monitor
Materializes to a FlowMonitor
that monitors messages flowing through or completion of the operators.
Signature
Source.monitor
Source.monitor
Flow.monitor
Flow.monitor
Description
Materializes to a FlowMonitor
that monitors messages flowing through or completion of the stream. Elements pass through unchanged. Note that the FlowMonitor
inserts a memory barrier every time it processes an event, and may therefore affect performance. The provided FlowMonitor
contains a state
field you can use to peek and get information about the stream.
Example
The example below uses the monitorMat
variant of monitor
. The only difference between the two operators is that monitorMat
has a combine
argument so we can decide which materialization value to keep. In the sample below be Keep.right
so only the FlowMonitor[Int]
is returned.
- Scala
-
source
val source: Source[Int, NotUsed] = Source.fromIterator(() => Iterator.from(0)) def printMonitorState(flowMonitor: FlowMonitor[Int]) = flowMonitor.state match { case FlowMonitorState.Initialized => println("Stream is initialized but hasn't processed any element") case FlowMonitorState.Received(msg) => println(s"Last element processed: $msg") case FlowMonitorState.Failed(cause) => println(s"Stream failed with cause $cause") case FlowMonitorState.Finished => println(s"Stream completed already") } val monitoredSource: Source[Int, FlowMonitor[Int]] = source.take(6).throttle(5, 1.second).monitorMat(Keep.right) val (flowMonitor, futureDone) = monitoredSource.toMat(Sink.foreach(println))(Keep.both).run() // If we peek in the monitor too early, it's possible it was not initialized yet. printMonitorState(flowMonitor) // Periodically check the monitor Source.tick(200.millis, 400.millis, "").runForeach(_ => printMonitorState(flowMonitor))
- Java
-
source
private static <T> void printMonitorState(FlowMonitorState.StreamState<T> state) { if (state == FlowMonitorState.finished()) { System.out.println("Stream is initialized but hasn't processed any element"); } else if (state instanceof FlowMonitorState.Received) { FlowMonitorState.Received msg = (FlowMonitorState.Received) state; System.out.println("Last message received: " + msg.msg()); } else if (state instanceof FlowMonitorState.Failed) { Throwable cause = ((FlowMonitorState.Failed) state).cause(); System.out.println("Stream failed with cause: " + cause.getMessage()); } else { System.out.println("Stream completed already"); } } Source<Integer, FlowMonitor<Integer>> monitoredSource = Source.fromIterator(() -> Arrays.asList(0, 1, 2, 3, 4, 5).iterator()) .throttle(5, Duration.ofSeconds(1)) .monitorMat(Keep.right()); Pair<FlowMonitor<Integer>, CompletionStage<Done>> run = monitoredSource.toMat(Sink.foreach(System.out::println), Keep.both()).run(actorSystem); FlowMonitor<Integer> monitor = run.first(); // If we peek in the monitor too early, it's possible it was not initialized yet. printMonitorState(monitor.state()); // Periodically check the monitor Source.tick(Duration.ofMillis(200), Duration.ofMillis(400), "") .runForeach(__ -> printMonitorState(monitor.state()), actorSystem);
When run, the sample code will produce something similar to:
Stream is initialized but hasn't processed any element
0
1
2
Last element processed: 2
3
4
5
Stream completed already
Reactive Streams semantics
emits when upstream emits an element
backpressures when downstream backpressures
completes when upstream completes