Cases3 cases

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1. IL-0436 A supremum lock left by ROLLBACK TO SAVEPOINT in a long transaction blocked inserts and filled Aurora MySQL connections

   freee2025

   Symptoms

   Connections to Aurora MySQL failed again and again in one freee service until its web pages could not be opened at all. The database did not respond, the web application timed out and could not open new database connections, and the whole service was down.

   Lock contention or deadlockService outage

   Trigger

   A long transaction inserted a row into the table after a SAVEPOINT and then ran ROLLBACK TO SAVEPOINT, which left it holding an exclusive lock on the supremum of the table's primary index.

   Root cause

   Software defect

   Component

   Database

   Contributing factor

   Unverified assumptionUnreviewed defaults

   Action items

   • As a rule stop issuing SAVEPOINT, split a transaction when a partial rollback is needed, and set an application-side timeout where a nested transaction is unavoidable
   • Teach developers to consider READ COMMITTED per session for use cases that can accept phantom rows
   • Monitor long transactions and keep working to remove them across the company

   Lessons

   ROLLBACK TO SAVEPOINT rolls back rows but does not release their locks, so rolling back one inserted row can leave a supremum lock that blocks every INSERT into the table. ORM frameworks, including Active Record for nested transactions, use SAVEPOINT internally without developers noticing. Locks held by long transactions often go unnoticed when few workloads are blocked, and this incident showed their hidden risk.

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2. IL-0511 MIT xPro stopped responding after a configuration deploy as pgbouncer logins to RDS failed with 'wrong password type'

   MIT Open Learning2023

   Symptoms

   On 30 October 2023 at 3:51PM, six minutes after a configuration update was deployed to the xPro production Heroku stack, Pingdom alerted the DevOps on-call engineer, who confirmed at 3:52PM that the site was not responding.

   Service outageCredentials unavailable

   Trigger

   A configuration update applied through Salt at 3:45PM found that the Vault lease of the database credentials was about to expire and could not be renewed, so Vault generated new credentials.

   Root cause

   Configuration change

   Component

   DatabaseSecrets store

   Contributing factor

   Unreviewed defaultsChange with a delayed effectConfiguration not in code

   Detection

   Pingdom alerted the DevOps on-call engineer at 3:51PM, and the alert went through OpsGenie; Pingdom closed it automatically at 4:25PM.

   Remediation

   • Created a new database parameter group and applied it to the running RDS instance
   • Cleared the cached credentials and re-applied the Heroku configuration through Salt

   Action items

   • Move the management of xPro resources, including the RDS instance, into Pulumi, where AWS default parameter groups are not used

   Lessons

   Almost none of the configuration for this stack, the RDS instance in particular, was managed as code, and that was one of the main reasons the fix took long. Under Pulumi the instance would not have used an AWS default parameter group, and the fix would have been a change to an attached group.

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3. IL-0218 GitLab.com's primary PostgreSQL data was wiped by mistake and, with no working backup, the site was down about 18 hours

   GitLab2017

   Symptoms

   From around 19:00 UTC on 31 January 2017, load on GitLab.com's PostgreSQL database rose, which GitLab suspected was spam, and many users could not post comments on issues and merge requests. Around 23:00 UTC the replication of the PostgreSQL secondary fell behind and then failed, because WAL segments it still needed had already been removed from the primary.

   Database overloadReplication lagData lossService outage

   Trigger

   While trying to restore replication to the secondary, an engineer ran the removal of the PostgreSQL data directory on the primary, db1.cluster.gitlab.com, believing it was the secondary.

   Root cause

   Operator error

   Component

   DatabaseDatabase replicationDatabase backups and snapshotsCloud block storage

   Contributing factor

   Untested backupSingle point of failureNo runbookPages not deliveredUnverified assumptionUnreviewed defaults

   Detection

   The engineer who ran the deletion on the primary noticed the mistake and stopped it a second or two later.

   Remediation

   • Raised max_wal_senders from 3 to 32 and lowered max_connections from 8000 to 2000 so PostgreSQL would restart
   • Stopped the deletion on the primary a second or two after noticing it
   • Restored the database from the LVM snapshot taken about 6 hours before the outage by copying the PostgreSQL data directory from staging
   • Restored the webhooks from a separate database built from the snapshot and imported them as a SQL dump
   • Incremented all database sequences by 100,000 so IDs used before the outage could not be reused
   • Re-enabled GitLab.com gradually
   • Rebuilt projects created after 17:20 UTC in basic form from their Git repositories on the NFS cluster

   Action items

   • Change the shell prompt (PS1) on all hosts to show more clearly which host and environment is in use
   • Monitor backups with Prometheus
   • Set PostgreSQL's max_connections to a sane value
   • Investigate point-in-time recovery and continuous archiving for PostgreSQL
   • Take LVM snapshots of the production databases every hour
   • Take Azure disk snapshots of the production databases
   • Move staging to the ARM environment
   • Recover the production replicas
   • Test recovery of PostgreSQL database backups automatically
   • Improve the PostgreSQL replication documentation and runbooks
   • Investigate pgbarman for PostgreSQL backups
   • Investigate WAL-E for database backup and realtime replication
   • Build a streaming database restore
   • Assign an owner for data durability
   • Set up multiple secondaries and balance load among them with pgpool-II
   • Provide a tool for developers to run and revert Rails migrations on staging
   • Build owned recovery procedures for the rest of the GitLab.com infrastructure, not only the database
   • Publish a backup monitoring dashboard and add more backup data to it
   • Improve the abuse reporting and response system so removal for spam does not hard delete
   • Redact engineers' names in future public incident documents

   Lessons

   GitLab split the incident into two problems: GitLab.com went down because the primary's data directory was removed by mistake during a manual, undocumented replication rebuild, and recovery took over 18 hours because the backups had failed silently, disk snapshots were not enabled and the only copy sat on slow staging disks. The backup procedure was never tested because no one owned it. GitLab chose to focus on fast, regularly practised recovery and on making it obvious which host an engineer is on, rather than on blocking commands, since a safer rm only protects against one of many ways to lose data.

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