YARN framework is an event driven framework. It uses event handlers to listen and trigger callbacks to handle various events sent by components to the event queue. Threads are also being used by some event handlers to run long running logic after receiving the event. eg. in ResourceLocalizationService, during the event loop handling, it initializes LocalizerRunner thread to download remote resources. NodeManager is a core component in YARN. It consists of the following services internally

NodeStatusUpdater
NodeLabelsProvider
NodeResourceMonitor
ContainerManagerImpl
NMStateStoreService
NodeHealthCheckerService
LocalDirsHandlerService

Yes, it has a WebServer internally too.

These services are initialized within the serviceInit method

 @Override
  protected void serviceInit(Configuration conf) throws Exception 

One of the internal services, ContainerManagerImpl extends CompositeService and implements ServiceStateChangeListener, ContainerManagementProtocol, and EventHandler interfaces. It contains event handlers called ResourceLocalizationService and ContainersLauncher.

public class ContainerManagerImpl extends CompositeService implements
    ServiceStateChangeListener, ContainerManagementProtocol,
    EventHandler<ContainerManagerEvent>

Both ResourceLocalizationService and ContainerLauncher are defined in ContainerManagerImpl as follows

 private final ResourceLocalizationService rsrcLocalizationSrvc;
 private final ContainersLauncher containersLauncher;

As you can see in the class definition of ResourceLocalizationService, it implements EventHandler interface to handle LocalizationEvent type.

public class ResourceLocalizationService extends CompositeService
    implements EventHandler<LocalizationEvent>, LocalizationProtocol {

ContainersLauncher is the event handler responsible for initialization, starting, launching, and termination of containers. It implements EventHandler interface to handle ContainersLauncherEvent type, eg. LAUNCH_CONTAINER, RECOVER_CONTAINER, CLEANUP_CONTAINER events.

public class ContainersLauncher extends AbstractService
    implements EventHandler<ContainersLauncherEvent> {

ContainerManagerImpl registers the above event handlers as shown below.

dispatcher.register(LocalizationEventType.class, rsrcLocalizationSrvc);
dispatcher.register(ContainersLauncherEventType.class, containersLauncher);

In addition, it also registers other event handlers to listen and handle ContainersMonitorEvent, ContainerEvent, ApplicationEvent, and AuxServicesEvent.

dispatcher.register(AuxServicesEventType.class, auxiliaryServices);
dispatcher.register(ContainersMonitorEventType.class, containersMonitor);
dispatcher.register(ContainerEventType.class, new ContainerEventDispatcher());
dispatcher.register(ApplicationEventType.class, new ApplicationEventDispatcher());

In NodeManager, it has an important class called DefaultContainerExecutor which extends ContainerExecutor.

ContainerExecutor exec = ReflectionUtils.newInstance(
        conf.getClass(YarnConfiguration.NM_CONTAINER_EXECUTOR,
          DefaultContainerExecutor.class, ContainerExecutor.class), conf);
    try {
      exec.init();
    } catch (IOException e) {
      throw new YarnRuntimeException("Failed to initialize container executor", e);
    }    

This DefaultContainerExecutor provides the methods to activate and launch container. This class is used by both ResourceLocalizationService and ContainersLauncher. Both ResourceLocalizationService and ContainersLauncher are instantiated in ContainerManagerImpl.

If you look into DefaultContainerExecutor, you will find methods like
activateContainer, launchContainer used by ContainersLauncher whereas method startLocalizer is called by ResourceLocalizationService.

After a container is created, the required artifacts for the job have to be downloaded first, this is what we refer to as localization in YARN. Localization is to download remote resources onto the local file system. You can check out this excellent blog post by Hortonworks about resource localization.
http://hortonworks.com/blog/resource-localization-in-yarn-deep-dive/

I will blog post about resource localization next time. Stay tuned.