As described earlier, when you use event-based communication, a microservice publishes an event when something notable happens, such as when it updates a business entity. Other microservices subscribe to those events. When a microservice receives an event, it can update its own business entities, which might lead to more events being published. This is the essence of the eventual consistency concept. This publish/subscribe system is usually performed by using an implementation of an event bus. The event bus can be designed as an interface with the API needed to subscribe and unsubscribe to events and to publish events. It can also have one or more implementations based on any inter-process or messaging communication, such as a messaging queue or a service bus that supports asynchronous communication and a publish/subscribe model.
You can use events to implement business transactions that span跨越 multiple services, which gives you eventual consistency between those services. An eventually consistent transaction consists of a series of distributed actions. At each action, the microservice updates a business entity and publishes an event that triggers the next action. Figure 6-18 below, shows a PriceUpdated event published through and event bus, so the price update is propagated to the Basket and other microservices.
Figure 6-18. Event-driven communication based on an event bus
This section describes how you can implement this type of communication with .NET by using a generic event bus interface, as shown in Figure 6-18.
There are multiple potential implementations, each using a different technology or infrastructure such as RabbitMQ, Azure Service Bus, or any other third-party open-source or commercial service bus.
The event bus
An event bus allows publish/subscribe-style communication between microservices without requiring the components to explicitly be aware of each other, as shown in Figure 6-19.
Figure 6-19. Publish/subscribe basics with an event bus
The above diagram shows that microservice A publishes to Event Bus, which distributes to subscribing microservices B and C, without the publisher needing to know the subscribers. The event bus is related to the Observer pattern and the publish-subscribe pattern.
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