Compatibility Changes in MongoDB 4.2¶

On this page

Removal of MMAPv1 Storage Engine
Removed or Deprecated Commands and Methods
Aggregation
Transactions
Change Streams
MongoDB Tools
Replica Set State Changes
4.2 Drivers Enable Retryable Writes by Default
General Changes
4.2 Feature Compatibility

Removal of MMAPv1 Storage Engine¶

MongoDB 4.2 removes support for 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 Commands and Methods¶

MongoDB removes the MMAPv1 specific options:

noPadding and usePowerOf2Sizes for collMod
verbose for collStats
flags for create
paddingFactor, paddingBytes, preservePadding for db.createCollection().

MongoDB ignores the MMAPv1 specific option async for fsync.

Removed or Deprecated Commands and Methods¶

Remove Support for the `group` Command¶

Starting in version 4.2, MongoDB removes the group command (deprecated since version 3.4) and its mongo shell helper db.collection.group().

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

Remove Support for the `eval` Command¶

Starting in version 4.2, MongoDB removes the eval command. eval command has been deprecated since version 3.0.

The associated MongoDB 4.2 mongo shell methods db.eval() and db.collection.copyTo() can only be run when connected to MongoDB 4.0 or earlier.

Remove Support for the `copydb` and `clone` Commands¶

Starting in version 4.2, MongoDB removes the deprecated copydb command and clone command.

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

As alternatives, users can use mongodump and mongorestore (with the mongorestore options --nsFrom and --nsTo) or write a script using the drivers.

For example, to copy the test database from a local instance running on the default port 27017 to the examples database on the same instance, you can:

Use mongodump to dump the test database to an archive mongodump-test-db:
copy
```
mongodump --archive="mongodump-test-db" --db=test
```
Use mongorestore with --nsFrom and --nsTo to restore (with database name change) from the archive:
copy
```
mongorestore --archive="mongodump-test-db" --nsFrom='test.*' --nsTo='examples.*'
```

Tip

Include additional options as necessary, such as to specify the uri or host, username, password and authentication database.

Alternatively, instead of using an archive file, you can mongodump the test database to the standard output stream and pipe into mongorestore:

copy

mongodump --archive --db=test | mongorestore --archive  --nsFrom='test.*' --nsTo='examples.*'

Remove Support for the `parallelCollectionScan` Command¶

Starting in version 4.2, MongoDB removes the parallelCollectionScan command.

Remove `maxScan`¶

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.

Remove Support for the `geoNear` Command¶

Starting in version 4.2, MongoDB removes the geoNear command. Use the $geoNear aggregation stage instead.

The options for $geoNear are similar to the removed geoNear command with the following exceptions:

The removed geoNear command includes in its output a field named dis that included the distance information.

For the $geoNear stage, specify the distance field name in distanceField.
The removed geoNear command accepts a boolean value for the includeLocs option to include the loc field.

For the $geoNear stage, specify the location field name in includeLocs.
The removed geoNear command includes the avgDistance and maxDistance of the returned results.

You can use the aggregation pipeline to return the avgDistance and maxDistance as well. Specifically, after the $geoNear stage, include a $group stage to calculate the avgDistance and maxDistance:
copy
```
db.places.aggregate([
   { $geoNear: { near: <...>, distanceField: "dis", includeLocs: "loc", spherical: true, ... } },
   { $group: { _id: null, objectsLoaded: { $sum: 1 }, maxDistance:
         { $max: "$dis" }, avgDistance: { $avg: "$dis" } } }
])
```
See also

Remove limit and num Options from $geoNear

Remove Support for the `repairDatabase` Command¶

Starting in version 4.2, MongoDB removes the repairDatabase command and its mongo shell helper db.repairDatabase() as well as the repairDatabase privilege.

As alternatives:

To compact data for a mongod, use the compact command. For details about the operation, see compact command.
To rebuild indexes on a standalone, use the reIndex command or its helper db.collection.reIndex(). For details about the operation, see reIndex command and db.collection.reIndex() reference pages.
To recover data for a standalone, use mongod --repair. See Recover a Standalone after an Unexpected Shutdown for details.

Remove Support for `getPrevError` Command¶

Starting in version 4.2, MongoDB removes the deprecated getPrevError command and its mongo shell helper db.getPrevError().

Deprecate Support for `cloneCollection`¶

MongoDB deprecates the cloneCollection command and its mongo shell helper db.cloneCollection()

As alternatives,

Users can use mongoexport and mongoimport.
Users can use the aggregation pipeline $out or $merge stages.
Write a script using the drivers.

