How to configure PostgreSQL declaratively (NixOS and non-NixOS)?


With my limited knowledge, I’m trying to figure out how to configure a PostgreSQL database in my dev environment, created via a shell.nix file. Currently, in shellHook, I edit postgresql.conf with sed right after calling initdb, but there has to be a better solution, especially if many options need to configured.

Tried passing the configuration options directly to pg_ctl but they don’t seem to take effect:

pg_ctl                              \                                                                                                                                                                                        
  -D $PGDATA                        \                                                                                                                                                                                 
  -l $PGDATA/postgres.log           \                                                                                                                                                                                              
  -o "-c log_destination='stderr'"  \                                                                                                                                                                                              
  -o "-c logging_collector=on"      \                                                                                                                                                                                              
  -o "-c log_directory=\'log\'"     \                                                                                                                                                                                             
  -o "-c log_filename=\'postgresql-%Y-%m-%d_%H%M%S.log\'" \                                                                                                                                                                 
  -o "-c log_min_messages=info"                           \                                                                                                                                                                                       
  -o "-c log_min_error_statement=info"                    \                                                                                                                                                                                     
  -o "-c log_connections=on"                              \                                                                                                                                                                                               

I know that on NixOS there is the services.postgresql.* family of options, with lots of examples (1, 2), but

  1. these are global options, so how would one use it with nix-shell on NixOS?
  2. these are NixOS-specific options, so what could one do on a non-NixOS system?

I’m trying to make this work on Ubuntu instance in Google Cloud Engine (GCE) but once I figured out NixOps, I hope to move to NixOS entirely.

I’m fairly sure that I also have profound misunderstandings regarding Nix and NixOS, therefore please don’t hesitate to correct me.

Thank you!

1 Like

Hey Attila,

this seems to work perfectly.
But this options won’t alter your postgresql.conf.
You need to verify the settings within the Database: e.g. SHOW logging_collector;

You are right, thank you!

I wasn’t expecting postgresql.conf to change, and checking options with postgres -C always returned off, but that was because I didn’t read the postgres man page careful enough (emphases mine):

-C name

Prints the value of the named run-time parameter, and exits. (See the -c option above for details.) This can be used on a running server, and returns values from postgresql.conf , modified by any parameters supplied in this invocation. It does not reflect parameters supplied when the cluster was started.

Testing with SHOW did work of course:

$ psql -h $PGDATA -U toraritte -d postgres -c 'SHOW log_connections'                                                                                                                                log_connections
(1 row)

Plus, I also checked whether the default log/ directory exists with a logfile, after invoking nix-shell, but I messed that up too. I was playing around with quotes so many times (thinking that they were the problem) that I left single quotes quoted (see first post), and ended up with log directory name "'log'"/ and "'postgresql-2019-09-17_135959.log'"… The correct invocation is

pg_ctl                              \                                                                                                                                                                                        
  -D $PGDATA                        \                                                                                                                                                                                 
  -l $PGDATA/postgres.log           \                                                                                                                                                                                              
  -o "-c log_destination='stderr'"  \                                                                                                                                                                                              
  -o "-c logging_collector=on"      \                                                                                                                                                                                              
  -o "-c log_directory='log'"     \                                                                                                                                                                                             
  -o "-c log_filename='postgresql-%Y-%m-%d_%H%M%S.log'" \                                                                                                                                                                 
  -o "-c log_min_messages=info"                           \                                                                                                                                                                                       
  -o "-c log_min_error_statement=info"                    \                                                                                                                                                                                     
  -o "-c log_connections=on"                              \                                                                                                                                                                                               

Hey, I do run Postgres using Nix on Ubuntu. My approach is container-based deployment and extra-container tool.

So, extra-container is:

self: super: {

    extra-container = super.stdenv.mkDerivation {
        name = "extra-container-amplicare";
        src = super.fetchurl {
            url = "";
            sha256 = "1gz7rl2hx8kbmy5wdyai75mbscr0v61da4h7paz3j3yzfk6p7wf9";
        propagatedBuildDeps = [ self.nixos-container ];
        buildCommand = ''
            unpackPhase && cd "$sourceRoot"
            # extra-container was designed for NixOS. We have to adapt it for Ubuntu
            sed -i 's|/etc/systemd-mutable|$((uname -a \|grep -q NixOS) \&\& echo /etc/systemd-mutable\|\| echo /usr/lib/systemd)|g' extra-container
            mkdir -p $out/bin
            cp extra-container $out/bin/extra-container
            patchShebangs $out/bin

