client: fall back to followers when leader is unreachable

[coocood]

Motivation

This PR makes client-rust compatible with the cold-region on-demand Raft behavior used by Cloud Storage Engine.

A cold Region can lose its leader while both PD and the client still cache the old leader. Two failure modes can then prevent the client from learning the new leader:

1. The cached leader Store is unreachable, so mapping the Region to a Store or dispatching the RPC fails before any Region response is returned.
2. The old leader rejects a read at the read-pool entrance with ServerIsBusy(0). The request never reaches the Raft leader check, so TiKV cannot return NotLeader even though the peer has already lost leadership.

Previously client-rust kept retrying through the cached Region route and depended on PD eventually publishing a new leader. For cold Regions, sending a normal leader request to another voter is useful: the request wakes the cold peer, and the peer can either accept it as the new leader or return NotLeader while starting request-driven recovery.

High-level behavior

The request remains a leader request throughout this flow. This PR does not set replica_read = true and does not enable follower reads.

locate Region
    |
    v
try cached leader  ------------------------------> success: return
    |
    | Store mapping or retryable transport failure
    v
try eligible voter followers sequentially
    |
    +--> accepted without Region error: update cached leader and return
    |
    +--> NotLeader / ServerIsBusy: continue to the remaining voter
    |
    +--> all candidates exhausted: invalidate the appropriate cache and retry

The candidate iterator is lazy and leader-first. In the normal case, only the cached leader is mapped and called; follower candidates are not prepared unless the leader attempt fails.

Learners and witnesses are excluded because they cannot become a useful leader candidate for cold-region recovery. Candidates are tried sequentially, and only one concurrency permit is held at a time.

Detailed changes

1. Candidate rounds

RetryableMultiRegion now executes one candidate round per shard:

• Start with the cached leader.
• If that candidate cannot be mapped or has a retryable routing failure, continue with the Region voter peers.
• A fallback NotLeader response does not stop the round and its leader hint is deliberately ignored. With the production three-replica topology, continuing the voter list is simpler and the actual new leader will accept the same normal request.
• A fallback ServerIsBusy response also does not hide a remaining healthy voter.
• If a fallback peer accepts the request without a Region error, update the shared leader cache before returning. A key-level error still proves that this peer accepted the request as leader, so the cache is updated before the key error is propagated.

If all voters answer NotLeader in a normal round, the cached peer list may itself be stale, so the Region cache is invalidated and reloaded from PD.

2. ServerIsBusy(0) follower probe

This follows the same suspect-old-leader idea as the client-go workaround:

• Count ServerIsBusy(0) responses from the same cached leader within one request retry state.
• After two responses, run one followers-only probe round.
• Keep replica_read unchanged, so a follower cannot serve data under follower-read semantics.
• Probe at most once per request. If the probe is inconclusive, restore the cached leader and resume the normal ServerIsBusy backoff path.
• A change of cached leader starts a new count for that peer.
• A non-zero estimated_wait_ms follows the ordinary ServerIsBusy path and does not trigger this probe.

Cloud Storage Engine currently constructs scheduler/read-pool busy errors with the protobuf default estimated_wait_ms = 0. In the relevant failure mode, read-pool admission happens before the Raft snapshot and leader check, which is why a follower probe is needed to recover routing.

This heuristic can issue one extra leader-semantics RPC during genuine overload, but it cannot introduce a stale read. Cold-region probe and campaign work is deduplicated on the TiKV side.

3. Error classification and cache handling

Failure	Action in the candidate round	Cache handling
map_region_to_store fails	Try the next voter	Invalidate that Store entry; invalidate the Region if every candidate in a normal round fails to map
Error::Grpc or a tonic status backed by a transport source	Try the next voter	Invalidate the failed Store; invalidate the Region route if the round is exhausted
gRPC Unavailable, Cancelled, Internal, or Unknown without a source	Treat as a retryable routing failure and try the next voter	Same Store/Region invalidation as above
gRPC DeadlineExceeded	Try the next voter	Preserve Store and Region caches because a deadline does not prove that routing is stale
Other source-less application gRPC status	Return the error	No cross-replica replay
Fallback NotLeader	Continue remaining voters	Reload the Region after exhausting a normal round
Fallback ServerIsBusy	Continue remaining voters	Preserve it only when no stronger routing failure requires a route reload
Fallback accepted without a Region error	Return the response	Update the cached leader to the accepting peer

map_region_to_store errors are named separately from PD errors because that operation includes both Store metadata resolution and opening the TiKV channel.

4. Replay safety

Fallback is only allowed where the existing plan semantics permit retry:

• Raw CAS and other plans marked terminal_on_dispatch_error return the first dispatch error instead of replaying on another peer, because the first RPC may have reached the server.
• Plans marked terminal_on_undetermined return an UndeterminedResult immediately. A later error from another shard or candidate cannot overwrite an unknown apply outcome.
• Ordinary retryable/idempotent transaction operations retain their existing retry behavior.
• Multi-shard result ordering and undetermined-error precedence are preserved.

5. Bounded TiKV connection setup

TikvConnect now applies its configured timeout while establishing the tonic channel. Without a TCP connection timeout, map_region_to_store could remain pending on an unreachable Store and never advance to the next voter.

The existing SecurityManager::connect behavior remains available for other callers; the TiKV Store client uses the new timeout-aware helper.

Test coverage

The added tests cover:

• unreachable cached leader falls back to a voter without setting replica_read;
• learners and witnesses are skipped;
• one failed follower mapping does not hide a later healthy voter;
• all mapping failures invalidate the stale Region and reload replacement peers;
• transport gRPC failures invalidate the failed Store and stale Region route;
• deadline statuses retry without cache churn;
• fallback NotLeader and ServerIsBusy responses do not hide remaining voters;
• follower leader hints are ignored while the voter round continues;
• a successful fallback and a fallback key error both update the shared leader cache;
• two ServerIsBusy(0) responses trigger one followers-only probe;
• the busy probe never sets replica_read and is one-shot;
• non-zero ServerIsBusy follows the normal retry path;
• non-idempotent dispatch errors and undetermined outcomes remain terminal where required;
• multi-shard output order and error precedence remain stable.

Validation

• cargo test request:: --lib
• cargo test --lib
• cargo test --doc
• cargo check --no-default-features
• cargo clippy --all-targets --all-features

Summary by CodeRabbit

• New Features

  • Connection establishment now honors the configured timeout, helping prevent indefinitely stalled connections.

• Bug Fixes

  • Improved request retry and routing behavior when cached region information is stale or a leader is unavailable.
  • Requests can fall back to eligible replica voters in supported failure scenarios.
  • Transient server-busy and routing errors are handled more reliably, including refreshing region information when necessary.

[coderabbitai]

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---

📝 Walkthrough

Walkthrough

RetryableMultiRegion now retries requests across eligible region peers, classifies gRPC errors, probes followers after repeated busy responses, and updates routing state. SecurityManager and TikvConnect now support connection timeouts. MockPdClient exposes routing hooks for fallback tests.

Changes

Multi-region retry and connection updates

Layer / File(s)	Summary
Connection timeout propagation src/common/security.rs, src/store/client.rs	SecurityManager adds timeout-aware connection setup. TikvConnect passes its configured timeout.
PD client routing hooks src/mock.rs	MockPdClient adds hooks for store mapping, leader updates, and region-cache invalidation.
Candidate-based retry pipeline src/request/plan.rs	Retry handling now evaluates the cached leader and eligible voters, classifies gRPC errors, probes followers after repeated zero-wait ServerIsBusy responses, and updates routing state.
Fallback and retry validation src/request/mod.rs, src/request/plan.rs	Tests cover peer fallback, mapping failures, cache invalidation, leader updates, busy-response probes, and gRPC status classification.

Estimated code review effort: 4 (Complex) | ~60 minutes

Merge Risk: ⚪ Minimal · up to c394a

The source-bearing gRPC classification test is reliable with the pinned dependency, so no concrete merge-blocking risk remains.

