Cases8 cases

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1. IL-0524 An index migration locked Open Collective's whole Transactions table and degraded the service for nearly an hour

   Open Collective2023

   Symptoms

   At 09:20 UTC on 4 April 2023, seven minutes after a deployment was triggered, Open Collective's service degraded: a migration in the deployment had locked the entire Transactions table in the production database.

   Lock contention or deadlockDatabase overload

   Trigger

   A deployment ran a migration that dropped indexes on the Transactions table and re-created them with CONCURRENTLY, excluding soft-deleted rows.

   Root cause

   Faulty deployment

   Component

   DatabaseSchema migration

   Contributing factor

   Missed in reviewUnrepresentative staging

   Detection

   The degradation of service was noticed at 09:20 UTC, seven minutes after the deployment was triggered.

   Remediation

   • Killed the migration, though the table remained locked
   • Put the frontend into maintenance mode to limit queries
   • Closed the other connections to the database
   • Cancelled the new index creation and restored the old index
   • Restored the frontend and confirmed the issue was resolved

   Action items

   • Add a script that simulates production load locally, using a tool like vegeta
   • Run this kind of migration during specific maintenance windows
   • Create the new index first and drop the old one afterwards in future migrations
   • Insist on a review from another engineer before merging such a pull request

   Lessons

   Dropping the old index before creating the new one was too optimistic under constant production queries, even with CONCURRENTLY, and an unreviewed pull request let it through. Tests locally and on staging did not show the problem because there were no tools to run migrations in stressed environments. Putting the frontend in maintenance mode limited the impact while showing users a clear message.

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2. IL-0385 GitHub MySQL read replicas hit a semaphore deadlock during a schema migration's table rename and kept crash-recovering

   GitHub2021

   Symptoms

   On 27 November 2021 at 20:40 UTC, a significant part of GitHub's MySQL read replicas entered a semaphore deadlock while a schema migration on a large MySQL table ran its final step, the rename that moves the updated table into place. Core GitHub services, including GitHub Actions, API Requests, Codespaces, Git Operations, Issues, GitHub Packages, GitHub Pages, Pull Requests and Webhooks, became degraded.

   Lock contention or deadlockDatabase overload

   Trigger

   The final rename step of a schema migration on a large MySQL table. Schema migrations are a common task at GitHub and often take weeks to complete.

   Root cause

   Software defect

   Component

   DatabaseSchema migration

   Contributing factor

   No headroomNo staged rollout

   Remediation

   • Promoted every healthy internal replica into the production path to add read capacity
   • Removed production traffic from the broken replicas until each had processed the table rename
   • Moved the recovered replicas back into production to restore capacity

   Action items

   • Continue functional partitioning so migrations can run in canary mode on a single shard
   • Update internal procedures to increase how much each cluster is over-provisioned
   • Pause schema migrations until it is known how to safeguard against this failure
   • Keep investigating the specific failure scenario
   • Test the migration tooling and classify ways to improve it for this kind of scenario

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3. IL-0391 GoCardless API requests timed out for about 15 seconds while a Postgres migration waited for an AccessExclusive lock

   GoCardless2015

   Symptoms

   Just after a planned Postgres schema migration started, GoCardless received alerts that API requests were timing out. The API was down for about 15 seconds, until the migration went through and the API came back.

   Lock contention or deadlockService outage

   Trigger

   A deploy with a schema migration that renamed the empty invoicing tables and dropped and recreated their foreign key constraints, which referenced some of the most heavily used tables.

   Root cause

   Faulty deployment

   Component

   DatabaseSchema migration

   Contributing factor

   Unverified assumption

   Detection

   Alerts about API requests timing out began just after the schema migration started.

   Lessons

   Avoiding renames of tables in use, table rewrites under an exclusive lock and expensive work while holding one was not enough for GoCardless: a short DDL statement can still cause downtime by waiting in the lock queue behind a long-running query. GoCardless recommends removing long-running queries and transactions from the application and running analytics on an asynchronously updated replica, setting log_min_duration_statement and log_lock_waits to find such queries, setting lock_timeout in migration scripts so a deploy aborts rather than taking the application down, splitting schema changes so locks are held for shorter times, and keeping Postgres up to date. Developers with a console on the production database can cause the same downtime by leaving a transaction open.

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