The container config is:


  mounts = {
    pg-monster = {
      mountPoint = "/db";
      hostPath = "/db/postgresql/pg-monster";
      isReadOnly = false;
    host-socket = {
      mountPoint = "/var/run/postgresql";
      hostPath = "/var/run/postgresql";
      isReadOnly = false;
    ssl-certificate = {
      mountPoint = "/etc/ssl/certs/ssl-cert-snakeoil.pem";
      hostPath = "/etc/ssl/certs/ssl-cert-snakeoil.pem";
    ssl-key = {
      mountPoint = "/etc/ssl/private/ssl-cert-snakeoil.key";
      hostPath = "/etc/ssl/private/ssl-cert-snakeoil.key";
    pg_stat_tmp = "/run/pg_stat_tmp";

in {

  imports = [ ./nix-config/host-base.nix ]; = {
    autoStart = true;
    tmpfs = [
    bindMounts = {
      inherit (mounts)

    config = { pkgs, config, lib, ... }: {

      imports = [

      services.postgresql = {
        enable = true;
        package = pkgs.postgresql_11;
        extraPlugins = with; [
        dataDir =;
        port = 5432;
        enableTCPIP = true;
        extraConfig = ''
          # CONFIG REFERENCE (with defaults)

          listen_addresses = '*'
          unix_socket_directories = '/tmp,${}'
          max_connections = 100
          shared_buffers = 10GB
          huge_pages = try
          temp_buffers = 64MB
          work_mem = 64MB
          maintenance_work_mem = 4GB
          autovacuum_work_mem = 128MB
          max_worker_processes = 24
          max_parallel_maintenance_workers = 8
          max_parallel_workers_per_gather = 8
          max_parallel_workers = 8

          synchronous_commit = off
          effective_cache_size = 300GB
          default_statistics_target = 1000
          jit = off # slows done things actually

          logging_collector = on
          log_line_prefix = '%t [%p]: [%l-1] user=%u,db=%d,app=%a,client=%h '
          log_min_duration_statement = 1
          log_filename = 'postgresql-%Y-%m-%d.log'
          log_checkpoints = on
          log_connections = on
          log_disconnections = on
          log_lock_waits = on
          log_temp_files = 0
          log_autovacuum_min_duration = 0
          log_error_verbosity = default
          #log_min_messages = debug5

          stats_temp_directory = '${mounts.pg_stat_tmp}'

          ssl = true
          ssl_cert_file = '${mounts.ssl-certificate.mountPoint}'
          ssl_key_file = '${mounts.ssl-key.mountPoint}'

          # those options should minimize WAL generation when inserting data
          # (WAL slows down everything, because it touches disk. But we still
          # cannot go with UNLOGGED tables, because those don't survive reboot)
          wal_level = minimal
          archive_mode = off
          max_wal_senders = 0
          wal_compression = on
          max_wal_size = 4GB
        authentication = ''
          # We trust ubuntu user, because he has sudo without password
          # He can connect/dump bypassing this authentication file
          local all postgres peer
          local all root peer
      }; = ''
        chown postgres:postgres -R ${mounts.pg_stat_tmp}
      ''; =
        let cfg =;
      in lib.mkAfter ''
          set -x
          export PGPORT=${toString cfg.port}
          export PGDATABASE=template1
          SUDO="${pkgs.sudo}/bin/sudo -u ${cfg.superUser} -E"

          function createUser {
             local user="$1"
             $SUDO psql -tAc "SELECT 1 FROM pg_roles WHERE rolname='$user'" | grep -q 1 || $SUDO createuser $user $@
          createUser root -s || true
          chmod 750 -R ${cfg.dataDir}

and container base is:

{ lib, ... }: {
    imports = [

    networking.firewall.enable = false;
    documentation.nixos.enable = false;

    # map Ubuntu's `postgres` user to NixOS
    # We want `postgres` user in container correspond to `postgres` user in host
    # So it's possible to do peer authentication.
    users.users.postgres.uid = lib.mkForce 114;
    users.groups.postgres.gid = lib.mkForce 120;

    # map Ubuntu's ssl-cert group to NixOS to allow read SSL private key
    users.users.postgres.extraGroups = [ "ssl-cert" ];
    users.groups.ssl-cert.gid = 119;

Host-base is:

{ lib, ... }: {
    fileSystems."/".device = lib.mkForce "nodev";
    boot.loader.grub.devices = lib.mkForce [ "nodev" ];
1 Like

Thank you @danbst, this looks awesome! Barely understand it, and haven’t used containers in NixOS (or at all) but this definitely seems a cleaner solution.

