Github user StefanRRichter commented on a diff in the pull request:
https://github.com/apache/flink/pull/3522#discussion_r113686064
--- Diff:
flink-runtime/src/main/java/org/apache/flink/runtime/state/CheckpointMetadataStreamFactory.java
---
@@ -0,0 +1,101 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements. See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership. The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.flink.runtime.state;
+
+import org.apache.flink.core.fs.FSDataOutputStream;
+
+import java.io.IOException;
+
+import static org.apache.flink.util.Preconditions.checkNotNull;
+
+/**
+ * The {@link CheckpointMetadataStreamFactory} creates streams to write
checkpoint <i>metadata</i>,
+ * similar as the {@link CheckpointStreamFactory} creates streams to write
checkpoint <i>data</i>.
+ */
+public interface CheckpointMetadataStreamFactory {
+
+ /**
+ * Creates a stream to write the metadata for the particular checkpoint
that this factory
+ * belongs to.
+ *
+ * @return The metadata output stream for this factory's checkpoint.
+ * @throws IOException Thrown, if the stream could not be opened.
+ */
+ CheckpointMetadataOutputStream createCheckpointStateOutputStream()
throws IOException;
+
+ /**
+ * Gets the location (as a string pointer) where the metadata factory
+ * The interpretation of the pointer is up to the implementation of the
state backend.
+ *
+ * <p>In case of high-availability setups, the target location pointer
is stored
+ * in the "ground-truth" store for checkpoint recovery.
+ *
+ * @return The checkpoint location pointer.
+ */
+ String getTargetLocation();
+
+ //
------------------------------------------------------------------------
+
+ /**
+ * A dedicated output stream for persisting checkpoint metadata. Upon
completion, this returns
+ * the state handle to the persisted metadata, plus the external
pointer that can be used
+ * to restore from that checkpoint.
+ */
+ abstract class CheckpointMetadataOutputStream extends
FSDataOutputStream {
+
+ /**
+ * Closes the metadata stream and gets the external pointer to
the checkpoint and a
+ * handle that can create an input stream producing the data
written to this stream.
+ *
+ * @return The pointer and state handle with access to the
written metadata.
+ * @throws IOException Thrown, if the stream cannot be closed.
+ */
+ public abstract StreamHandleAndPointer
closeAndGetPointerHandle() throws IOException;
+ }
+
+ //
------------------------------------------------------------------------
+
+ /**
+ * A combination of a {@code StreamStateHandle} and an external pointer
(in the form of a String).
+ */
+ final class StreamHandleAndPointer {
--- End diff --
Not entirely sure why we need this "AndPointer" part. The pointer in the
end is a string and the interpretation of the string is generic and up to the
backend that is confronted with it. So right now, the string is always a path,
which is already contained in the plain `StreamStateHandle` that happens to be
a `FileStateHandle` for the current backends. So similar to expecting a certain
string format, the backend could as well expect certain subclasses of
`StreamStateHandle`, like `FileStateHandle` and resolve the "pointer" from
there. Or maybe I am missing something fundamental here?
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