Writing into the group0 raft group on a client side involves locking
the state machine, choosing a state id and checking for its presence
after operation completes. The code that does it resides now in the
migration manager since the currently it is the only user of group0. In
the near future we will have more client for group0 and they all will
have to have the same logic, so the patch moves it to a separate class
raft_group0_client that any future user of group0 can use to write
into it.
Message-Id: <YoYAJwdTdbX+iCUn@scylladb.com>
Check that group 0 history grows iff a schema change does not throw
`group0_concurrent_modification`. Check that the CQL DDL statement retry
mechanism works as expected.
We perform a bunch of schema changes with different values of
`migration_manager::_group0_history_gc_duration` and check if entries
are cleared according to this setting.