Deprecated Plan Cache Commands/Methods¶

MongoDB deprecates the following:

PlanCache.getPlansByQuery() method/planCacheListPlans command.

To get the cached query plans for a shape, use the $planCacheStats aggregation stage instead. See Find Cache Entry Details for a Query Shape.
PlanCache.listQueryShapes() method/planCacheListQueryShapes command.

To list the cached query shapes, use the $planCacheStats aggregation stage instead. See List Query Shapes.

Aggregation¶

`$out` Stage Restrictions¶

`$out` and Views¶

The view definition pipeline cannot include the $out stage. If you already have an existing view that includes the $out stage, you can no longer create new views from this existing view.

For existing views that include the $out stage, you should either drop and recreate the views without the $out stage or use replace the view definition with a new pipeline that does not contain the $out stage.

`$out` and `$lookup`¶

The $lookup stage cannot include the $out stage in its nested pipeline field for the joined collection.

`$out` and `linearizable` Read Concern Level¶

The $out stage cannot be used in conjunction with read concern "linearizable".

`$out` and Explain¶

You cannot run the db.collection.explain() method (or the explain command) in executionStats mode or allPlansExecution mode if the aggregation pipeline contains the $out stage.

If the aggregation pipeline contains the $out stage, to view executionStats or allPlansExecution information, run explain without the $out stage in order to return explain results for the preceding stages.

Alternatively, you can run explain in queryPlanner mode for an aggregation pipeline that contains the $out stage.

`$out` and `majority` Read Concern Level¶

Starting in MongoDB 4.2, you can specify read concern level "majority" for an aggregation that includes an $out stage.

In MongoDB 4.0 and earlier, you cannot include the $out stage to use "majority" read concern for the aggregation.

Remove `limit` and `num` Options from `$geoNear`¶

Starting in version 4.2, MongoDB removes the limit and num options for the $geoNear stage as well as the default limit of 100 documents. To limit the results of $geoNear, use the $geoNear stage with the $limit stage.

For example, the following aggregation where the $geoNear stage contains the num option is no longer valid in 4.2.

db.places.aggregate([
   {
     $geoNear: {
        near: { type: "Point", coordinates: [ -73.99279 , 40.719296 ] },
        distanceField: "distance",
        num: 5,                 // Not supported in 4.2
        spherical: true
     }
   }
])

Instead, you can rewrite the aggregation to the following pipeline:

copy

db.places.aggregate([
   {
     $geoNear: {
        near: { type: "Point", coordinates: [ -73.99279 , 40.719296 ] },
        distanceField: "distance",
        spherical: true
     }
   },
   { $limit: 5 }
])

Transactions¶

Starting in MongoDB 4.2, you cannot specify killCursors as the first operation in a transaction.
Starting in MongoDB 4.2, you cannot write to capped collections in transactions. Reads from capped collections are still supported in transactions.
Starting in MongoDB 4.2, MongoDB removes the 16MB total size limit for a transaction. In version 4.2, MongoDB creates as many oplog entries as necessary to the encapsulate all write operations in a transaction. In previous versions, MongoDB creates a single entry for all write operations in a transaction, thereby imposing a 16MB total size limit for a transaction.

Change Streams¶

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).

Default Collation¶

Starting in MongoDB 4.2, change streams use simple binary comparisons unless an explicit collation is provided. In earlier versions, change streams opened on a single collection (db.collection.watch()) would inherit that collection’s default collation.

Resume Token Modification¶

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

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.

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`.
`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).

`--query` Options¶

Starting in version 4.2, the query option for mongodump --query and mongoexport --query must be in Extended JSON v2 format (relaxed or canonical/strict mode), including enclosing the field names and the operators in quotes, as in the following:

copy

mongoexport -d=test -c=records -q='{ "a": { "$gte": 3 }, "date": { "$lt": { "$date": "2016-01-01T00:00:00.000Z" } } }' --out=exportdir/myRecords.json

In earlier versions, the query options uses the Extended JSON v1 format and the field names and the operators do not need to be in quotes:

copy

mongoexport -d=test -c=records -q='{ a: { $gte: 3 }, date: { $lt: { "$date": "2016-01-01T00:00:00.000Z" } } }' --out=exportdir/myRecords.json

Replica Set State Changes¶

Primary Step Down¶

Starting in MongoDB 4.2, replSetStepDown (and replSetReconfig that results in a step down) no longer closes all client connections. However, writes that were in progress are killed.

In MongoDB 4.0 and earlier, replSetStepDown closes all client connections during the step down.

