Compatibility Changes in MongoDB 8.3

Stricter Wildcard Index Validation

Starting in MongoDB 8.3 (and 8.2.4, 8.0.18, 7.0.29), stricter validation rules apply to wildcardProjection specifications in compound wildcard indexes. Existing indexes that do not meet the new validation requirements continue to function, but you cannot create new indexes that don't meet these requirements.

For complete details on the validation rules, see Wildcard Index Restrictions.

Accessing Array Element Indexes in $map, $filter, and $reduce

MongoDB 8.3 improves access to array element indexes in $map, $filter, and $reduce aggregation expressions. You can use the new arrayIndexAs field to set a variable to store the index of an array element. You can also use the new $$IDX aggregation system variable to access the index of the current array element if you omit arrayIndexAs.

If you create a view or collection validation rule that use the new array element index feature, you must update or remove the view or rule before downgrading from MongoDB 8.3 to an earlier version.

2dsphereIndexVersion

Starting in MongoDB 8.3, 2dsphereIndexVersion is set to version 4 by default.

If you need to downgrade the FCV to anything below 8.3, you must first drop the 2dsphere version 4 indexes.

Expressions in Views

If you created views that use expressions introduced in MongoDB 8.3, those views may stop working after you downgrade to a version that does not support those expressions.

If you need to downgrade from 8.3, you must first update or drop any views that depend on expressions not supported in the target version.

Parameters and Variables in Views and Validators

If you created views or collection validators that use parameters or variables introduced in MongoDB 8.3, those objects may fail validation or evaluation after you downgrade.

If you need to downgrade from 8.3, you must first update or remove any views or validators that reference parameters or variables not supported in the target version.

Validated Collections

MongoDB 8.3 can mark collections as validated using new validation semantics that are not compatible with versions earlier than 8.3. If you attempt to downgrade while validated collections exist, the downgrade fails.

If you need to downgrade from 8.3, you must first change validated collections to a validation level supported in the target version or drop those collections.

$facet Memory-Limit Error Code

Starting in MongoDB 8.3, when a $facet stage constructs a document larger than the 100 megabyte limit, MongoDB returns the ExceededMemoryLimit error.

In earlier versions, stages that exceed the memory limit return the error code 4031700. If you have applications, drivers, or tools that explicitly check for 4031700, you can adjust your code to recognize ExceededMemoryLimit instead.

Subnormal Numbers in $convert Pipelines

Starting in MongoDB 8.3, the server is able to parse the full range of all representable double precision floating point numbers. This includes subnormal numbers where the most significant digit has leading zeroes and the exponent has the least possible value. In earlier versions of MongoDB, the server returns an error when you try to parse these numbers.

For details, see Subnormal Numbers.

Balancer Window Settings

When you downgrade from MongoDB 8.3 to an earlier version, the activeBalancerWindowDOW configuration is not available. On downgrade, the balancer defaults to using the activeWindow setting.

No manual downgrade steps are required, but any custom day-of-week (DOW) configurations made in 8.3 do not apply after downgrading.

$$CLUSTER_TIME Errors on Standalone Deployments

Starting in MongoDB 8.2, if you use the CLUSTER_TIME variable in a standalone deployment, the server returns an error.

$rankFusion on Views

Starting in MongoDB 8.2, $rankFusion can run on views. For more information, see $rankFusion.

orphanCleanupDelaySecs Change

Starting in MongoDB 8.2, the default value of orphanCleanupDelaySecs is 3600 (previously 900).

Remaining Resharding Time Threshold Change

Starting in MongoDB 8.2 (also available in 8.1.2 and 8.0.13), resharding enters the critical section when the estimated time to complete the remaining work is 500 ms or less.

$cpuNanos Change

Starting in MongoDB 8.2, cpuNanos is located in the $queryStats aggregation stage metrics and includes additional metrics related to CPU usage for query operations. The cpuNanos metric is only available on Linux systems.

In earlier versions, cpuNanos was located in operationMetrics.

Change Streams

In MongoDB 8.2.0, collectionUUID and updateDescription.disambiguatedPaths are included in applicable change events even if you do not set showExpandedEvents. In MongoDB versions earlier than 8.2.0 and in versions 8.2.1 and later, these fields are included only if you open the change stream with showExpandedEvents: true.

If you upgrade from MongoDB 8.2.0 to MongoDB 8.2.1 or later, collectionUUID and updateDescription.disambiguatedPaths no longer appear in change events unless you set showExpandedEvents when opening the stream.

Applications should not rely on commitTimestamp. It is exposed in MongoDB 8.2.0, but is not present by default in MongoDB 8.2.1 and later.

Removed Parameters

MongoDB 8.2 removes the catalogCacheIndexMaxEntries parameter.