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epriestley ebff07d019 Automatically sever databases after prolonged unreachability
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
2016-04-11 08:43:52 -07:00
bin Provide bin/nuance import and ngram indexes for sources 2016-03-08 10:30:24 -08:00
conf Mark some strings for translation 2015-06-09 23:06:52 +10:00
externals Improve Amazon SES code error handling behavior 2016-04-05 17:28:45 -07:00
resources When Phabricator is in read-only mode, explain why 2016-04-10 12:19:18 -07:00
scripts When cluster.databases is configured, read the master connection from it 2016-04-10 12:18:42 -07:00
src Automatically sever databases after prolonged unreachability 2016-04-11 08:43:52 -07:00
support Use AphrontRequestStream to read request input 2016-03-17 08:08:18 -07:00
webroot When Phabricator is in read-only mode, explain why 2016-04-10 12:19:18 -07:00
.arcconfig Use the configuration driven unit test engine 2015-08-11 07:57:11 +10:00
.arclint Begin adding test coverage to GitHub Events API parsers 2016-03-09 09:30:07 -08:00
.arcunit Use the configuration driven unit test engine 2015-08-11 07:57:11 +10:00
.editorconfig Fix text lint issues 2015-02-12 07:00:13 +11:00
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