoDB marks the following operations for termination (e.g. killOp on the operation):

Command mongo Shell Method Notes
aggregate db.collection.aggregate() Behavior only applies if the pipeline does not include $out and $merge
authenticate db.auth  
count  
distinct db.collection.distinct  
find  
getnonce    
isMaster    
listCollections  
listDatabases    
listIndexes db.collection.getIndexes  

Startup Warnings

In-Memory Storage Engines

Starting in version 4.2 (and 4.0.13 and 3.6.14 ), if a replica set member uses the in-memory storage engine (voting or non-voting) but the replica set has writeConcernMajorityJournalDefault set to true, the replica set member logs a startup warning.

mongo Shell

Starting in MongoDB 4.2 (and 4.0.13), the mongo shell displays a warning message when connected to non-genuine MongoDB instances as these instances may behave differently from the official MongoDB instances; e.g. missing or incomplete features, different feature behaviors, etc.

Map-Reduce

Starting in version 4.2, MongoDB deprecates:

  • The map-reduce option to create a new sharded collection as well as the use of the sharded option for map-reduce. To output to a sharded collection, create the sharded collection first. MongoDB 4.2 also deprecates the replacement of an existing sharded collection.
  • The explicit specification of nonAtomic: false option.

Rollback Time Limit

Starting in MongoDB 4.2, the rollback time limit is calculated between the first operation after the common point and the last point in the oplog for the member to roll back.

In MongoDB 4.0, the rollback time limit is calculated between the common point and the last point in the oplog for the member to roll back.

For more information, see Rollback Elapsed Time Limitations.

isInteractive()

MongoDB 4.2 adds a new mongo shell method isInteractive() that returns a boolean indicating whether the mongo shell is running in interactive or script mode.

Change to explain Output

Starting in MongoDB 4.2, the explain output can include a new optimizedPipeline field. For details, refer to optimizedPipeline.

Change to isMaster Output

Starting in MongoDB 4.2, the output for isMaster, and the db.isMaster() helper method, returns the connectionId for the mongod/mongos instance’s connection to the client.

Optimized Index Builds

MongoDB index builds against a populated collection require an exclusive read-write lock against the collection. Operations that require a read or write lock on the collection must wait until the mongod releases the lock. MongoDB uses an optimized build process that only holds the exclusive lock at the beginning and end of the index build. The rest of the build process yields to interleaving read and write operations.

For feature compatibility version (fcv) 4.2, MongoDB 4.2 index builds fully replace the index build processes supported in previous MongoDB versions. MongoDB ignores the background index build option if specified to createIndexes or its shell helpers createIndex() and createIndexes().

Requires featureCompatibilityVersion 4.2

For MongoDB clusters upgraded from 4.0 to 4.2, you must set the feature compatibility version (fcv) to 4.2 to enable the optimized build process. For more information on setting the fCV, see setFeatureCompatibilityVersion.

MongoDB 4.2 clusters running with fCV 4.0 only support 4.0 index builds.

For complete documentation on the index build process, see Index Builds on Populated Collections.

Changes Affecting Compatibility

Some changes can affect compatibility and may require user actions. For a detailed list of compatibility changes, see Compatibility Changes in MongoDB 4.2.

Upgrade Procedures

Feature Compatibility Version

To upgrade, the 4.0 instances must have featureCompatibilityVersion set to 4.0. To check the version:

db.adminCommand( { getParameter: 1, featureCompatibilityVersion: 1 } )

For specific details on verifying and setting the featureCompatibilityVersion as well as information on other prerequisites/considerations for upgrades, refer to the individual upgrade instructions:

If you need guidance on upgrading to 4.2, MongoDB offers major version upgrade services to help ensure a smooth transition without interruption to your MongoDB application.

Download

To download MongoDB 4.2, go to the MongoDB Download Center

Known Issues

In Version Issues Status
4.2.0 SERVER-43075: Missing storage.journal.commitIntervalMs Fixed in 4.2.1

Report an Issue

To report an issue, see https://github.com/mongodb/mongo/wiki/Submit-Bug-Reports for instructions on how to file a JIRA ticket for the MongoDB server or one of the related projects.