advanced webservices on nixos

31 Jan 2014

motivation

i’ve been using nixos on my webserver for two years now and i love the declarative approach [1] of describing services as well as webservices! describing a webservices in a declarative way is like using the object factory programming pattern and helps to make better abstractions. a good example is the mediawiki abstraction shown below. a user only has to fill the values into the declarative description and nixos-rebuild will automatically install apache-2.2.25, mysql-5.1.72, php-5.4.20 and of course the mediawiki sources (versions of course might differ).

although the declarative approach found in NixOS seems great, it is very unflexible as it forces a rigid dependency model on its users. for example one can only use exactly one php version which needs to be the same for all such services but here i present an idea how to solve this. instead of constraining all webapps to be run on the same webserver, why not have a ‘generic reverse proxy (apache)’ which acts as a frontend and each declaratively described webservice would then use its own, individual mix of webtools like webserver, CGI-based and static/dynamic webpages?

i would love to have your opinion/feedback/critisism to make this idea come true, so please write thoughts about this to ‘js@lastlog.de’.

declarative description of webservices as we have them in jan 2014

this section describes what we already have and how it works:

a declarative description of the mediawiki software

a webservice declaration (from [1]) which will will start the httpd service but also the serviceType = “mediawiki” on top of it:

services.postgresql.enable = true;
services.postgresql.package = pkgs.postgresql92;
services.httpd.enable = true;
services.httpd.adminAddr = "admin@example.org";
services.httpd.extraSubservices =
  [ { serviceType = "mediawiki";
      siteName = "My Wiki";
      logo = "http://www.example.org/wiki-logo.png"; # should be 135x135px
      extraConfig =
        ''
          # See http://www.mediawiki.org/wiki/Manual:Configuration_settings
          $wgEmailConfirmToEdit = true;
        '';
    }
  ];

when evaluating this service by running

nixos-rebuild switch

it was using this store paths:

/nix/store/fd1mqjpccqpasl0q462iqzx7d6jpl3h4-apache-httpd-2.2.25
/nix/store/n9c6ba46kyn0yakv3valyrhmlw0qy9a0-mysql-5.1.72
/nix/store/hy0mmhy6wd3dj1svyi6kxnslgkbbcb6y-php-5.4.20

and it would reload apache in case of a change.

using vhosts along with declarative descriptions

services.postgresql.enable = true;
services.postgresql.package = pkgs.postgresql92;

services.httpd = {
  enable = true;
  logPerVirtualHost = true;
  adminAddr="example@example.com";
  hostName = "example.com";

  virtualHosts =
  [ 
    {
      hostName = "www.example.com";
      serverAliases = ["www.example.com"];
      documentRoot = "/www";
    }
   { 
      # Note: do not forget to add a DNS entry for wiki.example.com in the DNS settings
      hostName = "wiki.example.com";
      extraConfig = ''
          RedirectMatch ^/$ /mywiki
        '';
      extraSubservices =
      [
        {
          serviceType = "mediawiki";
          id="wiki1";
          dbName="mediawiki_wiki1";

          siteName = "My Wiki";
          articleUrlPrefix = "/mywiki";
          #logo = "http://www.example.org/wiki-logo.png"; # should be 135x135px
          extraConfig =
            ''
              # See http://www.mediawiki.org/wiki/Manual:Configuration_settings
              $wgEmailConfirmToEdit = true;
            '';
        }
      ];
    }
    { 
      # Note: do not forget to add a DNS entry for wiki.example.com in the DNS settings
      hostName = "wiki2.example.com";
      extraConfig = ''
          RedirectMatch ^/$ /mywiki2
        '';
      extraSubservices =
      [
        {
          serviceType = "mediawiki";
          id="wiki2";
          dbName="mediawiki_wiki2";
          siteName = "wiki 2";

          siteName = "My Wiki";
          articleUrlPrefix = "/mywiki";
          #logo = "http://www.example.org/wiki-logo.png"; # should be 135x135px
          extraConfig =
            ''
              # See http://www.mediawiki.org/wiki/Manual:Configuration_settings
              $wgEmailConfirmToEdit = true;
            '';
        }
      ];
    }
  ];
};

declarative description of webservices (next generation)

advantages:

stable API for third party webservices
- each webservice can have its own mysql database server since connections between webserver, application and mysql could be done on the fly
- one can use different php versions on different webservers
- individual upgrade paths could be used to migrate an old webservice to a more recent one using database duplication/freezing
- webservices can be stopped and started independently (interesting when deploying new services)
will make nix-docker [3] even cooler
different security levels per webserver (sometimes you need to support webapps with old php versions which are NOT secure)
can be combined with LXC http://linuxcontainers.org/
optimization
- webservices like trac can be optimized for python usage while
- webservices like mediawiki can be optimized for php usage

disadvantages:

using individual ports for all each service (like the n’th mysql server) might complicate the design when not using LXC or similar
reverse proxies performance penalty
someone has to implement it…

with help of you i would like to start implementing it.