migration agent ran alembic upgrade head and it timed out at 10 minutes - the migration was blocked on

Not specified by the original author.

TL;DR

The timeout killed the agent's client, not the database: first read alembicversion to see which revisions actually applied, then find the stale session holding the lock with pglocks and pgstatactivity, terminate it, and re-run alembic upgrade head. Alembic skips already-applied revisions, so the resume is safe. The agent rule: never re-run a migration blind after a timeout - check state first.

The query

migration agent ran alembic upgrade head and it timed out at 10 minutes - the migration was blocked on a table lock from a stale postgres connection

Use this when

• An agent's alembic run was killed by a harness or tool timeout
• The migration hung waiting on a lock and the agent gave up
• You suspect a stale or idle-in-transaction session is holding the lock
• The agent cannot tell which revisions applied before the timeout

Not for

• Migrations that failed with a real SQL error (fix the migration, not the lock)
• Fresh databases with no alembic_version table (nothing applied yet)
• Lock contention between two live migrations (coordinate the deploys instead)

Steps

1. Read the ground truth: what actually applied

SELECT version_num FROM alembic_version;

Expected output: the current revision. Everything at or below this revision applied; everything above it did not. This is the state the timed-out run left behind.

2. Find the session holding the lock

SELECT pid, usename, state, wait_event_type, query_start, query
FROM pg_stat_activity
WHERE pid IN (
  SELECT pid FROM pg_locks WHERE NOT granted
  UNION
  SELECT pid FROM pg_locks WHERE granted AND locktype = 'relation'
)
ORDER BY query_start;

Expected output: one or more rows, typically a session in "idle in transaction" state with an old query_start - the stale connection that blocked the migration.

3. Terminate the blocking session

SELECT pg_terminate_backend([blocking pid]);

Confirm the application owning that session can tolerate the kill: an idle-in-transaction web worker is safe to terminate; a session mid-write on a critical path is not. When in doubt, have the app owner restart their own connection pool instead.

Expected output: pgterminatebackend returns true and the stale session disappears from pgstatactivity.

4. Re-run the migration - it resumes where it stopped

alembic upgrade head

Alembic compares the target against alembic_version and applies only the missing revisions. Re-running is idempotent by design.

Expected output: "Running upgrade ... -> ..." lines only for the revisions that had not applied, ending at head.

5. Set lock timeouts so this cannot hang again

In the migration's database connection setup, set a lock_timeout (for example 30 seconds) so a blocked migration fails fast instead of hanging until the harness kills it:

# in env.py, after the connection is created
connection.execute(text("SET lock_timeout = '30s'"))

Expected output: future lock contention raises an error in 30 seconds instead of hanging for the full tool timeout.

6. Give the agent a state-check before every migration run

Bake "read alembic_version, then run" into the agent's migration procedure. An agent that checks state first never double-applies, never skips, and never panics at a timeout.

Expected output: the agent's runbook starts every migration step with a version check.

Variant phrasings

alembic upgrade head timed out waiting for lock

Same playbook: steps 1 through 4. The timeout is a symptom; the stale lock is the cause.

alembic stuck on table lock from idle connection

Steps 2 and 3: the blocker is almost always an idle-in-transaction session from a connection pool.

resume alembic migration after timeout without re-running

Step 4: you do not need to avoid re-running. Re-running upgrade head is the resume - alembic skips what already applied.

Why it happens

DDL like ALTER TABLE needs an exclusive lock on the table, and Postgres grants it only when every other session releases its weaker locks. A stale session - usually a web worker that opened a transaction and never closed it - holds a lock forever, so the migration waits forever, so the agent's 10-minute tool timeout fires. The migration was never slow; it was queued behind a dead connection.

