Github user JoshRosen commented on a diff in the pull request:
https://github.com/apache/spark/pull/3801#discussion_r22449114
--- Diff:
streaming/src/test/scala/org/apache/spark/streaming/TestSuiteBase.scala ---
@@ -104,6 +106,77 @@ class TestOutputStreamWithPartitions[T:
ClassTag](parent: DStream[T],
}
/**
+ * This is an interface that can be used to block until certain events
occur, such as
+ * the start/completion of batches. This is much less brittle than
waiting on wall-clock time.
+ * Internally, this is implemented using a StreamingListener.
Constructing a new instance of this
+ * class automatically registers a StreamingListener on the given
StreamingContext.
+ */
+class StreamingTestWaiter(ssc: StreamingContext) {
+
+ // All access to this state should be guarded by
`StreamingTestWaiter.this.synchronized`
+ private var numCompletedBatches = 0
+ private var numStartedBatches = 0
+
+ private val listener = new StreamingListener {
+ override def onBatchStarted(batchStarted:
StreamingListenerBatchStarted): Unit =
+ StreamingTestWaiter.this.synchronized {
+ numStartedBatches += 1
+ StreamingTestWaiter.this.notifyAll()
+ }
+ override def onBatchCompleted(batchCompleted:
StreamingListenerBatchCompleted): Unit =
+ StreamingTestWaiter.this.synchronized {
+ numCompletedBatches += 1
+ StreamingTestWaiter.this.notifyAll()
+ }
+ }
+ ssc.addStreamingListener(listener)
+
+ def getNumCompletedBatches: Int = this.synchronized {
+ numCompletedBatches
+ }
+
+ def getNumStartedBatches: Int = this.synchronized {
+ numStartedBatches
+ }
+
+ /**
+ * Block until the number of completed batches reaches the given
threshold.
+ */
+ def waitForTotalBatchesCompleted(
--- End diff --
It occurred to me that this might be misleadingly-named since it waits
until _at least_ that many batches have been processed. To avoid this naming
issue, plus a proliferation of similar methods, I might be able to just
introduce a helper class that encapsulates this "synchronize on an object and
wait for a condition involving it to become true" pattern.
I'm imagining that it could look something vaguely like
```scala
def waitUntil[T](obj: T, condition: T => Boolean, timeout: Long): Unit = {
obj.synchronized {
while(!condition(obj)) {
[...] // do the wait() logic here
}
}
}
that encapsulates this wait / notify pattern, so could write something like
```scala
waitUntil(waiter, _.completedBatches > 2, timeout, seconds(10)
```
Or, with an implicit conversion, something like
```
waiter.waitUntil(_.completedBatches > 2, timeout=seconds(10))
```
which is a nice-looking syntax and avoids those issues of having to name
inequalities.
Similar to [your
suggestion](https://github.com/apache/spark/pull/3868/files#r22447509) on
another PR, we could add a `pollUntil` method that works for objects that don't
support monitor notification / synchronization.
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