Strong Consistency (Linearizability)

Strong Consistency (souvent Linearizability — propriété formelle plus stricte) garantit qu'après un write committé, all subsequent reads voient cette valeur (ou plus récente), donnant l'illusion d'un système single-node atomic. Plus strict que eventual consistency, sacrifices latency et availability en présence de partitions (CAP theorem).

Fonctionnement : (1) writes répliqués synchronement vers majority de replicas (quorum) avant ack au client ; (2) protocols de consensus (Paxos, Raft, ZAB) coordonnent ; (3) reads vont au leader ou utilisent quorum reads pour garantie ; (4) en cas de partition, minority side stops accepting writes pour préserver consistency.

Systèmes strong-consistent : (1) **Google Spanner** — global strong consistency via TrueTime API (atomic clocks + GPS), revolutionary ; (2) **CockroachDB** — Spanner-inspired open source, distributed SQL ACID ; (3) **YugabyteDB** ; (4) **etcd, ZooKeeper, Consul** — distributed config, Raft consensus ; (5) **MongoDB** linearizable read concern + majority write concern ; (6) **DynamoDB** strongly consistent reads (option, double cost vs eventual) ; (7) **Postgres synchronous replication** ; (8) **Aurora** (multi-AZ writer) ; (9) **CockroachDB**.

Trade-offs : (1) **higher latency** — wait for cross-region replication consensus, can be hundreds of ms ; (2) **lower availability** during partitions ; (3) **leadership election** overhead ; (4) **clock synchronization** crucial (Spanner uses TrueTime, others NTP best-effort).

Quand requis : (1) **financial transactions** (money movements) ; (2) **scarce inventory** (concert tickets, limited edition products) ; (3) **distributed locks** ; (4) **leader election** ; (5) **configuration data** that must be consistent across cluster ; (6) **medical records** writes ; (7) **regulatory compliance** requiring strict consistency guarantees.

Consensus algorithms : (1) **Paxos** (Lamport 1989) — theoretical foundation ; (2) **Raft** (2014) — popular implementation, easier to understand and implement, used by etcd, Consul, CockroachDB ; (3) **ZAB** (ZooKeeper Atomic Broadcast) ; (4) **EPaxos** — leaderless variant. Compétences DEA-C01, DP-203.