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PgBouncer Guide

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.

Installing PgBouncer

On Debian and Ubuntu:

sudo apt install pgbouncer

Restarting:

sudo systemctl restart pgbouncer

(Note that this guide assumes you aren't using Docker.)

Configuring PgBouncer

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';

Then \q to quit.

Configuring PgBouncer

PgBouncer has two config files: pgbouncer.ini and userlist.txt both in /etc/pgbouncer/. The first contains the configuration, whereas the second just contains a list of usernames and passwords.

Configuring userlist.txt

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 userlist.txt:

"mastodon" "md5d75bb2be2d7086c6148944261a00f605"

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 userlist.txt:

"mastodon" "md5d75bb2be2d7086c6148944261a00f605"
"pgbouncer" "md5a45753afaca0db833a6f7c7b2864b9d9"

In both cases the password is just password.

Configuring pgbouncer.ini

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

The listen_addr and listen_port tells PgBouncer which address/port to accept connections. The defaults are fine:

listen_addr = 127.0.0.1
listen_port = 6432

Put md5 as the auth_type (assuming you're using the md5 format in userlist.txt):

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 from session to transaction:

pool_mode = transaction

Next up, 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

In your .env.production file, first off make sure that this is set:

PREPARED_STATEMENTS=false

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 DB_PORT=5432 together with RAILS_ENV=production when calling the task, for example: RAILS_ENV=production DB_PORT=5432 bundle exec rails db:migrate (you can specify DB_HOST too if it's different, etc)

Administering PgBouncer

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:

RELOAD;

Then use \q to quit.

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