memcached: Add NixOS support

modules/module-list.nix

@@ -69,6 +69,7 @@
   ./services/backup/sitecopy-backup.nix
   ./services/databases/4store-endpoint.nix
   ./services/databases/4store.nix
+  ./services/databases/memcached.nix
   ./services/databases/mongodb.nix
   ./services/databases/mysql.nix
   ./services/databases/mysql55.nix

modules/services/databases/memcached.nix

@@ -0,0 +1,61 @@
+{ config, pkgs, ... }:
+
+with pkgs.lib;
+
+let
+
+  cfg = config.services.memcached;
+
+  memcached = pkgs.memcached;
+
+in
+
+{
+
+  ###### interface
+
+  options = {
+
+    services.memcached = {
+
+      enable = mkOption {
+        default = false;
+        description = "
+          Whether to enable Memcached.
+        ";
+      };
+
+      user = mkOption {
+        default = "memcached";
+        description = "The user to run Memcached as";
+      };
+
+    };
+
+  };
+
+  ###### implementation
+
+  config = mkIf config.services.memcached.enable {
+
+    users.extraUsers = singleton
+      { name = cfg.user;
+        description = "Memcached server user";
+      };
+
+    environment.systemPackages = [ memcached ];
+
+    systemd.services.memcached =
+      { description = "Memcached server";
+
+        wantedBy = [ "multi-user.target" ];
+        after = [ "network.target" ];
+
+        serviceConfig = {
+          ExecStart = "${memcached}/bin/memcached";
+          User = cfg.user;
+        };
+      };
+  };
+
+}