GCounters

GCounters, short for G-Counters, are a type of grow-only conflict-free replicated data type (CRDT) used to implement distributed counters that converge without requiring coordination. They are designed for environments with eventual consistency, where multiple replicas may update independently and later synchronize.

In a GCounters implementation, each replica maintains a vector (an array) of counters, with one entry per replica. The value at index i represents the total increments performed by replica i. The visible total counter is the sum of all vector entries. Operations are simple: increment on a replica i increases vector[i] by one. Importantly, GCounters are grow-only: there are no decrement operations.

Merging states between replicas is done by taking the componentwise maximum of the two vectors. This merge rule guarantees convergence: once all replicas have received all updates, every replica stores the same vector, and therefore the same total sum. Because individual entries never decrease, the overall counter is monotonic.

Limitations include the inability to perform decrements directly. To model decrements or more complex counting semantics, GCounters are often composed with other CRDTs, such as PN-Counters, which use a G-Counter for increments and a P-Counter for decrements. GCounters are commonly used to track tallies in distributed databases, collaborative applications, and systems where availability and partition tolerance are prioritized over strict synchronization.

Example: with three replicas A, B, and C, a state might be [2,0,0] after A increments twice. If B increments once, the states merge to [2,1,0], yielding a total count of 3.