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strong_consistency: add begin_read() to raft_server

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Add begin_read() method to raft_server that checks leadership for read
operations. Unlike begin_mutate(), it does not need to compute a
timestamp or interact with leader_info. It simply checks current_leader()
and returns one of three dispositions:

  - ok: this node is the leader, proceed with read_barrier() locally
  - raft::not_a_leader: redirect to the indicated leader
  - need_wait_for_leader: leader unknown, caller must wait and retry

This will be used by the read forwarding logic in subsequent commits.
This commit is contained in:
Wojciech Mitros
2026-04-21 03:26:06 +02:00
parent f8156702de
commit 1f91524547
2 changed files with 19 additions and 0 deletions
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11
service/strong_consistency/groups_manager.cc
View File

@@ -89,6 +89,17 @@ auto raft_server::begin_mutate(abort_source& as) -> begin_mutate_result {
return timestamp_with_term{new_ts, term};
}
auto raft_server::begin_read(abort_source& as) -> begin_read_result {
const auto leader = _state.server->current_leader();
if (!leader) {
return need_wait_for_leader{_state.server->wait_for_leader(&as)};
}
if (leader != _state.server->id()) {
return raft::not_a_leader{leader};
}
return ok{};
}
groups_manager::groups_manager(netw::messaging_service& ms,
raft_group_registry& raft_gr, cql3::query_processor& qp,
replica::database& db, service::migration_manager& mm, db::system_keyspace& sys_ks, gms::feature_service& features,

8
service/strong_consistency/groups_manager.hh
View File

@@ -222,6 +222,14 @@ public:
};
using begin_mutate_result = std::variant<timestamp_with_term, raft::not_a_leader, need_wait_for_leader>;
begin_mutate_result begin_mutate(abort_source&);
// Possible results:
// ok - this node is the leader, proceed with read_barrier() locally
// raft::not_a_leader - this node is not a leader, redirect to the leader
// need_wait_for_leader - the leader is unknown, the caller needs to wait and retry
struct ok {};
using begin_read_result = std::variant<ok, raft::not_a_leader, need_wait_for_leader>;
begin_read_result begin_read(abort_source&);
};
}