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Atlas App Services has several guidelines to keep in mind when architecting how your tools and clients interact with MongoDB through App Services. Keep this guidance in mind when deciding how to structure queries, selecting which CRUD and aggregation operations to use, and determining how to handle concurrent workloads.

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

For security reasons, only a subset of aggregation pipeline stages are supported in user functions.

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

When data is bulk/batch loaded into MongoDB Atlas, you may see a delay in data appearing on devices while App Services processes changes.

You can link a Federated database instance to your app as a MongoDB data source. However, there are some caveats to keep in mind when working with Atlas Data Federation:

Federated data sources do not support write operations.

You can only access a Federated data source from a system function.

You cannot connect to a Federated data source via the wire protocol.

You cannot define roles and permissions for a Federated data source.

You cannot set a read preference for a Federated data source.

You cannot create a database trigger on a Federated data source.

You cannot use a Federated data source as your app's Device Sync cluster.

You can link a serverless instance to your app as a MongoDB data source. However, serverless instances do not currently support change streams, so the following features are limited:

You cannot create a database trigger on a serverless instance.

You cannot use a serverless instance as your app's Device Sync cluster.

You cannot watch collections for changes data sources that are serverless MongoDB Atlas instances.

Atlas Device Sync does not currently support sharded MongoDB Atlas clusters. However, you can add a sharded cluster as a data source and access it from a function.

Time series collections do not yet support change streams. Therefore, you cannot define triggers on a time series collection.

You can use time series collections with Atlas Device Sync, but only Data Ingest is supported.

App Services does not currently support private endpoints.

App Services limits the total number of change streams open against a given cluster across all Apps based on the cluster's size. The following table lists the limitations for each cluster size:

App Services opens a single change stream on each collection that is associated with a Database Trigger or Device Sync operation.

To minimize the number of open change streams:

Start Sync operations only when necessary. Close any open Sync streams immediately when they're no longer necessary.

Avoid data models that require you to Sync an unbounded number of collections.

App Services 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 wire protocol. For a list of supported commands, see Database Commands.

When you intend to have many concurrent writers, have each writer work on a separate document.

After 30 days with no activity, Device Sync pauses. Refer to Resume Sync for information about how to re-enable Device Sync.

You can access most of the CRUD and Aggregation functionality of MongoDB version 3.6 with the MongoDB service; however, App Services does not support all operations and features available in standard tools and clients. For a list of specific MongoDB operations that are available when you connect to MongoDB through App Services, see the CRUD & Aggregation API reference.

Device Sync requires MongoDB Atlas clusters to run specific versions of MongoDB:

Partition-Based Sync requires MongoDB 4.4.0 or greater.

Flexible Sync requires MongoDB 5.0.0 or greater.

App Services supports all query options in system functions. App Services 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 App Services, see Query Options.

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

App Services limits request traffic to 10,000 concurrent requests. Any requests made beyond this limit return an HTTP response status code of 429 - Too Many Requests. You can request a higher limit by filing a support ticket.

Device Sync supports Sync rules. Device Sync does not support roles/field-level rules at this time.

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