Distributed Systems

Anti-entropy

Background process that reconciles divergent replicas.

In plain terms

Merkle-tree comparison or read-repair. Cassandra and Riak run anti-entropy continuously to converge eventually-consistent replicas.

Origin

Demers et al., "Epidemic Algorithms" (1987) used the term anti-entropy for full-state reconciliation between replicas. Modern implementations use Merkle trees for efficiency.

Where it shows up in production

Cassandra repair Merkle tree comparison detects divergent ranges; only the diff is streamed.
Riak Active anti-entropy runs continuously; key spaces reconcile in the background.

Sources & further reading

Paper Demers et al. — Epidemic Algorithms (1987)

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