yashmayya commented on code in PR #13801:
URL: https://github.com/apache/kafka/pull/13801#discussion_r1233861544


##########
connect/runtime/src/main/java/org/apache/kafka/connect/storage/ConnectorOffsetBackingStore.java:
##########
@@ -140,6 +142,9 @@ public static ConnectorOffsetBackingStore 
withOnlyConnectorStore(
     private final Optional<KafkaOffsetBackingStore> connectorStore;
     private final Optional<TopicAdmin> connectorStoreAdmin;
 
+    private boolean isEOSEnabled;

Review Comment:
   ```suggestion
       private boolean exactlyOnce;
   ```
   nit: similar to 
https://github.com/apache/kafka/blob/9b7f7e0fa09121e204540b46bd3d7340a0175e7d/connect/runtime/src/main/java/org/apache/kafka/connect/storage/KafkaOffsetBackingStore.java#L152



##########
connect/runtime/src/main/java/org/apache/kafka/connect/storage/ConnectorOffsetBackingStore.java:
##########
@@ -279,10 +284,61 @@ public Future<Void> set(Map<ByteBuffer, ByteBuffer> 
values, Callback<Void> callb
             throw new IllegalStateException("At least one non-null offset 
store must be provided");
         }
 
+        boolean containsTombstones = values.containsValue(null);
+
+        // If there are tombstone offsets, then the failure to write to 
secondary store will
+        // not be ignored. Also, for tombstone records, we first write to 
secondary store and
+        // then to primary stores.
+        if (secondaryStore != null && containsTombstones) {
+            AtomicReference<Throwable> secondaryStoreTombstoneWriteError = new 
AtomicReference<>();
+            Future<Void> secondaryWriteFuture = secondaryStore.set(values, 
(secondaryWriteError, ignored) -> {
+                try (LoggingContext context = loggingContext()) {
+                    if (secondaryWriteError != null) {
+                        log.warn("Failed to write offsets with tombstone 
records to secondary backing store", secondaryWriteError);
+                        secondaryStoreTombstoneWriteError.compareAndSet(null, 
secondaryWriteError);
+                    } else {
+                        log.debug("Successfully flushed tombstone offsets to 
secondary backing store");
+                    }
+                }
+            });

Review Comment:
   Why do we need to do this if we're anyway doing a get on the returned future 
immediately? Any producer error will be wrapped in an `ExecutionException` and 
thrown when we call get on the future returned from `KafkaOffsetBackingStore` 
(the same exception will also be used to complete the passed callback 
exceptionally) - 
https://github.com/apache/kafka/blob/546b912b831d46208499c6e1f4f785db6ddd9aa3/connect/runtime/src/main/java/org/apache/kafka/connect/storage/KafkaOffsetBackingStore.java#L344-L414



##########
connect/runtime/src/main/java/org/apache/kafka/connect/storage/ConnectorOffsetBackingStore.java:
##########
@@ -279,10 +284,61 @@ public Future<Void> set(Map<ByteBuffer, ByteBuffer> 
values, Callback<Void> callb
             throw new IllegalStateException("At least one non-null offset 
store must be provided");
         }
 
+        boolean containsTombstones = values.containsValue(null);
+
+        // If there are tombstone offsets, then the failure to write to 
secondary store will
+        // not be ignored. Also, for tombstone records, we first write to 
secondary store and
+        // then to primary stores.
+        if (secondaryStore != null && containsTombstones) {
+            AtomicReference<Throwable> secondaryStoreTombstoneWriteError = new 
AtomicReference<>();
+            Future<Void> secondaryWriteFuture = secondaryStore.set(values, 
(secondaryWriteError, ignored) -> {
+                try (LoggingContext context = loggingContext()) {
+                    if (secondaryWriteError != null) {
+                        log.warn("Failed to write offsets with tombstone 
records to secondary backing store", secondaryWriteError);
+                        secondaryStoreTombstoneWriteError.compareAndSet(null, 
secondaryWriteError);
+                    } else {
+                        log.debug("Successfully flushed tombstone offsets to 
secondary backing store");
+                    }
+                }
+            });
+            try {
+                // For EOS, there is no timeout for offset commit and it is 
allowed to take as much time as needed for
+                // commits. We still need to wait because we want to fail the 
offset commit for cases when
+                // tombstone records fail to be written to the secondary 
store. Note that while commitTransaction
+                // already waits for all records to be sent and ack'ed, in 
this case we do need to add an explicit
+                // blocking call. In case of ALOS, we wait for the same 
duration as `offset.commit.timeout.ms`

Review Comment:
   nit: I haven't seen the term ALOS used anywhere else in Connect. I'm 
presuming it means at least once delivery semantics? The Connect framework 
doesn't guarantee at least once semantics for all source connectors - depending 
on the specific connector plugin implementation, we could also have at most 
once delivery semantics. We could just phrase this as "If exactly-once support 
is disabled" instead.



