External State Store

How to integrate event-driven microservices with request-response APIs?

There are two types of external events:

• Autonomously Generated Events (analytical events)
• Reactively Generated Events (events from request-reply)

There are two approaches of processing and serving requests using stateful services:

• using internal state stores (with silly or with smart routing)
• using external state stores (with regular or with composite microservice)

Ways of handling requests within an event-driven workflow:

• Handling requests directly
• Turning requests into events (big latency)
• Processing events for user interfaces (a mix of two ways)

Approaches for request-response applications, their pros and cons:

Event-Driven Microservices need to materialize states. And it comes with the important things to think about. Two approaches to choose from:

• Internal state store
• or External state store

Both have pros and cons. Both have important scalability and recovery considerations.

Two approaches for changing data structures:

• Bebuilding
• and Migration

Transactions and how to emulate them in order to implement Effectively Once processing.

All of these is disclosed in the Chapter 7 of the book we are currently studying: