[GitHub] [hadoop] anuengineer commented on a change in pull request #882: HDDS-1624 : Refactor operations inside the bucket lock in OM key write.

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anuengineer commented on a change in pull request #882: HDDS-1624 : Refactor operations inside the bucket lock in OM key write.
URL: https://github.com/apache/hadoop/pull/882#discussion_r290136492
 
 

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 File path: hadoop-hdds/common/src/main/java/org/apache/hadoop/utils/UniqueId.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.hadoop.utils;
+
+import org.apache.hadoop.hdds.HddsUtils;
+
+/**
+ * This class uses system current time milliseconds to generate unique id.
+ */
+public final class UniqueId {
+    /*
+     * When we represent time in milliseconds using 'long' data type,
+     * the LSB bits are used. Currently we are only using 44 bits (LSB),
+     * 20 bits (MSB) are not used.
+     * We will exhaust this 44 bits only when we are in year 2525,
+     * until then we can safely use this 20 bits (MSB) for offset to generate
+     * unique id within millisecond.
+     *
+     * Year        : Mon Dec 31 18:49:04 IST 2525
+     * TimeInMillis: 17545641544247
+     * Binary Representation:
+     *   MSB (20 bits): 0000 0000 0000 0000 0000
+     *   LSB (44 bits): 1111 1111 0101 0010 1001 1011 1011 0100 1010 0011 0111
+     *
+     * We have 20 bits to run counter, we should exclude the first bit (MSB)
+     * as we don't want to deal with negative values.
+     * To be on safer side we will use 'short' data type which is of length
+     * 16 bits and will give us 65,536 values for offset.
+     *
+     */
+
+  private static volatile short offset = 0;
+
+  /**
+   * Private constructor so that no one can instantiate this class.
+   */
+  private UniqueId() {}
+
+  /**
+   * Calculate and returns next unique id based on System#currentTimeMillis.
+   *
+   * @return unique long value
+   */
+  public static synchronized long next() {
+    long utcTime = HddsUtils.getUtcTime();
+    if ((utcTime & 0xFFFF000000000000L) == 0) {
+      return utcTime << Short.SIZE | (offset++ & 0x0000FFFF);
+    }
 
 Review comment:
   Do we need this if check ? if we know by definition that UtcTime the top bits are always empty, should we really have this if check at all ? 

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