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
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
directory. Let's say you're working on an app that lives in
~/Projects/myapp. You'd like to access it at
http://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
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
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.
If you decide Pow's not for you, uninstallation is just as easy:
$ curl get.pow.cx/uninstall.sh | sh
Pow deals exclusively with Rack applications. For the purposes of this
document, a Rack application is a directory with a
rackup file (and optionally a
public subdirectory containing static
assets). For more information on rackup files, see the Rack::Builder
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
~/.pow directory. The name of the symlink tells Pow which
hostname you want to use to access the application. For example, a
myapp will be accessible at
Note: The Pow installer creates
~/.pow as a convenient symlink
~/Library/Application Support/Pow/Hosts, the actual location
from which virtual host symlinks are read.
Once a virtual host is installed, it's also automatically accessible
from all subdomains of the named host. For example, the
virtual host described above could also be accessed at
http://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
.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
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
Working with different versions of Ruby
For example, to instruct Pow to run an application with Ruby 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
config.ru file as long as it has a
You can tell Pow to restart an application the next time it's
accessed. Simply save a file named
restart.txt in the
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 tmp/restart.txt
Restarting an application will also reload any environment scripts
.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
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
tail -f log/development.log to see those.
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
pow process in OS X's Activity Monitor and clicking
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
$ cd pow $ npm install --dev
Run the test suite:
$ cake test
(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.