HBASE-2283 row level atomicity; add missing file WALEdit.java

git-svn-id: https://svn.apache.org/repos/asf/hadoop/hbase/trunk@925519 13f79535-47bb-0310-9956-ffa450edef68

core/src/main/java/org/apache/hadoop/hbase/regionserver/wal/WALEdit.java

@@ -0,0 +1,147 @@
+/**
+ * Copyright 2009 The Apache Software Foundation
+ *
+ * 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.hadoop.hbase.regionserver.wal;
+
+import java.io.DataInput;
+import java.io.DataOutput;
+import java.io.IOException;
+import java.util.ArrayList;
+import java.util.List;
+
+import org.apache.hadoop.hbase.KeyValue;
+import org.apache.hadoop.hbase.util.ClassSize;
+import org.apache.hadoop.io.Writable;
+
+/**
+ * WALEdit: Used in HBase's transaction log (WAL) to represent
+ * the collection of edits (KeyValue objects) corresponding to a 
+ * single transaction. The class implements "Writable" interface
+ * for serializing/deserializing a set of KeyValue items.
+ * 
+ * Previously, if a transaction contains 3 edits to c1, c2, c3 for a row R,
+ * the HLog would have three log entries as follows:
+ * 
+ *    <logseq1-for-edit1>:<KeyValue-for-edit-c1>
+ *    <logseq2-for-edit2>:<KeyValue-for-edit-c2>
+ *    <logseq3-for-edit3>:<KeyValue-for-edit-c3>
+ * 
+ * This presents problems because row level atomicity of transactions
+ * was not guaranteed. If we crash after few of the above appends make
+ * it, then recovery will restore a partial transaction.
+ *
+ * In the new world, all the edits for a given transaction are written
+ * out as a single record, for example:
+ * 
+ *   <logseq#-for-entire-txn>:<WALEdit-for-entire-txn>
+ * 
+ * where, the WALEdit is serialized as:
+ *   <-1, # of edits, <KeyValue>, <KeyValue>, ... >
+ * For example:
+ *   <-1, 3, <Keyvalue-for-edit-c1>, <KeyValue-for-edit-c2>, <KeyValue-for-edit-c3>>
+ *   
+ * The -1 marker is just a special way of being backward compatible with 
+ * an old HLog which would have contained a single <KeyValue>.
+ * 
+ * The deserializer for WALEdit backward compatibly detects if the record
+ * is an old style KeyValue or the new style WALEdit.
+ *
+ */
+public class WALEdit implements Writable {
+  
+  private final int VERSION_2 = -1;
+  
+  private List<KeyValue> kvs = new ArrayList<KeyValue>();
+  
+  public WALEdit() {
+  }
+  
+  public void add(KeyValue kv) {
+    this.kvs.add(kv);
+  }
+
+  public boolean isEmpty() {
+    return kvs.isEmpty();
+  }
+
+  public int size() {
+    return kvs.size();
+  }
+  
+  public List<KeyValue> getKeyValues() {
+    return kvs;
+  }
+
+  public void readFields(DataInput in) throws IOException {
+    
+    // ignore any old state in case caller is recycling an instance of this object.
+    kvs = new ArrayList<KeyValue>();
+
+    int versionOrLength = in.readInt();
+
+    if (versionOrLength == VERSION_2) {
+      // this is new style HLog entry containing multiple KeyValues.
+      int numEdits = in.readInt();
+      for (int idx = 0; idx < numEdits; idx++) {
+        KeyValue kv = new KeyValue();
+        kv.readFields(in);
+        this.add(kv);
+      }
+    } else {
+      // this is an old style HLog entry. The int that we just
+      // read is actually the length of a single KeyValye.
+      KeyValue kv = new KeyValue();
+      kv.readFields(versionOrLength, in);
+      this.add(kv);
+    }
+
+  }
+
+  public void write(DataOutput out) throws IOException {
+    out.writeInt(VERSION_2);
+    out.writeInt(kvs.size());
+    for (KeyValue kv : kvs) {
+      kv.write(out);
+    }
+  }
+
+  public long heapSize() {
+    long size = ClassSize.align(ClassSize.OBJECT +
+                                ClassSize.REFERENCE + 
+                                ClassSize.ARRAYLIST);
+    for (KeyValue kv : kvs) {
+      size += kv.heapSize();
+    }
+    
+    return size;
+  }
+
+  public String toString() {
+    StringBuilder sb = new StringBuilder();
+
+    sb.append("[#edits: " + kvs.size() + " = <");
+    for (KeyValue kv : kvs) {
+      sb.append(kv.toString());
+      sb.append("; ");
+    }
+    sb.append(">]");
+    return sb.toString();
+  }
+  
+}