Add PgBouncer guide

Running-Mastodon/PgBouncer-guide.md

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+PgBouncer Guide
+====
+
+The following guide explains how to use [PgBouncer](https://pgbouncer.github.io/) as an efficient database pooler on top of Postgres. For a bit of background, you might read ["Scaling Mastodon"](https://medium.com/@Gargron/scaling-mastodon-1becde463090) 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.
+
+Installing PgBouncer
+-----
+
+On Ubuntu:
+
+    sudo apt install pgbouncer
+
+Restarting:
+
+    sudo service pgbouncer restart
+
+(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:
+
+    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
+
+Add the `mastodon` user to the `userlist.txt`:
+
+    "mastodon" "md599dff7ae01ae2dc33ae052264bf22bf4"
+
+Here we're using the md5 scheme, where the md5 password is just the md5sum of `username + password` with the string `md5` prepended. For instance, to derive the hash for user `mastodon` with password `password`, you can do:
+
+```bash
+# ubuntu, debian, etc.
+echo -n "mastodonpassword" | md5sum
+# macOS, openBSD, etc.
+md5 -s "mastodonpassword"
+```
+
+Then just add `md5` to the beginning of that.
+
+You'll also want to create a `pgbouncer` admin user to log in to the admin database. So here's a sample `userlist.txt`:
+
+```
+"mastodon" "md599dff7ae01ae2dc33ae052264bf22bf4"
+"pgbouncer" "md509c950063a6cf1b217ee831d0f4c6771"
+```
+
+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 Postgres use the same username/password and database name to connect to the underlying Postgres database:
+
+```ini
+[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:
+
+```ini
+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`):
+
+```ini
+auth_type = md5
+```
+
+Make sure the `pgbouncer` user is an admin:
+
+```ini
+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`:
+
+```ini
+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:
+
+```ini
+max_client_conn = 100
+default_pool_size = 20
+```
+
+### 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.
+
+### Configuring Mastodon to talk to PgBouncer
+
+In your `.env.production` file, first off make sure that this is set:
+
+```bash
+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:
+
+```bash
+DB_HOST=localhost
+DB_USER=mastodon
+DB_NAME=mastodon_production
+DB_PASS=password
+DB_PORT=6432
+```
+
+### Administering PgBouncer
+
+The easiest way to reboot is:
+
+    sudo service pgbouncer restart
+
+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.
+
+### Resources
+
+- ["Scaling Mastodon"](https://medium.com/@Gargron/scaling-mastodon-1becde463090)
+- [PgBouncer documentation](https://pgbouncer.github.io/)
+- [Connection Pooling in PostgreSQL using pgbouncer](https://www.slideshare.net/sameerkasi200x/5th-pugs-meetupjuly2014pgbouncer)
+- [PgBouncer on the Postgres wiki](https://wiki.postgresql.org/wiki/PgBouncer)