Consistency Models

A consistency model defines the contract between a distributed system and its clients: which reads and writes are guaranteed to reflect which other reads and writes. The models form a spectrum from strongest (most expensive) to weakest (most performant).

Strongest ←──────────────────────────────────────────── Weakest

Linearizability > Sequential > Causal > Eventual
                                        + Session guarantees (practical middle ground)

Linearizability (Strong Consistency)

Linearizability is the strongest single-object consistency model. The system behaves as if there is one copy of the data and every operation takes effect at a single instant between its invocation and its response.

Properties:

• Operations have a global total order consistent with real time
• If operation A completes before operation B begins, B must observe A’s effects
• Every read returns the most recent write (from any client, anywhere)

sequenceDiagram
    participant C1 as Client 1
    participant C2 as Client 2
    participant S as System (single logical copy)

    Note over S: x = 0

    C1->>S: write x = 1
    S->>C1: ACK (write complete at t=10ms)

    Note over C1,C2: After t=10ms, any read must return x=1

    C2->>S: read x
    S->>C2: x = 1 ✓

    C1->>S: read x
    S->>C1: x = 1 ✓

    Note over C1,C2: No client can see x=0 after the write completes

Why it’s expensive: To make every read reflect the globally latest write, the system must either:

• Route all reads through a single primary (adds latency for remote readers), or
• Use a consensus protocol (Raft, Paxos) so a quorum confirms the value before responding

Used by: Google Spanner (TrueTime-bounded), etcd, ZooKeeper, CockroachDB, PostgreSQL with synchronous replication + routing reads to primary.

Common violation: Two replicas returning different values for the same key at the same logical instant. Classic symptom: user writes a value, reloads the page, and sees the old value — the read was served by a lagging replica.

Sequential Consistency

Sequential consistency is weaker than linearizability. Each process’s operations appear in their program order to all observers, but there is no requirement that the interleaving matches real-world time.

Properties:

• All processes agree on the same global ordering of operations
• Each process’s own operations appear in the order it issued them
• No real-time constraint — the agreed-upon order may not match wall-clock time

Process 1: write x=1 ─────────────── write y=2
Process 2:                     read y=2,  read x=0  ← sees y=2 but not yet x=1

Under linearizability: INVALID — x=1 happened before y=2; if you see y=2, you must see x=1
Under sequential consistency: VALID — P2's reads appear consistent with some interleaving
   Linearizable order: x=1, y=2, read-y, read-x
   Sequential order P2 sees: write-y, read-y, read-x, write-x (valid program order for P2)

In practice: Sequential consistency is rarely used as a standalone model in modern distributed databases. It appears in CPU memory models (x86 TSO, relaxed memory ordering) and some distributed shared-memory systems. Most systems skip directly from linearizability to causal consistency.

Causal Consistency

Causal consistency preserves the order of causally related operations. If write A happened before write B and B was influenced by A (A causally precedes B), all nodes must see A before B. Concurrent operations (neither caused the other) can be seen in any order.

Properties:

• Causally related writes are seen in the same order by all nodes
• Concurrent (unrelated) writes may be seen in different orders by different nodes
• Weaker than sequential: only partial ordering required

sequenceDiagram
    participant U1 as User 1
    participant U2 as User 2
    participant A as Node A
    participant B as Node B

    U1->>A: write "Question: 2+2?"
    A->>U1: ACK

    Note over U1: U1 reads question, then replies
    U1->>A: write "Answer: 4"  (causally after question)

    Note over A,B: Causally related: Question must arrive before Answer

    A->>B: replicate "Question: 2+2?" (arrives first)
    A->>B: replicate "Answer: 4" (arrives second)

    U2->>B: read
    B->>U2: "Question: 2+2?" then "Answer: 4" ✓

    Note over U1,U2: Concurrent writes (unrelated) can arrive in any order

Detecting causality: Systems use vector clocks or logical timestamps to track causal dependencies. A write carries the vector clock of its causal predecessors; a replica delays applying a write until all its causal predecessors have been applied.

