System Design Interview - Designing from Invariants

Designing from Invariants Software architecture is frequently treated as an exercise in connecting infrastructure components. We often reach for Kafka, Redis, or microservice boundaries as if they are the building blocks of the business logic itself. But when tools come before logic, the resulting design prioritizes infrastructure choices over the problem they are meant to solve. A high reliability system does not start with a distributed queue or a complex workflow engine. It starts with the core constraints—the invariants—that make the system reliable. If you start by choosing your infrastructure before you have defined the logic that keeps your data correct, you are building complexity on an undefined foundation. The Distribution Trap Most designs become unmanageable because they assume every step of a business process must be distributed across new infrastructure from the beginning. In this style of design, the business logic is spread across a database, a queue, and a workflow eng