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  1. IL-0261 Discord went fully offline twice as short network losses on Google Cloud sessions nodes wedged Erlang distribution
    Discord2017
    Symptoms

    On 17 and 18 November 2017 nodes of Discord's 'sessions' cluster on Google Cloud Platform lost network connectivity for short periods, 16 times in all. Engineers saw a service impact, a node whose service had died and restarted, and pages for low memory on other sessions nodes; twice the failure cascaded and Discord was completely offline for about 30 minutes.

    Network unreachableMemory pressureService outage
    Trigger

    A software change that Google rolled out to the Google Cloud Platform networking stack on Friday introduced a bug that made VMs in Discord's sessions cluster lose network connectivity for short periods.

    Root cause
    Cloud provider outage
    Component
    Service discovery
    Contributing factor
    Limited debug accessHarmful manual action
    Detection

    At 11:52 PST on 17 November engineers noticed a service impact and escalated it in the incident chat. Later affected nodes showed up as pages to the on-call engineer for low memory on sessions nodes, and on 18 November as a page for a networking incident on sessions-1-11 together with issue reports that the customer experience team escalated from support channels.

    Remediation
    • Forcefully terminated sessions nodes whose Erlang distribution port had failed
    • Escalated the Google support ticket to P1 and notified the Technical Account Manager
    • Restarted several core Discord services on 17 November, making clients reconnect over about 20 minutes
    • Began terminating and rebooting the entire sessions cluster after seeing that rebooted machines did not fail again
    • Disabled message sending globally on 18 November to help the service recover
    • Restarted all members of the sessions cluster
    • Re-enabled message sending after the initial all clear
    • Google rolled back the software change made to its networking stack
    Action items
    • Continue the project to improve how Discord's Erlang clusters handle node failure
    • Expand how upgrades to these services are load tested so the rollout can be completed safely
    • Replace the HTTP library used by Discord's Elixir/Erlang services
    • Fix the instability seen in Erlang's net_kernel component
    • Replace the pieces of the service discovery component that degraded with more performant implementations
    • Work with Google on the issues Discord met while working with Google on this incident
    • Expand the internal procedure for communicating with Google so the progress of P1 issues is tracked
    Lessons

    Discord concluded that performance and stability issues in its own system, in Erlang's net_kernel, the service discovery component and the HTTP library, were what turned short network losses on part of the fleet from isolated issues for some users into two full system outages.

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By root cause1

Cloud provider outage1 case

Component
Service discovery
Contributing factor
Limited debug accessHarmful manual action
Organisation
Discord
  1. IL-0261 Discord went fully offline twice as short network losses on Google Cloud sessions nodes wedged Erlang distribution
    Discord2017