What does Rails 3 session_store domain :all really do?

Question

Updated question to make it more clear

I understand that you can set the domain of your session_store to share sessions between subdomains like this

Answer 1

tl;dr: Use @Nader's code. BUT I found I needed add it into my conifg/environments/[production|development].rb and pass my dot-prefixed-domain as an argument. This is on Rails 3.2.11

Cookie sessions are usually stored only for your top level domain.

If you look in Chrome -> Settings -> Show advanced settings… -> Privacy/Content settings… -> All cookies and site data… -> Search {yourdomain.com} You can see that there will be separate entries for sub1.yourdomain.com and othersub.yourdomain.com and yourdomain.com

The challenge is to use the same session store file across all subdomains.

Step 1: Use @Nader's CustomDomainCookie code

This is where Rack Middleware comes in. Some more relevant rack & rails resources:

• Railscasts about Rack
• Railsguide for Rack
• Rack documentation for sesssions abstractly and for cookie sessions

Basically what this does is that it will map all of your cookie session data back onto the exact same cookie file that is equal to your root domain.

Step 2: Add To Rails Config

Now that you have a custom class in lib, make sure are autoloading it. If that meant nothing to you, look here: Rails 3 autoload

The first thing is to make sure that you are system-wide using a cookie store. In config/application.rb we tell Rails to use a cookie store.

# We use a cookie_store for session data
config.session_store :cookie_store,
                     :key => '_yourappsession',
                     :domain => :all

The reason this is here is mentioned here is because of the :domain => :all line. There are other people that have suggested to specify :domain => ".yourdomain.com" instead of :domain => :all. For some reason this did not work for me and I needed the custom Middleware class as described above.

Then in your config/environments/production.rb add:

config.middleware.use "CustomDomainCookie", ".yourdomain.com"

Note that the preceding dot is necessary. See "sub-domain cookies, sent in a parent domain request?" for why.

Then in your config/environments/development.rb add:

config.middleware.use "CustomDomainCookie", ".lvh.me"

The lvh.me trick maps onto localhost. It's awesome. See this Railscast about subdomains and this note for more info.

Hopefully that should do it. I honestly am not entirely sure why the process is this convoluted, as I feel cross subdomain sites are common. If anyone has any further insights into the reasons behind each of these steps, please enlighten us in the comments.