Sequence Diagram(s)

sequenceDiagram
  participant RetryableMultiRegion
  participant PdClient
  participant TiKVStore
  RetryableMultiRegion->>PdClient: Load cached region and candidate peers
  RetryableMultiRegion->>PdClient: Map candidate region to store
  RetryableMultiRegion->>TiKVStore: Execute request
  TiKVStore-->>RetryableMultiRegion: Return response or gRPC error
  RetryableMultiRegion->>PdClient: Invalidate cache or update leader
  RetryableMultiRegion->>TiKVStore: Retry another candidate

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🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 49.41% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 85 functions across 8 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the primary change: client fallback to eligible followers when the cached leader is unreachable.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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[coocood]

Review report for ca9cc47

Conclusion

I reviewed the change as production-critical request-routing code. I did not find a remaining known P1 correctness issue in the current revision. The implementation satisfies the original cold-region compatibility goal while preserving leader-read semantics and the existing replay-safety boundaries.

Scope reviewed

• leader-first candidate selection and fallback ordering;
• Store mapping and TiKV connection-establishment failures;
• tonic/gRPC error classification;
• Region and Store cache invalidation;
• fallback NotLeader, key error, and ServerIsBusy handling;
• leader-cache updates after a fallback peer accepts a request;
• raw CAS and undetermined-result replay safety;
• multi-shard result ordering and error precedence;
• concurrency-permit lifetime;
• the corresponding Cloud Storage Engine cold-region and ServerBusy paths;
• unit, doc, no-default-feature, and clippy validation.

Correctness checks

Leader semantics and stale-read safety

• Fallback rewrites only the target peer in the request context.
• It never sets replica_read = true.
• Learners and witnesses are excluded.
• A follower therefore cannot serve the request as a follower read. It must return a Region error or accept the request as leader.
• Candidate execution is sequential, so the client does not race duplicate RPCs against multiple peers.

Result: no stale-read path was introduced.

Normal fast path

• Candidate construction is lazy and starts with the cached leader.
• A successful leader request returns immediately.
• No follower is mapped and no additional connection is opened in the common path.

Result: the normal leader-success path retains leader-only work and does not pay eager peer-selection cost.

Mapping and connection failures

• map_region_to_store covers both Store metadata lookup and opening the TiKV channel, so the result is correctly classified as MapRegionToStoreError, not a PD-only error.
• A failed candidate invalidates its Store cache entry and does not hide a later healthy voter.
• If every candidate in a normal round fails to map, the Region cache is invalidated before retrying, preventing repeated use of a completely stale peer list.
• TiKV channel establishment now uses the configured connection timeout, so an unreachable Store cannot block the candidate round indefinitely.

Result: the fallback path makes bounded progress across voters and eventually reloads stale Region metadata.

Routing-error precedence

• Transport-like failures preserve the need to invalidate the Region route even if a later candidate returns ServerIsBusy or fails during mapping.
• A deadline retries another candidate without invalidating caches because it does not prove stale routing.
• Explicit application statuses that do not look transport-derived are returned instead of being replayed across all replicas.

Result: later weaker errors do not mask a routing failure, while deadline and application errors avoid unnecessary cache churn.

Fallback Region responses

• A fallback NotLeader response does not stop the voter round.
• Follower leader hints are intentionally ignored; the remaining voter is tried directly.
• If every voter answers NotLeader in a normal round, the Region is invalidated and reloaded.
• A fallback ServerIsBusy response also does not hide a remaining voter.
• If a fallback peer returns no Region error, its acceptance proves leadership. The shared leader cache is updated before returning either success or a key-level error.

Result: a stale leader cache can heal within the request without trusting potentially stale follower hints.

Replay safety

• Plans marked terminal_on_dispatch_error, including raw CAS, do not retry another peer after a dispatch error because the first request may have reached the server.
• Plans marked terminal_on_undetermined return the first unknown apply outcome immediately.
• An undetermined result cannot be overwritten by a later error from another shard.
• Retryable/idempotent transaction operations retain their previous retry behavior.

Result: follower fallback does not broaden replay for operations whose outcome may already be externally visible.

Cloud Storage Engine verification

I traced the current cloud-storage-engine implementation rather than relying only on client-go behavior.

