IL-0497 Buttondown · 2026-03-31database
Buttondown's database ran out of connections twice in one day and requests failed while health checks mostly passed
On 31 March 2026 Buttondown's database reached the maximum number of connections it was configured to accept, and once that ceiling was hit, new requests could not acquire a connection and failed. The database itself stayed healthy. The connection limit had been set a long time before and had not been revisited as Buttondown's traffic patterns changed. The health check endpoint returned a static 200 without checking that the process could get a database connection, and the external checker's requests reached workers that already held connections, so monitoring did not alert and the outage was found through user reports and manual observation.
In the first outage the team identified the connection exhaustion and killed active queries to free connections, with about seven minutes of downtime. The same root cause came back later that day, and this time no connection was free to kill queries with, so the database had to be restarted to force-kill the running queries; that outage lasted about thirteen minutes. Buttondown says the second outage lasted nearly twice as long because no connection was reserved for administrative access.
- Severity
- –
- Scope
- Buttondown's service: requests that needed a database connection failed, in two outages of about seven and about thirteen minutes
- Users affected
- –
- Region
- –
- Time to detect
- –
- Time to resolve
- –
Symptoms
On 31 March 2026 Buttondown was down twice, for about seven and about thirteen minutes. New requests could not get a database connection and failed, although the database itself was healthy: queries were fast, CPU and memory were fine and replication lag was nominal.
No automated alert fired. The health check endpoint returned 200 for most requests, because the health checker's requests happened to land on workers that already held open database connections. The team found out from user reports and its own manual observation.
In the first outage the team killed active queries to free connections. In the second outage the connection count was so saturated that the team could not connect to the database to kill queries, because the tool used in the first outage itself needed a database connection.
Root cause
Root cause Capacity exhaustion
- Trigger
- –
Detection
User reports and the team's own manual observation, not automated monitoring. The health check endpoint kept returning 200 for most requests, so no alert fired.
Remediation
- Killed active queries to free database connections in the first outage
- Restarted the database to force-kill the running queries in the second outage
Action items
- Reserve a small number of database connections for administrative access
- Monitor and alert on the database connection count as a percentage of the configured maximum
- Make the health check endpoint run a lightweight query so connection exhaustion shows in the health checker and the load balancer
- Add database connection limits to the quarterly capacity review
- Document and script a fallback that force-recycles workers to release connections without needing a database connection
Lessons
Buttondown notes that the tool used to recover from the first outage needed the same resource that was exhausted, which locked the team out during the second outage, and that a health check which cannot detect connection exhaustion, the most common failure mode, gives little protection. A configuration value set long ago drifted out of line with traffic because nobody reviewed it.
Sources
Related cases
IL-0450Oracle RAC commits behind order APIs kept waiting over 0.1 s on log file sync while average host CPU had headroomToss Securities · 2026same root cause · shared Database
IL-0553CircleCI's orchestration database filled its transaction log volume, and no workflows or jobs started for 50 minutesCircleCI · 2026same root cause · shared Database
IL-0379InfluxDB Cloud queries in Azure West Europe slowed and timed out as storage pods fell behind and kept restartingInfluxData · 2026same root cause · shared Database
IL-0309Preset's RDS PostgreSQL 14.7 to 17.9 upgrade in US East 1 ran out of storage and took Superset down for six hoursPreset · 2026same root cause · shared Database
IL-0435Error rates rose as Aurora MySQL killed connections and declined queries to avoid OOM while load metrics stayed flatCyberAgent · 2026same root cause · shared Database
IL-0288ZooKeeperWait rose to thousands of seconds on three redundant ClickHouse instances, degrading a CDN77 data pipelineCDN77 · 2026same root cause · shared Database
IL-0350Hosts in an IONOS TXL cluster lost connectivity when an InfiniBand fabric resweep failed on a full multicast tableIONOS Cloud · 2026same root cause · shared Database
IL-0305Replicas of one Intercom Vitess shard hit 100% CPU and fell up to 2.2 hours behind, serving stale inbox dataIntercom · 2026same root cause · shared Database
IL-0390PostHog US Cloud event ingestion slowed to delays of up to 2 days as writes to the Persons Postgres table stalledPostHog · 2025same root cause · shared Database
IL-0100PostHog's feature flags pods were packed onto Kubernetes nodes that were too small for their CPU requests, and CPU saturation above 90% caused three outages in ten days through Redis overload, connection pool exhaustion and slow queriesPostHog · 2025same root cause · shared Database
IL-0287One overloaded shard of MongoDB's internal monitoring database delayed Atlas metrics, alerts and cluster operationsMongoDB · 2025same root cause · shared Database
IL-0557Database transaction locks piled up on Storj's US1 satellite, and uploads and downloads failed with timeouts for 2 hoursStorj · 2025same root cause · shared Database
IL-0551Bitbucket Cloud's overloaded primary database failed over, and the website, API and Pipelines were slow for almost a dayAtlassian · 2025same root cause · shared Database
IL-0298Wikimedia edits failed for 38 minutes when sessionstore Cassandra nodes in both data centres crashed on full disksWikimedia Foundation · 2025same root cause · shared Database
IL-0312Gadget's main Postgres did not start again after a restart to raise max_connections, and the platform went downGadget · 2024same root cause · shared Database
IL-0544Wasabi's database cache stopped responding when its queueing service filled up, and S3 failed in all regionsWasabi · 2024same root cause · shared Database
IL-0447An every RDS for MySQL instance slowed badly when its gp2 EBS BurstBalance ran out and IOPS fell to the baselineevery · 2024same root cause · shared Database
IL-0375MongoDB Atlas cluster changes and alerts lagged for hours after a control plane metadata migration congested a databaseMongoDB · 2024same root cause · shared Database
IL-0110A Grafana Agent pod's high memory use and crash-looping Hangfire pods drove a Kubernetes node into system OOM and NotReady, twice in one dayDFDS · 2024same root cause · shared Database
IL-0550Bitbucket Cloud's database slowed until connection pools saturated and requests timed out, leaving Pipelines pendingAtlassian · 2024same root cause · shared Database
IL-0270Discord's ScyllaDB authentication cluster overloaded during an OS upgrade, and logins failed for about 50 minutesDiscord · 2023same root cause · shared Database
IL-0202A burst of queries and database writes exhausted connection pools and memory on Honeycomb's ingest hosts, and Kubernetes crash-loop back-off kept failed hosts offline so ingest capacity stayed too low to recoverHoneycomb · 2023 · sev1same root cause · shared Database
IL-0386After a move to Google Cloud, Postgres connection timeouts and memcached i/o timeouts spiked on GKE nodesincident.io · 2023same root cause · shared Database
IL-0326A retention clean-up job filled the disk of MailPace's managed Postgres, which stayed offline for just over two hoursMailPace · 2023same root cause · shared Database
IL-0546Buildkite jobs could not go to newly connected agents once agent IDs passed a 32-bit integer foreign key's limitBuildkite · 2023same root cause · shared Database
IL-0500UToronto JupyterHub logins and server starts returned 500 errors when the hub's sqlite database disk filled up2i2c · 2022same root cause · shared Database
IL-0301Basecamp 3's primary database went into total gridlock at the top of the hour and Basecamp 3 went down twice37signals · 2022same root cause · shared Database
IL-0206Consul agents in GitLab.com's GKE pods were repeatedly OOM-killed, so the Rails pods lost database replica discovery and sent every query to the primaryGitLab · 2022same root cause · shared Database
IL-0302Peak load on GitHub's mysql1 MySQL cluster maxed out its database proxy's connections and stopped writes in March 2022GitHub · 2022same root cause · shared Database
IL-0212Many Slack users could not connect because a Vitess keyspace for channel membership was overloaded by cache-miss queriesSlack · 2022same root cause · shared Database
IL-0456Inflearn went down six times during a January 2022 free-course event as slow queries piled up on Aurora PostgreSQLInflab · 2022same root cause · shared Database
IL-0321GCP snapshots on all Patroni nodes stalled GitLab.com's primary PostgreSQL disk, Patroni failed over and 5xx spikedGitLab · 2021same root cause · shared Database
IL-0496Ably's Cassandra hit 100% CPU in every region and queries timed out globally, and the load returned on two more eveningsAbly · 2020same root cause · shared Database
IL-0443A Repro Aurora MySQL reader restarted unexpectedly and writer insert latency rose about sevenfold for a day or moreRepro · 2019same root cause · shared Database
IL-0037Image registry rate-limited a whole region after a crash-looping DaemonSet with imagePullPolicy Always pulled through three NAT instancesDatadog · 2019same root cause · shared Database
IL-0297Mezmo users could not sign in and log indexing lagged after the auth cache's Redis servers were OOM killedMezmo · 2019same root cause · shared Database
IL-0211A Mandrill PostgreSQL shard shut down at transaction ID wraparound and email sending was impaired for about 40 hoursMailchimp · 2019same root cause · shared Database
IL-0064CPU-heavy pods packed onto one Google Cloud node drove it to kernel panics, and the crash moved from node to node until the site was downMoonlight · 2019same root cause · shared Database
IL-0300Basecamp 3 went read-only for almost five hours when its events table reached the ceiling of 2,147,483,64737signals · 2018same root cause · shared Database
IL-0334Instapaper's RDS MySQL bookmarks table hit the 2TB ext3 file size limit, inserts failed and the service went downInstapaper · 2017same root cause · shared Database
IL-0333A downsized RDS PostgreSQL database maxed out its CPU at US peak and failing ELB health checks took buildkite.com downBuildkite · 2016same root cause · shared Database
IL-0282DynamoDB in US-East reached about 55% request errors as storage servers overloaded its metadata serviceAmazon Web Services · 2015same root cause · shared Database
IL-0383PagerDuty's shared Cassandra cluster degraded during a repair, delaying notifications and losing incoming eventsPagerDuty · 2014 · sev1same root cause · shared Database
IL-0320Postmark's primary MySQL disk filled but MySQL kept running, so MySQL-MMM never failed over and sending was delayedPostmark · 2011same root cause · shared Database
Check your cluster for this
For AI tools and scripts
- This case as JSON: https://incidentlib-api.benchlet.com/cases/IL-0497
- The cases most like it: https://incidentlib-api.benchlet.com/cases/IL-0497/similar
- MCP server (Streamable HTTP, no key):
https://incidentlib-mcp.benchlet.com/mcp
The API needs no key. The MCP server offers search_incidents, get_incident, list_incidents, get_topic and vocab; both are explained on API and MCP.