Event-Driven Architecture

What is it?

Explaining first what it is not should help.

Most programmers start out building simple programs/websites/apps with a straight forward request/response flow.

Something like this, probably with a bit more layers:

sequenceDiagram
 actor User
 participant Cart
 participant Database
 
 
 User->>Cart: Put in cart
 Cart->>Database: Save in table
 Database-->>Cart: List of items in cart + 1
 Cart-->>User: Page + list of items in cart

Very straightforward, and in this flow the user waits in some form for the response to come back.

… to be continued…

What patterns are often used when applying an event-driven architecture?

Always publish an event as a result of a process

This is a good practice because, it

• lowers the friction when building new functionality
• allows for testing from a more behaviour driven approach to testing.
  • You can test from the outside in and don’t couple tests to implementation as easily.
  • Related pattern Decider pattern.

Saga pattern

This paragraph seems to align with Event-Carried State Transfer, see below.

• https://docs.particular.net/nservicebus/sagas/#avoid-external-resource-access-event-driven-data-collection

Event Notification vs Event-Carried State Transfer

When do we prefer one over the other?

• I should read this: https://medium.com/swlh/event-notification-vs-event-carried-state-transfer-2e4fdf8f6662
• Event-Carried State Transfer ! event-carried state transfer is more complex but can definitely be a good way to increase reliability and decrease coupling of separate services. - Storage is cheap.

Related:

• Message contracts
• Event Notification
• Event-Carried State Transfer

Source(s):

• https://event-driven.io/en/internal_external_events/
• https://www.youtube.com/watch?v=STKCRSUsyP0
• https://martinfowler.com/articles/201701-event-driven.html