tweise commented on a change in pull request #12759:
URL: https://github.com/apache/beam/pull/12759#discussion_r484618795



##########
File path: 
runners/flink/src/main/java/org/apache/beam/runners/flink/translation/wrappers/streaming/state/FlinkStateInternals.java
##########
@@ -111,19 +128,43 @@ public K getKey() {
   @Override
   public <T extends State> T state(
       StateNamespace namespace, StateTag<T> address, StateContext<?> context) {
+    if (globalWindowNamespace.equals(namespace)) {
+      // Take note of state bound to the global window for cleanup in 
clearGlobalState below.
+      globalWindowStateTags.add(address);
+    }
     return address.getSpec().bind(address.getId(), new 
FlinkStateBinder(namespace, context));
   }
 
-  public void clearBagStates(StateNamespace namespace, StateTag<? extends 
BagState> address)
-      throws Exception {
-    CoderTypeSerializer typeSerializer = new 
CoderTypeSerializer<>(VoidCoder.of());
-    flinkStateBackend.applyToAllKeys(
-        namespace.stringKey(),
-        StringSerializer.INSTANCE,
-        new ListStateDescriptor<>(address.getId(), typeSerializer),
-        (key, state) -> {
+  /**
+   * Allows to clear all state for the global watermark when the maximum 
watermark arrives. We do
+   * not clean up the global window state via timers which would lead to an 
unbounded number of keys
+   * and cleanup timers. Instead, the cleanup code below should be run when we 
finally receive the
+   * max watermark.
+   */
+  public void clearGlobalState() {
+    try {
+      for (StateTag stateTag : globalWindowStateTags) {
+        State state =
+            state(
+                globalWindowNamespace,
+                stateTag,
+                StateContexts.windowOnlyContext(GlobalWindow.INSTANCE));
+        // We collect all keys in the global window for a particular state
+        // Note that the alternative method applyToAllKeys(..) does the same 
internally.

Review comment:
       Is there a good reason to not use `applyToAllKeys`? A specific state 
backend may have a better implementation, overriding the naive generic key 
iteration here: 
https://github.com/apache/flink/blob/c1a12e925b6ef46ad5cf0e0a5723949572550e9b/flink-runtime/src/main/java/org/apache/flink/runtime/state/AbstractKeyedStateBackend.java#L242




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