• The relevant cold-region failure occurs when the entire Get is admitted to the read pool before Raft snapshot and leader validation.
• When that pool is full, CSE returns ServerIsBusy with reason scheduler is busy before it can return NotLeader.
• The current CSE tree does not assign estimated_wait_ms in any ServerIsBusy construction path, so server-generated busy responses currently use the protobuf default value 0.
• The ordinary CSE Get path does not rely on a full rfstore proposal queue returning raftstore is busy; the read-pool entrance is the important path for this feature.

This confirms that a ServerIsBusy(0) follower probe addresses a real pre-Raft rejection path.

ServerIsBusy(0) probe review

• The count is scoped to the same cached leader within one request retry state.
• Two zero-wait busy responses trigger a followers-only round.
• The probe is one-shot per request.
• The request remains a leader request.
• If the probe is inconclusive, the code restores the cached leader and resumes normal busy backoff.
• A non-zero estimated wait does not contribute to the probe count.

Result: the probe breaks the old-leader retry loop without becoming an unbounded follower scan.

Known non-blocking trade-offs

1. In the current CSE build, estimated_wait_ms = 0 is a default value, not a unique stale-leader signal. Genuine scheduler overload and deadline-derived Region busy errors can therefore contribute to the probe count. The consequence is a possible one-shot extra leader-semantics RPC and, in a cold peer with no leader hint, request-driven recovery activity. It still cannot return stale data.
2. Probe state is request-local. Concurrent client requests may independently reach the two-busy threshold, although CSE deduplicates Region-level cold recovery work.
3. Source-less tonic Internal and Unknown statuses are treated as routing failures because tonic can synthesize them for incomplete HTTP/2 responses. This is intentionally limited by the terminal-dispatch guard for non-replayable plans.

These are bounded availability/efficiency trade-offs rather than data-correctness issues, and they match the narrowly scoped cold-region objective.

Test coverage checked

• unreachable leader fallback without replica reads;
• voter filtering for learners and witnesses;
• partial and total Store mapping failure;
• Store and Region invalidation after routing failures;
• no cache churn on deadlines;
• fallback NotLeader, ignored hints, and remaining-voter continuation;
• fallback ServerIsBusy precedence;
• accepted-fallback leader-cache updates, including key-error responses;
• two-busy trigger, same-leader accounting, and one-shot probe behavior;
• non-zero busy behavior;
• raw CAS dispatch safety;
• undetermined-result precedence;
• multi-shard result ordering.

Validation completed:

• cargo test request:: --lib: 36 passed;
• cargo test --lib: 102 passed;
• cargo test --doc: 50 passed;
• cargo check --no-default-features: passed;
• cargo clippy --all-targets --all-features: passed;
• cargo fmt -- --check and git diff --check: passed.

Final assessment

No known P1 remains in this revision. The remaining documented risks are bounded false-positive probing and transport-classification trade-offs; neither changes consistency semantics or permits unsafe replay.

[coderabbitai]

Fixes Applied Successfully

Fixed 6 file(s) based on 1 failed pre-merge check.

Files modified:

• src/common/security.rs
• src/mock.rs
• src/request/plan.rs
• src/store/mod.rs
• src/transaction/buffer.rs
• src/transaction/transaction.rs

Commit: 7abac9c944a10434b54d39d15e68fb9d8c32ea52

The changes have been pushed to the coocood/probe-follower branch.

Time taken: 9m 29s

[coderabbitai]

Actionable comments posted: 2

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Inline comments:
In `@src/request/plan.rs`:
- Around line 1632-1640: Update the assertion for the transport_error passed to
grpc_error_action: with tonic 0.12.3 and Status::from_error, expect
GrpcErrorAction::Return rather than GrpcErrorAction::TryNextPeerAndInvalidate.

In `@src/transaction/transaction.rs`:
- Line 1225: Update check_allow_operation to explicitly reject
TransactionStatus::ReadOnly before write mutations are buffered, covering put,
insert, delete, and batch_mutate while preserving existing behavior for other
transaction statuses.

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Timeline:

• 2026-09-02 09:20:53.215571194 +0000 UTC m=+1271088.386665303: ☑️ agreed by overvenus.
• 2026-09-03 14:40:32.284249096 +0000 UTC m=+1376667.455343203: ☑️ agreed by pingyu.