##########
connect/runtime/src/main/java/org/apache/kafka/connect/storage/ConnectorOffsetBackingStore.java:
##########
@@ -279,10 +284,61 @@ public Future<Void> set(Map<ByteBuffer, ByteBuffer> 
values, Callback<Void> callb
             throw new IllegalStateException("At least one non-null offset 
store must be provided");
         }
 
+        boolean containsTombstones = values.containsValue(null);
+
+        // If there are tombstone offsets, then the failure to write to 
secondary store will
+        // not be ignored. Also, for tombstone records, we first write to 
secondary store and
+        // then to primary stores.
+        if (secondaryStore != null && containsTombstones) {
+            AtomicReference<Throwable> secondaryStoreTombstoneWriteError = new 
AtomicReference<>();
+            Future<Void> secondaryWriteFuture = secondaryStore.set(values, 
(secondaryWriteError, ignored) -> {
+                try (LoggingContext context = loggingContext()) {
+                    if (secondaryWriteError != null) {
+                        log.warn("Failed to write offsets with tombstone 
records to secondary backing store", secondaryWriteError);
+                        secondaryStoreTombstoneWriteError.compareAndSet(null, 
secondaryWriteError);
+                    } else {
+                        log.debug("Successfully flushed tombstone offsets to 
secondary backing store");
+                    }
+                }
+            });
+            try {
+                // For EOS, there is no timeout for offset commit and it is 
allowed to take as much time as needed for
+                // commits. We still need to wait because we want to fail the 
offset commit for cases when
+                // tombstone records fail to be written to the secondary 
store. Note that while commitTransaction
+                // already waits for all records to be sent and ack'ed, in 
this case we do need to add an explicit
+                // blocking call. In case of ALOS, we wait for the same 
duration as `offset.commit.timeout.ms`
+                // and throw that exception which would allow the offset 
commit to fail.
+                if (isEOSEnabled) {
+                    secondaryWriteFuture.get();
+                } else {
+                    secondaryWriteFuture.get(offsetFlushTimeoutMs, 
TimeUnit.MILLISECONDS);
+                }
+            } catch (InterruptedException e) {
+                log.warn("{} Flush of tombstone offsets to secondary store 
interrupted, cancelling", this);
+                secondaryStoreTombstoneWriteError.compareAndSet(null, e);
+            } catch (ExecutionException e) {
+                log.error("{} Flush of tombstone offsets to secondary store 
threw an unexpected exception: ", this, e);
+                secondaryStoreTombstoneWriteError.compareAndSet(null, e);
+            } catch (TimeoutException e) {
+                log.error("{} Timed out waiting to flush offsets with 
tombstones to secondary storage ", this);
+                secondaryStoreTombstoneWriteError.compareAndSet(null, e);
+            } catch (Exception e) {
+                log.error("{} Got Exception when trying to flush tombstone 
offsets to secondary storage", this);

Review Comment:
   Let's log the exception as well?



##########
connect/runtime/src/main/java/org/apache/kafka/connect/storage/ConnectorOffsetBackingStore.java:
##########
@@ -279,10 +284,61 @@ public Future<Void> set(Map<ByteBuffer, ByteBuffer> 
values, Callback<Void> callb
             throw new IllegalStateException("At least one non-null offset 
store must be provided");
         }
 
