ObjectTransaction forwards to the ring owner so one filer's per-path lock
arbitrates every writer of a key. But a ring change hands the key over before
the new owner has rebuilt the locks the prior owner still holds, so for the
cooling-off window both can grant it. LockRing.PriorOwner exists for exactly
this and nothing consulted it.
Route to the prior owner while that window is open. LockRing.WriteOwner
resolves prior-else-current under one read lock, so the pair cannot come from
different rings and name the same filer twice.
An unreachable owner fails the request rather than falling back to the current
one. gRPC reports a response lost in transit as Unavailable, indistinguishable
from a request the owner never saw, so re-sending elsewhere could re-apply what
the owner already committed; and an owner unreachable from here may be
partitioned rather than down, still serving the key to everyone else — which is
the split brain the routing exists to prevent. The window is bounded: once it
closes the ring hands the key to its new owner.
The owner resolution and the forward move into writeOwner/forwardToWriteOwner
so the next routed RPC reuses them rather than copying the block.
Claude-Session: https://claude.ai/code/session_01Fx1Hx8RqsJqHpbfbgTf4WJ
While the ring changed within the last snapshot interval, a fresh owner
asks the key's previous owner (LockRing.PriorOwner) whether it still
holds a conflicting lock before granting TRY_LOCK or answering GET_LK, so
it does not double-grant before re-assertion rebuilds its local state.
The probe is marked cooling_probe so the previous owner answers from
local state without recursing. PriorOwner uses the snapshot's prebuilt
ring rather than rebuilding a hash ring per call.
* dlm: replace modulo hashing with consistent hash ring
Introduce HashRing with virtual nodes (CRC32-based consistent hashing)
to replace the modulo-based hashKeyToServer. When a filer node is
removed, only keys that hashed to that node are remapped to the next
server on the ring, leaving all other mappings stable. This is the
foundation for backup replication — the successor on the ring is
always the natural takeover node.
* dlm: add Generation and IsBackup fields to Lock
Lock now carries IsBackup (whether this node holds the lock as a backup
replica) and Generation (a monotonic fencing token that increments on
each fresh acquisition, stays the same on renewal). Add helper methods:
AllLocks, PromoteLock, DemoteLock, InsertBackupLock, RemoveLock, GetLock.
* dlm: add ReplicateLock RPC and generation/is_backup proto fields
Add generation field to LockResponse for fencing tokens.
Add generation and is_backup fields to Lock message.
Add ReplicateLock RPC for primary-to-backup lock replication.
Add ReplicateLockRequest/ReplicateLockResponse messages.
* dlm: add async backup replication to DistributedLockManager
Route lock/unlock via consistent hash ring's GetPrimaryAndBackup().
After a successful lock or unlock on the primary, asynchronously
replicate the operation to the backup server via ReplicateFunc
callback. Single-server deployments skip replication.
* dlm: add ReplicateLock handler and backup-aware topology changes
Add ReplicateLock gRPC handler for primary-to-backup replication.
Revise OnDlmChangeSnapshot to handle three cases on topology change:
- Promote backup locks when this node becomes primary
- Demote primary locks when this node becomes backup
- Transfer locks when this node is neither primary nor backup
Wire up SetupDlmReplication during filer server initialization.
* dlm: expose generation fencing token in lock client
LiveLock now captures the generation from LockResponse and exposes it
via Generation() method. Consumers can use this as a fencing token to
detect stale lock holders.
* dlm: update empty folder cleaner to use consistent hash ring
Replace local modulo-based hashKeyToServer with LockRing.GetPrimary()
which uses the shared consistent hash ring for folder ownership.
* dlm: add unit tests for consistent hash ring
Test basic operations, consistency on server removal (only keys from
removed server move), backup-is-successor property (backup becomes
new primary when primary is removed), and key distribution balance.
* dlm: add integration tests for lock replication failure scenarios
Test cases:
- Primary crash with backup promotion (backup has valid token)
- Backup crash with primary continuing
- Both primary and backup crash (lock lost, re-acquirable)
- Rolling restart across all nodes
- Generation fencing token increments on new acquisition
- Replication failure (primary still works independently)
- Unlock replicates deletion to backup
- Lock survives server addition (topology change)
- Consistent hashing minimal disruption (only removed server's keys move)
* dlm: address PR review findings
1. Causal replication ordering: Add per-lock sequence number (Seq) that
increments on every mutation. Backup rejects incoming mutations with
seq <= current seq, preventing stale async replications from
overwriting newer state. Unlock replication also carries seq and is
rejected if stale.
