January 14, 2012

Preventing Rack::Session on a Per-Request Basis

This is admittedly a rare need, but the other day I found myself needing to conditionally enable and disable sessions in Sinatra on a per-request basis. Sinatra uses the Rack::Session middleware to manage sessions. I searched around for a while but couldn’t find out how to do it. There are options for the session that can be set per-request, like this:

get '/' do
  request.session_options[:renew] = true

The documentation shows a :defer option that seems to do what I want, but the terminology didn’t make it abundantly clear whether it did or not.

:defer will not set a cookie in the response.

Actually that does sound like what I want, sorta, but mentally I was thinking “turn off” or “disabling” and not “deferring the setting of a cookie.”

With most ruby projects, or for that matter, any interpreted language that isn’t compiled , I’ve found that I end up spending a decent amount of time reading the code from my dependencies. Some unscientific polling of ruby devs I know showed that to be a pretty standard practice.

Doing a little digging through the Rack::Session middleware showed an option that didn’t appear in the documentation:

:skip will not a set a cookie in the response nor update the session state

Now that matches the mental model of what I was looking for, and the behavior as well. Turns out it’s not in the docs because it was added recently from a Rails core team member to support the new asset pipeline in Rails 3.1, and the docs haven’t been updated yet.

So if, on a per-request basis, you want to disable using sessions entirely, simply do:

request.session_options[:skip] = true unless use_session?

(where “use_session?” is your method that figures out whether or not to use sessions)

What’s the diff between :defer and :skip, you ask? Well the answer is in the pull request description

This will not send a cookie back nor change the session state.

The :defer option did not send the cookie back but did change the session state in the backend.

This is useful for assets requests that still go through the rack stack but do not want to cause any change in the session (for example accidentally expiring flash messages).