+        boolean containsTombstones = values.containsValue(null);
+
+        // If there are tombstone offsets, then the failure to write to 
secondary store will
+        // not be ignored. Also, for tombstone records, we first write to 
secondary store and
+        // then to primary stores.
+        if (secondaryStore != null && containsTombstones) {
+            AtomicReference<Throwable> secondaryStoreTombstoneWriteError = new 
AtomicReference<>();
+            Future<Void> secondaryWriteFuture = secondaryStore.set(values, 
(secondaryWriteError, ignored) -> {
+                try (LoggingContext context = loggingContext()) {
+                    if (secondaryWriteError != null) {
+                        log.warn("Failed to write offsets with tombstone 
records to secondary backing store", secondaryWriteError);
+                        secondaryStoreTombstoneWriteError.compareAndSet(null, 
secondaryWriteError);
+                    } else {
+                        log.debug("Successfully flushed tombstone offsets to 
secondary backing store");
+                    }
+                }
+            });
+            try {
+                // For EOS, there is no timeout for offset commit and it is 
allowed to take as much time as needed for
+                // commits. We still need to wait because we want to fail the 
offset commit for cases when
+                // tombstone records fail to be written to the secondary 
store. Note that while commitTransaction
+                // already waits for all records to be sent and ack'ed, in 
this case we do need to add an explicit
+                // blocking call. In case of ALOS, we wait for the same 
duration as `offset.commit.timeout.ms`
+                // and throw that exception which would allow the offset 
commit to fail.
+                if (isEOSEnabled) {
+                    secondaryWriteFuture.get();
+                } else {
+                    secondaryWriteFuture.get(offsetFlushTimeoutMs, 
TimeUnit.MILLISECONDS);
+                }
+            } catch (InterruptedException e) {
+                log.warn("{} Flush of tombstone offsets to secondary store 
interrupted, cancelling", this);
+                secondaryStoreTombstoneWriteError.compareAndSet(null, e);
+            } catch (ExecutionException e) {
+                log.error("{} Flush of tombstone offsets to secondary store 
threw an unexpected exception: ", this, e);
+                secondaryStoreTombstoneWriteError.compareAndSet(null, e);
+            } catch (TimeoutException e) {
+                log.error("{} Timed out waiting to flush offsets with 
tombstones to secondary storage ", this);
+                secondaryStoreTombstoneWriteError.compareAndSet(null, e);
+            } catch (Exception e) {
+                log.error("{} Got Exception when trying to flush tombstone 
offsets to secondary storage", this);
+                secondaryStoreTombstoneWriteError.compareAndSet(null, e);
+            }
+            Throwable writeError = secondaryStoreTombstoneWriteError.get();
+            if (writeError != null) {
+                FutureCallback<Void> failedWriteCallback = new 
FutureCallback<>(callback);
+                failedWriteCallback.onCompletion(writeError, null);
+                return failedWriteCallback;
+            }
+        }
+
         return primaryStore.set(values, (primaryWriteError, ignored) -> {
-            if (secondaryStore != null) {
+            // Secondary store writes have already happened for tombstone 
records

Review Comment:
   ```suggestion
               // Secondary store writes have already happened if the offsets 
batch contains tombstone records
   ```
   nit: the existing comment could be interpreted as - `only tombstone records 
in the offsets batch have been written to the secondary store`, which is not 
the case.



##########
connect/runtime/src/main/java/org/apache/kafka/connect/storage/ConnectorOffsetBackingStore.java:
##########
@@ -279,10 +284,61 @@ public Future<Void> set(Map<ByteBuffer, ByteBuffer> 
values, Callback<Void> callb
             throw new IllegalStateException("At least one non-null offset 
store must be provided");
         }
 
+        boolean containsTombstones = values.containsValue(null);
+
+        // If there are tombstone offsets, then the failure to write to 
secondary store will
+        // not be ignored. Also, for tombstone records, we first write to 
secondary store and
+        // then to primary stores.
+        if (secondaryStore != null && containsTombstones) {
+            AtomicReference<Throwable> secondaryStoreTombstoneWriteError = new 
AtomicReference<>();
+            Future<Void> secondaryWriteFuture = secondaryStore.set(values, 
(secondaryWriteError, ignored) -> {
+                try (LoggingContext context = loggingContext()) {
+                    if (secondaryWriteError != null) {
+                        log.warn("Failed to write offsets with tombstone 
records to secondary backing store", secondaryWriteError);
+                        secondaryStoreTombstoneWriteError.compareAndSet(null, 
secondaryWriteError);
+                    } else {
+                        log.debug("Successfully flushed tombstone offsets to 
secondary backing store");

Review Comment:
   This log line (and multiple subsequent others) seems to indicate that the 
whole offsets batch consists of tombstone records which need not necessarily be 
the case since we're only checking whether the batch contains at least one 
tombstone record. Maybe we could reword it to something like - `Successfully 
flushed tombstone(s)-containing offsets batch to secondary backing store` 
instead, WDYT?



