cadonna commented on a change in pull request #11765:
URL: https://github.com/apache/kafka/pull/11765#discussion_r809336171



##########
File path: 
streams/src/main/java/org/apache/kafka/streams/processor/internals/DefaultStateUpdater.java
##########
@@ -0,0 +1,390 @@
+package org.apache.kafka.streams.processor.internals;
+
+import org.apache.kafka.common.TopicPartition;
+import org.apache.kafka.common.utils.Time;
+import org.apache.kafka.streams.errors.StreamsException;
+import org.apache.kafka.streams.errors.TaskCorruptedException;
+import org.apache.kafka.streams.processor.TaskId;
+
+import java.time.Duration;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.HashSet;
+import java.util.LinkedList;
+import java.util.List;
+import java.util.Queue;
+import java.util.Set;
+import java.util.concurrent.BlockingQueue;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.ConcurrentMap;
+import java.util.concurrent.LinkedBlockingQueue;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.concurrent.locks.Condition;
+import java.util.concurrent.locks.Lock;
+import java.util.concurrent.locks.ReentrantLock;
+import java.util.function.Supplier;
+import java.util.stream.Collectors;
+
+public class DefaultStateUpdater implements StateUpdater {
+
+    private final static String BUG_ERROR_MESSAGE = "This indicates a bug. " +
+        "Please report at https://issues.apache.org/jira/projects/KAFKA/issues 
or to the dev-mailing list (https://kafka.apache.org/contact).";
+
+    private class StateUpdaterThread extends Thread {
+
+        private final ChangelogReader changelogReader;
+        private final ConcurrentMap<TaskId, Task> updatingTasks = new 
ConcurrentHashMap<>();
+        private final AtomicBoolean isRunning = new AtomicBoolean(true);
+        private final java.util.function.Consumer<Set<TopicPartition>> 
offsetResetter;
+
+        public StateUpdaterThread(final String name,
+                                  final ChangelogReader changelogReader,
+                                  final 
java.util.function.Consumer<Set<TopicPartition>> offsetResetter) {
+            super(name);
+            this.changelogReader = changelogReader;
+            this.offsetResetter = offsetResetter;
+        }
+
+        @Override
+        public void run() {
+            try {
+                while (isRunning.get()) {
+                    try {
+                        performActionsOnTasks();
+                        restoreTasks();
+                        waitIfAllChangelogsCompletelyRead();
+                    } catch (final InterruptedException interruptedException) {
+                        return;
+                    }
+                }
+            } catch (final Throwable anyOtherError) {
+                // ToDo: log that the thread died unexpectedly
+            } finally{
+                clear();
+            }
+        }
+
+        private void performActionsOnTasks() throws InterruptedException {
+            tasksAndActionsLock.lock();
+            try {
+                for (final TaskAndAction taskAndAction : getTasksAndActions()) 
{
+                    final Task task = taskAndAction.task;
+                    final Action action = taskAndAction.action;
+                    switch (action) {
+                        case ADD:
+                            addTask(task);
+                            break;
+                        case REMOVE:
+                            removeTask(task);
+                            break;
+                        case RECYCLE:
+                            ;
+                            break;
+
+                    }
+                }
+            } finally {
+                tasksAndActionsLock.unlock();
+            }
+        }
+
+        private void restoreTasks() throws InterruptedException {
+            try {
+                // ToDo: Prioritize restoration of active tasks over standby 
tasks
+                //                changelogReader.enforceRestoreActive();
+                changelogReader.restore(updatingTasks);
+            } catch (final TaskCorruptedException taskCorruptedException) {
+                handleTaskCorruptedException(taskCorruptedException);
+            } catch (final StreamsException streamsException) {
+                handleStreamsException(streamsException);
+            }
+            final Set<TopicPartition> completedChangelogs = 
changelogReader.completedChangelogs();
+            final List<Task> activeTasks = 
updatingTasks.values().stream().filter(Task::isActive).collect(Collectors.toList());
+            for (final Task task : activeTasks) {
+                endRestorationIfChangelogsCompletelyRead(task, 
completedChangelogs);
+            }
+        }
+
+        private void endRestorationIfChangelogsCompletelyRead(final Task task,
+                                                              final 
Set<TopicPartition> restoredChangelogs) {
+            catchException(() -> {
+                final Collection<TopicPartition> taskChangelogPartitions = 
task.changelogPartitions();
+                if (restoredChangelogs.containsAll(taskChangelogPartitions)) {
+                    task.completeRestoration(offsetResetter);
+                    addTaskToRestoredTasks((StreamTask) task);
+                    updatingTasks.remove(task.id());
+                }
+                return null;
+            }, task);
+        }
+
+        private void waitIfAllChangelogsCompletelyRead() throws 
InterruptedException {
+            if (isRunning.get() && changelogReader.allChangelogsCompleted()) {
+                tasksAndActionsLock.lock();
+                try {
+                    while (tasksAndActions.isEmpty()) {
+                        tasksAndActionsCondition.await();
+                    }
+                } finally {
+                    tasksAndActionsLock.unlock();
+                }
+            }
+        }
+
+        private List<TaskAndAction> getTasksAndActions() {
+            final List<TaskAndAction> tasksAndActionsToProcess = new 
ArrayList<>(tasksAndActions);
+            tasksAndActions.clear();
+            return tasksAndActionsToProcess;
+        }
+
+        private void addTask(final Task task) {

Review comment:
       Actually, this was my original thought, but then I tried a couple of 
other things. If we agree on the invariant of not having a task at the same 
time in `Tasks` and the state updater, I think we can go back to my original 
thought and what you propose here of keeping the state transition logic out of 
the state updater.




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