Leader election - synapse

lib/synapse/haproxy.rb

@@ -631,7 +631,7 @@ def generate_backend_stanza(watcher, config)
         config.map {|c| "\t#{c}"},
         watcher.backends.shuffle.map {|backend|
           backend_name = construct_name(backend)
-          "\tserver #{backend_name} #{backend['host']}:#{backend['port']} #{watcher.haproxy['server_options']}" }
+          "\tserver #{backend_name} #{backend['host']}:#{backend['port']} #{watcher.haproxy['server_options']}#{' backup' if backend['backup']}" }
       ]
     end
 

lib/synapse/service_watcher/base.rb

@@ -14,6 +14,7 @@ def initialize(opts={}, synapse)
 
       @name = opts['name']
       @discovery = opts['discovery']
+      @leader_election = opts['leader_election'] || false
 
       # the haproxy config
       @haproxy = opts['haproxy']

lib/synapse/service_watcher/zookeeper.rb

@@ -53,7 +53,7 @@ def discover
             server_port = @server_port_override ? @server_port_override : port
 
             log.debug "synapse: discovered backend #{name} at #{host}:#{server_port} for service #{@name}"
-            new_backends << { 'name' => name, 'host' => host, 'port' => server_port}
+            new_backends << { 'name' => name, 'host' => host, 'port' => server_port, 'backup' => @leader_election }
           end
         end
       rescue ZK::Exceptions::NoNode
@@ -72,6 +72,26 @@ def discover
       else
         log.info "synapse: discovered #{new_backends.length} backends for service #{@name}"
         @backends = new_backends
+
+        if @leader_election
+          log.info "synapse: electing a leader from the discovered backends"
+          leader_election
+        end
+      end
+    end
+
+    #sort the list of servers based on their sequence, and the server with lowest sequence will be the leader
+    def leader_election
+      begin
+        #Integer parsing added to check that a sequence has been appended to the node key
+        @backends = @backends.sort { |x, y| Integer(x['name'].gsub(/^.*-(0)*/, '')) <=> Integer(y['name'].gsub(/^.*-(0)*/, '')) }
+        @backends[0]['backup'] = false
+        log.debug "synapse: electing leader, updated backends #{@backends}"
+      rescue ArgumentError, NoMethodError
+        raise "'sequential' should be enabled in nerve configuration for service " \
+          "#{@name} to perform leader election, please enable 'sequential' in nerve" \
+          " configuration for all servers of #{@name} service,  or disable " \
+          "'leader_election' in synapse configuration"
       end
     end