added flash plugin

+ added flash plugin, and the session plugin required by flash
+ set the DB name as an environment variable
+ set @message variable where it needs to be set to show on the UI

app.rb

@@ -4,11 +4,16 @@ require 'json'
 require 'sequel'
 
 class App < Roda
-  DB = Sequel.sqlite('db/url_shortener.db')
-  links = DB[:links]
+  plugin :sessions, secret: ENV.delete('APP_SESSION_SECRET')
   plugin :render, escape: true
+  plugin :flash
+
+  DB = Sequel.sqlite("db/#{ENV['DB_NAME']}")
+  links = DB[:links]
+
   route do |r|
     r.root do
+      @message = flash['message'] || "Enter a URL"
       view :home
     end
 
@@ -19,11 +24,16 @@ class App < Roda
 
     r.post "create" do
       url = r.params['url']
-      if url.empty? then r.redirect '/' end
+      if url.empty? 
+        flash['message'] = "Please enter a valid URL";
+        r.redirect '/'
+      end
       if nil == links.filter(:url => url).first
         code = SecureRandom.urlsafe_base64 4
         links.insert(url: url, code: code) 
+        @message = "Link created"
       end
+      @message ||= "Link exists"
       @new_link = 'http://' + request.env['HTTP_HOST'] + '/' + links.filter(:url => url).first[:code]
       view :create   
     end