Plack Handbook - Reloading applications

Day 4: Reloading applications

Yesterday I introduced the basics of plackup and its command line options. Today I'll show you more!

Reload the application as necessary

During development you often change your Perl code, saved in .psgi or .pm files. Because the Plack server launched by the plackup command is a persistent process you need to restart your server whenever your code changes. This is a little painful.

So there's an option to watch for changes to files under your working directory and reload the application as needed: -r (or --reload).

 plackup -r hello.psgi

It will watch files under the current directory by default, but you can change it to watch additional locations by using the -R option (note the uppercase).

 plackup -R lib,/path/to/scripts hello.psgi

As you can see, multiple paths can be monitored by combining them with , (comma).

By default plackup uses a dumb timer to scan the whole directory, but if you're on Linux and have Linux::Inotify2 installed or on Mac OS and have Mac::FSEvents installed filesystem notification is used so it's more efficient.

-r vs Server auto-detection

In Day 3 I told you that plackup's automatic server detection is smart enough to tell if PSGI application uses an event modules such as AnyEvent or Coro and choose the correct backend. Be aware that this automatic selection doesn't work if you use the -r option because plackup uses a delayed loading technique to reload apps in forked processes. It's recommended that you explicitly set the server with the -s option when using the -r option.

Reloading sucks? Shotgun!

Reloading a module or application in a persistent Perl process can cause problems. For instance, module package variables could be redefined or overwritten and then get stuck in a bad state.

Plack now has the Shotgun loader, inspired by Rack's shotgun, which solves the reloading problem by loading the app on every request in a forked child environment.

Using the Shotgun loader is easy:

 > plackup -L Shotgun myapp.psgi

This will delay the compilation of your application to runtime. When a request is received it will fork off a new child process to compile your app and return the PSGI response over the pipe. You can also preload modules in the parent process that are not likely to be updated to reduce the time needed to compile your application.

For instance, if your application uses Moose and DBIx::Class then use the following options:

 > plackup -MMoose -MDBIx::Class -L Shotgun myapp.psgi

This speeds up the time required to compile your application in the runtime.