有一个项目用来负责调度集群中的"cron任务",比如一个application中可以配置N个定时任务,这些任务信息最终注册到zookeeper上,并开发了一系列代码用于维护这些任务的"活性";当applicaton中一个server故障,那么这个server上接管的任务,需要迁移到其他server上,如果多个server存活的话,还需要这些任务能够"均衡"的分布.
其中"负载均衡",很好理解,比如有6个任务,3个server,那么就需要每个server上尽可能的运行2个任务;其实这个事情想起来很简单,但是做起来似乎有些不得不考虑的问题:
1) "相对平均"怎么设计
2) 迁移任务时,是否会丢失任务的触发时机;比如一个任务凌晨3点执行,刚好此时运行了一次"均衡",任务在原来的server上没有触发,在新的server上又过了时间..
3) 迁移任务时,还需要考虑"最少移动"次数,不能大面积迁移任务;只能从"负载高"的server上迁移到"负载低"的.
例如:
sid1: w1 w2 w3 w4
sid2: w5
sid3:w6期望迁移之后:
sid1:w1 w2
sid2:w5 w3
sid3:w4 w6
而不是(这种结果迁移的面积太大,只需要把"多余"的任务迁移出去即可,而不是重新洗牌再均衡)
sid1:w6 w5
sid2:w2 w3
sid3:w1 w4
经过提取,"相对平均"的设计代码如下,仅作备忘:
Java代码
package com.test.demo.zookeeper;
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.*;
public class WorkersBalanceMain {
private List<String> servers = new ArrayList<String>();
private Map<String, List<String>> current = new HashMap<String, List<String>>();
private Set<String> workers = new HashSet<String>();
public static void main(String[] args) {
BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
String line;
Set<String> servers = new HashSet<String>();
WorkersBalanceMain balancer = new WorkersBalanceMain();
try {
while ((line = br.readLine()) != null) {
if (line.startsWith("addWorker")) {
balancer.addWorkers(line);
} else if (line.startsWith("addServer")) {
balancer.addServers(line);
} else {
System.out.println("???");
continue;
}
balancer.rebalance();
}
} catch (Exception e) {
e.printStackTrace();
}
System.out.println("--END---");
}
public void addServers(String source) {
int index = source.indexOf(" ");
if (index == -1) {
return;
}
String[] values = source.substring(index + 1).split(" ");
if (values == null || values.length == 0) {
return;
}
for (String server : values) {
servers.add(server);
if(current.get(server) == null){
current.put(server,new ArrayList<String>());
}
}
}
public void addWorkers(String source) {
int index = source.indexOf(" ");
if (index == -1) {
return;
}
String[] values = source.substring(index + 1).split(" ");
if (values == null || values.length == 0) {
return;
}
//当有新的worker提交时,将咱有一台机器接管
String sid = servers.get(0);
List<String> sw = current.get(sid);
if(sw == null){
current.put(sid,new ArrayList<String>());
}
for (String worker : values) {
workers.add(worker);
sw.add(worker);
}
}
public void rebalance() {
try {
if (workers.isEmpty()) {
return;
}
for (String sid : servers) {
if (current.get(sid) == null) {
current.put(sid, new ArrayList<String>());
}
}
//根据每个sid上的worker个数,整理成一个排序的map
TreeMap<Integer, List<String>> counterMap = new TreeMap<Integer, List<String>>();
for (Map.Entry<String, List<String>> entry : current.entrySet()) {
int total = entry.getValue().size();
List<String> sl = counterMap.get(total);
if (sl == null) {
sl = new ArrayList<String>();
counterMap.put(total, sl);
}
sl.add(entry.getKey());//sid
}
int totalWorkers = workers.size();
int totalServers = current.keySet().size();
int avg = totalWorkers / totalServers;//每个server实例可以接管任务的平均数
while (true) {
Map.Entry<Integer, List<String>> gt = counterMap.higherEntry(avg); //大于平均数的列表, >= avg + 1
Map.Entry<Integer, List<String>> lt = counterMap.lowerEntry(avg); //与平均数差值为2的 <= arg - 1
//允许任务个数与avg上线浮动1各个,不是绝对的平均
if (gt == null || lt == null) {
break;
}
Integer gtKey = gt.getKey();
Integer ltKey = lt.getKey();
if (gtKey - ltKey < 2) {
break;
}
if (gt.getValue().size() == 0) {
counterMap.remove(gt.getKey());
}
if (lt.getValue().size() == 0) {
counterMap.remove(lt.getKey());
}
Iterator<String> it = gt.getValue().iterator(); //sid列表
while (it.hasNext()) {
String _fromSid = it.next();
List<String> _currentWorkers = current.get(_fromSid);
if (_currentWorkers == null || _currentWorkers.isEmpty()) {
it.remove();
current.remove(_fromSid);
continue;
}
List<String> _ltServers = lt.getValue();
if (_ltServers.isEmpty()) {
counterMap.remove(ltKey);
break;
}
//取出需要交换出去的任务id
int _currentWorkersSize = _currentWorkers.size();
String _wid = _currentWorkers.get(_currentWorkersSize - 1);
String _toSid = _ltServers.get(0);
//从_fromSid的worker列表中移除低workerId
//注意:移除最后一个,和_ltWorkers.add(_wid)对应,_ltWorkers将新任务添加到list的尾部
//即从尾部移除,从尾部添加,基本保证"原任务,最少迁移次数"
_currentWorkers.remove(_currentWorkersSize - 1);
it.remove();
_ltServers.remove(0);
//将此workerId添加到_toSid的worker列表中
List<String> _ltWorkers = current.get(_toSid);
if (_ltWorkers == null) {
_ltWorkers = new ArrayList<String>();
current.put(_toSid, _ltWorkers);
}
_ltWorkers.add(_wid);
//将gt的key降低一个数字
List<String> _next = counterMap.get(gtKey - 1);
if (_next == null) {
_next = new ArrayList<String>();
counterMap.put(gtKey - 1, _next);
}
_next.add(_fromSid);
//将lt的key提升一个数字
List<String> _prev = counterMap.get(ltKey + 1);
//从lt的countMap中移除,因为它将被放置在key + 1的新位置
Iterator<String> _ltIt = _ltServers.iterator();
while (_ltIt.hasNext()) {
if (_ltIt.next().equalsIgnoreCase(_toSid)) {
_ltIt.remove();
break;
}
}
if (_prev == null) {
_prev = new ArrayList<String>();
counterMap.put(ltKey + 1, _prev);
}
_prev.add(_toSid);
}
}
//dump info
for (Map.Entry<String, List<String>> entry : current.entrySet()) {
System.out.println("Sid:" + entry.getKey());
System.out.println(entry.getValue().toString());
}
} catch (Exception e) {
e.printStackTrace();
}
}
}