`ROLLBACK` State¶

Starting in version 4.2, MongoDB kills all in-progress user operations when a member enters the ROLLBACK state.

4.2 Drivers Enable Retryable Writes by Default¶

The official MongoDB 3.6 and 4.0-compatible drivers required including the retryWrites=true option in the connection string to enable retryable writes for that connection.

The official MongoDB 4.2-compatible drivers enable Retryable Writes by default. Applications upgrading to the 4.2-compatible drivers that require retryable writes may omit the retryWrites=true option. Applications upgrading to the 4.2-compatible drivers that require disabling retryable writes must include retryWrites=false in the connection string.

Important

The local database does not support retryable writes. Applications which write to the local database will encounter write errors upon upgrading to a 4.2-series driver unless retryable writes are explicitly disabled.

General Changes¶

Indexes¶

Stronger Restrictions on `reIndex`¶

MongoDB implements a stronger restriction on running reIndex command and db.collection.reIndex() shell helper against a collection in a sharded cluster by disallowing reIndex and db.collection.reIndex() on a mongos.

Restriction on `db.collection.dropIndex()` Method¶

You cannot specify db.collection.dropIndex("*") to drop all non-_id indexes. Use db.collection.dropIndexes() instead.

Duplicate Index Creation Attempt Error Message¶

MongoDB changes the returned response if you create an index with one name, and then try to create the index again with another name.

Starting in version 4.2, the createIndexes command and the mongo shell helpers db.collection.createIndex() and db.collection.createIndexes() report an error if you create an index with one name, and then try to create the same index again but with another name.

copy

{
   "ok" : 0,
   "errmsg" : "Index with name: x_1 already exists with a different name",
   "code" : 85,
   "codeName" : "IndexOptionsConflict"
}

In previous versions, MongoDB did not create the index again, but would return a response object with ok value of 1 and a note that implied that the index was not recreated. For example:

copy

{
   "numIndexesBefore" : 2,
   "numIndexesAfter" : 2,
   "note" : "all indexes already exist",
   "ok" : 1
}

Hashed Index on PowerPC¶

For hashed indexes, MongoDB 4.2 ensures that the hashed value for the floating point value 2⁶³ on PowerPC is consistent with other platforms. In previous versions, the hashed value for the floating point value 2⁶³ on PowerPC is inconsistent with other platforms.

Although hashed indexes on a field that may contain floating point values greater than 2⁵³ is an unsupported configuration, clients may still insert documents where the indexed field has the value 2⁶³.

To list all hashed indexes for your deployment, see PowerPC and 263.

If the current MongoDB 4.0 sharded cluster on PowerPC contains hashed values for 2⁶³ as part of the shard key, additional considerations must be taken before upgrading the sharded cluster to 4.2. See Upgrade a Sharded Cluster to 4.2.

min()/max()¶

Starting in MongoDB 4.2, when specifying min()/max() for a db.collection.find() operation, you must explicitly specify the index for min()/max() with the cursor.hint() method unless the find() query is an equality condition on the _id field { _id: <value> }.

Similarly, when specifying min/max in the find command, you must also explicitly specify the hint for the min/max index.

In previous versions, you could run min()/max() (or the corresponding min/max fields in the command) with or without explicitly hinting the index regardless of the query condition. If run without the hint in 4.0 and earlier, MongoDB selects the index using the fields in the indexBounds; however, if multiple indexes exist on same fields with different sort orders, the selection of the index may be ambiguous.

CurrentOp¶

When reporting on "getmore" operations, the $currentOp aggregation stage, along with currentOp command and the db.currentOp() helper, now returns the originatingCommand field as a nested field in the new cursor field. In previous versions, the originatingCommand was a top-level field for the associated "getmore" document. See also 4.2 currentOp Changes.

Server Status¶

The serverStatus and db.serverStatus method return opcounters and opcountersRepl metrics as 64-bit integers (i.e. NumberLong) instead of 32-bit integers (i.e. NumberInt).

Logging¶

When logging to syslog, the format of the message text includes the component. For example:
copy
```
...  ACCESS   [repl writer worker 5] Unsupported modification to roles collection ...
```
Previously, the syslog message text did not include the component. For example:
copy
```
... [repl writer worker 1] Unsupported modification to roles collection ...
```
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 version 4.2, MongoDB logs the debug verbosity level. For example, if verbosity level is 2, MongoDB logs D2.

In previous versions, MongoDB log messages only specified D for Debug level.

Wire Protocol¶

MongoDB no longer supports the deprecated internal OP_COMMAND and the corresponding OP_COMMANDREPLY wire protocol.

