recover

Allow sending of one last element downstream when a failure has happened upstream.

Error handling

Signature

Source.recoverSource.recover Flow.recoverFlow.recover

Description

recover allows you to emit a final element and then complete the stream on an upstream failure. Deciding which exceptions should be recovered is done through a PartialFunction. If an exception does not have a matching case match defined the stream is failed.

Recovering can be useful if you want to gracefully complete a stream on failure while letting downstream know that there was a failure.

Throwing an exception inside recover will be logged on ERROR level automatically.

Reactive Streams semantics

emits when the element is available from the upstream or upstream is failed and pf returns an element

backpressures when downstream backpressures, not when failure happened

completes when upstream completes or upstream failed with exception pf can handle

Below example demonstrates how recover gracefully complete a stream on failure.

Scala
sourceSource(0 to 6)
  .map(n =>
    // assuming `4` and `5` are unexpected values that could throw exception
    if (List(4, 5).contains(n)) throw new RuntimeException(s"Boom! Bad value found: $n")
    else n.toString)
  .recover {
    case e: RuntimeException => e.getMessage
  }
  .runForeach(println)
Java
sourceSource.from(Arrays.asList(0, 1, 2, 3, 4, 5, 6))
    .map(
        n -> {
          // assuming `4` and `5` are unexpected values that could throw exception
          if (Arrays.asList(4, 5).contains(n))
            throw new RuntimeException(String.format("Boom! Bad value found: %s", n));
          else return n.toString();
        })
    .recover(
        new PFBuilder<Throwable, String>()
            .match(RuntimeException.class, Throwable::getMessage)
            .build())
    .runForeach(System.out::println, system);

This will output:

Scala
source0
1
2
3                         // last element before failure
Boom! Bad value found: 4  // first element on failure
Java
source0
1
2
3                         // last element before failure
Boom! Bad value found: 4  // first element on failure

The output in the line before failure denotes the last successful element available from the upstream, and the output in the line on failure denotes the element returns by partial function when upstream is failed.