Commit 32d9b81a authored by Eugen Rochko's avatar Eugen Rochko

Add scaling up information

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## Transaction pooling with pgBouncer ## Transaction pooling with pgBouncer
### 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
### 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.
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 userlist.txt
Edit `/etc/pgbouncer/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:
```bash
# 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
Edit `/etc/pgbouncer/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:
```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
```
Don't forget to reload or restart pgbouncer after making your changes:
sudo systemctl 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:
```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
```
> **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.
## Separate Redis for cache ## Separate Redis for cache
Redis is used widely throughout the application, but some uses are more important than others. Home feeds, list feeds, and Sidekiq queues as well as the streaming API are backed by Redis and that's important data you wouldn't want to lose (even though the loss can be survived, unlike the loss of the PostgreSQL database - never lose that!). However, Redis is also used for volatile cache. If you are at a stage of scaling up where you are worried if your Redis can handle everything, you can use a different Redis database for the cache. In the environment, you can specify `CACHE_REDIS_URL` or individual parts like `CACHE_REDIS_HOST`, `CACHE_REDIS_PORT` etc. Unspecified parts fallback to the same values as without the cache prefix.
As far as configuring the Redis database goes, basically you can get rid of background saving to disk, since it doesn't matter if the data gets lost on restart and you can save some disk I/O on that. You can also add a maximum memory limit and a key eviction policy, for that, see this guide: [Using Redis as an LRU cache](https://redis.io/topics/lru-cache)
## Read-replicas ## Read-replicas
To reduce the load on your Postgresql server, you may wish to setup hot streaming replication (read replica). [See this guide for an example](https://cloud.google.com/community/tutorials/setting-up-postgres-hot-standby). You can make use of the replica in Mastodon in these ways:
- The streaming API server does not issue writes at all, so you can connect it straight to the replica. But it's not querying the database very often anyway so the impact of this is little.
- Use the Makara driver in the web and sidekiq processes, so that writes go to the master database, while reads go to the replica. Let's talk about that.
You will have to edit the `config/database.yml` file and replace the `production` section as follows:
```yml
production:
<<: *default
adapter: postgresql_makara
prepared_statements: false
makara:
id: postgres
sticky: true
connections:
- role: master
blacklist_duration: 0
url: postgresql://db_user:db_password@db_host:db_port/db_name
- role: slave
url: postgresql://db_user:db_password@db_host:db_port/db_name
```
Make sure the URLs point to wherever your PostgreSQL servers are. You can add multiple replicas. You could have a locally installed pgBouncer with configuration to connect to two different servers based on database name, e.g. "mastodon" going to master, "mastodon_replica" going to the replica, so in the file above both URLs would point to the local pgBouncer with the same user, password, host and port, but different database name. There are many possibilities how this could be setup! For more information on Makara, [see their documentation](https://github.com/taskrabbit/makara#databaseyml).
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