Github user marmbrus commented on a diff in the pull request:
https://github.com/apache/spark/pull/17179#discussion_r105991219
--- Diff:
sql/core/src/main/scala/org/apache/spark/sql/streaming/KeyedState.scala ---
@@ -61,25 +65,50 @@ import
org.apache.spark.sql.catalyst.plans.logical.LogicalKeyedState
* - After that, if `update(newState)` is called, then `exists()` will
again return `true`,
* `get()` and `getOption()`will return the updated value.
*
+ * Important points to note about using `KeyedStateTimeout`.
+ * - The timeout type is a global param across all the keys (set as
`timeout` param in
+ * `[map|flatMap]GroupsWithState`, but the exact timeout duration is
configurable per key
+ * (by calling `setTimeout...()` in `KeyedState`).
+ * - When the timeout occurs for a key, the function is called with no
values, and
+ * `KeyedState.isTimingOut()` set to true.
+ * - The timeout is reset for key every time the function is called on
the key, that is,
+ * when the key has new data, or the key has timed out. So the user has
to set the timeout
+ * duration every time the function is called, otherwise there will not
be any timeout set.
+ * - Guarantees provided on processing-time-based timeout of key, when
timeout duration is D ms:
+ * - Timeout will never be called before real clock time has advanced
by D ms
+ * - Timeout will be called eventually when there is a trigger in the
query
+ * (i.e. after D ms). So there is a no strict upper bound on when the
timeout would occur.
+ * For example, the trigger interval of the query will affect when
the timeout is actually hit.
+ * If there is no data in the stream (for any key) for a while, then
their will not be
+ * any trigger and timeout will not be hit until there is data.
+ *
* Scala example of using KeyedState in `mapGroupsWithState`:
* {{{
* // A mapping function that maintains an integer state for string keys
and returns a string.
* def mappingFunction(key: String, value: Iterator[Int], state:
KeyedState[Int]): String = {
- * // Check if state exists
- * if (state.exists) {
- * val existingState = state.get // Get the existing state
- * val shouldRemove = ... // Decide whether to remove the state
+ *
+ * if (state.isTimingOut) { // If called when timing out,
remove the state
+ * state.remove()
+ *
+ * } else if (state.exists) { // If state exists, use it
for processing
+ * val existingState = state.get // Get the existing state
+ * val shouldRemove = ... // Decide whether to remove
the state
* if (shouldRemove) {
- * state.remove() // Remove the state
+ * state.remove() // Remove the state
+ *
* } else {
* val newState = ...
- * state.update(newState) // Set the new state
+ * state.update(newState) // Set the new state
* }
+ *
* } else {
* val initialState = ...
- * state.update(initialState) // Set the initial state
+ * state.update(initialState) // Set the initial state
* }
- * ... // return something
+ * state.setTimeoutDuration("1 hour") // Set the timeout
--- End diff --
Hmm, I wonder if we should throw an exception here. I was pretty confused
what this function would do just looking at it.
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