Overview
This page is the atomic definition. The ACID alternative consistency model is defined at base.
Definition
Eventual consistency is a consistency model used in distributed systems that guarantees replicas will converge to identical state given sufficient time and no new writes, but allows temporary divergence. It is the weakest consistency guarantee that is still useful: the system does not promise what value a read returns, only that it will not diverge forever. Eventual consistency is a practical trade-off in distributed databases (DynamoDB, Cassandra, CouchDB) where strong consistency would require coordination across nodes, introducing latency and reducing availability. In CAP theorem terms, eventually consistent systems prioritize availability and partition tolerance (AP) over consistency (CP). The time to convergence (convergence window) depends on replication topology, network latency, and conflict resolution strategy. Conflict resolution mechanisms include last-write-wins (LWW, uses timestamps), vector clocks, and CRDTs (Conflict-free Replicated Data Types). PostgreSQL’s logical replication and streaming replication are also eventually consistent because replication-lag means replicas temporarily lag the primary.
When it applies
Accept eventual consistency for: shopping carts, social media counts, non-financial notifications, and analytics aggregations where slight staleness is acceptable. Require strong consistency for: financial transactions, inventory, authentication state, and any data where reading a stale value causes business harm.
Example
A social media app shows “1,024 likes” on a post. Writes go to one datacenter; reads serve from a regional cache. A user in another region sees “1,022 likes” for a few seconds until the replica catches up. The count eventually converges; no business harm results.
Related concepts
- replication-lag - the mechanism that makes PostgreSQL replicas eventually consistent.
- base - the distributed-systems consistency model that accepts eventual consistency explicitly.
- acid - the strong consistency model that eventual consistency relaxes.
- sharding - cross-shard operations often operate under eventual consistency.
- postgres - PostgreSQL’s logical replication delivers eventual consistency for cross-region topologies.
Citing this term
See Eventual Consistency (llmbestpractices.com/glossary/eventual-consistency).