Used by: MongoDB (causal sessions using operationTime and clusterTime), some CRDT-based systems, DynamoDB with vector clocks for conflict detection.

The benefit: Causal consistency avoids the coordination cost of linearizability while preserving the intuitive ordering that users expect (e.g., seeing a reply before the original post would be disorienting).

Eventual Consistency

Eventual consistency is the weakest useful model. If all writes stop, all replicas will eventually converge to the same value. There is no guarantee of when, or what a read returns in the meantime.

Properties:

• No ordering or recency guarantee on reads
• Replicas may diverge arbitrarily during active writes
• Convergence happens through background anti-entropy (Merkle tree comparison, gossip)

Timeline:
  t=0: Write x=1 to Node A
  t=1: Write x=2 to Node B (concurrent — neither knows about the other)
  t=2: Read x from Node C → could return 0 (hasn't received any write yet), 1, or 2
  t=10: Anti-entropy runs → all nodes reconcile
         Last-write-wins: x=2 (if t=1 is newer) — Node A's write is overwritten
  t=∞: All nodes eventually agree on x=2 (or x=1, depending on conflict resolution)

Conflict resolution strategies (because concurrent writes produce conflicts):

Used by: Cassandra (ONE consistency level), DynamoDB (eventually consistent reads), DNS propagation, S3 cross-region replication lag window.

Session Guarantees

Session guarantees are client-scoped consistency properties — they hold within a single client session but not globally across all clients. They are the practical middle ground between eventual and linearizability.

Read Your Writes

After a client writes a value, all subsequent reads by that client see that value or a more recent value.

Client 1 writes x=1 → sees x=1 on next read ✓
Client 2 (independent session) may still see x=0 for a short window ← acceptable

Implementation: Track the write’s LSN (log sequence number) per session. Route subsequent reads to replicas that have applied at least that LSN. See Read Replicas & Replication Lag for implementation patterns.

Monotonic Reads

If a client reads a value at version V, all subsequent reads by that client return version V or a more recent version. Reads never go backward in time.

Client reads x=2 (from Replica A, which applied writes up to t=50)
Client's next read hits Replica B (which only applied writes up to t=30)

Without monotonic reads: client sees x=1 ← time travels backward ✗
With monotonic reads:    router detects Replica B is behind → routes to Replica A ✓

Implementation: Sticky replica routing — hash the client ID to a specific replica. If that replica fails, re-hash; brief inconsistency during failover is acceptable.

Monotonic Writes

All writes from a single client are applied in the order they were issued. If client writes x=1 then x=2, no replica applies x=2 before x=1.

Implementation: Sequence numbers per client session. Replicas buffer writes until the preceding sequence number has been applied.

Writes Follow Reads

If a client reads value V and then issues a write, that write is guaranteed to be applied after the state that produced V. The write “follows” the read causally.

Client reads x=5 (from replica at state S)
Client writes y=x+1=6 (causally depends on reading x=5)

Writes follow reads guarantees: y=6 is applied after state S
Without this: y=6 could be applied before x=5 was set → y reads as x+1 but x is still 0

Implementation: The client passes the read timestamp/LSN with the write; the write is applied only after that state is reached.

Consistency Models in Practice

What Consistency Model Should You Choose?

The choice is per-data-type, not per-system:

Same application, different consistency requirements:

User feed posts:        Eventual consistency (EL) — stale is fine
User's own profile:     Read-your-writes (session guarantee) — must see own changes
Account balance:        Linearizability (EC) — must be accurate, always
Inventory count:        Linearizability (EC) — overselling costs money
Like counts:            Eventual consistency (EL) — off by a few is acceptable
Idempotency key check:  Linearizability (EC) — must not process duplicate payments
Session token valid:    Read-your-writes or linearizability — logout must take effect
Search index:           Eventual consistency (EL) — seconds of lag is fine