PostgreSQL

PostgreSQL also known as Postgres, is a free and open-source relational database management system (RDBMS) emphasizing extensibility and SQL compliance.

This article extends the documentation in the NixOS manual.

Getting started

To try out Postgresql add the following minimal example to your NixOS module:

{
  # ...
  config.services.postgresql = {
    enable = true;
    ensureDatabases = [ "mydatabase" ];
    authentication = pkgs.lib.mkOverride 10 ''
      #type database  DBuser  auth-method
      local all       all     trust
    '';
  };
}

This will setup Postgresql with a default DB superuser/admin "postgres", a database "mydatabase" and let every DB user have access to it without a password through a "local" Unix socket "/var/run/postgresql" (TCP/IP is disabled by default because it's less performant and less secure).

• Available NixOS Postgresql service options

It's also possible to setup PostgreSQL with Nix Darwin similar to how you would on NixOS, see the options.

Verify setup

You can use psql that comes with Postgres in the terminal to verify that the DB setup is as expected:

 $ sudo -u postgres psql
psql
Type "help" for help.

postgres=#

We have to switch to a system user like "postgres" with sudo -u postgres, because by default psql logs you into the DB user of the same name as the current Linux/system user. By default, NixOS creates a system and DB user names "postgres". So the line postgres=# shows that we are now logged-in as DB user "postgres".

Inside psql here the most frequent commands are:

List all databases running on this Postgres instance with \l:

postgres=# \l
                                   List of databases
    Name    |  Owner   | Encoding |   Collate   |    Ctype    |   Access privileges    
------------+----------+----------+-------------+-------------+------------------------
 mydatabase | postgres | UTF8     | en_US.UTF-8 | en_US.UTF-8 | =Tc/postgres          +
            |          |          |             |             | postgres=CTc/postgres +
            |          |          |             |             | rustnixos=CTc/postgres
 postgres   | postgres | UTF8     | en_US.UTF-8 | en_US.UTF-8 | 
 template0  | postgres | UTF8     | en_US.UTF-8 | en_US.UTF-8 | =c/postgres           +
            |          |          |             |             | postgres=CTc/postgres
 template1  | postgres | UTF8     | en_US.UTF-8 | en_US.UTF-8 | =c/postgres           +
            |          |          |             |             | postgres=CTc/postgres
(4 rows)

List all DB users (also called "roles" in Postgres) with \du:

postgres=# \du
                                   List of roles
 Role name |                         Attributes                         | Member of 
-----------+------------------------------------------------------------+-----------
 postgres  | Superuser, Create role, Create DB, Replication, Bypass RLS | {}

List all authentication rules (called an "pg_hba.conf" file in Postgres ) with table pg_hba_file_rules;:

postgres=# table pg_hba_file_rules;
 line_number | type  | database | user_name | address | netmask | auth_method | options | error 
-------------+-------+----------+-----------+---------+---------+-------------+---------+-------
           1 | local | {all}    | {all}     |         |         | peer        |         | 
(1 row)

• Official Postgres authentication pg_hba.conf documentation

Allow TCP/IP connections

This example shows how to roll out a database with a default user and password:

services.postgresql = {
  enable = true;
  ensureDatabases = [ "mydatabase" ];
  enableTCPIP = true;
  # port = 5432;
  authentication = pkgs.lib.mkOverride 10 ''
    #type database DBuser origin-address auth-method
    local all      all     trust
    # ... other auth rules ...

    # ipv4
    host  all      all     127.0.0.1/32   trust
    # ipv6
    host  all      all     ::1/128        trust
  '';
  initialScript = pkgs.writeText "backend-initScript" ''
    CREATE ROLE nixcloud WITH LOGIN PASSWORD 'nixcloud' CREATEDB;
    CREATE DATABASE nixcloud;
    GRANT ALL PRIVILEGES ON DATABASE nixcloud TO nixcloud;
  '';
};

This will allow "host" based authentification only from other webservices on the same computer ("127.0.0.1"), although any DB user will have access to any database.

Set the Postgresql versions

By default, NixOS uses whatever Postgres version shipped as default for your system.stateVersion. To use a different or more recent version, you can set it manually:

services.postgresql = {
  enable = true;
  package = pkgs.postgresql_15;
  # ...
};

Note that changing the package version does not trigger any automatic migrations of your existing databases: if you update Postgres you should be ready to migrate any existing databases manually.

• Available Nixpkgs Postgresql versions

Security

Letting every system and DB user have access to all data is dangerous. Postgres supports several layers of protection. One is to prefer "local" connections using Unix sockets, that aren't accessible from the internet, whenever Postgres and your client app run on the same server.

Harden authentication

We can limit what system user can connect.

Postgres supports "user name maps", which limit which system users can log in as which DB user:

services.postgresql = {
  enable = true;
  ensureDatabases = [ "mydatabase" ];
 
  identMap = ''
    # ArbitraryMapName systemUser DBUser
       superuser_map      root      postgres
       superuser_map      postgres  postgres
       # Let other names login as themselves
       superuser_map      /^(.*)$   \1
  '';
};

This map can have an arbitrary name and defines which system user can login as which DB user. Every other user and combination is rejected. For example, with the above mapping if we are logged-in as system user "root" but want enter the DB as DB user "postgres" we would be allowed:

root$ psql -U postgres
# ok

Limit Access

Once logged-in we can limit what DB users can access. With the authentication we can limit what DB user can access which databases. A good default setting is as follows:

services.postgresql = {
  enable = true;
  ensureDatabases = [ "mydatabase" ];
  authentication = pkgs.lib.mkOverride 10 ''
    #type database  DBuser  auth-method optional_ident_map
    local sameuser  all     peer        map=superuser_map
  '';
};

With "sameuser" Postgres will allow DB user access only to databases of the same name. E.g. DB user "mydatabase" will get access to database "mydatabase" and nothing else. The part map=superuser_map is optional. One exception is the DB user "postgres", which by default is a superuser/admin with access to everything.

