1 files changed, 99 insertions, 0 deletions
diff --git a/lib/gitlab/database/load_balancing/host_list.rb b/lib/gitlab/database/load_balancing/host_list.rb
new file mode 100644
index 00000000000..24800012947
--- /dev/null
+++ b/lib/gitlab/database/load_balancing/host_list.rb
@@ -0,0 +1,99 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module Database
+    module LoadBalancing
+      # A list of database hosts to use for connections.
+      class HostList
+        # hosts - The list of secondary hosts to add.
+        def initialize(hosts = [])
+          @hosts = hosts.shuffle
+          @pools = Set.new
+          @index = 0
+          @mutex = Mutex.new
+          @hosts_gauge = Gitlab::Metrics.gauge(:db_load_balancing_hosts, 'Current number of load balancing hosts')
+
+          set_metrics!
+          update_pools
+        end
+
+        def hosts
+          @mutex.synchronize { @hosts.dup }
+        end
+
+        def shuffle
+          @mutex.synchronize do
+            unsafe_shuffle
+          end
+        end
+
+        def length
+          @mutex.synchronize { @hosts.length }
+        end
+
+        def host_names_and_ports
+          @mutex.synchronize { @hosts.map { |host| [host.host, host.port] } }
+        end
+
+        def manage_pool?(pool)
+          @pools.include?(pool)
+        end
+
+        def hosts=(hosts)
+          @mutex.synchronize do
+            @hosts = hosts
+            unsafe_shuffle
+            update_pools
+          end
+
+          set_metrics!
+        end
+
+        # Sets metrics before returning next host
+        def next
+          next_host.tap do |_|
+            set_metrics!
+          end
+        end
+
+        private
+
+        def unsafe_shuffle
+          @hosts = @hosts.shuffle
+          @index = 0
+        end
+
+        # Returns the next available host.
+        #
+        # Returns a Gitlab::Database::LoadBalancing::Host instance, or nil if no
+        # hosts were available.
+        def next_host
+          @mutex.synchronize do
+            break if @hosts.empty?
+
+            started_at = @index
+
+            loop do
+              host = @hosts[@index]
+              @index = (@index + 1) % @hosts.length
+
+              break host if host.online?
+
+              # Return nil once we have cycled through all hosts and none were
+              # available.
+              break if @index == started_at
+            end
+          end
+        end
+
+        def set_metrics!
+          @hosts_gauge.set({}, @hosts.length)
+        end
+
+        def update_pools
+          @pools = Set.new(@hosts.map(&:pool))
+        end
+      end
+    end
+  end
+end