Raft是由Stanford提出的一种更易理解的一致性算法,意在取代目前广为使用的Paxos算法。目前,在各种主流语言中都有了一些开源实现,比如本文中将使用的基于JGroups的Raft协议实现。关于Raft的原理,强烈推荐 动画版Raft讲解。
在Raft中,每个结点会处于下面三种状态中的一种:
当follower在选举超时时间(election timeout)内未收到leader的心跳消息(append entries),则变成candidate状态。 为了避免选举冲突,这个超时时间是一个150~300ms之间的随机数。
成为candidate的结点发起新的选举期(election term)去“拉选票”:
如果接收结点在新term内没有投过票那它就会投给此candidate,并重置它自己的选举超时时间。candidate拉到大部分选票就会成为leader,并定时发送心跳—— Append Entries消息,去重置各个follower的计时器。当前Term会继续直到某个follower接收不到心跳并成为candidate。
如果不巧两个结点同时成为candidate都去“拉票”怎么办? 这时会发生Splite Vote情况。两个结点可能都拉到了同样多的选票,难分胜负,选举失败,本term没有leader。之后又有计时器超时的follower会变成candidate,将term加一并开始新一轮的投票。
当发生改变时,leader会复制日志给follower结点,这也是通过Append Entries心跳消息完成的。前面已经列举了Log Replication的过程,这里就不重复了。
Raft能够正确地处理网络分区(“脑裂”)问题。假设A~E五个结点,B是leader。如果发生“脑裂”,A、B成为一个子分区,C、D、E成 为一个子分区。此时C、D、E会发生选举,选出C作为新term的leader。这样我们在两个子分区内就有了不同term的两个leader。这时如果 有客户端写A时,因为B无法复制日志到大部分follower所以日志处于uncommitted未提交状态。而同时另一个客户端对C的写操作却能够正确 完成,因为C是新的leader,它只知道D和E。
当网络通信恢复,B能够发送心跳给C、D、E了,却发现“改朝换代”了, 因为C的term值更大,所以B自动降格为follower。然后A和B都回滚未提交的日志,并从新leader那里复制最新的日志。但这样是不是就会丢失更新?
JGroups是Java里比较流行的网络通信框架,近期顺应潮流,它也推出了Raft基于JGroups的实现。简单试用了一下,还比较容易上 手,底层Raft的内部机制都被API屏蔽掉了。下面就通过一个分布式计数器的实例来学习一下Raft协议在JGroups中的实际用法。
Maven依赖如下:
org.jgroups
jgroups-raft
0.2
其实JGroups-raft的Jar包中已经自带了一个 Counter的Demo,但仔细看了一下,有的地方写的有些麻烦,不太容易把握住Raft这根主线。所以这里就参照官方的例子,进行了简写,突出Raft协议的基本使用方法。JGroups-raft目前资料不多,InfoQ上的 这篇文章很不错,还有 官方文档。
使用JGroups-raft时,我们一般会实现两个接口: RAFT.RoleChange和StateMachine:
典型单点服务实现方式就是:
JChannel ch = null;
RaftHandle handle = new RaftHandle(ch, this);
handle.addRoleListener(role -> {
if(role == Role.Leader)
// start singleton services
else
// stop singleton services
});
jgroups-raft.jar中已经带了一个raft.xml配置文件,作为实例程序我们可以直接使用它。
简要解释一下最核心的几个配置项,参照 GitHub上的文档:
...
实例很简单,只有JGroupsRaftTest和CounterService两个类组成。JGroupsRaftTest是测试启动类,而CounterService就是利用Raft协议实现的分布式计数服务类。
JGroupsRaftTest的职责主要有三个:
目前简单实现了几种操作包括:初始化计数器、加一、减一、读取计数器、查看Raft日志、做Raft快照(用于压缩日志文件)等。其中对计数器的操作,因为要与其他Raft成员进行分布式通信,所以当前集群必须要多于一个结点时才能进行操作。 如果要支持单结点时的操作,需要做特殊处理。
import org.jgroups.JChannel;
import org.jgroups.protocols.raft.RAFT;
import org.jgroups.util.Util;
/**
* Test jgroups raft algorithm implementation.
