Add <worker n> section to set a configuration for a specific worker

example/worker_section.conf

@@ -0,0 +1,36 @@
+<system>
+  workers 4
+  root_dir /path/fluentd/root
+</system>
+
+<source> # top-level sections works on all workers in parallel
+  @type forward
+  port 24224
+</source>
+
+<match all> # this sections also works on all workers in parallel
+  @type stdout
+  <inject>
+    worker_id_key worker_id
+  </inject>
+</match>
+
+<worker 0> # this section works only on first worker process
+  <source>
+    @type tail
+    format none
+    path /var/log/fluentd_test.log
+    pos_file /var/log/fluentd_test.pos
+    tag tail
+    rotate_wait 5
+    read_from_head true
+    refresh_interval 60
+  </source>
+
+  <match tail>
+    @type stdout
+    <inject>
+      worker_id_key worker_id
+    </inject>
+  </match>
+</worker>

lib/fluent/agent.rb

@@ -62,6 +62,7 @@ def configure(conf)
 
       # initialize <match> and <filter> elements
       conf.elements('filter', 'match').each { |e|
+        next if e.for_another_worker?
         pattern = e.arg.empty? ? '**' : e.arg
         type = e['@type']
         raise ConfigError, "Missing '@type' parameter on <#{e.name}> directive" unless type
@@ -121,7 +122,8 @@ def lifecycle(desc: false)
     end
 
     def add_match(type, pattern, conf)
-      log.info :worker0, "adding match#{@context.nil? ? '' : " in #{@context}"}", pattern: pattern, type: type
+      log_type = conf.for_this_worker? ? :default : :worker0
+      log.info log_type, "adding match#{@context.nil? ? '' : " in #{@context}"}", pattern: pattern, type: type
 
       output = Plugin.new_output(type)
       output.context_router = @event_router
@@ -142,7 +144,8 @@ def add_match(type, pattern, conf)
     end
 
     def add_filter(type, pattern, conf)
-      log.info :worker0, "adding filter#{@context.nil? ? '' : " in #{@context}"}", pattern: pattern, type: type
+      log_type = conf.for_this_worker? ? :default : :worker0
+      log.info log_type, "adding filter#{@context.nil? ? '' : " in #{@context}"}", pattern: pattern, type: type
 
       filter = Plugin.new_filter(type)
       filter.context_router = @event_router

lib/fluent/config/element.rb

@@ -35,10 +35,13 @@ def initialize(name, arg, attrs, elements, unused = nil)
 
         # it's global logger, not plugin logger: deprecated message should be global warning, not plugin level.
         @logger = defined?($log) ? $log : nil
+
+        @target_worker_id = nil
       end
 
       attr_accessor :name, :arg, :unused, :v1_config, :corresponding_proxies, :unused_in
       attr_writer :elements
+      attr_reader :target_worker_id
 
       RESERVED_PARAMETERS_COMPAT = {
         '@type' => 'type',
@@ -213,6 +216,25 @@ def self.unescape_parameter(v)
         v.each_char { |c| result << LiteralParser.unescape_char(c) }
         result
       end
+
+      def set_target_worker_id(worker_id)
+        @target_worker_id = worker_id
+        @elements.each { |e|
+          e.set_target_worker_id(worker_id)
+        }
+      end
+
+      def for_every_workers?
+        @target_worker_id == nil
+      end
+
+      def for_this_worker?
+        @target_worker_id == Fluent::Engine.worker_id
+      end
+
+      def for_another_worker?
+        @target_worker_id != nil && @target_worker_id != Fluent::Engine.worker_id
+      end
     end
   end
 end

lib/fluent/engine.rb

@@ -31,6 +31,7 @@ def initialize
       @root_agent = nil
       @default_loop = nil
       @engine_stopped = false
+      @_worker_id = nil
 
       @log_event_router = nil
       @log_emit_thread = nil
@@ -97,12 +98,21 @@ def run_configure(conf)
                     else
                       "section <#{e.name}> is not used in <#{parent_name}>"
                     end
-          $log.warn :worker0, message
+          if e.for_every_workers?
+            $log.warn :worker0, message
+          elsif e.for_this_worker?
+            $log.warn message
+          end
           next
         end
         unless e.name == 'system'
           unless @without_source && e.name == 'source'
-            $log.warn :worker0, "parameter '#{key}' in #{e.to_s.strip} is not used."
+            message = "parameter '#{key}' in #{e.to_s.strip} is not used."
+            if e.for_every_workers?
+              $log.warn :worker0, message
+            elsif e.for_this_worker?
+              $log.warn message
+            end
           end
         end
       }
@@ -128,7 +138,7 @@ def configure(conf)
 
