[yordis]

• When the advisory lock pool dies while Postgres refuses new connections, the lock server can wait forever on a request nothing will answer, so every subscription of the event store crashes on its lock call and none can subscribe again until the lock server or the event store restarts.
• While the lock pool is down for its restart backoff, lock calls block instead of failing, so subscriptions stall rather than retrying on their own schedule.

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PR Summary

High Risk
Changes clustered subscription ownership, schema migration requirements, and how last_seen is written—incorrect fencing or upgrade ordering could stall subscriptions or allow duplicate processing until all nodes upgrade.

Overview
Adds subscription lock fencing so checkpoint writes cannot advance progress after another node takes the advisory lock. Each lock claim bumps lock_generation on the subscription row (schema v1.5.0); acks must carry a FencingToken and are rejected with :fenced when the generation is stale, while last_seen only moves forward via GREATEST.

Advisory lock server gains try_subscription_lock/3 (returns a lease with token), non-blocking release/1, call timeouts mapped to {:error, :timeout}, and errors when the lock pool exits instead of hanging. Startup tolerates a down lock pool; duplicate acquires from the same owner reuse one reference.

Subscriptions acquire the subscription lock path, track fencing_token, disconnect and purge in-flight work on fence, and docs note migrate-before-roll and idempotent handlers in clusters.

Extensive tests (property, churn, TCP proxy) cover fencing, upgrades, and unmigrated schema behavior.

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Walkthrough

Subscription locks now return leases with fencing tokens and committed progress. Acknowledgements are accepted only for the current lock generation and cannot move progress backward. The subscription FSM uses lease data to resume, and it disconnects when storage reports a fenced checkpoint. A schema migration adds the generation column.

Changes

Subscription lock fencing

Layer / File(s)	Summary
Persist subscription lock generations lib/event_store/sql/*, lib/event_store/storage*, lib/event_store/subscriptions/fencing_token.ex, lib/event_store/storage/lock/lease.ex, lib/event_store/tasks/migrations.ex, priv/event_store/migrations/*, test/storage/*, test/migrated_event_store_test.exs, test/list_event_store_migrations_test.exs, test/migrate_event_store_test.exs, mix.exs	The schema and migration add lock_generation. Lock claims return a lease and generation-bound token. Acknowledgements use the token, distinguish fenced and missing subscriptions, and preserve the greater stored progress. Storage, migration, compatibility, and property tests cover these changes.
Manage lock leases and pool failures lib/event_store/advisory_locks.ex, lib/event_store/supervisor.ex, test/advisory_locks_test.exs	The supervisor configures a dedicated monitor for the advisory-lock pool. AdvisoryLocks adds subscription lease acquisition and reference-based release, reuses matching owner locks, and converts query exits to connection errors. Tests cover releases, timeouts, pool failure, and startup while the pool is down.
Apply lease fencing to subscriptions lib/event_store/subscriptions/*, guides/Architecture.md, guides/Cluster.md, guides/Upgrades.md, test/subscriptions/subscription_fsm_test.exs, test/subscriptions/subscription_locking_test.exs	The FSM stores a lease’s token and uses its last_seen value when initializing or reconnecting. Checkpoints pass the token to storage. A fenced result releases the lock and disconnects the FSM. Tests and guides describe takeover, resumption, and migration behavior.
Validate migration and lock takeover test/subscriptions/subscription_churn_property_test.exs, test/subscriptions/subscription_fencing_test.exs, test/subscriptions/subscription_unmigrated_schema_test.exs, test/subscriptions/subscription_zombie_lock_test.exs, test/support/tcp_proxy.ex	Integration and property tests cover subscription churn, lock takeover, stale acknowledgements, migration readiness, and recovery after connection loss. The TCP proxy supports controlled connection failure scenarios.

Priority: Normal

Estimated code review effort: 5 (Critical) | ~90 minutes

Change: Bug fix

Sequence Diagram(s)

sequenceDiagram
  participant Subscriber
  participant SubscriptionFSM
  participant AdvisoryLocks
  participant PostgreSQL
  Subscriber->>SubscriptionFSM: acknowledge event
  SubscriptionFSM->>PostgreSQL: write checkpoint with fencing token
  PostgreSQL-->>SubscriptionFSM: acknowledged or fenced
  SubscriptionFSM->>AdvisoryLocks: release lock after fencing
  AdvisoryLocks->>PostgreSQL: unlock current session

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Possibly related PRs

• TrogonStack/commanded_eventstore#27: Both PRs change advisory-lock behavior when the lock connection or pool fails.

Suggested reviewers: slashdotdash

Merge Risk: Low · up to 7aafa

If a lock release cannot check its session, the advisory lock can stay held until the next reconnect, which delays another node from taking the subscription. This is a narrow edge case with a one-line fix, so the change can merge with that follow-up.

