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Chapter 2: Consistency Models

Overview​

Consistency models define the rules for how data is shared and synchronized across distributed systems. Understanding these models is crucial for choosing the right approach for your application.

Strong Consistency​

Strong consistency ensures that all nodes see the same data at the same time. This model provides the most intuitive behavior but comes with performance and availability trade-offs.

Characteristics​

  • Linearizability: Operations appear to execute atomically
  • Sequential Consistency: Operations appear to execute in some sequential order
  • Causal Consistency: Causally related operations maintain their order

Trade-offs​

  • Pros: Simple to reason about, predictable behavior
  • Cons: Higher latency, reduced availability during partitions

Eventual Consistency​

Eventual consistency allows replicas to temporarily diverge but guarantees they will eventually converge to the same state.

Characteristics​

  • Convergence: All replicas eventually reach the same state
  • Availability: Operations can proceed even during network partitions
  • Performance: Lower latency and higher throughput

Trade-offs​

  • Pros: High availability, better performance
  • Cons: Temporary inconsistencies, more complex application logic

CRDTs and Consistency​

CRDTs provide a structured way to achieve eventual consistency by ensuring that:

  1. Concurrent operations commute: Operations can be applied in any order
  2. Operations are idempotent: Applying the same operation multiple times has no effect
  3. State converges: All replicas eventually reach the same final state

Choosing the Right Model​

The choice between consistency models depends on your application requirements:

  • Strong Consistency: When data accuracy is critical and some latency is acceptable
  • Eventual Consistency: When availability and performance are more important than immediate consistency
  • CRDTs: When you need eventual consistency with automatic conflict resolution

Next Steps​

In the next chapter, we'll dive into the fundamentals of CRDTs and how they work.