diff --git a/lib/perfectqueue/backend/rdb.rb b/lib/perfectqueue/backend/rdb.rb
index 0a16ed8..b256df6 100644
--- a/lib/perfectqueue/backend/rdb.rb
+++ b/lib/perfectqueue/backend/rdb.rb
@@ -6,6 +6,7 @@ module PerfectQueue::Backend
   class RDBBackend
     MAX_RETRY = ::PerfectQueue::Backend::RDBCompatBackend::MAX_RETRY
     DELETE_OFFSET = ::PerfectQueue::Backend::RDBCompatBackend::DELETE_OFFSET
+    EVENT_HORIZON = ::PerfectQueue::Backend::RDBCompatBackend::EVENT_HORIZON
     class Token < Struct.new(:key)
     end
 
@@ -48,7 +49,7 @@ def submit(id, data, time=Process.clock_gettime(Process::CLOCK_REALTIME, :second
 
     def cancel(id, delete_timeout=3600, now=Process.clock_gettime(Process::CLOCK_REALTIME, :second))
       connect {
-        n = @db["UPDATE `#{@table}` SET timeout=?, created_at=NULL, resource=NULL WHERE id=? AND created_at IS NOT NULL;", now+delete_timeout-DELETE_OFFSET, id].update
+        n = @db["UPDATE `#{@table}` SET timeout=?, created_at=NULL, resource=NULL WHERE id=? AND #{EVENT_HORIZON} < timeout;", now+delete_timeout-DELETE_OFFSET, id].update
         return n > 0
       }
     end
diff --git a/lib/perfectqueue/backend/rdb_compat.rb b/lib/perfectqueue/backend/rdb_compat.rb
index 9b46f94..9b02d25 100644
--- a/lib/perfectqueue/backend/rdb_compat.rb
+++ b/lib/perfectqueue/backend/rdb_compat.rb
@@ -284,7 +284,7 @@ def finish(task_token, retention_time, options)
         key = task_token.key
 
         connect {
-          n = @db["UPDATE `#{@table}` SET timeout=?, created_at=NULL, resource=NULL WHERE id=? AND created_at IS NOT NULL", delete_timeout, key].update
+          n = @db["UPDATE `#{@table}` SET timeout=?, created_at=NULL, resource=NULL WHERE id=? AND #{EVENT_HORIZON} < timeout", delete_timeout, key].update
           if n <= 0
             raise IdempotentAlreadyFinishedError, "task key=#{key} does not exist or already finished."
           end
@@ -292,7 +292,7 @@ def finish(task_token, retention_time, options)
         nil
       end
 
-      # => nil
+      # => next_timeout
       def heartbeat(task_token, alive_time, options)
         now = (options[:now] || Time.now).to_i
         next_timeout = now + alive_time
@@ -305,8 +305,14 @@ def heartbeat(task_token, alive_time, options)
           sql << ", data=?"
           params << compress_data(data.to_json, options[:compression])
         end
-        sql << " WHERE id=? AND created_at IS NOT NULL"
-        params << key
+        if last_heartbeat = options[:last_heartbeat]
+          sql << " WHERE id=? AND timeout=?"
+          params << key
+          params << last_heartbeat
+        else
+          sql << " WHERE id=? AND #{EVENT_HORIZON} < timeout"
+          params << key
+        end
 
         connect {
           n = @db[*params].update
@@ -315,13 +321,15 @@ def heartbeat(task_token, alive_time, options)
             if row == nil
               raise PreemptedError, "task key=#{key} does not exist or preempted."
             elsif row[:created_at] == nil
-              raise PreemptedError, "task key=#{key} preempted."
+              raise PreemptedError, "task key=#{key} is finished or canceled"
+            elsif options[:last_heartbeat] && row[:timeout] != options[:last_heartbeat]
+              raise PreemptedError, "task key=#{key} is preempted by another worker."
             else # row[:timeout] == next_timeout
               # ok
             end
           end
         }
-        nil
+        next_timeout
       end
 
