Eventuous.Projections.MongoDB 0.8.2

Eventuous MongoDB projections

This package adds MongoDB projections to applications built with Eventuous.

There are two main components provided by this package:

• MongoCheckpointStore, implements ICheckpointStore
• MongoProjection, implements IEventHandler

Using checkpoint store

You can register MongoCheckpointStore so it will be used by all subscriptions that need to store checkpoints (normally, EventStoreDB catch-up subscriptions).

services.AddSingleton<ICheckpointStore, MongoCheckpointStore>();

It will add a checkpoint collection to your Mongo database. Each subscription will have its own checkpoint document. The document id would be the subscription id. Make sure to use unique subscription ids across different applications if they use the same Mongo database.

Using projections

Create your own projection class that inherits MongoProjection<T> abstract class. Here, T is the document type, which must be a record. Your document type should inherit from ProjectedDocument record.

Use the On<TEvent> function to register event projection handlers:

public class ProjectWithTasksProjection : MongoProjection<ProjectWithTasks> {
    public ProjectWithTasksProjection(
        IMongoDatabase  database,
        ILoggerFactory? loggerFactory
    ) : base(database, QuerySubscription.Id, loggerFactory) { 
        On<V1.ProjectRegistered>(
            evt => evt.ProjectId, 
            (evt, update) => update.SetOnInsert(x => x.ProjectName, evt.Name)
        );
        On<V1.TaskCreated>(
            evt => evt.ProjectId,
            (evt, update) => update.AddToSet(
                x => x.Tasks,
                new ProjectTaskRecord(evt.TaskId, evt.Description)
            )
        );
    }
}

There's an overload for On<TEvent> that gives you direct access to the consume context, so you can have more advanced operations.

On<V1.ProjectRegistered>(ctx => { ... });

The function provided as an argument for On in this case needs to return ValueTask<Operation<T>>. To make things easier, we provide a function called UpdateOperationTask.

The UpdateOperationTask function has two overloads. The one used in the example allows passing the document id as the first argument. Another overload allows specifying a custom filter as (filter, evt) => filter.Eq(x => x.Field == evt.Value).

You can also use the UpdateOperation, which allows you to perform asynchronous operations inside the update function.

There is also a handler registration function for building updates asynchronously called OnAsync, which works similar t On:

OnAsync<V1.ProjectRegistered>(
    evt => evt.ProjectId, 
    async (evt, update) => update.SetOnInsert(
        x => x.ProjectName, 
        await getExternalData(evt.Name)
    )
);

You also have an option of not using pre-registered typed handlers by overriding the GetUpdate function. By default it returns NoOp, so if you override it, don't call base.GetUpdate. Eventuous will only call GetUpdate if there's no registered handler for a received event.

The GetUpdate function must return an instance of the Operation<T> record wrapped in ValueTask. UpdateOperation and UpdateOperationTask return ValueTask<UpdateOperation<T>>. You can use one of the supplied record types:

• UpdateOperation<T> - includes a filter and an update operation
• CollectionOperation<T> - allows executing any operation on the collection
• OtherOperation<T> - allows executing anything as it will be just awaited