Known Limitations

Known Limitations

Not Simulated

Feature	Reason
Authentication / Authorization	ReadConcern/WriteConcern/ReadPreference stored but not enforced
Replica set topology	Single-node in-memory, no failover/election simulation
Sharding	No shard key routing or chunk migration
WiredTiger internals	No storage engine simulation, no journaling
RU throttling	Not applicable to MongoDB (Cosmos DB concept)
Consistency levels	Always strong consistency
Connection pooling	Single in-memory instance, no network simulation
Client-Side Field Level Encryption (CSFLE)	Requires mongocryptd or crypt_shared native libraries
Queryable Encryption	Requires server-side support
mongocryptd / crypt_shared	Native binaries, not simulatable in-process
Query plan optimization	All queries do full scan; indexes stored but not used for query optimization
Aggregation memory limits	No 100MB stage memory limit (unconstrained)
Cursor timeout	In-memory cursors never expire from idle

Approximations

Feature	Behavior
TTL eviction	Lazy — expired documents removed on query, not by background thread
Collation	Uses .NET CultureInfo, may differ from ICU on edge cases
Collection/index statistics	Synthetic values (not from a real storage engine)
Session tokens	Simplified (not opaque like real MongoDB)
Diagnostics	Stub implementation
$search / $searchMeta	Basic substring search (not Atlas Search)
$vectorSearch	Brute-force exact computation (no ANN index)
Geospatial	Real geometric calculations via NetTopologySuite

Operations Not Supported

Operation	Behavior
MapReduce	Throws NotSupportedException (deprecated in MongoDB 5.0+)
Explain()	Returns stub { "executionSuccess": true, "nReturned": count }
RetryReads / RetryWrites	Accepted but never trigger (no transient network errors)

Command Monitoring / Cluster Events

ClusterConfigurator event subscribers (CommandStartedEvent, CommandSucceededEvent, CommandFailedEvent) are not fired. Production code that registers event handlers via MongoClientSettings.ClusterConfigurator will not receive events. APM/structured logging that depends on command events should be tested against real MongoDB.

Concurrency Notes

• Multi-document operations are not fully isolated from concurrent direct CRUD
• Per-document locking prevents lost updates on single-document operations

See Also

• Features — What is supported
• Troubleshooting — Common issues and solutions
• Feature Comparison — Comparison with alternatives