code: public <K, N> InternalTimerService<N> getInternalTimerService(
  			String name,
  			TypeSerializer<N> namespaceSerializer,
  			Triggerable<K, N> triggerable) {
  
  		checkTimerServiceInitialization();
  
  		// the following casting is to overcome type restrictions.
  		KeyedStateBackend<K> keyedStateBackend = getKeyedStateBackend();
  		TypeSerializer<K> keySerializer = keyedStateBackend.getKeySerializer();
  		InternalTimeServiceManager<K> keyedTimeServiceHandler = (InternalTimeServiceManager<K>) timeServiceManager;
  		TimerSerializer<K, N> timerSerializer = new TimerSerializer<>(keySerializer, namespaceSerializer);
  		return keyedTimeServiceHandler.getInternalTimerService(name, timerSerializer, triggerable);
  	}
docs: Returns a {@link InternalTimerService} that can be used to query current processing time
  and event time and to set timers. An operator can have several timer services, where
  each has its own namespace serializer. Timer services are differentiated by the string
  key that is given when requesting them, if you call this method with the same key
  multiple times you will get the same timer service instance in subsequent requests.
  
  <p>Timers are always scoped to a key, the currently active key of a keyed stream operation.
  When a timer fires, this key will also be set as the currently active key.
  
  <p>Each timer has attached metadata, the namespace. Different timer services
  can have a different namespace type. If you don't need namespace differentiation you
  can use {@link VoidNamespaceSerializer} as the namespace serializer.
  
  @param name The name of the requested timer service. If no service exists under the given
  name a new one will be created and returned.
  @param namespaceSerializer {@code TypeSerializer} for the timer namespace.
  @param triggerable The {@link Triggerable} that should be invoked when timers fire
  
  @param <N> The type of the timer namespace.