Implement multi store

lib/gitlab/redis/multi_store.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module Redis
+    class MultiStore
+      attr_reader :primary_store, :secondary_store
+
+      READ_COMMANDS = %i(
+        get
+        mget
+        smembers
+      ).freeze
+
+      WRITE_COMMANDS = %i(
+        setnx
+        setex
+        sadd
+        srem
+        del
+        pipelined
+        flushdb
+      ).freeze
+
+      def initialize(primary_store_options, secondary_store_options)
+        @primary_store = ::Redis::Store.new(primary_store_options)
+        @secondary_store = ::Redis::Store.new(secondary_store_options)
+      end
+
+      # This is needed because of Redis::Rack::Connection is requiring Redis::Store
+      # https://github.com/redis-store/redis-rack/blob/a833086ba494083b6a384a1a4e58b36573a9165d/lib/redis/rack/connection.rb#L15
+      # Done similarly in https://github.com/lsegal/yard/blob/main/lib/yard/templates/template.rb#L122
+      def is_a?(klass)
+        return true if klass == ::Redis::Store
+
+        super(klass)
+      end
+
+      # TODO: Add Feature flags, by default read only from the secondary store,
+      # by enabling the FF, read from the primary, and fallback to read from the secondary.
+      READ_COMMANDS.each do |name|
+        define_method(name) do |*args, **kwargs, &block|
+          if @instance
+            send_command(@instance, name, *args, **kwargs, &block)
+          else
+            value = send_command(primary_store, name, *args, **kwargs, &block)
+            # TODO: Add logger to detect for which key we fallback to shared_steate_store
+            value ||= send_command(secondary_store, name, *args, **kwargs, &block)
+
+            value
+          end
+        end
+      end
+
+      # TODO: Add proper error handling if primary store fails, ensuring that we execute at least on secondary store
+      # TODO: Add Feature flags, by default write only on the secondary store (SharedState), by enabling the FF, write to the primary as well.
+      WRITE_COMMANDS.each do |name|
+        define_method(name) do |*args, **kwargs, &block|
+          if @instance
+            send_command(@instance, name, *args, **kwargs, &block)
+          else
+            send_command(primary_store, name, *args, **kwargs, &block)
+            send_command(secondary_store, name, *args, **kwargs, &block)
+          end
+        end
+      end
+
+      # TEST: try to avoid Deprecation Toolkit issues
+      # See the pipelines of the POC for the example
+      # We call set there https://github.com/redis-store/redis-rack/blob/v2.1.3/lib/rack/session/redis.rb#L49
+      # With the meta-definition like there, we have hash <-> kwargs Ruby 2.7 issues as the downstream is defined like:
+      # https://github.com/redis/redis-rb/blob/master/lib/redis.rb#L846
+      def set(...)
+        if @instance
+          send_command(@instance, :set, ...)
+        else
+          send_command(primary_store, :set, ...)
+          send_command(secondary_store, :set, ...)
+        end
+      end
+
+      private
+
+      def send_command(redis_instance, name, *args, **kwargs, &block)
+        if block_given?
+          redis_instance.send(name, *args, **kwargs) do |*args| # rubocop:disable GitlabSecurity/PublicSend
+            with_instance(redis_instance, *args, &block)
+          end
+        else
+          redis_instance.send(name, *args, **kwargs) # rubocop:disable GitlabSecurity/PublicSend
+        end
+      end
+
+      def with_instance(instance, *args)
+        @instance = instance
+        yield(*args)
+      ensure
+        @instance = nil
+      end
+
+      def method_missing(...)
+        # TODO: Add logger here to log for which key and command we did fallback to the shared_state_store
+        secondary_store.send(...) # rubocop:disable GitlabSecurity/PublicSend
+      end
+
+      def respond_to_missing?(method_name, include_private = false)
+        true
+      end
+    end
+  end
+end