Mongodb Knowledge Patch

MongoDB knowledge patch

Use this patch before changing MongoDB deployment compatibility, upgrade or downgrade procedures, aggregation pipelines, geospatial indexes, sharding behavior, encryption, validation, search, WiredTiger settings, or operational telemetry. Check the deployed binary and maintenance release before applying version-dependent guidance.

Reference index

Breaking compatibility and patch-level hazards

Avoid the Linux kernel 6.19 combination

MongoDB 8.0 and newer can crash during startup on Linux kernel 6.19 because of the bundled TCMalloc version. The risk applies to official packages, package-manager installations, and containers. Do not deploy this combination until the MongoDB build contains the patched allocator.

Treat encrypted string-query data as disposable

Queryable Encryption prefix, suffix, and substring queries are public preview functionality. Do not use them in production or expect data compatibility with the GA design. A collection that enables the preview will have to be dropped.

For mongosh, provide the Automatic Encryption Shared Library 8.2 or newer:

mongosh --cryptSharedLibPath /path/to/mongo_crypt_v1.so

Route around affected mongocryptd releases

• On Windows, mongocryptd 8.2.0 does not start with --logpath NUL. This also breaks the .NET/C# driver's default launch. Use a real log path, use the Automatic Encryption Shared Library, or upgrade to 8.2.1 or newer.
• Releases 8.2.4 and 8.2.5 limit mongocryptd messages to 16 KiB. Skip those releases or use the shared library when encrypted commands can be larger.

Mitigate the 8.2.0 spill-session crash

MongoDB 8.2.0 can crash when more than 1000 memory-intensive queries are spilling at once. Raise spillWiredTigerSessionMax as the documented mitigation, and prefer a fixed maintenance release when possible.

Enforce the trim character-set limit

Starting in 8.2.8, the chars argument to $trim, $ltrim, and $rtrim cannot exceed 4096 characters. Validate generated character sets before sending a pipeline.

Upgrade and downgrade guardrails

Before upgrading an 8.0 deployment to 8.2, ensure the feature compatibility version is 8.0:

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

Apply all of these constraints:

• Only a single-version downgrade from 8.2 to 8.0 is supported.
• Community Edition does not support binary downgrades.
• FCV cannot be downgraded to or from a minor release.
• After changing FCV, an Enterprise binary downgrade requires support assistance.

Changed defaults and operational semantics

Protect secondary reads during orphan cleanup

terminateSecondaryReadsOnOrphanCleanup defaults to true. During migrated-range cleanup, MongoDB terminates secondary reads that started before the chunk migration committed and are still active before orphaned documents are deleted. This avoids the older behavior in which those reads could silently omit the documents.

The default orphanCleanupDelaySecs is therefore 3600, increased from 900. Account for the longer retention window when sizing storage and planning cleanup.

Rebuild affected geospatial indexes

When a document contains both GeoJSON and legacy numeric coordinates, geospatial index key generation prioritizes GeoJSON. Rebuild affected indexes and verify query results if existing keys depended on legacy numeric coordinates appearing first.

Size initial-sync index builds deliberately

In 8.2, 8.0.13, and 7.0.26, initial-sync index builds use 10% of available RAM by default, bounded by 200 MB and 16 GB. Tune the policy with:

• initialSyncIndexBuildMemoryPercentage
• initialSyncIndexBuildMemoryMinMB
• initialSyncIndexBuildMemoryMaxMB

High-value query and aggregation features

Use server time in a pipeline

$currentDate returns the current time on the server:

db.events.aggregate([
  { $project: { observedAt: { $currentDate: {} } } }
])

Accumulate arrays directly

Use $concatArrays or $setUnion as accumulators:

db.events.aggregate([
  {
    $group: {
      _id: "$tenant",
      all: { $concatArrays: "$values" },
      unique: { $setUnion: "$values" }
    }
  }
])

Allow nullable $merge keys only with the right index

Fields named by $merge.on may be missing or null when the supporting index is non-sparse. A sparse supporting index does not permit this behavior. Inspect the actual index before depending on nullable merge keys.

Inspect the cluster catalog

Use $listClusterCatalog as a database-level aggregation stage:

db.aggregate([{ $listClusterCatalog: {} }])

Use Search on eligible standard views

Search-index create, update, drop, and list operations work on standard views only when the view pipeline contains $addFields, $set, or $match wrapping $expr. Eligible views can run $search, $searchMeta, and $vectorSearch; explain output includes execution statistics for these stages.

Storage and compression configuration

WiredTiger can size its cache as a percentage. Configure either --wiredTigerCacheSizePct or storage.wiredTiger.engineConfig.cacheSizePct. storage.wiredTiger.engineConfig.zstdCompressionLevel accepts values from -7 through 22; negative levels exchange compression ratio for faster compression and decompression.

storage:
  wiredTiger:
    engineConfig:
      cacheSizePct: 50
      zstdCompressionLevel: -3

Connection and cache-warming controls

Enable ingress connection-establishment limiting when connection surges threaten CPU:

• ingressConnectionEstablishmentRateLimiterEnabled
• ingressConnectionEstablishmentRatePerSec
• ingressConnectionEstablishmentBurstCapacitySecs
• ingressConnectionEstablishmentMaxQueueDepth

Use the related serverStatus connection and ingress-session queue fields to observe admissions, rejections, exemptions, disconnects, queue time, and tokens.

mirrorReads.targetedMirroring can select tagged nodes, including hidden nodes. Mirroring may originate from a primary or a secondary, allowing selected node caches to be warmed.

Validation and API additions

• validationAction: "errorAndLog" rejects an invalid insert or update and logs the violation.
• renameCollection and db.collection.renameCollection() are part of Stable API V1.
• One $lookup can reference multiple encrypted collections under Client-Side Field Level Encryption or Queryable Encryption.
• setQuerySettings accepts a comment so the operational reason travels with the setting.

db.runCommand({
  collMod: "orders",
  validationAction: "errorAndLog"
})

Observability checkpoints

• $queryStats includes shapes for count and distinct, plus execution-ticket delinquency counts and durations.
• Slow-log durationMillis includes authorization and command-parsing time.
• Low disk space causes disk-spilling queries to fail; log and explain spill field names differ as detailed in the observability reference.
• $currentOp.versionContext exposes operation FCV information.
• connectionStatus.authInfo.UUID identifies the current client connection.
• validate.indexDetails includes index specifications, and validate.repairMode reports attempted inconsistency repairs.
• serverStatus includes expanded query, plan-cache, spilling, replication, transaction, connection, network-timing, and WiredTiger metrics.

Verification checklist

• Confirm exact server, shell, encryption-library, and kernel versions before deployment.
• Verify FCV before an upgrade and recheck edition-specific downgrade support before rollback.
• Rebuild and test mixed-format geospatial indexes.
• Load-test initial sync, query spilling, and connection admission with production-like limits.
• Treat preview encrypted string-query collections as temporary and disposable.
• Verify index sparsity before allowing missing or null $merge.on values.
• Recheck telemetry parsers for expanded fields and changed duration semantics.
• Open the matching topic reference before changing a parameter or relying on a new output field.