UserNames (MongoDB)

Here is a projection that handles UserCreated and UserDeleted events using MongoDB as datastore. Currently, for MongoDB no transactionally aware projection implementation is provided.

Configuration

Before creating the Projection class we need to configure the com.mongodb.client.MongoClient and create an instance of MongoDatabase.

Constructing

Create a subscribed projection by extending AbstractMongoDbSubscribedProjection and passing a MongoDatabase object to the constructor. The getScopedName() helper generates a prefix based on the class name and its current revision, ensuring a fresh collection is created whenever the revision is bumped. Optionally provide a document class (here UserSchema) when requesting the collection so that MongoDB returns typed objects directly.

@Component
@ProjectionMetaData(revision = 1)
public class UserNames extends AbstractMongoDbSubscribedProjection {

  private final MongoCollection<UserSchema> userCollection;

  public UserNames(@NonNull MongoDatabase mongoDatabase) {
    super(mongoDatabase);
    String scopedName = getScopedName().with("userNames").asString();
    userCollection = mongoDatabase.getCollection(scopedName, UserSchema.class);
  }

  // ...

  @Data
  @Builder
  @AllArgsConstructor
  @NoArgsConstructor
  @Accessors(fluent = false)
  public final class UserSchema {
    @BsonProperty("id")
    private UUID id;
    private String firstName;
    private String lastName;
  }
}

FactStreamPosition and lock management are automatically handled by AbstractMongoDbSubscribedProjection. For this purpose, two shared collections named locks and states are created.

Updating the projection

Applying Events

Received events are processed inside methods annotated with @Handler.

@Handler
void apply(UserCreatedV1 e) {
  userCollection.insertOne(
      UserSchema.builder()
          .id(e.aggregateId())
          .firstName(e.firstName())
          .lastName(e.lastName())
          .build());
}

@Handler
void apply(UserChangedV1 e) {
  userCollection.replaceOne(
      new Document("id", e.aggregateId()),
      UserSchema.builder()
          .id(e.aggregateId())
          .firstName(e.firstName())
          .lastName(e.lastName())
          .build());
}

Full Example

A full example for both subscribed and managed projections can be found here.