分散式ZAB協議
阿新 • • 發佈：2020-07-15
提到ZAB，恐怕大家第一時間就會想到Zookeeper，然後由Zookeeper又會聯想到Paxos。這之間的聯絡是不是因為有本暢銷書叫《從Paxos到Zookeeper分散式一致性原理與實踐》，使得大家常常把Zookeeper和Paxos關聯起來，畢竟“買了就是讀了”，開個玩笑，ZAB和Paxos的確也是有很多相似之處的，理解了Paxos也的確對學習其它分散式一致性或共識演算法非常有幫助。不過Paxos的內容我們以後再說，在這裡只討論ZAB。
ZAB的全稱為Zookeeper Atomic Broadcast，其實大家看到“Atomic”和“Broadcast”兩個詞，應該就能大概明白ZAB的主要工作方式了。他是一個為Zookeeper量身定製的支援崩潰恢復的原子廣播協議，用於保障Zookeeper各副本之間在正常或異常情況下的資料一致性。
// 碼 1 QuorumPeer.java：節點狀態列舉
public enum ServerStatepublic enum ServerState {
LOOKING,
FOLLOWING,
LEADING,
OBSERVING
}// 碼 2 Vote.java：選票結構
public class Vote {
private final int version;
private final long id; //伺服器ID
private final long zxid; //Epoch + Counter
private final long electionEpoch; //選舉輪次
private final long peerEpoch; //被推舉的Leader所在的選舉輪次
private final ServerState state; //當前伺服器狀態
}// 碼 3 ZxidUtils.java：Zxid結構
public class ZxidUtils {
public static long getEpochFromZxid(long zxid) {
return zxid >> 32L;
}
public static long getCounterFromZxid(long zxid) {
return zxid & 0xffffffffL;
}
public static long makeZxid(long epoch, long counter) {
return (epoch << 32L) | (counter & 0xffffffffL);
}
public static String zxidToString(long zxid) {
return Long.toHexString(zxid);
}
}// 碼 4 FastLeaderElection.java：選票對比
protected boolean totalOrderPredicate(long newId, long newZxid, long newEpoch, long curId, long curZxid, long curEpoch) {
LOG.debug(
"id: {}, proposed id: {}, zxid: 0x{}, proposed zxid: 0x{}",
newId,
curId,
Long.toHexString(newZxid),
Long.toHexString(curZxid));
if (self.getQuorumVerifier().getWeight(newId) == 0) {
return false;
}
/*
* We return true if one of the following three cases hold:
* 1- New epoch is higher
* 2- New epoch is the same as current epoch, but new zxid is higher
* 3- New epoch is the same as current epoch, new zxid is the same
* as current zxid, but server id is higher.
*/
return ((newEpoch > curEpoch)
|| ((newEpoch == curEpoch)
&& ((newZxid > curZxid)
|| ((newZxid == curZxid)
&& (newId > curId)))));
}