MemoryEventBus

MemoryEventBus is a class that implements an event bus using EventEmitter as the in-memory message broker. Since the message broker has no access to anything outside the EventEmitter, the bus can only consume events that are published by it. This bus can be used for consuming webhooks. See this example.

constructor

The constructor takes MemoryEventBusConfig as the only argument:

type EventBusSubscriberCb = (props: {
  topic: string;
  payload: {};
  _fullData: {};
  metadata: GraphQLEventbusMetadata;
}) => Promise<unknown>;

export type MemoryEventBusConfig = {
  schema: GraphQLSchema;
  subscriber?: {
    cb: EventBusSubscriberCb;
    queries: DocumentNode;
  };
  plugins?: EventBusPlugin[];
  allowInvalidTopic?: boolean;
};

schema

This is the GraphQLSchema that is used by the publish to publish the events and the subscriber to consume the events. The reason MemoryEventBus uses the same schema for publishing and consuming is because it can only consume events that it publishes. This is different from a bus that uses a message broker that lives outside the process memory: RabbitMQ for instance.

subscriber

This field must be specified when consuming events.

cb

Required field. This is the event handler callback that is invoked for each event being consumed as specfied by the queries field. topic is the name of the topic, payload is the object that has fields queried by the consumer in queries field.

queries

Required field. The GraphQL DocumentNode that specifies all the events being published as GraphQL queries. For instance, the document below subscribes for UserCreatedEvent and UserDeletedEvent and queries for specific fields from the payload.

query UserCreatedEvent {
  UserCreatedEvent {
    userId
    userName
    userType
    eventId
  }
}
query UserDeletedEvent {
  UserDeletedEvent {
    userId
  }
}

plugins

This is an optional list of EventBusPlugin.

allowPublishNonExistingTopic

This is a boolean field (default false). If true, the bus does not throw an error when publishing a topic that does not exist in the schema. This option could be useful when we don't control the events that we publish and want to avoid error being thrown in production. An event that's not in the schema will be simply ignored.