What is weak consistency?

Weak consistency is a consistency model in distributed systems that offers the weakest guarantee regarding the synchronization of data across multiple replicas. In a weakly consistent system, there are no strict requirements on when or if replicas will converge to a consistent state. Weak consistency allows for significant divergence between replicas and permits temporary inconsistencies to exist.

Key characteristics of weak consistency include:

• Delayed Propagation: Updates made to data items may not be immediately propagated to all replicas in the system. There can be delays in disseminating updates across the distributed environment.
• Lack of Synchronization: Unlike stronger consistency models such as strong consistency or eventual consistency, weak consistency does not enforce synchronization mechanisms to ensure that all replicas converge to the same value over time.
• Potential for Stale Reads: Clients accessing data may observe stale or outdated values, as replicas may diverge or lag behind the latest updates. Stale reads are acceptable within the weak consistency model.
• Eventual Convergence Not Guaranteed: While eventual consistency aims for all replicas to converge to a consistent state eventually, weak consistency does not provide such a guarantee. Replicas may remain divergent indefinitely, and convergence is not mandated.

Consistency in system design refers to the property of ensuring that all nodes in a distributed system have the same view of the data at any given point in time, despite possible concurrent operations and network delays. In simpler terms, it means that when multiple clients access or modify the same data concurrently, they all see a consistent state of that data.