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Summary: Ref T4571. When a database goes down briefly, we fall back to replicas. However, this fallback is slow (not good for users) and keeps sending a lot of traffic to the master (might be bad if the root cause is load-related). Keep track of recent connections and fully degrade into "severed" mode if we see a sequence of failures over a reasonable period of time. In this mode, we send much less traffic to the master (faster for users; less load for the database). We do send a little bit of traffic still, and if the master recovers we'll recover back into normal mode seeing several connections in a row succeed. This is similar to what most load balancers do when pulling web servers in and out of pools. For now, the specific numbers are: - We do at most one health check every 3 seconds. - If 5 checks in a row fail or succeed, we sever or un-sever the database (so it takes about 15 seconds to switch modes). - If the database is currently marked unhealthy, we reduce timeouts and retries when connecting to it. Test Plan: - Configured a bad `master`. - Browsed around for a bit, initially saw "unrechable master" errors. - After about 15 seconds, saw "major interruption" errors instead. - Fixed the config for `master`. - Browsed around for a while longer. - After about 15 seconds, things recovered. - Used "Cluster Databases" console to keep an eye on health checks: it now shows how many recent health checks were good: {F1213397} Reviewers: chad Reviewed By: chad Maniphest Tasks: T4571 Differential Revision: https://secure.phabricator.com/D15677 |
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