       def release(task_token, alive_time, options)
diff --git a/lib/perfectqueue/multiprocess/thread_processor.rb b/lib/perfectqueue/multiprocess/thread_processor.rb
index eb2cde7..718f55b 100644
--- a/lib/perfectqueue/multiprocess/thread_processor.rb
+++ b/lib/perfectqueue/multiprocess/thread_processor.rb
@@ -96,8 +96,9 @@ def run_loop
               @finish_flag.wait(@poll_interval)
             else
               begin
+                last_heartbeat = Time.now.to_i
                 while task = tasks.shift
-                  process(task)
+                  process(task, last_heartbeat)
                 end
               ensure
                 # TODO do not call release! because rdb_compat backend
@@ -120,11 +121,11 @@ def run_loop
         @tm.stop
       end
 
-      def process(task)
+      def process(task, last_heartbeat=Time.now.to_i)
         @log.info "acquired task task=#{task.key} id=#{@processor_id}: #{task.inspect}"
         begin
           r = @runner.new(task)
-          @tm.set_task(task, r)
+          @tm.set_task(task, r, last_heartbeat)
           begin
             r.run
           ensure
diff --git a/lib/perfectqueue/task.rb b/lib/perfectqueue/task.rb
index 1723fae..84e28f7 100644
--- a/lib/perfectqueue/task.rb
+++ b/lib/perfectqueue/task.rb
@@ -72,7 +72,7 @@ def initialize(client, key, attributes, task_token)
     end
 
     def heartbeat!(options={})
-      @client.heartbeat(@task_token, options)
+      @attributes[:timeout] = @client.heartbeat(@task_token, options.merge(last_heartbeat: timeout))
     end
 
     def finish!(options={})
diff --git a/lib/perfectqueue/task_metadata.rb b/lib/perfectqueue/task_metadata.rb
index 2caad60..fad3032 100644
--- a/lib/perfectqueue/task_metadata.rb
+++ b/lib/perfectqueue/task_metadata.rb
@@ -50,11 +50,7 @@ def created_at
     end
 
     def timeout
-      if t = @attributes[:timeout]
-        return Time.at(t)
-      else
-        return nil
-      end
+      @attributes[:timeout]
     end
 
     def finished?
diff --git a/lib/perfectqueue/task_monitor.rb b/lib/perfectqueue/task_monitor.rb
index dced0ef..d34f682 100644
--- a/lib/perfectqueue/task_monitor.rb
+++ b/lib/perfectqueue/task_monitor.rb
@@ -52,15 +52,20 @@ def join
       @thread.join
     end
 
-    def set_task(task, runner)
+    def set_task(task, runner, last_heartbeat=Time.now.to_i)
       task.extend(TaskMonitorHook)
       task.log = @log
       task.task_monitor = self
       task.runner = runner
       @mutex.synchronize {
         @task = task
-        @last_task_heartbeat = Time.now.to_i
+        @last_task_heartbeat = last_heartbeat
+        @cond.broadcast
       }
+      now = Time.now.to_i
+      while @task && @last_task_heartbeat + @task_heartbeat_interval < now
+        sleep 1
+      end
     end
 
     def stop_task(immediate)
@@ -101,7 +106,6 @@ def external_task_heartbeat(task, &block)
       @mutex.synchronize {
         if task == @task
           ret = block.call if block
-          @last_task_heartbeat = Time.now.to_i
         end
         ret
       }
@@ -118,7 +122,6 @@ def run
             next_task_heartbeat = @last_task_heartbeat + @task_heartbeat_interval
             next_time = [next_child_heartbeat, next_task_heartbeat].min
           else
-            next_task_heartbeat = nil
             next_time = next_child_heartbeat
           end
 
