Gateway

The definition of the Gateway is part of the NPM package @tmorin/ceb-messaging-core.

The Gateway is a set of interfaces acting as an entry point to the messaging world. It can be used to send or handle messages but also to observe the implementations' behavior.

A Gateway provides accessors to the main buses: the CommandBus, the QueryBus and the EventBus. A remaining accessor provides an observable view point of the Gateway.

A Gateway should be ready to used once created. Its end-of-life is triggered with its method dispose(). Its purpose is to release all stateful stuff. So that, once invoked, the Gateway cannot be used anymore.

Commands

The CommandBus provides methods to interact with the Point-to-Point Channel which handles the commands. The CommandBus supports the one-way or two-way conversations.

On the receiver side, the handled commands are registered using the method handle(). The method expects the type of the command to handle as well as its handler. The handler is a callback function which will be invoked once a command is received. A handler may return an output which may be composed of an optional Result and/or an optional set of Events. If expected, the Result will be sent back to the sender. About the set of Events, they will be published on the EventBus. At any time, a handler can be unregistered using the Removable object returned by the method handler().

On the sender side, those requiring a one-way conversation must use the method executeAndForget(). Within this conversation style, the command sender doesn't expect a Result from the receiver side. So that, the receiver and server sides don't need to handle the Request-Reply pattern.

However, senders requiring a two-way conversation must use the method execute().

Queries

The QueryBus provides methods to interact with the Point-to-Point Channel which handles the queries. The QueryBus supports only the two-way conversations.

On the receiver side, the handled queries are registered using the method handle(). At any time, a handler can be unregistered using the Removable object returned by the method handler().

On the sender side, the queries are sent to the bus and the replies received using the method execute().

Events

The EventBus provides methods to interact with the Publish-Subscribe Channel which handles the events.

The publisher must use the method publish() to publish events. On the other side, the subscribers must use the method subscribe() to subscribe on published events. At any time, a subscription can be disposed using the Removable object returned by the method subscribe().

Observer

The Observer is a view side of the Control Bus. It provides an entry point to observe the main buses: the CommandBus, the QueryBus and the EventBus.