MongoDB

Your read models live in MongoDB, and every tenant has its own database. Wiring a client, choosing the database per request, mapping concepts and GUIDs to BSON, and turning a change stream into a live query is the same code in every service. @cratis/arc.mongodb supplies it: your model declares its fields once, and the collection maps them to BSON and returns instances of your model.

Source preview

@cratis/arc.mongodb is optional and not published to npm. It uses the MongoDB 6 driver. The capability reference has its status and the checks behind it.

What it provides

Capability	Page
Register collections with builder.withMongoDB(...), or from Cratis:MongoDB configuration, and inject them into queries	Get started
Choose a database, or a server, per tenant	Tenancy
Map decorated fields, concepts, GUIDs, and dates to BSON	Serializers
Match Arc on .NET’s property and collection naming	Naming policies
Count, sort, and page in the database	Paging
Turn a change stream into an observable query	Observing collections
Combine two or three live collections	Joined observation
React to raw collection changes	Change-stream watcher
Store GeoJSON Point, LineString, and Polygon fields	Geospatial types
Load a read model by command key	Command context

withMongoDB registers a read-model resolver for the models you list in readModels, so a command can declare @inject(commandReadModel(TaskRecord)) and receive the document whose identity equals the command key. Do not also register another integration, such as Chronicle, as the owner of the same type; build() fails when two claim one type.

Storage does not authorize a caller

Arc selects a tenant from the execution context, and the collection selects that tenant’s database. Your authentication and authorization still have to verify that the caller may use that tenant and read those documents. Never pass untrusted request JSON directly to a MongoDB filter.

Low-level helper

The original MongoReadModels<T, I> remains for low-level defineQuery users. It takes a caller-owned client, databaseForTenant, and a trusted filterFor(input, context). Its queryPage accepts Arc sorting only for fields listed in sortableFields, and caps pages at 100 by default. It has no change streams or field codecs; use the model-bound collection for those.

MongoReadModels returns raw driver documents. When they hold a protected Chronicle read model, set readModel to that class so Arc can release them. The class must also be registered with withChronicle; otherwise Arc has no release interceptor for it and serves the documents as stored. See Release raw MongoDB documents.

Current boundaries

This integration does not supply cross-store transactions or a durable change-stream checkpoint. The watcher shares a stream within a tenant scope, not across the process. Recognized transient reads retry at most twice; writes are not retried. Arc-owned clients expose OpenTelemetry MongoDB metrics, but caller-owned clients are not instrumented. Do not infer .NET’s process-wide watcher or general-purpose resilience interceptors from these narrower guarantees. The capability reference has the parity details.

Start with Get started.