Pre-merge checks: 4 · 1

Failed checks: 1 warning

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 17.19% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 64 functions across 30 files. (6 skipped:…	Write docstrings for the functions missing them to satisfy the coverage threshold.

Passed checks: 4 passed

Check name	Status	Explanation
Title check		The title clearly identifies the main fix: preventing advisory-lock queries from waiting indefinitely when the lock pool fails.
Description check		The description directly explains the lock-pool failure and restart-backoff issues addressed by the changes.
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

Full details: Docstring Coverage

Explanation

Docstring coverage is 17.19% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 64 functions across 30 files. (6 skipped: 6 unsupported.)

Resolution

Write docstrings for the functions missing them to satisfy the coverage threshold.

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Actionable comments posted: 1

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Review comments at @lib/event_store/advisory_locks.ex:
- Around line 279-294: Move the `MonitoredServer.whereis/1` call inside the
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[vheathen]

I ran the script from #331 and my own script for the 120 s refusal case from the issue against the PR head (20e5c0a). #331 no longer reproduces.

The issue script ran twice and the 120 s case with the default retry interval three times. AdvisoryLocks never got stuck in DBConnection.Holder.checkout_call/5, the subscriptions stayed alive and held their locks again once the lock pool was back, and a new subscription got its lock. On Hex 1.4.10 the same scripts still end with AdvisoryLocks in checkout_call/5 and a new subscription unable to subscribe.

The full suite passes at the head in two runs (500 passed, 14 excluded). I also ran the two tests from the first commit (fdb00e0) against #24, and both fail there: the in-flight query gets no reply within 1 s, and the call made while the pool is down exits on its 1 s timeout.

For #330 the results match #24: the script from the issue is still fixed, and when the reconnect hangs the subscription still stays subscribed without its lock for 15 s, which #27 addresses.

One thing stays open at this point in the stack, unchanged from main: try_advisory_lock/3 lets the GenServer.call timeout exit the caller. I hit it with a subscription that started while a proxy was dropping all traffic on its store's lock connection, after which pg_terminate_backend ended that lock session. In all four runs the subscription process exited after 15 s with {:timeout, {GenServer, :call, [..., {:try_advisory_lock, 3, ...}, 15000]}}. #30 catches this exit at its current head (2bf7322), so it only stays if #25 merges without #30.

One smaller point is in an inline comment.

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[yordis]

@vheathen cr 🙏🏻 🙏🏻

[vheathen]

Re-checked at c12fe17: the library changes are the same as in the version I reviewed, the test now releases the holder in its after block, and the full suite passes in two runs (503 passed, 14 excluded).

The #331 script gives the same timeline as at 20e5c0a: the lock pool dies twice while new connections are refused, AdvisoryLocks stays in its receive loop with an empty mailbox at every sample, and the subscriptions take their locks again after the refusals end.

One of three runs of advisory_locks_test.exs failed in the disconnect test from #24, with the same timeout I described there.

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Inline comments:
Review comments at @lib/event_store/advisory_locks.ex:
- Around line 95-100: Handle the absent-pool case in the AdvisoryLocks
initialization path around DBConnection.disconnect_all: treat only the :noproc
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Review comments at @lib/event_store/advisory_locks.ex:
- Around line 448-450: Update the `release_held_lock/2` error branch so that
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• guides/Architecture.md
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• lib/event_store/advisory_locks.ex
• lib/event_store/sql/init.ex
• lib/event_store/sql/statements.ex
• lib/event_store/sql/statements/claim_subscription_lock.sql.eex
• lib/event_store/sql/statements/subscription_ack.sql.eex
• lib/event_store/storage.ex
• lib/event_store/storage/lock.ex
• lib/event_store/storage/lock/lease.ex
• lib/event_store/storage/subscription.ex
• lib/event_store/subscriptions/fencing_token.ex
• lib/event_store/subscriptions/subscription.ex
• lib/event_store/subscriptions/subscription_fsm.ex
• lib/event_store/subscriptions/subscription_state.ex
• lib/event_store/tasks/migrations.ex
• mix.exs
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• test/advisory_locks_test.exs
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• test/migrated_event_store_test.exs
• test/storage/subscription_fencing_property_test.exs
• test/storage/subscription_lock_test.exs
• test/storage/subscription_persistence_test.exs
• test/storage/subscription_previous_version_test.exs
• test/subscriptions/subscription_churn_property_test.exs
• test/subscriptions/subscription_fencing_test.exs
• test/subscriptions/subscription_fsm_test.exs
• test/subscriptions/subscription_locking_test.exs
• test/subscriptions/subscription_unmigrated_schema_test.exs
• test/subscriptions/subscription_zombie_lock_test.exs
• test/support/tcp_proxy.ex

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