.NET Architecture & System Design Interview Question #99

What is eventual consistency between services?

Transactions, CQRS, Event Sourcing & Distributed Consistency Mid-Level Intermediate

Quick Interview Answer

Eventual consistency means separate services or data stores may temporarily hold different views of the same business change, but they converge as asynchronous propagation completes.

Detailed Explanation

Eventual consistency means separate services or data stores may temporarily hold different views of the same business change, but they converge as asynchronous propagation completes.

The design must define acceptable staleness, user experience, retries, reconciliation, and what happens when propagation fails. Not every invariant can safely be eventual; critical rules should be enforced within an appropriate consistency boundary.

Code Example

Order Service: Paid
      |
event propagation
      v
Shipping Service: Pending
      |
later
      v
Shipping Service: Ready