*/
public class JGroupsRaftTest {
private static final String CLUSTER_NAME = "ctr-cluster";
private static final String COUNTER_NAME = "counter";
private static final String RAFT_XML = "raft.xml";
public static void main(String[] args) throws Exception {
JChannel ch = new JChannel(RAFT_XML).name(args[0]);
CounterService counter = new CounterService(ch);
try {
doConnect(ch, CLUSTER_NAME);
doLoop(ch, counter);
} finally {
Util.close(ch);
}
}
private static void doConnect(JChannel ch, String clusterName) throws Exception {
ch.connect(clusterName);
}
private static void doLoop(JChannel ch, CounterService counter) {
boolean looping = true;
while (looping) {
int key = Util.keyPress("\n[0] Create [1] Increment [2] Decrement [3] Dump log [4] Snapshot [x] Exit\n" +
"first-applied=" + ((RAFT) ch.getProtocolStack().findProtocol(RAFT.class)).log().firstApplied() +
", last-applied=" + counter.lastApplied() +
", commit-index=" + counter.commitIndex() +
", log size=" + Util.printBytes(counter.logSize()) + ": ");
if ((key == '0' || key == '1' || key == '2') && !counter.isLeaderExist()) {
System.out.println("Cannot perform cause there is no leader by now");
continue;
}
long val;
switch (key) {
case '0':
counter.getOrCreateCounter(COUNTER_NAME, 1L);
break;
case '1':
val = counter.incrementAndGet(COUNTER_NAME);
System.out.printf("%s: %s\n", COUNTER_NAME, val);
break;
case '2':
val = counter.decrementAndGet(COUNTER_NAME);
System.out.printf("%s: %s\n", COUNTER_NAME, val);
break;
case '3':
counter.dumpLog();
break;
case '4':
counter.snapshot();
break;
case 'x':
looping = false;
break;
case '\n':
System.out.println(COUNTER_NAME + ": " + counter.get(COUNTER_NAME) + "\n");
break;
}
}
}
}
CounterService是我们的核心类,利用Raft实现了分布式的计数器操作,它的API主要由四部分组成:
import org.jgroups.Channel;
import org.jgroups.protocols.raft.RAFT;
import org.jgroups.protocols.raft.Role;
import org.jgroups.protocols.raft.StateMachine;
import org.jgroups.raft.RaftHandle;
import org.jgroups.util.AsciiString;
import org.jgroups.util.Bits;
import org.jgroups.util.ByteArrayDataInputStream;
import org.jgroups.util.ByteArrayDataOutputStream;
import org.jgroups.util.Util;
import java.io.DataInput;
import java.io.DataOutput;
import java.io.IOException;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.HashMap;
import java.util.Map;
/**
* Distribute counter service based on Raft consensus algorithm.
*/
class CounterService implements StateMachine, RAFT.RoleChange {
private RaftHandle raft;
private final Map counters;
private enum Command {
CREATE, INCREMENT_AND_GET, DECREMENT_AND_GET, GET, SET
}
public CounterService(Channel ch) {
this.raft = new RaftHandle(ch, this);
this.counters = new HashMap<>();
raft.raftId(ch.getName())
.addRoleListener(this);
}
// ===========================================
// Raft Status API
// ===========================================
public int lastApplied() {
return raft.lastApplied();
}
public int commitIndex() {
return raft.commitIndex();
}
public int logSize() {
return raft.logSize();
}
public void dumpLog() {
System.out.println("\nindex (term): command\n---------------------");
raft.logEntries((entry, index) -> {
StringBuilder log = new StringBuilder()
.append(index)
.append(" (").append(entry.term()).append("): ");
if (entry.command() == null ) {
System.out.println(log.append(""));
return;
} else if (entry.internal()) {
System.out.println(log.append(""));
return;
}
ByteArrayDataInputStream in = new ByteArrayDataInputStream(
entry.command(), entry.offset(), entry.length()
);
try {
Command cmd = Command.values()[in.readByte()];
String name = Bits.readAsciiString(in).toString();
switch (cmd) {
case CREATE:
log.append(cmd)
.append("(").append(name).append(", ")
.append(Bits.readLong(in))
.append(")");
break;
case GET:
case INCREMENT_AND_GET:
