The following guide explains how to use PgBouncer as an efficient connection pooler on top of Postgres. For a bit of background, you might read "Scaling Mastodon" which briefly describes this approach.
Why you might need PgBouncer
If you start running out of available Postgres connections (the default is 100) then you may find PgBouncer to be a good solution. This document describes some common gotchas as well as good configuration defaults for Mastodon.
Note that you can check "PgHero" in the administration view to see how many Postgres connections are currently being used. Typically Mastodon uses as many connections as there are threads both in Puma, Sidekiq and the streaming API combined.
On Debian and Ubuntu:
sudo apt install pgbouncer
sudo systemctl restart pgbouncer
(Note that this guide assumes you aren't using Docker.)
Setting a password
First off, if your
mastodon user in Postgres is set up wthout a password, you will need to set a password. There seems to be no way to use PgBouncer with an empty password.
Here's how you might reset the password:
psql -p 5432 -U mastodon mastodon_production -w
Then (obviously, use a different password than the word "password"):
ALTER USER mastodon WITH PASSWORD 'password';
\q to quit.
PgBouncer has two config files:
userlist.txt both in
/etc/pgbouncer/. The first contains the configuration, whereas the second just contains a list of usernames and passwords.
As long as you specify a user/password in pgbouncer.ini later, the values in userlist.txt do not have to correspond to real PostgreSQL roles. You can arbitrarily define users and passwords, but you can reuse the "real" credentials for simplicity's sake. Add the
mastodon user to the
Here we're using the md5 scheme, where the md5 password is just the md5sum of
password + username with the string
md5 prepended. For instance, to derive the hash for user
mastodon with password
password, you can do:
# ubuntu, debian, etc. echo -n "passwordmastodon" | md5sum # macOS, openBSD, etc. md5 -s "passwordmastodon"
Then just add
md5 to the beginning of that.
You'll also want to create a
pgbouncer admin user to log in to the PgBouncer admin database. So here's a sample
"mastodon" "md5d75bb2be2d7086c6148944261a00f605" "pgbouncer" "md5a45753afaca0db833a6f7c7b2864b9d9"
In both cases the password is just
Add a line under
[databases] listing the Postgres databases you want to connect to. Here we'll just have PgBouncer use the same username/password and database name to connect to the underlying Postgres database:
[databases] mastodon_production = host=127.0.0.1 port=5432 dbname=mastodon_production user=mastodon password=password
listen_port tells PgBouncer which address/port to accept connections. The defaults are fine:
listen_addr = 127.0.0.1 listen_port = 6432
md5 as the
auth_type (assuming you're using the md5 format in
auth_type = md5
Make sure the
pgbouncer user is an admin:
admin_users = pgbouncer
This next part is very important! The default pooling mode is session-based, but for Mastodon we want transaction-based. In other words, a Postgres connection is created when a transaction is created and dropped when the transaction is done. So you'll want to change the
pool_mode = transaction
max_client_conn defines how many connections PgBouncer itself will accept, and
default_pool_size puts a limit on how many Postgres connections will be opened under the hood. (In PgHero the number of connections reported will correspond to
default_pool_size because it has no knowledge of PgBouncer.)
The defaults are fine to start, and you can always increase them later:
max_client_conn = 100 default_pool_size = 20
Don't forget to reload or restart pgbouncer after making your changes:
service pgbouncer reload
Debugging that it all works
You should be able to connect to PgBouncer just like you would with Postgres:
psql -p 6432 -U mastodon mastodon_production
And then use your password to log in.
You can also check the PgBouncer logs like so:
tail -f /var/log/postgresql/pgbouncer.log
Configuring Mastodon to talk to PgBouncer
.env.production file, first off make sure that this is set:
Since we're using transaction-based pooling, we can't use prepared statements.
Next up, configure Mastodon to use port 6432 (PgBouncer) instead of 5432 (Postgres) and you should be good to go:
DB_HOST=localhost DB_USER=mastodon DB_NAME=mastodon_production DB_PASS=password DB_PORT=6432
Gotcha: You cannot use pgBouncer to perform db:migrate tasks. But this is easy to work around. If your postgres and pgbouncer are on the same host, it can be as simple as defining
RAILS_ENV=productionwhen calling the task, for example:
RAILS_ENV=production DB_PORT=5432 bundle exec rails db:migrate(you can specify
DB_HOSTtoo if it's different, etc)
The easiest way to reboot is:
sudo systemctl restart pgbouncer
But if you've set up a PgBouncer admin user, you can also connect as the admin:
psql -p 6432 -U pgbouncer pgbouncer
And then do:
\q to quit.