`killCursors` Changes¶

Transactions¶

Starting in MongoDB 4.2, you cannot specify killCursors as the first operation in a transaction.

Privileges¶

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 in MongoDB 4.2+.

In MongoDB 3.6.3 through MongoDB 4.0.x, users required the killCursors privilege in order to kill their own cursors when access control is enabled.

Removes `AsyncRequestsSenderUseBaton` Parameter¶

In MongoDB 4.2+ deployment, MongoDB removes the AsyncRequestsSenderUseBaton parameter and always enables the performance enhancement controlled by the parameter.

Stricter Validation of `count` Syntax¶

Starting in version 4.2, MongoDB implements a stricter validation of the option names for the count command. The command now errors if you specify an unknown option name.

In previous versions, MongoDB ignores invalid option names.

Causal Consistency Sessions¶

Starting in MongoDB 4.2, the following commands no longer support afterClusterTime:

dbHash command
mapReduce command
validate command

As such, these operations cannot be associated with causally consistent sessions.

Removes `fastmodinsert` Metric¶

MongoDB 4.2 removes the deprecated fastmodinsert metric from various outputs, including the explain executionStats, the profiler output, 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.

Balancer State and Autosplit¶

Starting in MongoDB 4.2:

The balancerStart command and the mongo shell helper methods sh.startBalancer() and sh.setBalancerState(true) also enable auto-splitting for the sharded cluster.

To disable auto-splitting when the balancer is enabled, you can use sh.disableAutoSplit().
The balancerStop command and the mongo shell helper methods sh.stopBalancer() and sh.setBalancerState(false) also disable auto-splitting for the sharded cluster.

To enable auto-splitting when the balancer is disabled, you can use sh.enableAutoSplit().

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

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:

serverStatus command and db.serverStatus() method.
$currentOp aggregation pipeline stage, currentOp command, and db.currentOp() method.

`findAndModify` Query/Sort/Projection Argument Validation¶

Starting in MongoDB 4.2 (and 4.0.12+ and 3.6.14+), the findAndModify command and its associated mongo shell methods error if the specified query, sort, or projection argument is not a document.

In earlier versions, the operation treated non-document query or sort argument as an empty document {}.

See:

`dropDatabase` and `movePrimary`¶

Starting in MongoDB 4.2,

If you drop a database and create a new database with the same name, either:
- Restart all mongos instances and mongod shard members; or
- Use the flushRouterConfig command on all mongos instances and mongod shard members before reading or writing to that database.
If you use the movePrimary command to move unsharded collections, either:
- Restart all mongos instances and mongod shard members; or
- Use the flushRouterConfig command on all mongos instances and mongod shard members before reading or writing to that database.

This ensures that mongos and shard instances refresh their metadata cache. Otherwise, the you may miss data on reads, and may not write data to the correct shard. To recover, you must manually intervene.

In earlier versions, you only need to restart or run flushRouterConfig on the mongos instances.

For more information, see dropDatabase and movePrimary.

`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.

Connection Pooling and LDAP Server¶

Starting in version 4.2, MongoDB changes the ldapUseConnectionPool default values to:

true on Windows.
true on Linux where MongoDB Enterprise binaries are linked against libldap_r.

That is, on those systems, MongoDB, by default, uses connection pooling to connect to the LDAP server for authentication/authorization.

In earlier versions (versions 4.0.9+), MongoDB uses false as the default value for ldapUseConnectionPool. That is, MongoDB, by default, does not use connection pooling to connect to the LDAP server for authentication/authorization.

See ldapUseConnectionPool for details.

Removal of `system.indexes` and `system.namespaces` Collections¶

Starting in version 4.2, MongoDB removes the system.indexes and system.namespaces collections (deprecated since v3.0).

With the removal of these collections, built-in roles clusterManager, clusterMonitor, dbAdmin, read, restore, and other roles that inherit from these roles no longer provide privileges to directly access system.indexes and system.namespaces collections.

Arbiter Downgrade Requires Clearing Data Directory¶

MongoDB 4.2 arbiter data files are incompatible with MongoDB 4.0. Downgrading from MongoDB 4.2 to 4.0 requires deleting arbiter data files as an intermediary step. Running a MongoDB 4.0 arbiter against MongoDB 4.2 data files may result in unexpected behavior.

The downgrade instructions for replica sets and sharded clusters include specific steps for downgrading arbiters from 4.2 to 4.0:

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.

