Eventual consistency is a consistency model used in distributed systems, particularly in NoSQL databases, to achieve high availability and scalability. It guarantees that, given enough time and in the absence of new updates, all replicas of a data item will eventually converge to the same value. This model is often contrasted with strong consistency, where all operations appear to be executed in a strict order, ensuring that all replicas always have the same value at any given time.
Key Concepts of Eventual Consistency
Replication Delay: When a change is made to a data item on one node, it takes time for this change to propagate to all other nodes in the system. During this propagation period, different nodes may have different values for the same data item, leading to temporary inconsistencies[1][2][6].
Convergence: Despite the temporary inconsistencies, eventual consistency ensures that all replicas will eventually converge to the same value if no new updates are made. This means that, over time, all nodes will reflect the most recent update[6][7].
High Availability and Low Latency: Eventual consistency allows for high availability and low-latency reads because nodes can respond to read requests immediately without waiting for consensus from other nodes. This is particularly useful in distributed systems where network partitions or node failures can occur[2][4][5].
BASE Model: Eventual consistency is often associated with the BASE (Basically Available, Soft state, Eventually consistent) model, which contrasts with the ACID (Atomicity, Consistency, Isolation, Durability) properties of traditional relational databases. The BASE model prioritizes availability and partition tolerance over immediate consistency[6].
Examples and Use Cases
Microblogging Sites: In a globally distributed microblogging site, a user in Japan might like a post, increasing the like count from 100 to 101. A user in America might still see the like count as 100 until the update propagates to the American servers. Eventually, all users will see the like count as ...