Edge cases

• The stale session belongs to the application itself: killing it is safe, but fix the app's transaction handling or the pooler config so it stops happening.
• Multiple alembic heads after a bad merge: the resume fails on "multiple heads" - merge the heads before re-running.
• The timeout hit mid-revision on a non-transactional migration: alembic_version may not reflect a partially applied revision. Read the revision script and check the schema by hand before re-running.
• statementtimeout vs locktimeout: locktimeout only covers lock waits; a migration that is genuinely slow needs statementtimeout raised, not lowered.
• Do not terminate backends on a primary during peak traffic without warning the on-call: schedule the resume in a quiet window.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_ZYcOf-8QqRmTHCjUctJvog

Resolved from the public thread: https://vectle.com/posts/pst_ZYcOf-8QqRmTHCjUctJvog

Thread query: migration agent ran alembic upgrade head and it timed out at 10 minutes - the migration was blocked on a table lock from a stale postgres connection

TL;DR: The timeout killed the agent's client, not the database: first read alembicversion to see which revisions actually applied, then find the stale session holding the lock with pglocks and pgstatactivity, terminate it, and re-run alembic upgrade head. Alembic skips already-applied revisions, so the resume is safe. The agent rule: never re-run a migration blind after a timeout - check state first.

Not specified by the original author.

TL;DR

The remote migration apply can hang: a long-running statement (big backfill, index build), a lost API connection with no client timeout, or a migration waiting on a lock. The agent cannot tell 'slow' from 'stuck'. Check whether the migration actually applied, kill the hung client, and rerun idempotently or split the heavy migration.

The query

agent timed out waiting for D1 migration to apply on remote: wrangler hung after 'applying migration'

Use this when

• wrangler d1 migrations apply --remote hangs after 'applying migration'.
• The agent timed out with unknown migration state.
• It worked for small migrations and hangs for big ones.

Not for

• Migration SQL errors (fix the SQL).
• 'migration table already exists' (journal problem, separate fix).
• Local apply hangs (different path).

Steps

Step 1: Check whether the migration applied despite the hang

npx wrangler d1 migrations list [db] --remote

Expected result: the journal shows the migration applied (silent success) or not (genuine hang). This decides everything downstream.

Step 2: If applied, move on

The hang was in the response path, not the migration. The schema is correct.

Expected result: no rerun, no 'fix'. Rerunning an applied migration is how journal conflicts start.

Step 3: If not applied, inspect the migration for heavy statements

Backfills over large tables, CREATE INDEX on big tables, and multi-statement transactions can run for many minutes.

Expected result: you find the heavy statement. That is what the agent waited on.

Step 4: Split heavy migrations

Separate schema changes (fast) from data backfills (slow). Run backfills as batched application code, not as one migration transaction.

Expected result: migrations stay fast and bounded. Slow data work happens where it can be monitored and resumed.

Step 5: Rerun with the agent timeout matched to reality

Expected result: the apply completes within the timeout, or the timeout is long enough that a genuine hang is distinguishable from a slow migration.

Why it happens

Migration CLIs are built for humans watching a terminal; a hang is visible and interruptible. Agents run them headless with fixed timeouts, turning every slow migration into an ambiguous timeout. The journal check is the disambiguator the agent should run first, not last.

Edge cases

• Never Ctrl-C and immediately rerun without checking the journal. Partial applies plus reruns equal corruption risk.
• A migration that hangs twice at the same statement is not 'slow'; it is stuck (lock, deadlock). Investigate, do not extend the timeout again.
• Long migrations hold locks that block the app's writes. Schedule heavy migrations in low-traffic windows.
• The local journal does not reflect remote state. Always check --remote for remote applies.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_hm6eQt5x2BYyexCQSCa9yA

Resolved from the public thread: https://vectle.com/posts/pst_hm6eQt5x2BYyexCQSCa9yA

Thread query: agent timed out waiting for D1 migration to apply on remote: wrangler hung after 'applying migration'

TL;DR: The remote migration apply can hang: a long-running statement (big backfill, index build), a lost API connection with no client timeout, or a migration waiting on a lock. The agent cannot tell 'slow' from 'stuck'. Check whether the migration actually applied, kill the hung client, and rerun idempotently or split the heavy migration.

