Make Redis an optional dependency. Handle URL resolution gracefully i…

…f Redis is unavailable.

README.md

@@ -16,6 +16,8 @@ To deploy to Kubernetes, see [kubernetes/README.md](kubernetes/README.md).
 
 A Docker image is available on [Docker Hub](https://hub.docker.com/r/stefansundin/rssbox) and [Amazon ECR](https://gallery.ecr.aws/stefansundin/rssbox).
 
+**Note:** Redis is now an optional dependency! It is only used for the URL resolution feature (turned off on the public Heroku instance).
+
 #### Heroku
 
 If you need to re-provision redis, or you didn't use the deploy button above to provision the app initially, then you need to make sure to set the maxmemory policy:

app.rb

@@ -1562,11 +1562,7 @@
 end
 
 get "/health" do
-  if $redis.ping != "PONG"
-    return [500, "Redis error"]
-  end
-rescue Redis::CannotConnectError => e
-  return [500, "Redis connection error"]
+  return [200, ""]
 end
 
 if ENV["GOOGLE_VERIFICATION_TOKEN"]

app/url.rb

@@ -2,7 +2,7 @@
 
 module App
   class URL
-    URL_RESOLUTION_DISABLED = (ENV["URL_RESOLUTION_DISABLED"] == "true")
+    URL_RESOLUTION_DISABLED = ($redis.nil? || ENV["URL_RESOLUTION_DISABLED"] == "true")
 
     @@cache = {}
 
@@ -51,6 +51,8 @@ def self.resolve(urls, force=false)
       end
       threads.map(&:join)
       return nil
+    rescue Redis::BaseConnectionError
+      return nil
     end
 
     private

config/initializers/20-redis.rb

@@ -1,7 +1,46 @@
 # frozen_string_literal: true
 
-begin
-  $redis = Redis.new
-rescue
-  puts "Failed to connect to redis!"
+# Monkeypatch redis to throttle it from attempting to connect more often than once a second
+
+class Redis
+  class ThrottledConnectError < BaseConnectionError
+  end
+
+  class Client
+    @@last_connect_error = nil
+
+    # https://github.com/redis/redis-rb/blob/v4.6.0/lib/redis/client.rb#L379-L399
+    def establish_connection
+      if @@last_connect_error && Time.now < @@last_connect_error + 1
+        raise ThrottledConnectError, "Throttled connection attempt to Redis on #{location}"
+      end
+
+      server = @connector.resolve.dup
+
+      @options[:host] = server[:host]
+      @options[:port] = Integer(server[:port]) if server.include?(:port)
+
+      @connection = @options[:driver].connect(@options)
+      @pending_reads = 0
+    rescue TimeoutError,
+           SocketError,
+           Errno::EADDRNOTAVAIL,
+           Errno::ECONNREFUSED,
+           Errno::EHOSTDOWN,
+           Errno::EHOSTUNREACH,
+           Errno::ENETUNREACH,
+           Errno::ENOENT,
+           Errno::ETIMEDOUT,
+           Errno::EINVAL => error
+
+      @@last_connect_error = Time.now
+      raise CannotConnectError, "Error connecting to Redis on #{location} (#{error.class})"
+    end
+  end
+end
+
+if ENV.has_key?("REDIS_URL")
+  $redis = Redis.new({
+    reconnect_attempts: 0,
+  })
 end