2. Demote-after-handoff: OnDlmChangeSnapshot now transfers the lock to
the new primary first and only demotes to backup after a successful
TransferLocks RPC. If the transfer fails, the lock stays as primary
on this node.
3. SetSnapshot candidateServers leak: Replace the candidateServers map
entirely instead of appending, so removed servers don't linger.
4. TransferLocks preserves Generation and Seq: InsertLock now accepts
generation and seq parameters. After accepting a transferred lock,
the receiving node re-replicates to its backup.
5. Rolling restart test: Add re-replication step after promotion and
assert survivedCount > 0. Add TestDLM_StaleReplicationRejected.
6. Mixed-version upgrade note: Add comment on HashRing documenting that
all filer nodes must be upgraded together.
* dlm: serve renewals locally during transfer window on node join
When a new node joins and steals hash ranges from surviving nodes,
there's a window between ring update and lock transfer where the
client gets redirected to a node that doesn't have the lock yet.
Fix: if the ring says primary != self but we still hold the lock
locally (non-backup, matching token), serve the renewal/unlock here
rather than redirecting. The lock will be transferred by
OnDlmChangeSnapshot, and subsequent requests will go to the new
primary once the transfer completes.
Add tests:
- TestDLM_NodeDropAndJoin_OwnershipDisruption: measures disruption
when a node drops and a new one joins (14/100 surviving-node locks
disrupted, all handled by transfer logic)
- TestDLM_RenewalDuringTransferWindow: verifies renewal succeeds on
old primary during the transfer window
* dlm: master-managed lock ring with stabilization batching
The master now owns the lock ring membership. Instead of filers
independently reacting to individual ClusterNodeUpdate add/remove
events, the master:
1. Tracks filer membership in LockRingManager
2. Batches rapid changes with a 1-second stabilization timer
(e.g., a node drop + join within 1 second → single ring update)
3. Broadcasts the complete ring snapshot atomically via the new
LockRingUpdate message in KeepConnectedResponse
Filers receive the ring as a complete snapshot and apply it via
SetSnapshot, ensuring all filers converge to the same ring state
without intermediate churn.
This eliminates the double-churn problem where a rapid drop+join
would fire two separate ring mutations, each triggering lock
transfers and disrupting ownership on surviving nodes.
* dlm: track ring version, reject stale updates, remove dead code
SetSnapshot now takes a version parameter from the master. Stale
updates (version < current) are rejected, preventing reordered
messages from overwriting a newer ring state. Version 0 is always
accepted for bootstrap.
Remove AddServer/RemoveServer from LockRing — the ring is now
exclusively managed by the master via SetSnapshot. Remove the
candidateServers map that was only used by those methods.
* dlm: fix SelectLocks data race, advance generation on backup insert
- SelectLocks: change RLock to Lock since the function deletes map
entries, which is a write operation and causes a data race under RLock.
- InsertBackupLock: advance nextGeneration to at least the incoming
generation so that after failover promotion, new lock acquisitions
get a generation strictly greater than any replicated lock.
- Bump replication failure log from V(1) to Warningf for production
visibility.
* dlm: fix SetSnapshot race, test reliability, timer edge cases
- SetSnapshot: hold LockRing lock through both version update and
Ring.SetServers() so they're atomic. Prevents a concurrent caller
from seeing the new version but applying stale servers.
- Transfer window test: search for a key that actually moves primary
when filer4 joins, instead of relying on a fixed key that may not.
- renewLock redirect: pass the existing token to the new primary
instead of empty string, so redirected renewals work correctly.
- scheduleBroadcast: check timer.Stop() return value. If the timer
already fired, the callback picks up latest state.
- FlushPending: only broadcast if timer.Stop() returns true (timer
was still pending). If false, the callback is already running.
- Fix test comment: "idempotent" → "accepted, state-changing".
* dlm: use wall-clock nanoseconds for lock ring version
The lock ring version was an in-memory counter that reset to 0 on
master restart. A filer that had seen version 5 would reject version 1
from the restarted master.
Fix: use time.Now().UnixNano() as the version. This survives master
restarts without persistence — the restarted master produces a
version greater than any pre-restart value.
* dlm: treat expired lock owners as missing
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
* dlm: reject stale lock transfers
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
* dlm: order replication by generation
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
* dlm: bootstrap lock ring on reconnect
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
---------
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Chris Lu