Github user squito commented on a diff in the pull request:
https://github.com/apache/spark/pull/17902#discussion_r119420304
--- Diff:
common/kvstore/src/main/java/org/apache/spark/kvstore/LevelDB.java ---
@@ -0,0 +1,303 @@
+/*
+ * 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.File;
+import java.io.IOException;
+import java.util.HashMap;
+import java.util.Iterator;
+import java.util.Map;
+import java.util.NoSuchElementException;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.ConcurrentMap;
+import java.util.concurrent.atomic.AtomicReference;
+import static java.nio.charset.StandardCharsets.UTF_8;
+
+import com.google.common.annotations.VisibleForTesting;
+import com.google.common.base.Objects;
+import com.google.common.base.Preconditions;
+import com.google.common.base.Throwables;
+import org.fusesource.leveldbjni.JniDBFactory;
+import org.iq80.leveldb.DB;
+import org.iq80.leveldb.Options;
+import org.iq80.leveldb.WriteBatch;
+
+/**
+ * Implementation of KVStore that uses LevelDB as the underlying data
store.
+ */
+public class LevelDB implements KVStore {
+
+ @VisibleForTesting
+ static final long STORE_VERSION = 1L;
+
+ @VisibleForTesting
+ static final byte[] STORE_VERSION_KEY = "__version__".getBytes(UTF_8);
+
+ /** DB key where app metadata is stored. */
+ private static final byte[] METADATA_KEY = "__meta__".getBytes(UTF_8);
+
+ /** DB key where type aliases are stored. */
+ private static final byte[] TYPE_ALIASES_KEY =
"__types__".getBytes(UTF_8);
+
+ final AtomicReference<DB> _db;
+ final KVStoreSerializer serializer;
+
+ private final ConcurrentMap<String, byte[]> typeAliases;
--- End diff --
this `typeAliases` thing is pretty confusing. IIUC, the idea is to replace
a long fully qualified type name with a shorter numeric id, and this holds the
mapping? I'd include a comment about it. maybe even rename `typetoID`.
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