Pow: Zero-configuration Rack server for Mac OS X

Pow is a zero-configuration Rack server for Mac OS X. It makes developing Rails and Rack applications as frictionless as possible. You can install it in ten seconds and have your first app up and running in under a minute. No mucking around with /etc/hosts, no compiling Apache modules, no editing configuration files or installing preference panes. And running multiple apps with multiple versions of Ruby is trivial.

How does it work? A few simple conventions eliminate the need for tedious configuration. Pow runs as your user on an unprivileged port, and includes both an HTTP and a DNS server. The installation process sets up a firewall rule to forward incoming requests on port 80 to Pow. It also sets up a system hook so that all DNS queries for a special top-level domain (.dev) resolve to your local machine.

To serve a Rack app, just symlink it into your ~/.pow directory. Let's say you're working on an app that lives in ~/Projects/myapp. You'd like to access it at https://myapp.dev/. Setting it up is as easy as:

$ cd ~/.pow
$ ln -s ~/Projects/myapp

That's it! The name of the symlink (myapp) determines the hostname you use (myapp.dev) to access the application it points to (~/Projects/myapp).

Installation

Pow requires Mac OS X version 10.6 or newer. To install or upgrade Pow, just open a terminal and run this command:

$ curl get.pow.cx | sh

You can review the install script yourself before running it, if you'd like. Always a good idea.

The installer unpacks the latest Pow version into ~/Library/Application Support/Pow/Versions and points the ~/Library/Application Support/Pow/Current symlink there. It also installs launchd scripts for your user (the Pow server itself) and for the system (to set up the ipfw rule), if necessary. Then it boots the server.

Note: The firewall rule installed by Pow redirects all incoming traffic on port 80 to port 20559, where Pow runs. This means if you have another web server running on port 80, like the Apache that comes with Mac OS X, it will be inaccessible without either disabling the firewall rule or updating that server's configuration to listen on another port.

From Source

To install pow from source, you will need node >=0.4 and npm >=1.0.

$ git clone https://github.com/37signals/pow.git
$ cd pow
$ npm --global install
$ npm --global run-script pow restart

Uninstalling Pow

If you decide Pow's not for you, uninstallation is just as easy:

$ curl get.pow.cx/uninstall.sh | sh

(Review the uninstall script.)

Managing Applications

Pow deals exclusively with Rack applications. For the purposes of this document, a Rack application is a directory with a config.ru rackup file (and optionally a public subdirectory containing static assets). For more information on rackup files, see the Rack::Builder documentation.

Pow automatically spawns a worker process for an application the first time it's accessed, and will keep up to two workers running for each application. Workers are automatically terminated after 15 minutes of inactivity.

Using virtual hosts and the .dev domain

A virtual host specifies a mapping between a hostname and an application. To install a virtual host, symlink a Rack application into your ~/.pow directory. The name of the symlink tells Pow which hostname you want to use to access the application. For example, a symlink named myapp will be accessible at https://myapp.dev/.

Note: The Pow installer creates ~/.pow as a convenient symlink to ~/Library/Application Support/Pow/Hosts, the actual location from which virtual host symlinks are read.

Subdomains

Once a virtual host is installed, it's also automatically accessible from all subdomains of the named host. For example, the myapp virtual host described above could also be accessed at https://www.myapp.dev/ and https://assets.www.myapp.dev/. You can override this behavior to, say, point www.myapp.dev to a different application — just create another virtual host symlink named www.myapp for the application you want.

Multiple virtual hosts

You might want to serve the same application from multiple hostnames. In Pow, an application may have more than one virtual host. Multiple symlinks that point to the same application will share the same worker processes.

Customizing environment variables

Pow lets you customize the environment in which worker processes run. Before an application boots, Pow attempts to execute two scripts — first .powrc, then .powenv — in the application's root. Any environment variables exported from these scripts are passed along to Rack.

For example, if you wanted to adjust the Ruby load path for a particular application, you could modify RUBYLIB in .powrc:

export RUBYLIB="app:lib:$RUBYLIB"

Choosing the right environment script

Pow supports two separate environment scripts with the intention that one may be checked into your source control repository, leaving the other free for any local overrides. If this sounds like something you need, you'll want to keep .powrc under version control, since it's loaded first.

Working with different versions of Ruby

Pow offers full support for running multiple applications under different versions of Ruby with rvm. Just add a project .rvmrc file to your application's root directory and you're good to go.

For example, to instruct Pow to run an application with Ruby 1.9.2, use this .rvmrc file:

rvm 1.9.2

If an application has an .rvmrc file but rvm isn't installed, Pow will show an error message without booting the app.

Serving static files

Pow automatically serves static files in the public directory of your application. It's possible to serve a completely static site without a config.ru file as long as it has a public directory.

Restarting applications

You can tell Pow to restart an application the next time it's accessed. Simply save a file named restart.txt in the tmp directory of your application (you'll need to create the directory first if it doesn't exist). The easiest way to do this is with the touch command:

$ touch tmp/restart.txt

Restarting an application will also reload any environment scripts (.powrc, .powenv, or .rvmrc) before booting the app, so don't forget to touch restart.txt if you make changes to these scripts.

It's also fine to kill worker processes manually — they'll restart the next time you access the virtual host. A handy way to do this is with OS X's Activity Monitor. Select "All Processes, Hierarchically" from the dropdown at the top of the Activity Monitor window. Then find the pow process, expand the disclosure triangle, find the Ruby worker process you want to kill, and choose "Quit Process." (You can click "Inspect" on a worker process and choose "Open Files and Ports" to determine which application the process is serving.)

Viewing log files

Pow stores log files in the ~/Library/Logs/Pow directory so they can be viewed easily with OS X's Console application. Each incoming request URL is logged, along with its hostname and HTTP method, in the access.log file. The stdout and stderr streams for each worker process are captured and logged to the apps directory in a file matching the name of the application.

Note: Rails logger output does not appear in Pow's logs. You'll want to tail -f log/development.log to see those.

Configuring Pow

Pow is designed so that most people will never need to configure it. Sometimes you can't avoid having to adjust a setting or two, though. When Pow boots, it executes the .powconfig script in your home directory if it's present. You can use this script to set environment variables that will override Pow's default settings.

For example, this ~/.powconfig file tells Pow to kill idle applications after 5 minutes (300,000 ms) and spawn 3 workers per app:

export POW_TIMEOUT=300000
export POW_WORKERS=3

See the Configuration class documentation for a full list of settings that you can change.

Note: After modifying a setting in ~/.powconfig, you'll need to restart Pow for the change to take effect. You can do this by finding the pow process in OS X's Activity Monitor and clicking "Quit Process".

Contributing

Pow is written in Node.js with CoffeeScript. You can read the annotated source code to learn about how it works internally. Please report bugs on the GitHub issue tracker.

If you're interested in contributing to Pow, first start by cloning a copy of the Git repository:

$ git clone https://github.com/37signals/pow.git

Install the required JavaScript dependencies with npm (you'll need version 1.0 or higher):

$ cd pow
$ npm install --dev

Run the test suite:

$ cake test

License

(The MIT License)

Copyright © 2011 Sam Stephenson, 37signals

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.