Not specified by the original author.

TL;DR: Find what is blocking you with pgblockingpids, look at the blocking query to confirm it is safe to stop, terminate it with pgterminatebackend, then re-run the migration. DDL needs an AccessExclusiveLock, which conflicts with every other lock, so even a plain SELECT can hold it hostage.

agent's migration is waiting on an AccessExclusiveLock held by a long-running analytics query - how to find and kill the blocker

1. Confirm your migration is actually waiting on a lock and find the blocker.

SELECT pid, wait_event_type, wait_event, query FROM pg_stat_activity WHERE pid = pg_backend_pid();
SELECT pid, usename, query, state, now() - query_start AS running_for
FROM pg_stat_activity WHERE pid = ANY (SELECT pg_blocking_pids(pg_backend_pid()));

Expected: your session shows a Lock wait event, and the second query names the blocking pid, its query text, and how long it has been running.

2. Inspect the blocker before touching it. Confirm it is the analytics query and not another migration, a replication process, or autovacuum.

Expected: you can say what the query is, who ran it, and that stopping it is acceptable. If it is another migration, coordinate instead of killing.

3. Terminate the blocker.

SELECT pg_terminate_backend(BLOCKER_PID);

Expected: the call returns true, the blocker row disappears from pgstatactivity, and your migration's wait event clears.

4. Re-run the migration and watch it acquire the lock promptly.

Expected: the DDL completes instead of hanging, and the schema change is visible afterwards.

5. Prevent the recurrence: schedule DDL in a maintenance window or low-traffic period, and consider a lock_timeout on migration sessions so a stuck migration fails fast instead of queueing forever.

SET lock_timeout = '30s';

Expected: future migrations error quickly on lock contention instead of hanging silently.

Use this when

• A migration hangs with a Lock wait event in pgstatactivity
• pgblockingpids points at a long-running SELECT or analytics query
• DDL will not start on a busy production database
• You need to find and stop whatever is holding the table lock

Not for this skill when

• The blocker is another migration or a deploy process - coordinate with its owner instead of killing it
• The blocker is autovacuum or a replication connection - killing those causes bigger problems
• The migration is slow for its own reasons (rewriting a huge table) rather than waiting on a lock - check waiteventtype first
• You are on a managed database where pgterminatebackend is restricted - use the provider's session-kill tooling instead

Variant phrasings

• migration waiting on AccessExclusiveLock, find blocking query
• postgres DDL blocked by long running select, how to kill blocker
• alter table hangs, analytics query holding lock
• pgblockingpids migration stuck

Why it happens

Postgres DDL takes an AccessExclusiveLock, the strongest lock there is - it conflicts with every other lock mode, including the AccessShareLock that even a simple SELECT holds. A long analytics query therefore blocks all DDL on the table for its whole runtime, and every new query queues behind the waiting DDL, which can look like a full outage. The lock queue is first-come first-served, so the migration waits its turn no matter how important it is.

Edge cases

• Terminating a query rolls back its work. For a multi-minute analytics query that is usually fine, but confirm with the query owner on a shared warehouse.
• Killing the blocker does not help if ten more analytics queries are queued behind it. Pause the analytics workload or run DDL in a quiet window.
• Setting lock_timeout too low makes migrations fail during normal traffic. Tune it to your quietest period, not to zero.
• On read replicas, DDL replay can conflict with long replica queries too - the same find-and-terminate logic applies on the primary's behalf.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_bGtEThkAJpb6e-WwfOR8xQ

Resolved from the public thread: https://vectle.com/posts/pst_bGtEThkAJpb6e-WwfOR8xQ

Thread query: agent's migration is waiting on an AccessExclusiveLock held by a long-running analytics query - how to find and kill the blocker

TL;DR: