IL-0297 Mezmo · 2019-05-15database
Mezmo users could not sign in and log indexing lagged after the auth cache's Redis servers were OOM killed
A tool that tracks batches of logs was not sized for a traffic spike and overwhelmed the Redis servers of Mezmo's authentication cache until they failed. The Redis servers were OOM killed and left pid files that blocked automatic restarts, so Redis kept rebooting. Logs and their metadata backed up in the process queue until they overwhelmed the log processing nodes, and the failover from Redis to MongoDB sent so many authentication requests that MongoDB failed as well, so users could not sign in or reach their logs. The postmortem, signed by the LogDNA team, says the batch tracking instrumentation worked as intended and that the traffic increase was just enough to push the pipeline over the edge.
The first fix resized specific parts of the cluster to take a much higher load. By 18:26 UTC on 15 May the root cause had been identified and a fix deployed, and the incident was resolved at 00:12 UTC on 16 May.
- Severity
- –
- Scope
- Sign-in and access to logs in the web application, log indexing for live tail and search, and log ingestion through the agent, REST API, code libraries, Heroku and syslog
- Users affected
- –
- Region
- –
- Time to detect
- –
- Time to resolve
- –
Symptoms
On 15 May 2019, alerts and customer reports showed that the Mezmo web application did not let users sign in, and the status page reported an indexing delay: incoming logs reached live tail and search late.
None of the Redis servers of the small, distributed, disk-backed cache that stores and looks up authentication information were running. They had been OOM killed and had left pid files behind, which prevented automatic restarts, and Redis went into a reboot loop. When Redis fails the applications fall back to MongoDB, but the volume of authentication requests took MongoDB down too, and the application could not be reached at all to retrieve logs.
Symptom Service outageOOMKilledDatabase overloadThroughput degradation
Root cause
Root cause Capacity exhaustion
- Trigger
- A traffic spike that the tool tracking batches of logs was not sized to handle.
Detection
Alerts and customer reports in the early morning hours triggered the escalation procedure and woke the on-call engineers.
Remediation
- Resized specific parts of the cluster to handle a much higher load
Action items
- Retool the batch tracking system to reduce its direct impact on the Redis clusters
- Add observability instrumentation around the batch tracking feature set
- Improve monitoring for Redis
- Remove the bottlenecks in the pipeline that were involved in this incident
Lessons
Mezmo took from the incident that deep instrumentation, tracing and observability across the platform matter more as features scale up.
Sources
- https://status.mezmo.com/incidents/lyyjmgr8dxz4 · official-status
Related cases
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, In-memory cache
IL-0544Wasabi's database cache stopped responding when its queueing service filled up, and S3 failed in all regionsWasabi · 2024same root cause · shared Database, In-memory cache
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, In-memory cache
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, In-memory cache
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, In-memory cache
IL-0432AutoReserve notifications silently went unsent as Memorystore for Redis 3.2 rejected writes with OOM below its limitHello Inc. · 2026same root cause · shared In-memory cache
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-0290GitLab.com's Redis Sidekiq cluster ran out of memory and lost quorum, causing 500 errors and a 12 million job backlogGitLab · 2026 · sev1same root cause · shared In-memory cache
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-0497Buttondown's database ran out of connections twice in one day and requests failed while health checks mostly passedButtondown · 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-058129CM product display servers lost pods and saturated Netty event loops on the first day of the Igu Week saleMUSINSA · 2025same root cause · shared In-memory cache
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-0067Redis filled its persistent volume, corrupted its append-only file and crash-looped on every restartBC Government · 2025 · sev1same root cause · shared In-memory cache
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-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-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-0453Hakuna API latency spiked every day during the ElastiCache Redis backup as memory filled and the backup went forklessHyperconnect · 2022same root cause · shared In-memory cache
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-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-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-0520logs.cloud.gov trapped users in a login redirect loop because its full Redis session store stopped saving sessionscloud.gov · 2016same root cause · shared In-memory cache
IL-0296Transloadit's us-east Redis cluster lost queued jobs and heartbeats, raising WORKER_JOB_ERROR errors for 81 customersTransloadit · 2016same root cause · shared In-memory cache
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-0297
- The cases most like it: https://incidentlib-api.benchlet.com/cases/IL-0297/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.