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# A Field Guide to System Design Trade-offs

Every system design decision is a trade-off. Here are the ones that actually matter, explained through the lens of "what breaks when."

## Consistency vs. Availability (it's not just CAP)

**The real question:** When two users read the same data at the same time, must they see the same thing?

- **Strong consistency** (e.g., single Postgres primary): Both readers always agree. Breaks when the primary is down — writes stop entirely.
- **Eventual consistency** (e.g., DynamoDB, Cassandra): Readers might briefly disagree. In exchange, the system stays w