Strongly consistent reads currently call read_barrier() on whichever
replica happens to process the request. When a follower runs
read_barrier(), it sends an RPC to the leader to get the current read
index, then waits for its local apply index to catch up. If the follower
is behind, this wait can be significant.
By forwarding linearizable reads to the leader, we don't need an RPC from replica to leader to get the index to wait for apply -- it's available locally.
Note that read_barrier() is still required on the leader to confirm it
is still the leader and guarantee linearizability. A future optimization
would be to implement leases in the raft library, which could eliminate
read_barrier() on the leader entirely.
The CL-to-behavior mapping is isolated in a single parse_consistency_level()
function:
- CL=(LOCAL_)QUORUM -> linearizable: forwarded to the raft leader
- CL=(LOCAL_)ONE -> non-linearizable: existing behavior (no read_barrier()/forwarding, may return stale results)
- All other CLs -> invalid request
Read forwarding reuses the same CQL-layer bounce_to_node() mechanism
that write forwarding already uses. The transport layer's existing
requests_forwarded_* metrics automatically count forwarded reads.
Coordinator-level metrics (linearizable_reads, non_linearizable_reads,
writes) are added for visibility into the strong consistency workload.
Fixes: SCYLLADB-1157
Closesscylladb/scylladb#29575
* github.com:scylladb/scylladb:
strong_consistency: test read forwarding to leader
strong_consistency: skip read_barrier() for non-linearizable reads
strong_consistency: split coordinator-level read latency metrics
strong_consistency: forward linearizable reads to raft leader
strong_consistency: classify reads by consistency level
strong_consistency: add begin_read() to raft_server
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