Github user squito commented on a diff in the pull request:
https://github.com/apache/spark/pull/17902#discussion_r119411351
--- Diff:
common/kvstore/src/main/java/org/apache/spark/kvstore/KVStore.java ---
@@ -0,0 +1,122 @@
+/*
+ * 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.spark.kvstore;
+
+import java.io.Closeable;
+import java.util.Iterator;
+import java.util.Map;
+
+/**
+ * Abstraction for a local key/value store for storing app data.
+ *
+ * <p>
+ * There are two main features provided by the implementations of this
interface:
+ * </p>
+ *
+ * <h3>Serialization</h3>
+ *
+ * <p>
+ * Data will be serialized to and deserialized from the underlying data
store using a
+ * {@link KVStoreSerializer}, which can be customized by the application.
The serializer is
+ * based on Jackson, so it supports all the Jackson annotations for
controlling the serialization
+ * of app-defined types.
+ * </p>
+ *
+ * <p>
+ * Data is also automatically compressed to save disk space.
+ * </p>
+ *
+ * <h3>Automatic Key Management</h3>
+ *
+ * <p>
+ * When using the built-in key management, the implementation will
automatically create unique
+ * keys for each type written to the store. Keys are based on the type
name, and always start
+ * with the "+" prefix character (so that it's easy to use both manual and
automatic key
+ * management APIs without conflicts).
+ * </p>
+ *
+ * <p>
+ * Another feature of automatic key management is indexing; by annotating
fields or methods of
+ * objects written to the store with {@link KVIndex}, indices are created
to sort the data
+ * by the values of those properties. This makes it possible to provide
sorting without having
+ * to load all instances of those types from the store.
+ * </p>
+ *
+ * <p>
+ * KVStore instances are thread-safe for both reads and writes.
+ * </p>
+ */
+public interface KVStore extends Closeable {
+
+ /**
+ * Returns app-specific metadata from the store, or null if it's not
currently set.
+ *
+ * <p>
+ * The metadata type is application-specific. This is a convenience
method so that applications
+ * don't need to define their own keys for this information.
+ * </p>
+ */
+ <T> T getMetadata(Class<T> klass) throws Exception;
+
+ /**
+ * Writes the given value in the store metadata key.
+ */
+ void setMetadata(Object value) throws Exception;
+
+ /**
+ * Read a specific instance of an object.
+ */
+ <T> T read(Class<T> klass, Object naturalKey) throws Exception;
--- End diff --
add that key cannot be null, and if the key does not exist, you throw a
NoSuchElementException (otherwise I might think you'd return null)
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