@@ -126,7 +129,7 @@ def run
           @cond.wait(next_wait) if next_wait > 0
 
           now = Time.now.to_i
-          if @task && next_task_heartbeat && next_task_heartbeat <= now
+          if @task && @last_task_heartbeat + @task_heartbeat_interval <= now
             task_heartbeat
             @last_task_heartbeat = now
           end
diff --git a/spec/multiprocess/child_process_spec.rb b/spec/multiprocess/child_process_spec.rb
index f87499b..6537f2e 100644
--- a/spec/multiprocess/child_process_spec.rb
+++ b/spec/multiprocess/child_process_spec.rb
@@ -73,7 +73,7 @@
   end
 
   describe '#process' do
-    let (:task){ double('task', key: double) }
+    let (:task){ double('task', key: double, timeout: Time.now.to_i) }
     before do
       expect(runner_insntace).to receive(:run)
     end
diff --git a/spec/rdb_compat_backend_spec.rb b/spec/rdb_compat_backend_spec.rb
index 214496a..27f5ebb 100644
--- a/spec/rdb_compat_backend_spec.rb
+++ b/spec/rdb_compat_backend_spec.rb
@@ -242,9 +242,13 @@
         expect(ary[3]).to be_an_instance_of(AcquiredTask)
         expect(ary[4]).to be_an_instance_of(AcquiredTask)
 
-        now1 = Time.at(now + alive_time)
-        expect(now1).to receive(:to_time).exactly(5).times.and_call_original
-        db.list({}){|task| expect(task.timeout).to eq now1.to_time }
+        now1 = now + alive_time
+        i = 0
+        db.list({}) do |task|
+          expect(task.timeout).to eq now1
+          i += 1
+        end
+        expect(i).to eq(5)
       end
     end
   end
@@ -294,15 +298,18 @@
     let (:delete_timeout){ now + retention_time }
     let (:options){ {now: now} }
     before{ allow(STDERR).to receive(:puts) }
-    context 'have a queueuled task' do
+    context 'have a queued task' do
       before do
         db.submit(key, 'test', nil, {})
       end
       it 'returns nil if next_run_time is not updated' do
-        expect(db.heartbeat(task_token, 0, {now: now})).to be_nil
+        expect(db.heartbeat(task_token, 0, {now: now})).to be_a(Integer)
       end
       it 'returns nil even if next_run_time is updated' do
-        expect(db.heartbeat(task_token, 1, {})).to be_nil
+        expect(db.heartbeat(task_token, 1, {})).to be_a(Integer)
+      end
+      it 'raises PreemptedError if last_heartbeat is not matched' do
+        expect{db.heartbeat(task_token, 1, {last_heartbeat: now-100})}.to raise_error(PreemptedError)
       end
     end
     context 'no tasks' do
@@ -319,6 +326,11 @@
         expect{db.heartbeat(task_token, 0, {})}.to raise_error(PreemptedError)
       end
     end
+    context 'stolen task' do
+      it 'raises PreemptedError if the task has unpexpected last_heartbeat' do
+        expect{db.heartbeat(task_token, 0, last_heartbeat: 0)}.to raise_error(PreemptedError)
+      end
+    end
   end
 
   context '#connect' do
diff --git a/spec/task_metadata_spec.rb b/spec/task_metadata_spec.rb
index 1619a3d..3d5a6db 100644
--- a/spec/task_metadata_spec.rb
+++ b/spec/task_metadata_spec.rb
@@ -60,10 +60,10 @@
   end
 
   describe 'timeout' do
-    it 'returns a time of given timeout' do
+    it 'returns given timeout' do
       epoch = 72
       tm = TaskMetadata.new(double, double, timeout: epoch)
-      expect(tm.timeout).to eq(Time.at(epoch))
+      expect(tm.timeout).to eq(epoch)
     end
   end
 end
diff --git a/spec/task_monitor_spec.rb b/spec/task_monitor_spec.rb
index 975cec0..7423afa 100644
--- a/spec/task_monitor_spec.rb
+++ b/spec/task_monitor_spec.rb
@@ -23,7 +23,6 @@
       ret = double('ret')
       tm.instance_variable_set(:@task, task)
       expect(tm.external_task_heartbeat(task){ret}).to eq(ret)
-      expect(tm.instance_variable_get(:@last_task_heartbeat)).to eq(epoch)
     end
   end
 