Monitoring

A Prometheus exporter is available to export metrics to Prometheus-compatible storage.

services.prometheus.exporters.postgres = {
    enable = true;
    listenAddress = "0.0.0.0";
    port = 9187;
};

See all available options for services.prometheus.exporters.postgres

TLS

To turn TLS on in recent versions of postgres it's pretty easy. Their docs are pretty good.

Create a simple cert just to make it work. If you are doing this in production, you need to provide your own server.crt and server.key in the main PGDATA dir (~postgres).

In a shell:

cd ~postgres
sudo -u postgres openssl req -new -x509 -days 365 -nodes -text -out server.crt  -keyout server.key -subj "/CN=dbhost.yourdomain.com"
chmod og-rwx server.key

Then in your nix configuration:

  services.postgresql = {
    enable = true;
    package = pkgs.postgresql_16;
    enableTCPIP = true;
    ensureDatabases = [ "tootieapp" ];
    settings = {
        ssl = true;
    };
    authentication = pkgs.lib.mkOverride 10 ''
      #type database  DBuser  auth-method
      local all       all     trust
      host  sameuser    all     127.0.0.1/32 scram-sha-256
      host  sameuser    all     ::1/128 scram-sha-256
    '';
  };

the `sameuser` mentioned in the authentication section requires the database name be the same as the username, which you may not want, you can change that to `all` to allow an authenticated user the ability to connect to any database.

`scram-sha-256` is the require a password option, but you can authenticate a variety of different ways, see the official docs for other options as part of pg_hba.conf.

user creation and permissions are best described in the PG manual under `CREATE ROLE` and `GRANT` for example:

CREATE USER tootieapp WITH PASSWORD 'BIGLONGRANDOMSTRINGHERE';
GRANT ALL PRIVILEGES ON DATABASE tootieapp TO tootieapp;

Debugging with psql

To debug the SQL statements futher, one can use systemctl cat postgresql and see the ExecStartPost=/nix/store/rnv1v95bbf2lsy9ncwg7jdwj2s71sqra-unit-script/bin/postgresql-post-start line. Then open it with `cat` on the shell and see the psql command.

Then execute the complete statement on the shell, as:

/nix/store/3mqha1naji34i6iv78i90hc20dx0hld9-sudo-1.8.20p2/bin/sudo -u postgres psql -f "/nix/store/az5nglyw7j94blxwkn2rmpi2p6z9fbmy-backend-initScript" --port=5432 -d postgres
psql:/nix/store/az5nglyw7j94blxwkn2rmpi2p6z9fbmy-backend-initScript:1: ERROR:  syntax error at or near "-"
LINE 1: CREATE ROLE nixcloud-admin WITH LOGIN PASSWORD 'nixcloud' CR...
                            ^
psql:/nix/store/az5nglyw7j94blxwkn2rmpi2p6z9fbmy-backend-initScript:2: ERROR:  database "nixcloud-db1" already exists
psql:/nix/store/az5nglyw7j94blxwkn2rmpi2p6z9fbmy-backend-initScript:3: ERROR:  syntax error at or near "-"
LINE 1: ...ALL PRIVILEGES ON DATABASE "nixcloud-db1" TO nixcloud-admin;
                                                                ^

Troubleshooting

Connection rejected with "Role does not exist"

$ psql
psql: error: connection to server on socket "/run/postgresql/.s.PGSQL.5432" failed: FATAL:  role "root" does not exist

You are trying to login as a system user ("root" in this example) that has no DB user of the same name. Try psql -U postgres or sudo -u postgres psql to log in as a different DB user.

Connection rejected with "Peer authentication failed"

root$ psql -U postgres
psql: error: connection to server on socket "/run/postgresql/.s.PGSQL.5432" failed: FATAL:  Peer authentication failed for user "postgres"

You are trying to login as a DB user ("postgres" in this example) for which your current system user ("root" in this example) has no permission to switch to. Check your "user name map" in the identMap section.

WARNING: database "XXX" has a collation version mismatch

The complete error which appears in the system log might look similar to this

WARNING:  database "outline" has a collation version mismatch
DETAIL:  The database was created using collation version 2.35, but the operating system provides version 2.38.
HINT:  Rebuild all objects in this database that use the default collation and run ALTER DATABASE outline REFRESH COLLATION VERSION, or build PostgreSQL with the right library version.

To fix it, run following commands in the psql console. Replace the database name outline with the name of the database which you want to migrate

sudo -u postgres psql
postgres=# \c outline;
outline=# REINDEX DATABASE outline;
outline=# ALTER DATABASE outline REFRESH COLLATION VERSION;

Major upgrades

If you're using NixOS' modules for PostgreSQL and find yourself in a boot/switch after a major bump of it, you'll need to upgrade your cluster.

Let the service successfully start once, and then stop it. Upon completion, proceed with the following command, substituting the numbers 15 and 16 with the respective versions you previously used and the more recent one:

sudo -u postgres pg_upgrade -b "$(nix build --no-link --print-out-paths nixpkgs#postgresql_15.out)/bin" -B /run/current-system/sw/bin -d /var/lib/postgresql/15 -D /var/lib/postgresql/16

See also

• Available NixOS service options