Cases33 cases

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1. IL-0305 Replicas of one Intercom Vitess shard hit 100% CPU and fell up to 2.2 hours behind, serving stale inbox data

   Intercom2026

   Symptoms

   From 17:03 UTC on January 27, 2026, a small subset of Intercom customers, fewer than 1%, saw inconsistent latency and stale data when reading from the platform. At 17:25 UTC an alarm on an excessive message backlog in a worker declared an incident automatically.

   Replication lagDatabase overloadHigh latency

   Trigger

   An internal app deletion process, running more concurrent deletion steps per app than usual with no rate limiting, and the ETL data pipeline both put heavy load on the same database shard at the same time.

   Root cause

   Capacity exhaustion

   Component

   DatabaseDatabase replication

   Contributing factor

   No headroom

   Detection

   An alarm on an excessive message backlog in a worker fired at 17:25 UTC and declared an incident automatically.

   Remediation

   • Disabled ETL processing to reduce load on the affected shard
   • Scaled the app deletion workers to zero
   • Re-enabled ETL carefully once monitoring showed stable metrics

   Action items

   • Increase the capacity of shards to leave headroom for unexpected load spikes
   • Add rate limiting and resource controls to the app deletion process so it backs off when database load is high
   • Review how background work such as ETL and maintenance tasks competes with customer-facing operations for database capacity
   • Alert on replication lag earlier and link it to specific workloads
   • Communicate proactively with affected customers in incidents with a limited blast radius

   Lessons

   Because data is spread across many independent shards, the overload stayed on one shard and reached fewer than 1% of customers instead of the whole platform.

   View case
2. IL-0368 PlanetScale's control plane went down and some database branches failed queries during the AWS us-east-1 outage

   PlanetScale2025

   Symptoms

   At 7:13 UTC on 20 October 2025, PlanetScale's continuous tests in production started failing because they could not create new database branches. The service that creates, resizes and configures database branches was unavailable, a near-total control plane outage, while existing database branches kept their capacity and connectivity.

   Service outageScale-up failureNetwork unreachableHigh error rateReplication lagDNS resolution failures

   Trigger

   The incident started with a DNS misconfiguration at one of PlanetScale's service providers. An Amazon DynamoDB outage impacted AWS STS, and several hours of capacity constraints and network instability in AWS us-east-1 followed.

   Root cause

   Cloud provider outage

   Component

   DatabaseDatabase replicationDatabase proxy and connection poolerObject storage

   Contributing factor

   –

   Detection

   PlanetScale's continuous testing in production started failing and alerted engineers at 7:13 UTC. At 10:05 UTC an alert fired for a Kubernetes operator in a large single-tenant customer installation exhausting its resources.

   Remediation

   • Disallowed new databases in us-east-1 and made us-east-2 the default region for new databases
   • Delayed new backups and cancelled pending backups that were waiting for an EC2 instance
   • Advised PlanetScale Managed customers with vtgate autoscaling to shed load
   • Paused draining and terminating EC2 instances older than 30 days
   • Kept vacant EC2 instances for reuse instead of terminating them
   • Bin-packed vtgate processes more tightly for customers with autoscaling
   • Sent manual reparent requests to move primaries to healthier availability zones
   • Restarted the edge load balancer and vtgate processes that did not recover after the partitions healed

   Action items

   • Understand and become resilient to the failure modes of SaaS dependencies for CI/CD, SSO, web application hosting and incident communication
   • Investigate more ambitious ways to reduce runtime dependence on internal and AWS services
   • Look into using all six us-east-1 availability zones to tolerate zonal outages and partitions

   Lessons

   Keeping the control plane separate from the data plane meant the control plane outage did not affect customer databases, and redundancy, automated failover and zonal traffic routing limited the effect of the network partitions. Network partitions are hard to reason about, test and tolerate: with three availability zones a system survives losing one zone only if the other two can still reach each other reliably.

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3. IL-0242 Both PostgreSQL servers behind matrix.org were lost during a disk expansion, and the homeserver was down for 24 hours

   Matrix.org Foundation2025

   Symptoms

   On 2 September 2025 the matrix.org homeserver went down at 17:45 UTC and stayed down until 18:00 UTC the next day. Both machines of its PostgreSQL cluster were out of action: the original primary had lost its RAID array during a disk expansion, and the data directory of the new primary had been deleted by mistake.

   Service outageData loss

   Trigger

   During maintenance to add drives, an existing drive disappeared from the primary database server's RAID 10 array at 11:17 UTC, which led to a failover and a rebuild of that server. The outage itself began when the data directory of the new primary on db-01 was deleted while clearing a failed restore on db-02.

   Root cause

   Operator error

   Component

   DatabaseDatabase backups and snapshotsDatabase replication

   Contributing factor

   Overlapping failuresHarmful manual actionUnpatched known bugNo safeguard on destructive change

   Detection

   Monitoring fired when the drive disappeared from db-02's RAID array at 11:17 UTC, and Mythic Beasts confirmed the issue. The team realised it had deleted db-01's data directory, and the backend team was then paged, an incident was opened and an emergency was declared.

   Remediation

   • Created a fresh RAID 10 array on db-02
   • Unblocked WAL archiving from the new primary on db-01 to S3
   • Started restoring db-02 from the S3 backups with wal-g
   • Shut down Synapse and the damaged primary database
   • Remounted the data partition of db-01 read-only
   • Copied the WALs missing from S3 from db-01 to db-02
   • Updated wal-g and patched it to continue from the partial restore
   • Configured a restore_command wrapper that fetched WALs from S3 and the local files, and replayed them in standby mode
   • Promoted db-02 to primary and restarted Synapse
   • Ran backups against the primary while no replica existed
   • Rebuilt db-01 as a replica from a base backup of db-02

   Action items

   • Run pg_repack to reclaim free space
   • Take incremental backups more often, since they restore faster than WAL replay
   • Reduce Synapse's storage requirements
   • Consider changing the terminal background colour by the database's role, for example by watching for the standby.signal file
   • Consider wrapper scripts around sensitive commands, such as an alias for rm, and automating operations such as starting a base backup from primary to secondary
   • Shorten the recovery time, with infrastructure from service providers
   • Upgrade wal-g on all servers and repeat disaster recovery testing with it
   • Explore ZFS local snapshots at the next hardware refresh
   • Make status.matrix.org the source of truth during incidents and keep social media updated

   Lessons

   Most of the difficulty came from the size of the database: restoring 51TB and replaying WALs took far longer than hoped, and incremental backups restore much faster than WAL replay. Running a destructive command on the wrong server was the turning point, and Matrix.org looks for better tooling and processes rather than blaming the individual; working as a trio did not catch the mistake, and server names that suggest a role caused confusion because a primary and a secondary swap roles. The recovery lost no data, but the recovery time needs to be shorter, and the wal-g versions on the disaster recovery server and the production servers had drifted apart.

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4. IL-0527 OpenStreetMap.org made no replication diffs for 16 hours as PostgreSQL logical decoding failed on a memory alloc error

   OpenStreetMap Foundation2025

   Symptoms

   At 21:40 UTC on 25 June 2025, Alertmanager alerted the OpenStreetMap operations team: minutely replication feed delayed - 6m 2s. Since 21:33 UTC every run of the osmdbt timer that produces the minutely replication diffs had failed with ERROR: invalid memory alloc request size 1243650064.

   Replication lag

   Root cause

   Software defect

   Component

   Database replicationDatabase

   Contributing factor

   Pages not delivered

   Detection

   Alertmanager sent the first alert at 21:40 UTC, minutely replication feed delayed - 6m 2s, and several more followed, but none was a pager or SMS alert. A community report followed at 00:37 UTC and a social media report at 01:13 UTC.

   Remediation

   • Stopped the osmdbt timer
   • Put OpenStreetMap.org into a read-only maintenance window to suppress edits
   • Ran osmdbt by hand with fakelog to skip ahead of the PostgreSQL replication condition
   • Restarted the osmdbt timer for minutely diffs
   • Ended the read-only maintenance window

   Action items

   • Upgrade PostgreSQL to 17.x
   • Raise an issue with the PostgreSQL developers

   Lessons

   Communication with the osmdbt developer was crucial for working out the workaround, and the operations team's understanding of osmdbt was limited.

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5. IL-0376 A maintenance update set off needless full syncs between Upstash Redis replicas in eu-west-1, slowing the databases

   Upstash2025

   Symptoms

   On 25 February 2025, regional Upstash Redis databases in AWS eu-west-1 slowed down after a maintenance update, with very high CPU load on databases that have a replica in that region. Latency stayed high until about 21:23 UTC, even after the affected databases were unthrottled at 15:06 UTC.

   High latencySustained high node CPU

   Trigger

   A maintenance update to the regional Upstash Redis databases in AWS eu-west-1.

   Root cause

   Software defect

   Component

   In-memory cacheDatabase replication

   Contributing factor

   Automation amplification

   Remediation

   • Added more resources to the regional eu-west-1 cluster
   • Unthrottled the affected databases at 15:06 UTC and enabled more throughput

   Action items

   • Deploy a fix that stops regional databases from starting unnecessary full synchronisations
   • Contact regional database users about migrating to Upstash Global, which does not have this issue

   View case

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