ApsaraDB for MongoDB:Features of MongoDB 8.3

Array index access

$map, $filter, and $reduce now expose the current element's index. Bind it to a variable with arrayIndexAs, or use the built-in $$IDX.

// Add a 1-based "rank" field to each element
db.scores.aggregate([
  { $project: {
      ranked: { $map: {
        input: "$items",
        as: "item",
        arrayIndexAs: "i",
        in: { value: "$$item", rank: { $add: ["$$i", 1] } }
      }}
  }}
])

New expressions

Broader type conversions

• $toString now handles objects, arrays, regular expressions, MaxKey, MinKey, and timestamps.

• $convert adds a base parameter for base-2, -8, -10, and -16 string/number conversions.

• New $toArray (string-to-array, binData-to-numeric-array) and $toObject (string-to-object) conversions. See Convert binData to Array.

Granular shard removal

removeShard is deprecated. Four new commands break shard removal into discrete, reversible stages:

You can pause and resume draining at any time -- useful during maintenance windows or unexpected load spikes.

DDL routing requirement

DDL operations and applyOps must now go through mongos. Running them directly on a shard node is no longer allowed. This prevents metadata inconsistencies in sharded clusters.

In-progress slow query logging

Slow queries are now logged while still running, not only after completion. Each query is logged once when it exceeds the threshold.

Set the threshold with --defaultSlowInProgMS at startup, or at runtime:

db.setProfilingLevel(1, { slowms: 100, slowinprogms: 3000 })

Each log entry includes an originalQueryShapeHash that links the shard-level log back to the original request on mongos.

Per-operation memory tracking

Two new fields appear in $currentOp, db.currentOp(), the profiler, slow query logs, explain output, and PlanCache.list():

Use these fields to identify memory-heavy queries and set informed limits.

queryStats on shard servers

When queryStats is enabled, shard servers now include queries forwarded from mongos. In earlier versions, these queries were invisible at the shard level.

Selective serverStatus output

New metrics cover aggregation extensions, shard critical sections, TTL operations, vector search, replication oplog fetcher latency, and execution queue admission.

Use the new none: 1 flag to suppress all optional sections and opt in selectively:

db.runCommand({ serverStatus: 1, none: 1, locks: 1 })

Use lockContentionMetrics: 1 to include lock-contention data on demand.

FTDC retention increase

The default FTDC diagnostic directory cap grows from 200 MB to 500 MB. connPoolStats is now collected for mongod.

Pre-authentication resource caps

Three new parameters limit resources consumed by unauthenticated connections:

These settings protect instances from resource exhaustion caused by connection-flooding attacks.

Automatic cache pressure recovery

cachePressureQueryPeriodMilliseconds controls how often the storage engine checks cache pressure. When pressure is detected, the oldest transactions are automatically aborted to free cache.

Overload-aware server selection

When overloadAwareServerSelectionEnabled is on, a client that receives SystemOverloadedError automatically retries against a different node.

Five parameters fine-tune retry backoff for internal mongos-to-mongod connections:

Ingress rate-limit exemptions

ingressRequestRateLimiterApplicationExemptions exempts specific applications (by appName) from ingress rate limits. Set the app name in the connection string:

mongodb://server:27017/db?appname=mongodump

Profiler impact control

Two parameters prevent the profiler from degrading production performance:

Monitor profiler overhead with profiler.totalAbandonedWrites and profiler.dbsPastThreshold in serverStatus.

2dsphere default version upgrade

The default 2dsphereVersion moves from 3 to 4.

GeoJSON priority in index generation

When a document contains both GeoJSON and legacy numeric coordinates, index generation now prefers GeoJSON. If you rely on legacy coordinates in existing indexes, rebuild those indexes after upgrading.