##########
connect/runtime/src/main/java/org/apache/kafka/connect/storage/ConnectorOffsetBackingStore.java:
##########
@@ -279,10 +284,61 @@ public Future<Void> set(Map<ByteBuffer, ByteBuffer> 
values, Callback<Void> callb
             throw new IllegalStateException("At least one non-null offset 
store must be provided");
         }
 
+        boolean containsTombstones = values.containsValue(null);
+
+        // If there are tombstone offsets, then the failure to write to 
secondary store will
+        // not be ignored. Also, for tombstone records, we first write to 
secondary store and
+        // then to primary stores.
+        if (secondaryStore != null && containsTombstones) {
+            AtomicReference<Throwable> secondaryStoreTombstoneWriteError = new 
AtomicReference<>();
+            Future<Void> secondaryWriteFuture = secondaryStore.set(values, 
(secondaryWriteError, ignored) -> {
+                try (LoggingContext context = loggingContext()) {
+                    if (secondaryWriteError != null) {
+                        log.warn("Failed to write offsets with tombstone 
records to secondary backing store", secondaryWriteError);
+                        secondaryStoreTombstoneWriteError.compareAndSet(null, 
secondaryWriteError);
+                    } else {
+                        log.debug("Successfully flushed tombstone offsets to 
secondary backing store");
+                    }
+                }
+            });
+            try {
+                // For EOS, there is no timeout for offset commit and it is 
allowed to take as much time as needed for
+                // commits. We still need to wait because we want to fail the 
offset commit for cases when
+                // tombstone records fail to be written to the secondary 
store. Note that while commitTransaction
+                // already waits for all records to be sent and ack'ed, in 
this case we do need to add an explicit
+                // blocking call. In case of ALOS, we wait for the same 
duration as `offset.commit.timeout.ms`
+                // and throw that exception which would allow the offset 
commit to fail.
+                if (isEOSEnabled) {
+                    secondaryWriteFuture.get();
+                } else {
+                    secondaryWriteFuture.get(offsetFlushTimeoutMs, 
TimeUnit.MILLISECONDS);
+                }
+            } catch (InterruptedException e) {
+                log.warn("{} Flush of tombstone offsets to secondary store 
interrupted, cancelling", this);
+                secondaryStoreTombstoneWriteError.compareAndSet(null, e);
+            } catch (ExecutionException e) {
+                log.error("{} Flush of tombstone offsets to secondary store 
threw an unexpected exception: ", this, e);
+                secondaryStoreTombstoneWriteError.compareAndSet(null, e);
+            } catch (TimeoutException e) {
+                log.error("{} Timed out waiting to flush offsets with 
tombstones to secondary storage ", this);
+                secondaryStoreTombstoneWriteError.compareAndSet(null, e);
+            } catch (Exception e) {
+                log.error("{} Got Exception when trying to flush tombstone 
offsets to secondary storage", this);
+                secondaryStoreTombstoneWriteError.compareAndSet(null, e);
+            }
+            Throwable writeError = secondaryStoreTombstoneWriteError.get();
+            if (writeError != null) {
+                FutureCallback<Void> failedWriteCallback = new 
FutureCallback<>(callback);
+                failedWriteCallback.onCompletion(writeError, null);
+                return failedWriteCallback;
+            }
+        }
+
         return primaryStore.set(values, (primaryWriteError, ignored) -> {
-            if (secondaryStore != null) {
+            // Secondary store writes have already happened for tombstone 
records
+            if (secondaryStore != null && !containsTombstones) {
                 if (primaryWriteError != null) {
-                    log.trace("Skipping offsets write to secondary store 
because primary write has failed", primaryWriteError);
+                    log.trace("Skipping offsets write to secondary store for 
non tombstone offsets because primary write has failed", primaryWriteError);

Review Comment:
   This log line would look pretty confusing to most users, maybe we can just 
leave it as is since the `non tombstone offsets` bit isn't too relevant here?



##########
connect/runtime/src/test/java/org/apache/kafka/connect/storage/OffsetStorageWriterTest.java:
##########
@@ -192,6 +197,220 @@ public void testCancelAfterAwaitFlush() throws Exception {
         flushFuture.get(1000, TimeUnit.MILLISECONDS);
     }
 
+    @Test

Review Comment:
   I think it makes more sense to directly test the 
`ConnectorOffsetBackingStore::set` functionality in a 
`ConnectorOffsetBackingStore`-specific test class rather than indirectly 
testing through `OffsetStorageWriter::doFlush`?



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