Eventual Consistency

What is Eventual Consistency?

Eventual Consistency is a consistency model where, given enough time without new updates, all replicas will converge to the same value. It's the most common consistency model in large-scale distributed systems.

The Promise

"If no new updates are made to a given data item, eventually all accesses to that item will return the last updated value."

This means:

• Updates propagate asynchronously
• Temporary inconsistencies are allowed
• System will eventually become consistent

How It Works

Time ──────────────────────────────────────────────►

           Write X=1
Client ────────┼─────────────────────────────────────
               │
               ▼
Server A    [X=1]──────────────────────────────────
               │
               └──── propagate ────►
                                    │
Server B    [X=0]────────────────[X=1]─────────────
               │                    │
               └──── propagate ──────────►
                                          │
Server C    [X=0]──────────────────────[X=1]───────

            ◄── Inconsistent window ──►  ◄── Consistent ──►

Key Properties

Consistency Window

The consistency window is the time between a write and when all replicas reflect it.

Factors affecting the window:

• Network latency between replicas
• Replication strategy (sync vs async)
• System load
• Geographic distribution

Typical windows:

• Same datacenter: milliseconds
• Cross-region: seconds to minutes

Conflict Resolution

When replicas receive updates in different orders, conflicts occur.

Last-Write-Wins (LWW)

Use timestamps to determine the "winner":

Server A receives: X=1 at T1, X=2 at T3
Server B receives: X=2 at T3, X=1 at T1

Final value on both: X=2 (T3 > T1)

Problem: Clock synchronization issues can cause data loss

Version Vectors

Track causality to detect conflicts:

Server A: X=1 with vector [A:1, B:0]
Server B: X=2 with vector [A:0, B:1]

These are concurrent updates (neither happened before the other)
Conflict detected → application must resolve

CRDTs (Conflict-free Replicated Data Types)

Data structures that automatically merge without conflicts:

• G-Counter: Only increments, always mergeable
• PN-Counter: Increments and decrements
• G-Set: Grow-only set
• OR-Set: Add and remove from sets
• LWW-Register: Last-write-wins value

When to Use Eventual Consistency

Good Fit

Poor Fit

Real-World Examples

Amazon DynamoDB

• Default: eventually consistent reads
• Optional: strongly consistent reads (2x latency, 2x cost)
• Use case: Shopping cart persists across sessions

Apache Cassandra

• Tunable consistency levels
• QUORUM for stronger, ONE for faster
• Use case: Time-series data, user activity

DNS

• Classic eventual consistency example
• TTL-based propagation
• Updates take hours to propagate globally

Implementation Patterns

Read Repair

Fix inconsistencies during reads:

1. Read from multiple replicas
2. Compare values
3. If different, update stale replicas
4. Return most recent value

Anti-Entropy

Background process to synchronize replicas:

1. Periodically compare data between replicas
2. Use Merkle trees for efficient comparison
3. Sync only the differences

Hinted Handoff

Handle temporarily unavailable nodes:

1. Write intended for Node B
2. Node B is down
3. Store "hint" on Node A
4. When B recovers, A sends the hint

Trade-offs

Interview Tips

• Explain the window: Consistency happens over time, not instantly
• Discuss conflicts: How do you handle concurrent updates?
• Know real systems: DynamoDB, Cassandra, DNS examples
• Mention CRDTs: Show awareness of advanced techniques
• Trade-offs: Availability and latency vs consistency

Summary

Eventual consistency is the backbone of large-scale distributed systems. It sacrifices immediate consistency for:

• Higher availability: Always respond to users
• Lower latency: Don't wait for all replicas
• Better partition tolerance: Work through network issues

The key is understanding your application's tolerance for temporary inconsistency and implementing proper conflict resolution strategies.