Strong Consistency

Overview

Strongly consistent operations in Riak are implemented using the riak_ensemble library (see its Git Repo and Wiki for more details). These page describes the integration points with Riak KV. The main ones are the riak_kv_ensembles module that manages ensembles in the cluster, and the riak_kv_ensemble_backend module that connects the ensemble peers to KV vnodes and the AAE system.

Managing Ensembles

See riak_kv_ensembles

An ensemble is created for each primary preference list. Unlike eventually consistent Riak, fallback vnodes never participate in consistent operations. For example, if there are buckets with a replication factor (n_val) of 2 and some with 3, two ensembles will be created for each partition of the ring. For partition zero, for example, we will have {0, 2} and {0, 3}.

The riak_kv_ensembles process polls the ring for changes at regular intervals (currently every 10 seconds). This happens only on the claimant node.

Routing consistent operations

Consistent operations are handled by riak_client, just like regular operations. If the bucket as the {consistent, true} property, it will be routed through the riak_ensemble_client. Notice that legacy buckets (that is, in the default bucket type) are not allowed to have this property. This is done through the bucket property validators in riak_kv_bucket.