         unmatched_tags = Fluent::Log.event_tags.select{|t| !@log_event_router.match?(t) }
         unless unmatched_tags.empty?
-          $log.warn :worker0, "match for some tags of log events are not defined (to be ignored)", tags: unmatched_tags
+          $log.warn "match for some tags of log events are not defined (to be ignored)", tags: unmatched_tags
         end
       rescue ArgumentError # ArgumentError "#{label_name} label not found"
         # use default event router if <label @FLUENT_LOG> is missing in configuration
@@ -139,7 +149,7 @@ def configure(conf)
 
           unmatched_tags = Fluent::Log.event_tags.select{|t| !@log_event_router.match?(t) }
           unless unmatched_tags.empty?
-            $log.warn :worker0, "match for some tags of log events are not defined (to be ignored)", tags: unmatched_tags
+            $log.warn "match for some tags of log events are not defined (to be ignored)", tags: unmatched_tags
           end
         end
       end
@@ -200,10 +210,6 @@ def log_event_loop
     end
 
     def run
-      # if ENV doesn't have SERVERENGINE_WORKER_ID, it is a worker under --no-supervisor or in tests
-      # so it's (almost) a single worker, worker_id=0
-      worker_id = (ENV['SERVERENGINE_WORKER_ID'] || 0).to_i
-
       begin
         $log.info "starting fluentd worker", pid: Process.pid, ppid: Process.ppid, worker: worker_id
         start
@@ -251,8 +257,15 @@ def push_log_event(tag, time, record)
       @log_event_queue.push([tag, time, record])
     end
 
-    private
+    def worker_id
+      return @_worker_id if @_worker_id
+      # if ENV doesn't have SERVERENGINE_WORKER_ID, it is a worker under --no-supervisor or in tests
+      # so it's (almost) a single worker, worker_id=0
+      @_worker_id = (ENV['SERVERENGINE_WORKER_ID'] || 0).to_i
+      @_worker_id
+    end
 
+    private
     def start
       @root_agent.start
     end

lib/fluent/plugin/base.rb

@@ -51,6 +51,9 @@ def fluentd_worker_id
       end
 
       def configure(conf)
+        if conf.respond_to?(:for_this_worker?) && conf.for_this_worker?
+          system_config_override(workers: 1)
+        end
         super
         @_state ||= State.new(false, false, false, false, false, false, false, false, false)
         @_state.configure = true

lib/fluent/root_agent.rb

@@ -64,11 +64,34 @@ def initialize(log:, system_config: SystemConfig.new)
     attr_reader :labels
 
     def configure(conf)
+      # initialize <worker> elements
+      conf.elements(name: 'worker').each do |e|
+        target_worker_id_str = e.arg
+        if target_worker_id_str.empty?
+          raise ConfigError, "Missing worker id on <worker> directive"
+        end
+
+        target_worker_id = target_worker_id_str.to_i
+        if target_worker_id < 0 || target_worker_id > (Fluent::Engine.system_config.workers - 1)
+          raise ConfigError, "worker id #{target_worker_id} specified by <worker> directive is not allowed. Available worker id is between 0 and #{(Fluent::Engine.system_config.workers - 1)}"
+        end
+
+        e.elements.each do |elem|
+          unless ['source', 'match', 'filter', 'label'].include?(elem.name)
+            raise ConfigError, "<worker> section cannot have <#{elem.name}> directive"
+          end
+          elem.set_target_worker_id(target_worker_id)
+        end
+        conf += e
+      end
+      conf.elements.delete_if{|e| e.name == 'worker'}
+
       error_label_config = nil
 
       # initialize <label> elements before configuring all plugins to avoid 'label not found' in input, filter and output.
       label_configs = {}
       conf.elements(name: 'label').each { |e|
+        next if e.for_another_worker?
         name = e.arg
         raise ConfigError, "Missing symbol argument on <label> directive" if name.empty?
 
@@ -90,6 +113,7 @@ def configure(conf)
         log.info :worker0, "'--without-source' is applied. Ignore <source> sections"
       else
         conf.elements(name: 'source').each { |e|
+          next if e.for_another_worker?
           type = e['@type']
           raise ConfigError, "Missing '@type' parameter on <source> directive" unless type
           add_source(type, e)
@@ -235,7 +259,8 @@ def suppress_interval(interval_time)
     end
 
     def add_source(type, conf)
-      log.info :worker0, "adding source", type: type
+      log_type = conf.for_this_worker? ? :default : :worker0
+      log.info log_type, "adding source", type: type
 
       input = Plugin.new_input(type)
       # <source> emits events to the top-level event router (RootAgent#event_router).