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Tech Glossary

Eventual Consistency

Eventual consistency is a consistency model used in distributed computing systems, especially in large-scale, highly-available databases like NoSQL (e.g., Amazon DynamoDB, Apache Cassandra). This model ensures that, in a distributed system, all replicas of a particular data item will eventually converge to the same value, given enough time, even if they are temporarily inconsistent.

In an eventually consistent system, when a data change (such as an update or delete operation) occurs, it is not immediately propagated to all replicas. Instead, the system allows a period of inconsistency while the changes propagate asynchronously to different nodes. During this period, different replicas might return different versions of the data. However, the system guarantees that, given sufficient time without additional updates, all replicas will converge to the same consistent state.

Eventual consistency is often chosen in distributed systems that prioritize availability and partition tolerance over strong consistency, as defined by the CAP theorem. In highly-available systems, ensuring that a read or write operation can succeed even when some nodes are offline or slow is critical. Eventual consistency enables this by allowing temporary inconsistencies rather than sacrificing availability.

One of the most common examples of eventual consistency is seen in content delivery networks (CDNs) and social media platforms. For instance, when a user updates their profile picture, it may not immediately reflect across all devices or servers due to replication lag. However, over time, all instances will eventually show the updated profile picture.

There are trade-offs to using eventual consistency. The major drawback is that clients may observe outdated or stale data during the inconsistency period, which can be problematic for applications that require real-time data accuracy. To address this, some systems offer tunable consistency models, allowing developers to choose between strong consistency (where data is immediately consistent across all replicas) and eventual consistency based on their application's requirements.

In summary, eventual consistency is a consistency model that sacrifices immediate consistency for higher availability and performance in distributed systems. It ensures that all data replicas eventually converge to a consistent state while allowing temporary inconsistencies, making it suitable for large-scale, fault-tolerant systems.

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