Capistrano to deploy rails application - how to handle long migrations?

You should put up a maintenance page if the application is not going to be available for a while. I use this Capistrano task:

namespace :deploy do  namespace :web do    desc <<-DESC      Present a maintenance page to visitors. Disables your application's web \      interface by writing a "maintenance.html" file to each web server. The \      servers must be configured to detect the presence of this file, and if \      it is present, always display it instead of performing the request.      By default, the maintenance page will just say the site is down for \      "maintenance", and will be back "shortly", but you can customize the \      page by specifying the REASON and UNTIL environment variables:        $ cap deploy:web:disable \\              REASON="a hardware upgrade" \\              UNTIL="12pm Central Time"      Further customization will require that you write your own task.    DESC    task :disable, :roles => :web do      require 'erb'      on_rollback { run "rm #{shared_path}/system/maintenance.html" }      reason = ENV['REASON']      deadline = ENV['UNTIL']            template = File.read('app/views/admin/maintenance.html.erb')      page = ERB.new(template).result(binding)      put page, "#{shared_path}/system/maintenance.html", :mode => 0644    end  endend

The app/views/admin/maintenance.html.erb file should contain:

<p>We’re currently offline for <%= reason ? reason : 'maintenance' %> as of <%= Time.now.utc.strftime('%H:%M %Z') %>.</p><p>Sorry for the inconvenience. We’ll be back <%= deadline ? "by #{deadline}" : 'shortly' %>.</p>

The final step is to configure the Apache virtual host with some directives to look for the maintenance.html file and redirect all requests to it if it's present:

<IfModule mod_rewrite.c>  RewriteEngine On  # Redirect all requests to the maintenance page if present  RewriteCond %{REQUEST_URI} !\.(css|gif|jpg|png)$  RewriteCond %{DOCUMENT_ROOT}/system/maintenance.html -f  RewriteCond %{SCRIPT_FILENAME} !maintenance.html  RewriteRule ^.*$ /system/maintenance.html [L]</IfModule>

To put the application into maintenance mode, run cap deploy:web:disable and to make it live again do cap deploy:web:enable.

My production deploys generally follow this process:

1. cap production deploy:web:disable which directs all requests to a static maintenance page
2. cap production deploy
3. migrations etc, testing each of the servers individually to make sure things are OK
4. cap production deploy:web:enable to make the site work as it should

John Topley's response gives you some good in depth info here.