As mentioned, the microservice architectural style is an approach for developing an application as a suite of discrete yet self-sufficient services built around specific business capabilities. Microservices-centric applications are being enabled to be event-driven. There are a few interesting architectural patterns quickly emerging and evolving. In this section, we will look at the event stream pattern.
Like polyglot and decentralized persistence, the decentralized polyglot messaging method should be a key to achieving the intended success in microservices architectures. This allows different groups of services to be developed in their own cadence. Furthermore, it minimizes the need for highly coordinated and risky big-bang releases. The microservices approach gives more flexibility to developers for choosing the optimal messaging middleware solution. Every use case will have its own specific needs mandating different messaging technologies such as Apache Kafka, RabbitMQ, or even event-driven NoSQL data grids, such as Apache Geode / Pivotal GemFire.
In the MSA approach, a common architecture pattern is event sourcing using an append-only event stream such as Kafka or MapR Streams, which implements Kafka. With MapR Streams, events are grouped into logical collections of events called topics. Topics are partitioned for parallel processing. We can think of a partitioned topic as a queue. Events are delivered in the order they are received. Unlike a queue, events are persisted, even after they are delivered they remain on the partition, available to other consumers. Older messages are automatically deleted based on the stream's time-to-live setting; if the setting is zero, then they will never be deleted. Messages are not deleted from topics when read, and topics can have multiple different consumers. This allows processing of the same messages by different consumers for different purposes.
Pipelining is also possible where a consumer enriches an event and publishes it to another topic:
With the faster proliferation, penetration, and participation of the microservices architecture, there will be fresh patterns to address its growing and prevailing issues and limitations. Furthermore, existing patterns can be seamlessly combined to come out with bigger and better patterns for enabling the realization of microservices-centric applications. We have detailed the architecture and design patterns in Chapter 9, Microservices Architecture Patterns.