case DECREMENT_AND_GET:
log.append(cmd)
.append("(").append(name).append(")");
break;
default:
throw new IllegalArgumentException("Command " + cmd + "is unknown");
}
System.out.println(log);
}
catch (IOException e) {
throw new IllegalStateException("Error when dump log", e);
}
});
System.out.println();
}
public void snapshot() {
try {
raft.snapshot();
} catch (Exception e) {
throw new IllegalStateException("Error when snapshot", e);
}
}
public boolean isLeaderExist() {
return raft.leader() != null;
}
// ===========================================
// Raft API
// ===========================================
@Override
public void roleChanged(Role role) {
System.out.println("roleChanged to: " + role);
}
@Override
public byte[] apply(byte[] data, int offset, int length) throws Exception {
ByteArrayDataInputStream in = new ByteArrayDataInputStream(data, offset, length);
Command cmd = Command.values()[in.readByte()];
String name = Bits.readAsciiString(in).toString();
System.out.println("[" + new SimpleDateFormat("HH:mm:ss.SSS").format(new Date())
+ "] Apply: cmd=[" + cmd + "]");
long v1, retVal;
switch (cmd) {
case CREATE:
v1 = Bits.readLong(in);
retVal = create0(name, v1);
return Util.objectToByteBuffer(retVal);
case GET:
retVal = get0(name);
return Util.objectToByteBuffer(retVal);
case INCREMENT_AND_GET:
retVal = add0(name, 1L);
return Util.objectToByteBuffer(retVal);
case DECREMENT_AND_GET:
retVal = add0(name, -1L);
return Util.objectToByteBuffer(retVal);
default:
throw new IllegalArgumentException("Command " + cmd + "is unknown");
}
}
@Override
public void readContentFrom(DataInput in) throws Exception {
int size = in.readInt();
System.out.println("ReadContentFrom: size=[" + size + "]");
for (int i = 0; i < size; i++) {
AsciiString name = Bits.readAsciiString(in);
Long value = Bits.readLong(in);
counters.put(name.toString(), value);
}
}
@Override
public void writeContentTo(DataOutput out) throws Exception {
synchronized (counters) {
int size = counters.size();
System.out.println("WriteContentFrom: size=[" + size + "]");
out.writeInt(size);
for (Map.Entry entry : counters.entrySet()) {
AsciiString name = new AsciiString(entry.getKey());
Long value = entry.getValue();
Bits.writeAsciiString(name, out);
Bits.writeLong(value, out);
}
}
}
// ===========================================
// Counter API
// ===========================================
public void getOrCreateCounter(String name, long initVal) {
Object retVal = invoke(Command.CREATE, name, false, initVal);
counters.put(name, (Long) retVal);
}
public long incrementAndGet(String name) {
return (long) invoke(Command.INCREMENT_AND_GET, name, false);
}
public long decrementAndGet(String name) {
return (long) invoke(Command.DECREMENT_AND_GET, name, false);
}
public long get(String name) {
return (long) invoke(Command.GET, name, false);
}
private Object invoke(Command cmd, String name, boolean ignoreRetVal, long... values) {
ByteArrayDataOutputStream out = new ByteArrayDataOutputStream(256);
try {
out.writeByte(cmd.ordinal());
Bits.writeAsciiString(new AsciiString(name), out);
for (long val : values) {
Bits.writeLong(val, out);
}
byte[] rsp = raft.set(out.buffer(), 0, out.position());
return ignoreRetVal ? null : Util.objectFromByteBuffer(rsp);
}
catch (IOException ex) {
throw new RuntimeException("Serialization failure (cmd="
+ cmd + ", name=" + name + ")", ex);
}
catch (Exception ex) {
throw new RuntimeException("Raft set failure (cmd="
+ cmd + ", name=" + name + ")", ex);
}
}
// ===========================================
// Counter Native API
// ===========================================
public synchronized Long create0(String name, long initVal) {
counters.putIfAbsent(name, initVal);
return counters.get(name);
}
public synchronized Long get0(String name) {