4.2 Feature Compatibility¶

Some features in 4.2 require not just the 4.2 binaries but the featureCompatibilityVersion (fCV) set to 4.2. These features include:

Distributed transactions.
Removal of Index Key Limit for MongoDB versions with fCV set to 4.2+. In concert with the removal of this limit, the failIndexKeyTooLong parameter has no effect for MongoDB versions with fCV set to 4.2+ and only applies for MongoDB 2.6 through MongoDB versions with fCV set to "4.0" or earlier.
Removal of Index Name Length for MongoDB versions with fCV set to 4.2+.
New internal format for unique indexes. The new format applies to both existing unique indexes as well as newly created/rebuilt unique indexes.
Starting in MongoDB 4.2, users can no longer use the query filter $type: 0 as a synonym for $exists:false. To query for null or missing fields, see Query for Null or Missing Fields.
MongoDB 4.2 adds wildcard indexes to support workloads where users query against custom fields or a large variety of fields in a collection.

← Release Notes for MongoDB 4.2 Upgrade a Standalone to 4.2 →

Compatibility Changes in MongoDB 4.2¶

Removal of MMAPv1 Storage Engine¶

MMAPv1 Specific Configuration Options¶

MMAPv1 Specific Parameters¶

MMAPv1 Specific Command¶

MMAPv1 Specific Options for Commands and Methods¶

Removed or Deprecated Commands and Methods¶

Remove Support for the group Command¶

Remove Support for the eval Command¶

Remove Support for the copydb and clone Commands¶

Remove Support for the parallelCollectionScan Command¶

Remove maxScan¶

Remove Support for the geoNear Command¶

Remove Support for the repairDatabase Command¶

Remove Support for getPrevError Command¶

Deprecate Support for cloneCollection¶

Deprecated Plan Cache Commands/Methods¶

Aggregation¶

$out Stage Restrictions¶

$out and Views¶

$out and $lookup¶

$out and linearizable Read Concern Level¶

$out and Explain¶

$out and majority Read Concern Level¶

Remove limit and num Options from $geoNear¶

Transactions¶

Change Streams¶

Availability¶

Default Collation¶

Resume Token Modification¶

MongoDB Tools¶

FIPS Mode¶

Extended JSON v2¶

--query Options¶

Replica Set State Changes¶

Primary Step Down¶

ROLLBACK State¶

4.2 Drivers Enable Retryable Writes by Default¶

General Changes¶

Indexes¶

Stronger Restrictions on reIndex¶

Restriction on db.collection.dropIndex() Method¶

Duplicate Index Creation Attempt Error Message¶

Hashed Index on PowerPC¶

min()/max()¶

CurrentOp¶

Server Status¶

Logging¶

Wire Protocol¶

killCursors Changes¶

Transactions¶

Privileges¶

Removes AsyncRequestsSenderUseBaton Parameter¶

Stricter Validation of count Syntax¶

Causal Consistency Sessions¶

Removes fastmodinsert Metric¶

Map-Reduce¶

Balancer State and Autosplit¶

Lock Diagnostics Reporting¶

findAndModify Query/Sort/Projection Argument Validation¶

dropDatabase and movePrimary¶

libldap and libldap_r¶

Connection Pooling and LDAP Server¶

Removal of system.indexes and system.namespaces Collections¶

Arbiter Downgrade Requires Clearing Data Directory¶

Sharded Collections and Replace Documents¶

4.2 Feature Compatibility¶

Remove Support for the `group` Command¶

Remove Support for the `eval` Command¶

Remove Support for the `copydb` and `clone` Commands¶

Remove Support for the `parallelCollectionScan` Command¶

Remove `maxScan`¶

Remove Support for the `geoNear` Command¶

Remove Support for the `repairDatabase` Command¶

Remove Support for `getPrevError` Command¶

Deprecate Support for `cloneCollection`¶

`$out` Stage Restrictions¶

`$out` and Views¶

`$out` and `$lookup`¶

`$out` and `linearizable` Read Concern Level¶

`$out` and Explain¶

`$out` and `majority` Read Concern Level¶

Remove `limit` and `num` Options from `$geoNear`¶

`--query` Options¶

`ROLLBACK` State¶

Stronger Restrictions on `reIndex`¶

Restriction on `db.collection.dropIndex()` Method¶

`killCursors` Changes¶

Removes `AsyncRequestsSenderUseBaton` Parameter¶

Stricter Validation of `count` Syntax¶

Removes `fastmodinsert` Metric¶

`findAndModify` Query/Sort/Projection Argument Validation¶

`dropDatabase` and `movePrimary`¶

`libldap` and `libldap_r`¶

Removal of `system.indexes` and `system.namespaces` Collections¶