LiveListenerBus

LiveListenerBus is used to announce application-wide events in a Spark application. It asynchronously passes listener events to registered Spark listeners.

Figure 1. LiveListenerBus, SparkListenerEvents, and Senders

LiveListenerBus is a single-JVM SparkListenerBus that uses listenerThread to poll events. Emitters are supposed to use post method to post SparkListenerEvent events.

Note	The event queue is java.util.concurrent.LinkedBlockingQueue with capacity of 10000 `SparkListenerEvent` events.

Note	An instance of `LiveListenerBus` is created and started when `SparkContext` is initialized.

Creating `LiveListenerBus` Instance

Caution

FIXME

Starting `LiveListenerBus` — `start` method

start(sc: SparkContext): Unit

start starts processing events.

Internally, it saves the input SparkContext for later use and starts listenerThread. It makes sure that it only happens when LiveListenerBus has not been started before (i.e. started is disabled).

If however LiveListenerBus has already been started, a IllegalStateException is thrown:

[name] already started!

Posting SparkListenerEvent Events — `post` method

post(event: SparkListenerEvent): Unit

post puts the input event onto the internal eventQueue queue and releases the internal eventLock semaphore. If the event placement was not successful (and it could happen since it is tapped at 10000 events) onDropEvent method is called.

The event publishing is only possible when stopped flag has been enabled.

Caution

FIXME Who’s enabling the stopped flag and when/why?

If LiveListenerBus has been stopped, the following ERROR appears in the logs:

ERROR [name] has already stopped! Dropping event [event]

Event Dropped Callback — `onDropEvent` method

onDropEvent(event: SparkListenerEvent): Unit

onDropEvent is called when no further events can be added to the internal eventQueue queue (while posting a SparkListenerEvent event).

It simply prints out the following ERROR message to the logs and ensures that it happens only once.

ERROR Dropping SparkListenerEvent because no remaining room in event queue. This likely means one of the SparkListeners is too slow and cannot keep up with the rate at which tasks are being started by the scheduler.

Note	It uses the internal `logDroppedEvent` atomic variable to track the state.

Stopping `LiveListenerBus` — `stop` method

stop(): Unit

stop releases the internal eventLock semaphore and waits until listenerThread dies. It can only happen after all events were posted (and polling eventQueue gives nothing).

It checks that started flag is enabled (i.e. true) and throws a IllegalStateException otherwise.

Attempted to stop [name] that has not yet started!

stopped flag is enabled.

`listenerThread` for Event Polling

LiveListenerBus uses SparkListenerBus single-daemon thread that ensures that the polling events from the event queue is only after the listener was started and only one event at a time.

Caution

FIXME There is some logic around no events in the queue.

Settings

Table 1. Spark Properties
Spark Property	Default Value	Description
`spark.extraListeners`	(empty)	The comma-separated list of fully-qualified class names of Spark listeners that should be registered (when `SparkContext` is initialized)

LiveListenerBus