@@ -39,23 +38,99 @@
   end
 
   describe '#task_heartbeat' do
-    let (:tm){ PerfectQueue::TaskMonitor.new(logger: double('logger').as_null_object) }
+    let (:config){ {type: 'rdb_compat', url: 'mysql2://root:@localhost/perfectqueue_test', table: 'test_queues', alive_time: 11} }
+    let (:client){ Client.new(config) }
+    let (:tm){ PerfectQueue::TaskMonitor.new(logger: double('logger').as_null_object, task_heartbeat_interval: 1) }
     let (:err){ StandardError.new('heartbeat preempted') }
-    before do
-      task = double('task')
-      allow(task).to receive(:heartbeat!){ raise err }
-      tm.set_task(task, double('runner'))
+    let (:now){ Time.now.to_i }
+    let (:task){ double('task', timeout: now) }
+    let (:runner){ double('runner') }
+    context 'timeout' do
+      before do
+        tm.set_task(task, double('runner'))
+      end
+      it 'calls kill_task($!) on heartbeat error' do
+        allow(task).to receive(:heartbeat!){ raise err }
+        expect(tm).to receive(:kill_task).with(err).exactly(:once)
+        tm.__send__(:task_heartbeat)
+      end
+    end
+    context 'normal' do
+      before do
+        client.backend.db.tap{|s| s.tables.each{|t| s.drop_table(t) } }
+        client.init_database
+        client.submit('key', 'test1', {'foo' => 1}, {now: now-90,compression: 'gzip'})
+        tm.start
+      end
+      after do
+        tm.stop
+        tm.join
+      end
+      it 'update timeout' do
+        tasks = client.acquire(now: now-80)
+        task = tasks[0]
+        expect(task.timeout).to eq(now-80+config[:alive_time])
+        allow(Time).to receive(:now).and_return(now-50)
+        tm.set_task(task, runner, now-80)
+        expect(task.timeout).to eq(now-50+config[:alive_time])
+      end
     end
-    it 'calls kill_task($!) on heartbeat error' do
-      expect(tm).to receive(:kill_task).with(err).exactly(:once)
-      tm.__send__(:task_heartbeat)
+    context 'stolen' do
+      before do
+        client.backend.db.tap{|s| s.tables.each{|t| s.drop_table(t) } }
+        client.init_database
+        client.submit('key', 'test1', {'foo' => 1}, {now: now-90,compression: 'gzip'})
+        tm.start
+      end
+      after do
+        tm.stop
+        tm.join
+      end
+      it 'raise error' do
+        now1 = now - 80
+        tasks = client.acquire(now: now1)
+        task1 = tasks[0]
+        expect(task1.timeout).to eq(now1+config[:alive_time])
+
+        now2 = now - 50
+        tasks = client.acquire(now: now2)
+        task2 = tasks[0]
+        expect(task2.timeout).to eq(now2+config[:alive_time])
+
+        now3 = now - 20
+        allow(Time).to receive(:now).and_return(now3)
+        expect(runner).to receive(:kill)
+        tm.set_task(task1, runner, now1)
+      end
+    end
+    context 'timeout but can acquire' do
+      before do
+        client.backend.db.tap{|s| s.tables.each{|t| s.drop_table(t) } }
+        client.init_database
+        client.submit('key', 'test1', {'foo' => 1}, {now: now-90,compression: 'gzip'})
+        tm.start
+      end
+      after do
+        tm.stop
+        tm.join
+      end
+      it 'raise error' do
+        tasks = client.acquire(now: now-80)
+        task1 = tasks[0]
+        expect(task1.timeout).to eq(now-80+config[:alive_time])
+
+        allow(Time).to receive(:now).and_return(now-50)
+        tm.set_task(task1, runner, now-50)
+
+        expect(task1.runner).to eq(runner)
+      end
     end
   end
 end
 
 describe PerfectQueue::TaskMonitorHook do
   let (:task) do
-    obj = AcquiredTask.new(double(:client).as_null_object, 'key', {}, double)
+    obj = AcquiredTask.new(double(:client).as_null_object, 'key', {timeout: Time.now.to_i}, double)
     tm = TaskMonitor.new(logger: double('logger').as_null_object)
     tm.set_task(obj, double('runner'))
     obj