Once you start sending message-based communication either with commands or events , you should avoid mixing message-based communication with synchronous HTTP communication. When the commands come from client applications, they can be implemented as HTTP synchronous commands. Use message-based commands when you need higher scalability or when you're already in a message-based business process.
That way, additional subscribers can be added in the future without the need to modify the sender service. When using asynchronous event-driven communication, a microservice publishes an integration event when something happens within its domain and another microservice needs to be aware of it, like a price change in a product catalog microservice. Additional microservices subscribe to the events so they can receive them asynchronously.
When that happens, the receivers might update their own domain entities, which can cause more integration events to be published. The event bus can be designed as an abstraction or interface, with the API that's needed to subscribe or unsubscribe to events and to publish events. If a system uses eventual consistency driven by integration events, it's recommended that this approach is made clear to the end user.
The system shouldn't use an approach that mimics integration events, like SignalR or polling systems from the client. The end user and the business owner have to explicitly embrace eventual consistency in the system and realize that in many cases the business doesn't have any problem with this approach, as long as it's explicit. This approach is important because users might expect to see some results immediately and this aspect might not happen with eventual consistency.
As noted earlier in the Challenges and solutions for distributed data management section, you can use integration events to implement business tasks that span multiple microservices.
Thus, you'll have eventual consistency between those services. An eventually consistent transaction is made up of a collection of distributed actions. At each action, the related microservice updates a domain entity and publishes another integration event that raises the next action within the same end-to-end business task.
An important point is that you might want to communicate to multiple microservices that are subscribed to the same event. The goal is to have eventual consistency between multiple data sources across your distributed system.
In asynchronous event-driven communication, one microservice publishes events to an event bus and many microservices can subscribe to it, to get notified and act on it. Your implementation will determine what protocol to use for event-driven, message-based communications. AMQP can help achieve reliable queued communication. When you use an event bus, you might want to use an abstraction level like an event bus interface based on a related implementation in classes with code using the API from a message broker like RabbitMQ or a service bus like Azure Service Bus with Topics.
The messaging technologies available for implementing your abstract event bus are at different levels. The event is a message sent to the publishing infrastucture where consumers may later retrieve it. Query — a message sent to a component to obtain some information from it. A Query is a special type of message because it must include a sender address so the target component can reply. Reply — the response to a query. A component handling a Query message will produce a Reply message and send it to the sender address specified in the query message.
Lightbend Who Is Lightbend? Who Uses Lightbend? Why Get A Subscription? What is Reactive? Event-Driven Edit this Page. Message Driven vs Event Driven Reactive Systems rely on asynchronous message-passing to establish a boundary between components that ensures loose coupling, isolation and location transparency. Reactive Manifesto. Message vs Event A Message is some data sent to a specific address.
An Event is some data emitted from a component for anyone listening to consume. The reasons why that happen are mainly technical in nature and a handler must be able to cope with this behavior. That is no matter how many times a message is processed, the outcome must be the same.
This is called idempotency. Invoking a handler that assigns 3 to a variable will have the same result no matter how many times is done.
But if, let's say, the variable is incremented then each invoke will get a different result. For these cases, the message handler must take additional steps to ensure its idempotency. Ensuring idempotency is an app specific action, but usually is done by having in the database a unique key for the processed message.
For example, inserting a new Product will also insert the message id. If the operation is a duplicate the db will refuse it. In this case the datastore is the actual enforcer of idempotency. This example also shows that idempotency is a concept that goes beyond the message handler. These are the basics of message driven architecture. In the next post I'll talk about the cool stuff. In a nuthsell things goes like this: Module sends a message to the ServiceBus. ServicesBus delivers the message to one or many configured destinations.
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