On the other hand, why would you recommend using containers instead of nix-shell? Is it because nix-shell kind of like chroot?

Forgive my ignorance, still fairly new to this, and right now I only use NixOS and Nix for development as setting up a dev environment is snap, but I assume the proper way to move forward in production is using containers.

Noting couple resources to myself:
+ chroot vs Docker
+ containers vs virtual machines
+ Extra-container - Run declarative containers without full system rebuilds - NixOS Discourse thread
+ NixOS container limitations - NixOS Discourse thread
+ NixOS manual - Chapter 39. Container Management
+ r/haskell - What are your deployment workflows/tools
+ NixOPS Container

there are two reasons why.

  1. Kinda isolation. Which is great for reproducibility
  2. It allows NixOS-style declarative config

Note, that there is another project to extend nix-shell into NixOS config: nixos-shell by Chris. It was nice, but it requires now some love to work on Ubuntu.

It would be really nice to combine extra-container and nixos-shell tools together (ideally embed into stock nixos-container).

Is it because nix-shell kind of like chroot ?

nix-shell is nowhere like chroot. nix-shell --pure is closer.

but I assume the proper way to move forward in production is using containers.

You can run without both containers and systemd if you want, There exists a thing called “runner”:

$ cat runner.nix
with import <nixpkgs/nixos> { configuration = {
  imports = [ ./configuration-test.nix ];
  boot.isContainer = true;
}; };

$ cat configuration-test.nix
{ config, pkgs, ... }: {
   services.postgresql.enable = true;
   services.postgresql.dataDir = "/tmp/datadir";
   services.postgresql.port = 5431;
   system.stateVersion = "18.09";

$ sudo -u postgres $(nix-build runner.nix --no-out-link)
these derivations will be built:
building '/nix/store/jpgy6m37c9lpzj8pq234410sl8nl3q8n-unit-script-postgresql-post-start.drv'...
building '/nix/store/y2md2wq9gw49v74cxh1510lyins4wlxc-postgresql.conf.drv'...
building '/nix/store/k3qja76c9jq5wjf2j7vdbiv3qa97ipfk-unit-script-postgresql-start.drv'...
building '/nix/store/gvshan2phnxh6p74iigvy50crvdx6dpk-postgresql-runner.drv'...
perl: warning: Setting locale failed.
perl: warning: Please check that your locale settings:
        LANGUAGE = (unset),
        LC_ALL = "en_US.UTF-8",
        LC_CTYPE = "en_US.UTF-8",
        LANG = "en_US.UTF-8"
    are supported and installed on your system.
perl: warning: Falling back to the standard locale ("C").
running ExecStartPre: /nix/store/8jr8k2nnhal3rsi75pws0nyjlvafzijb-unit-script-postgresql-pre-start
running ExecStart: /nix/store/n44a8135hq4kp6fvp5dxl67i8y29z6zr-unit-script-postgresql-start
running ExecStartPost: /nix/store/blpjf150p5j98fvm989gk0kp8qk11c2b-unit-script-postgresql-post-start
LOG:  database system was shut down at 2019-09-18 19:08:03 GMT
LOG:  MultiXact member wraparound protections are now enabled
LOG:  database system is ready to accept connections
LOG:  autovacuum launcher started

I don’t use it myself.

PS. In general, NixOS (not Nix, NixOS!) experience on Ubuntu is bad. I’ve recorded my current setup in NixOS-like experience on Ubuntu in AWS. So if you find containers difficult to use on Ubuntu, then don’t bother with that and use what is easiest for you.

1 Like

You just gave me a couple weeks worth of reading, and now I understand the reason behind choosing NixOS containers at least.

On local machines I always use NixOS, but have a couple projects running on Google Cloud Engine, and choose Ubuntu only to get started (quick and dirty) until I figure out NixOps to spin up NixOS instances.

Thank you also for the detailed examples!

You may try I don’t have GCE machines, but for Hetzner and some AWS (which are not listed as supported) it works.

  • you launch Ubuntu node
  • then build NixOS files on that node (nixos-infect covers this)
  • then replace bootloader and reboot (nixos-infect covers this)
  • new bootloader does “swap” and launches NixOS (NIXOS_LUSTRATE covers this)

Probably the only important thing is to not forget to imports = [ <nixpkgs/nixos/modules/virtualisation/google-compute-config.nix> ]; in desired NixOS configuration.

1 Like