[220130] 분산시스템을 위한 uid 만들기(깊게 알아보기|코드포함)

import java.net.NetworkInterface;
import java.security.SecureRandom;
import java.time.Instant;
import java.util.Enumeration;

/**
 * Distributed Sequence Generator.
 * Inspired by Twitter snowflake: https://github.com/twitter/snowflake/tree/snowflake-2010
 *
 * This class should be used as a Singleton.
 * Make sure that you create and reuse a Single instance of SequenceGenerator per node in your distributed system cluster.
 */
public class SequenceGenerator {
  private static final int UNUSED_BITS = 1;  // Sign bit, Unused (always set to 0)
  private static final int EPOCH_BITS = 41;
  private static final int NODE_ID_BITS = 10;
  private static final int SEQUENCE_BITS = 12;

  private static final int maxNodeId = (int)(Math.pow(2, NODE_ID_BITS) - 1);
  private static final int maxSequence = (int)(Math.pow(2, SEQUENCE_BITS) - 1);

  // Custom Epoch (January 1, 2015 Midnight UTC = 2014-01-01T00:00:00Z)
  private static final long CUSTOM_EPOCH = 1420070400000L;

  private final int nodeId;

  private volatile long lastTimestamp = -1L;
  private volatile long sequence = 0L;

  // Create SequenceGenerator with a nodeId
  // 머신의 노드아이디를 입력해서 제너레이터를 만든다.
  public SequenceGenerator(int nodeId) {
    if (nodeId < 0 || nodeId > maxNodeId) {
      throw new IllegalArgumentException(String.format("NodeId must be between %d and %d", 0, maxNodeId));
    }
    this.nodeId = nodeId;
  }
  // 머신의 노드아이디가 없을 때 하나 만든다.
  // Let SequenceGenerator generate a nodeId
  public SequenceGenerator() {
    this.nodeId = createNodeId();
  }

  // 현재 네트워크 정보를 hashCode로 바꾼다. 
  // 예외가 나면 Random함수를 수행한다.
  public int createNodeId() {
    int nodeId;
    try {
      StringBuilder sb = new StringBuilder();
      Enumeration<NetworkInterface> networkInterfaces = NetworkInterface.getNetworkInterfaces();

      while(networkInterfaces.hashMoreElements()) {
	NetworkInterface networkInterface = networkInterfaces.nextElement();
	byte[] mac = networkInterface.getHardwareAddress();
	if (mac != null) {
	  for (int i = 0; i < mac.length; i++) {
	    sb.append(String.format("%02X", mac[i]));
	  }
	}
      }
      nodeId = sb.toString().hashCode();
    } catch (Exception ex) {
      nodeId = (new SecureRandom().nextInt());
    }

    nodeId = nodeId & maxNodeId;
    return nodeId;
  }

  // 일단 락을 걸고 있다.
  public synchronized long nextId() {
    long currentTimestamp = timestamp();

    if (currentTimestamp < lastTimestamp) {
      throw new IllegalStateException("Invalid System Clock!");
    }

    if (currentTimestamp == lastTimestamp) {
      sequence = (sequence + 1) & maxSequence;
      if (sequence == 0) {
	// Sequence Exhausted, wait till next millisecond.
	currentTimestamp = waitNextMillis(currentTimestamp);
      }
    } else {
      // reset sequence to start with zero for the next millisecond
      sequence = 0;
    }

    lastTimestamp = currentTimestamp;

    long id = currentTimestamp << (NODE_ID_BITS + SEQUENCE_BITS);
    id |= (nodeId << SEQUENCE_BITS);
    id |= sequence;  // id = id | sequence;

    return id;
  }

  // Get current timestamp in milliseconds, adjust for the custom epoch.
  // 밀리초단위의 현재 타임스탬프를 가져온다. 그리고 커스텀 epoch로 조절한다.
  private static long timestamp() {
    return Instant.now().toEpochMilli() - CUSTOM_EPOCH;
  }

  // Block and wait till next millisecond
  // 다음 밀리초까지 블록하고 기다린다.
  private long waitNextMillis(long currentTimestamp) {
    while (currentTimestamp == lastTimestamp) {
      currentTimestamp = timestamp();
    }
    return currentTimestamp;
  }
}

currentTimestamp = 1528538400000 - 1420070400000 // 108468000000 (Adjust for custom epoch)

id = currentTimestamp << (10 + 12)

// 10 bit의 node ID
// node 786
id |= nodeId << 12
// 12 bit의 counter
// sequence 3450
id |= sequence // 454947766275219456

CREATE OR REPLACE FUNCTION insta5.next_id(OUT result bigint) AS $$
DECLARE
    our_epoch bigint := 1314220021721;
    seq_id bigint;
    now_millis bigint;
    shard_id int := 5;
BEGIN
    SELECT nextval('insta5.table_id_seq') %% 1024 INTO seq_id;
    SELECT FLOOR(EXTRACT(EPOCH FROM clock_timestamp()) * 1000) INTO now_millis;
    result := (now_millis - our_epoch) << 23;
    result := result | (shard_id <<10);
    result := result | (seq_id);
END;
    $$ LANGUAGE PLPGSQL;

CREATE TABLE insta5.our_table (
    "id" bigint NOT NULL DEFAULT insta5.next_id(),
    ...rest of table schema...
  )

select (1992325551 << 23);
-----------------------------
 -679477248

select pg_typeof(1992325551);
-----------------------------
integer

select pg_typeof(1992325551);
-----------------------------
integer

select (1992325551::bigint << 23);
-----------------------------
16712838055723008