The Core Tradeoff
In distributed systems, partitions are unavoidable. That means the real choice is usually consistency versus availability.
The Three Properties
- Consistency: reads return the latest write
- Availability: requests keep getting responses
- Partition tolerance: the system keeps working through network failure
Operation-Level Thinking
A single product can use different consistency models for different operations.
- Booking a seat needs strong consistency
- Showing an event page can tolerate staleness
Common Models
Know the basic spectrum:
- Strong consistency
- Causal consistency
- Read-your-own-writes
- Eventual consistency
PACELC
CAP only covers partitions. PACELC adds the everyday tradeoff between latency and consistency even when the network is healthy.
Final Shape
The clean interview answer is: explain the C vs A tradeoff per operation, not per system, and choose the weakest model that still protects the product rule.