[GitHub] [accumulo] keith-turner commented on a change in pull request #1151: WIP Initial stab at #816 abstracting accumulo metadata persistence

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keith-turner commented on a change in pull request #1151: WIP Initial stab at #816 abstracting accumulo metadata persistence
URL: https://github.com/apache/accumulo/pull/1151#discussion_r284752899
 
 

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 File path: server/base/src/main/java/org/apache/accumulo/server/metadata/MetadataMutator.java
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+/*
+ * 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.accumulo.server.metadata;
+
+import org.apache.accumulo.core.data.TableId;
+import org.apache.accumulo.core.dataImpl.KeyExtent;
+import org.apache.accumulo.core.metadata.schema.DataFileValue;
+import org.apache.accumulo.fate.zookeeper.ZooLock;
+import org.apache.accumulo.server.fs.FileRef;
+import org.apache.accumulo.server.master.state.TServerInstance;
+import org.apache.hadoop.io.Text;
+
+/**
+ * An abstraction layer for mutating Accumulo's persistent metadata. This metadata may be stored in
+ * Zookeeper or Accumulo's own metadata table.
+ *
+ * <p>
+ * Mutators created by this interface may only queue changes for persisting. There is no guarantee
+ * that any change is persisted until this object is closed.
+ *
+ * <p>
+ * This interface seeks to satisfy the following goals.
+ *
+ * <UL>
+ * <LI>The root tablet persists its data in Zookeeper. Metadata tablets persist their data in root
+ * tablet. All other tablets persist their data in the metadata table. This interface abstracts how
+ * and where information for a tablet is actually persisted.
+ * <LI>Before the creation of this interface, many concurrent metadata table updates resulted in
+ * separate synchronous RPCs. The design of this interface allows batching of metadata table updates
+ * within a tablet server for cluster wide efficiencies. Batching is not required by
+ * implementations, but the design of the interface makes it possible.
+ * <LI>Make code that updates Accumulo persistent metadata more concise. Before this interface
+ * existed, there was a lot of redundant and verbose code for updating metadata.
+ * <LI>Reduce specialized code for the root tablet. Currently there is specialized code to manage
+ * the root tablets files that is different from all other tablets. This interface is the beginning
+ * of an effort to remove this specialized code. See #936
+ * </UL>
+ */
+public interface MetadataMutator extends AutoCloseable {
+
+  public interface Factory {
 
 Review comment:
   It may not be needed.  My thinking was that it could make testing easier.  I will add TODO about removing.

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