Service Limitations


You can access most of the CRUD and Aggregation functionality of MongoDB version 3.6 with the MongoDB service; however, MongoDB Realm does not support all operations and features available in standard tools and clients. This page provides a high-level overview of MongoDB feature availability in Realm. For a list of specific MongoDB operations that are available when you connect to MongoDB through Realm, see the CRUD & Aggregation API reference.

Query Results

MongoDB queries executed through Realm can return a maximum of 50,000 documents. If you need to return more documents, consider paginating your query.

Query Options

Realm supports all query options in system functions. Realm does not support certain options for CRUD operations run from user functions. For a list of specific options that are available when you connect to MongoDB through Realm, see Query Options.


Realm supports all aggregation pipeline stages in system functions except for $currentOp and $indexStats.

In user functions, supported aggregation pipeline stages are reduced for security reasons.

For a list of pipeline stages that are available and their allowed function context, see Aggregation.

Database Commands

Realm does not support any database commands in the Client SDKs or Functions. You can, however, call a limited subset of database commands when when connected to a MongoDB cluster over the Realm wire protocol. For a list of wire protocol supported commands, see Database Commands.

Change Streams

Realm limits the total number of change streams open against a given cluster across all Realm apps based on the cluster’s size. It also limits the number of client applications that can concurrently listen for changes using Watch and Mobile Sync. The following table lists the limitations for each cluster size:

Cluster Size Maximum Number of Change Streams Maximum Number of Concurrent Listening Clients
Free Tier (M0) 5 250
Shared Clusters (M2/M5) 10 500
Small, Dedicated Clusters (M10/M20) 100 1000
Standard Clusters (M30/M40) 1000 10000
Standard Clusters (M50 - M90) 1000 50000
High-Power Clusters (M100+) 1000 100000 (Contact Support for Additional Usage)


Realm opens a single change stream on each collection that is associated with a Database Trigger, Collection Sync, or Watch operation.

Usage Recommendation

To minimize the number of concurrent listening clients and open change streams:

  • Start Watch and Sync operations only when necessary. Close any open Watch and Sync streams immediately when they’re no longer necessary.
  • Avoid data models that require you to Watch or Sync an unbounded number of collections.

Static Hosting

Realm enforces a 25MB maximum file size constraint on static hosting.