3.3 Consistency Models

Consistency models define the guarantees a distributed system makes about the ordering and visibility of reads and writes.

The Consistency Spectrum

Strongest <--------------------------------------> Weakest

Linearizability > Sequential > Causal > Read-Your-Writes > Monotonic > Session > Eventual

Strong Consistency (Linearizability)

Every read returns the result of the most recent completed write. The system behaves as if there is only a single copy of the data.

T1: Client A writes x = 1 (succeeds)
T2: Client B reads x
T3: Client B MUST see x = 1 (not 0)

Systems: etcd, ZooKeeper, Google Spanner, CockroachDB, MongoDB (linearizable readConcern)

Cost: Write 5-20ms, Read 5-20ms, Cross-region 100-200ms

Eventual Consistency

If no new writes are made, eventually all replicas will converge to the same value. No guarantee about WHEN.

T1: Client A writes x = 1 (succeeds on Node A)
T2: Client B reads x from Node B -> sees x = 0 (stale)
T3: Replication propagates x = 1 to Node B
T4: Client B reads x -> sees x = 1 (up-to-date)

Systems: Cassandra (ONE), DynamoDB (default), Riak, CouchDB

Causal Consistency

Operations that are causally related are seen in order by all nodes. Concurrent operations may be seen in different orders.

T1: User posts comment (C1)
T2: User replies to C1 (C2)

Causal consistency: Everyone who sees C2 MUST also see C1
(Because C2 was caused by C1)

Read-Your-Writes Consistency

A user always sees their own writes, even if other users might see stale data.

T1: User updates profile name to "Alice"
T2: User refreshes profile page
T3: User MUST see "Alice" (their own write)
Other users might still see "Bob" briefly.

Monotonic Read Consistency

Once a user reads a value, they will never see an older value in subsequent reads.

Without monotonic reads:
  T1: Read profile -> "Alice" (new)
  T2: Read profile -> "Bob" (old from different replica) - WENT BACKWARDS!

With monotonic reads:
  T1: Read profile -> "Alice"
  T2: Read profile -> "Alice" or later (never older)

Session Consistency

Combines read-your-writes + monotonic reads within a single session.

Comparison

Interview Tips

"Linearizability is the gold standard but expensive. Most systems use a mix: linearizable for critical operations, eventual for everything else."

"Read-your-writes is the most practical model for user-facing applications. Users expect to see their own changes immediately."