https://danbooru.donmai.us/forum_topics/9127?page=283#forum_post_160508
There was a recent outage that was caused by the read replica
(yukinoshita.donmai.us) being temporarily unavailable. The pg driver in
rails got hardstuck trying to connect to the replica, which brought down
the whole site. The app servers stopped responding and could only be
brought down with SIGKILL. Even try to boot the rails console didn't
work.
We only really used this to calculate tag counts inside Post.fast_count,
which wasn't really beneficial since the read replica is slower than the
main database.
Use git clean to delete stale files from an earlier checkout. These
files interfered with various things (specifically, a comment vote test
failed because there was an old comment_votes/create.json.erb template
hanging around that changed the behavior of the POST /comment_votes endpoint).
Upgrade procedure:
$ echo 2.6.3 > .ruby-version
# upgrade ruby-build to know about the latest ruby versions.
# see also: github.com/rkh/rbenv-update
$ cd ~/.rbenv/plugins/ruby-build && git pull
# install the version of ruby specified in .ruby-version.
# see also: github.com/capistrano/rbenv/issues/83
$ rbenv install --skip-existing
# update the shell to use the latest version of ruby by default.
$ rbenv global 2.6.3
# update rubygems and bundler
$ gem update --system
# update the lockfile to use the latest version of bundler
$ bundle update --bundler
