(Note: Work-in-progress documentation here)
riak_ensemble is a consensus library that supports creating multiple
consensus groups (ensembles). Each ensemble is a separate Multi-Paxos
instance with its own leader, set of members, and state.
Each ensemble also supports an extended API that provides consistent key/value operations. Conceptually, this is identical to treating each key as a separate Paxos entity. However, this isn't accomplished by having each key maintain its own Paxos group. Instead, an ensemble emulates per-key consensus through a combination of per-key and per-ensemble state.
As mentioned, riak_ensemble supports multiple independent consensus
groups. Ensembles are created dynamically, allowing applications to
use riak_ensemble in whatever way best fits that
application. Each ensemble also supports dynamic ensemble membership,
using joint consensus to guarantee consistent membership transitions.
A given ensemble is configured to use a particular "ensemble backend".
An ensemble backend is an implemented behavior that defines how a given
peer actually stores/retrieves data. For example, riak_ensemble
ships with the riak_ensemble_basic_backend which stores data as an
orddict that is saved to disk using term_to_binary, whereas
riak_kv implements the riak_kv_ensemble_backend that is used to
interface with Riak's vnodes.
Better documentation is coming soon. For now, this talk from RICON
West 2013 discusses the design of riak_ensemble in the context of
adding consistency to Riak 2.0:
http://www.youtube.com/watch?v=gXJxbhca5Xg