Deprecate controller level force_ssl

Today there are two common ways for Rails developers to force their
applications to communicate over HTTPS:

* `config.force_ssl` is a setting in environment configurations that
  enables the `ActionDispatch::SSL` middleware. With this middleware
  enabled, all HTTP communication to your application will be redirected
  to HTTPS. The middleware also takes care of other best practices by
  setting HSTS headers, upgrading all cookies to secure only, etc.
* The `force_ssl` controller method redirects HTTP requests to certain
  controllers to HTTPS.

As a consultant, I've seen many applications with misconfigured HTTPS
setups due to developers adding `force_ssl` to `ApplicationController`
and not enabling `config.force_ssl`. With this configuration, many
application requests can be served over HTTP such as assets, requests
that hit mounted engines, etc. In addition, because cookies are not
upgraded to secure only in this configuration and HSTS headers are not
set, it's possible for cookies that are meant to be secure to be sent
over HTTP.

The confusion between these two methods of forcing HTTPS is compounded
by the fact that they share an identical name. This makes finding
documentation on the "right" method confusing.

HTTPS throughout is quickly becomming table stakes for all web sites.
Sites are expected to operate over HTTPS for all communication,
sensitive or otherwise. Let's encourage use of the broader-reaching
`ActionDispatch::SSL` middleware and elminate this source of user
confusion. If, for some reason, applications need to expose certain
endpoints over HTTP they can do so by properly configuring
`config.ssl_options`.

1 files changed, 11 insertions, 0 deletions

diff --git a/guides/source/upgrading_ruby_on_rails.md b/guides/source/upgrading_ruby_on_rails.md
index d5dfaef591..c2fe012eeb 100644
--- a/guides/source/upgrading_ruby_on_rails.md
+++ b/guides/source/upgrading_ruby_on_rails.md
@@ -66,6 +66,17 @@ Overwrite /myapp/config/application.rb? (enter "h" for help) [Ynaqdh]
 
 Don't forget to review the difference, to see if there were any unexpected changes.
 
+Upgrading from Rails 5.2 to Rails 6.0
+-------------------------------------
+
+### Force SSL
+
+The `force_ssl` method on controllers has been deprecated and will be removed in
+Rails 6.1. You are encouraged to enable `config.force_ssl` to enforce HTTPS
+connections throughout your application. If you need to exempt certain endpoints
+from redirection, you can use `config.ssl_options` to configure that behavior.
+
+
 Upgrading from Rails 5.1 to Rails 5.2
 -------------------------------------