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The book recommends gem 'rack-flash' for session-based notifications, but rack-flash doesn't work with up-do-date versions of rack. The sinatra-authentication gem also requires rack-flash.
Downgrade to a version of Rack pre 1.3.6
Problem: It sets you up for more dependency problems.
The author says the downside is that it does not work across different kinds of rack apps.
Sinatra could include this functionality itself?
tl;dr Maybe the best short-term solution for the book is just to post a warning about rack-flash, or recommend 'sinatra/flash' instead.
alternate for that is rack-flash3, check this out nakajima/rack-flash#8
Yeah, I had tried that and ran into something undesirable - I don't remember what. I ended up using sinatra/flash on recommendation of one of the core team. I guess that is what the book should be recommending.
For the time being, you can use rack-flash3 or sinatra-flash gems.