return counters.getOrDefault(name, 0L);
}
public synchronized Long add0(String name, long delta) {
Long oldVal = counters.getOrDefault(name, 0L);
return counters.put(name, oldVal + delta);
}
}
我们分别以A、B、C为参数,启动三个JGroupsRaftTest服务。这样会自动在C:\Users\cdai\AppData\Local\Temp下生成A.log、B.log、C.log三个日志文件夹。
[email protected] /cygdrive/c/Users/cdai/AppData/Local/Temp
$ tree A.log/ B.log/ C.log/
A.log/
|-- 000005.sst
|-- 000006.log
|-- CURRENT
|-- LOCK
|-- LOG
|-- LOG.old
`-- MANIFEST-000004
B.log/
|-- 000003.log
|-- CURRENT
|-- LOCK
|-- LOG
`-- MANIFEST-000002
C.log/
|-- 000003.log
|-- CURRENT
|-- LOCK
|-- LOG
`-- MANIFEST-000002
0 directories, 17 files
首先A创建计数器,B“加一”,C“减一”。可以看到尽管我们是分别在A、B、C上执行这三个操作, 但三个结点都先后(leader提交日志后通知follower)通过apply()方法收到消息,并在本地的计数器Map上同步执行操作,保证了数据的一致性。最后停掉A服务,可以看到B通过roleChanged()得到消息,提升为新的Leader,并与C一同继续提供服务。
A的控制台输出:
-------------------------------------------------------------------
GMS: address=A, cluster=ctr-cluster, physical address=2001:0:9d38:6abd:cbb:1f78:3f57:50f6:50100
-------------------------------------------------------------------
[0] Create [1] Increment [2] Decrement [3] Dump log [4] Snapshot [x] Exit
first-applied=0, last-applied=0, commit-index=0, log size=0b:
roleChanged to: Candidate
roleChanged to: Leader
0
[14:16:00.744] Apply: cmd=[CREATE]
[0] Create [1] Increment [2] Decrement [3] Dump log [4] Snapshot [x] Exit
first-applied=0, last-applied=1, commit-index=1, log size=1b:
[14:16:07.002] Apply: cmd=[INCREMENT_AND_GET]
[14:16:14.264] Apply: cmd=[DECREMENT_AND_GET]
3
index (term): command
---------------------
1 (29): CREATE(counter, 1)
2 (29): INCREMENT_AND_GET(counter)
3 (29): DECREMENT_AND_GET(counter)
B的控制台输出:
-------------------------------------------------------------------
GMS: address=B, cluster=ctr-cluster, physical address=2001:0:9d38:6abd:cbb:1f78:3f57:50f6:50101
-------------------------------------------------------------------
[0] Create [1] Increment [2] Decrement [3] Dump log [4] Snapshot [x] Exit
first-applied=0, last-applied=0, commit-index=0, log size=0b:
[14:16:01.300] Apply: cmd=[CREATE]
1
counter: 2
[0] Create [1] Increment [2] Decrement [3] Dump log [4] Snapshot [x] Exit
first-applied=0, last-applied=2, commit-index=1, log size=2b:
[14:16:07.299] Apply: cmd=[INCREMENT_AND_GET]
[14:16:14.304] Apply: cmd=[DECREMENT_AND_GET]
roleChanged to: Candidate
roleChanged to: Leader
C的控制台输出:
-------------------------------------------------------------------
GMS: address=C, cluster=ctr-cluster, physical address=2001:0:9d38:6abd:cbb:1f78:3f57:50f6:55800
-------------------------------------------------------------------
[0] Create [1] Increment [2] Decrement [3] Dump log [4] Snapshot [x] Exit
first-applied=0, last-applied=0, commit-index=0, log size=0b:
[14:16:01.300] Apply: cmd=[CREATE]
[14:16:07.299] Apply: cmd=[INCREMENT_AND_GET]
2
counter: 3
[0] Create [1] Increment [2] Decrement [3] Dump log [4] Snapshot [x] Exit
first-applied=0, last-applied=3, commit-index=2, log size=3b:
[14:16:14.304] Apply: cmd=[DECREMENT_AND_GET]
在只有B和C的集群中,我们执行了一次“加一”。当我们重新启动A服务时,它会自动执行这条日志,保持与B和C的一致。从日志的index能够看出,69是一个Term,也就是A为Leader时的“任期”,而70也就是B为Leader时。
A的控制台输出:
-------------------------------------------------------------------
GMS: address=A, cluster=ctr-cluster, physical address=2001:0:9d38:6abd:cbb:1f78:3f57:50f6:53237
-------------------------------------------------------------------
[0] Create [1] Increment [2] Decrement [3] Dump log [4] Snapshot [x] Exit
first-applied=0, last-applied=3, commit-index=3, log size=3b:
[14:18:45.275] Apply: cmd=[INCREMENT_AND_GET]
[14:18:45.277] Apply: cmd=[GET]
3
index (term): command
---------------------
1 (69): CREATE(counter, 1)
2 (69): INCREMENT_AND_GET(counter)
3 (69): DECREMENT_AND_GET(counter)
4 (70): INCREMENT_AND_GET(counter)
5 (70): GET(counter)