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" To ensure consistency of schema and topology changes, Scylla needs a linearizable storage for this data available at every member of the database cluster. The series introduces such storage as a service, available to all Scylla subsystems. Using this service, any other internal service such as gossip or migrations (schema) could persist changes to cluster metadata and expect this to be done in a consistent, linearizable way. The series uses the built-in Raft library to implement a dedicated Raft group, running on shard 0, which includes all members of the cluster (group 0), adds hooks to topology change events, such as adding or removing nodes of the cluster, to update group 0 membership, ensures the group is started when the server boots. The state machine for the group, i.e. the actual storage for cluster-wide information still remains a stub. Extending it to actually persist changes of schema or token ring is subject to a subsequent series. Another Raft related service was implemented earlier: Raft Group Registry. The purpose of the registry is to allow Scylla have an arbitrary number of groups, each with its own subset of cluster members and a relevant state machine, sharing a common transport. Group 0 is one (the first) group among many. " * 'raft-group-0-v12' of github.com:scylladb/scylla-dev: raft: (server) improve tracing raft: (metrics) fix spelling of waiters_awaken raft: make forwarding optional raft: (service) manage Raft configuration during topology changes raft: (service) break a dependency loop raft: (discovery) introduce leader discovery state machine system_keyspace: mark scylla_local table as always-sync commitlog system_keyspace: persistence for Raft Group 0 id and Raft Server Id raft: add a test case for adding entries on follower raft: (server) allow adding entries/modify config on a follower raft: (test) replace virtual with override in derived class raft: (server) fix a typo in exception message raft: (server) implement id() helper raft: (server) remove apply_dummy_entry() raft: (test) fix missing initialization in generator.hh