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How do I enable FastCGI?

CGI vs FastCGI

Normal CGI scripts are initialized, run, and destroyed for every single browser request. The advantage of this model is that it is very simple, and thus robust.

If performance or latency becomes an issue, you can enable FastCGI. FastCGI is a web server module that speeds up your applications by keeping one interpreter running for multiple requests, instead of launching a separate interpreter for each request made while loading your page.

When FastCGI can help

This can lead to improvements in latency (responsiveness), for example if:

  • The language you are using takes a long time to start an interpreter, or to load libraries you are importing
  • You have a script which has significant repetitive launching overhead
  • Your scripts make the same, very large, database requests which (while cached by MySQL) still have to be read from memory and possibly cross an internal network
  • You are using a web framework like CherryPy, Turbogears, Pylons, or Ruby on Rails which prefers to run like a daemon

Caveats

There are some complications however:

  • The script in not guaranteed to die unless it kills itself
  • If you update your fcgi script, the old process (using the old script) may still be running with outdated code
  • The Scripts service automatically load-balances scripts across servers, just like regular CGI; new scripts may not even be launched on the servers the old scripts are running on, and thus cannot “kill all older versions of myself” since they may be on different servers

The canonical way around these issues is to have the script watch the timestamps of the files it has loaded, and kill itself if a modification timestamp changes. If you are using a framework such as CherryPy, this is already handled for you.

(Minor note: This method lends itself to race conditions on very short timescales during an update. It is also expensive if you are watching many files. Other solutions include updating a single “watch me” file with a version or hash after every update, or using transactional semantics. This is almost never an issue however.)

Enabling FastCGI

FastCGI is enabled by default for files with the .fcgi extension. To enable FastCGI for another extension, such as .rb, add the following lines to a plain text file named .htaccess in the top directory of your website:

<Files *.rb>
  SetHandler fcgid-script
</Files>

To enable FastCGI for another languages, change rb to your language’s extension (e.g., php, pl, etc.)

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