这通常用于那种对集群中机器状态，机器在线率有较高要求的场景，能够快速对集群中机器变化作出响应。这样的场景中，往往有一个监控系统，实时检测集群机器是否存活。
利用ZooKeeper有两个特性（读可监控，临时节点），就可以实现一种集群机器存活性监控系统：
1. 客户端在节点 x 上注册一个Watcher，那么如果x的子节点变化了，会通知该客户端
2. 创建EPHEMERAL类型的节点，一旦客户端和服务器的会话结束或过期，那么该节点就会消失
利用这两个特性，可以分别实现对客服端的状态变化、上下线进行监控。
例如，监控系统在 /Monitor 节点上注册一个Watcher，以后每动态加机器，那么就往 /Monitor 下创建一个 EPHEMERAL类型的节点：/Monitor/{hostname}. 这样，监控系统就能够实时知道机器的增减情况，至于后续处理就是监控系统的业务了。
在分布式环境中，有些业务逻辑只需要集群中的某一台机器进行执行，其他的机器可以共享这个结果，这样可以大大减少重复计算，提高性能，于是就需要进行master选举。
利用ZooKeeper的强一致性，能够保证在分布式高并发情况下节点创建的全局唯一性，即：同时有多个客户端请求创建 /currentMaster 节点，最终一定只有一个客户端请求能够创建成功。利用这个特性，就能很轻易的在分布式环境中进行集群选举了。
此外，也可以利用Zookeeper的EPHEMERAL_SEQUENTIAL节点，实现动态选举：每个客户端都在/Master/下创建一个EPHEMERAL_SEQUENTIAL节点,由于ZooKeeper保证SEQUENTIAL的有序性，因此我们可以简单的把节点号最小的作为Master,就完成了选主。
假设我们要监控集群中的一群活动的业务进程，同时会在这群进程中选取一个进程作为监控的Master进程。每个进程使用IP地址加进程号标识，即{ip:pid}.当新的业务进程上线时，该进程会到/Monitor下创建一个临时有序（EPHEMERAL_SEQUENTIAL）的节点.并获取/Monitor下的子节点列表，如果发现自己创建的节点最小，则提升自己为Master进程，否则仍是业务进程。当进程退出时该节点会自动删除，其他进程则会尝试选主，保证当Master进程退出后，会提升一个新的Master进程。
举个例子，假设集群中一开始没有进程，
执行情况如下表所示：
| A1 | A2 | A3 | A4 |
|---|---|---|---|
| create,show_list(M) | |||
| show_list(M) | create | ||
| show_list(M) | - | create | |
| killed | show_list(M) | - | |
| - | show_list(M) | - | create |
首先是获取本机的IP已经当前进程的进程号PID，并通过ip_pid返回。
void getlocalhost(char *ip_pid,int len)
{
char hostname[64] = {0};
struct hostent *hent ;
gethostname(hostname,sizeof(hostname));
hent = gethostbyname(hostname);
char * localhost = inet_ntoa(*((struct in_addr*)(hent->h_addr_list[0])));
snprintf(ip_pid,len,"%s:%lld",localhost,getpid());
}
选主函数，获取path下的所有子节点，选择序号最小的一个，取出它的ip_pid,如果和本进程相同，则本进程被选为Master。如果当前进程被选为Master，则进程中的全局变量g_mode会被赋值为MODE_MONITOR,否则不变。
void choose_mater(zhandle_t *zkhandle,const char *path)
{
struct String_vector procs;
int i = 0;
int ret = zoo_get_children(zkhandle,path,1,&procs);
if(ret != ZOK || procs.count == 0){
fprintf(stderr,"failed to get the children of path %s!\n",path);
}else{
char master_path[512] ={0};
char ip_pid[64] = {0};
int ip_pid_len = sizeof(ip_pid);
char master[512]={0};
char localhost[512]={0};
getlocalhost(localhost,sizeof(localhost));
strcpy(master,procs.data[0]);
for(i = 1; i < procs.count; ++i){
if(strcmp(master,procs.data[i])>0){
strcpy(master,procs.data[i]);
}
}
sprintf(master_path,"%s/%s",path,master);
ret = zoo_get(zkhandle,master_path,0,ip_pid,&ip_pid_len,NULL);
if(ret != ZOK){
fprintf(stderr,"failed to get the data of path %s!\n",master_path);
}else if(strcmp(ip_pid,localhost)==0){
g_mode = MODE_MONITOR;
}
}
for(i = 0; i < procs.count; ++i){
free(procs.data[i]);
procs.data[i] = NULL;
}
}
show_list为Master进程函数，所做的任务为打印path目录下所有子节点的ip_pid.
void show_list(zhandle_t *zkhandle,const char *path)
{
struct String_vector procs;
int i = 0;
char localhost[512]={0};
getlocalhost(localhost,sizeof(localhost));
int ret = zoo_get_children(zkhandle,path,1,&procs);
if(ret != ZOK){
fprintf(stderr,"failed to get the children of path %s!\n",path);
}else{
char child_path[512] ={0};
char ip_pid[64] = {0};
int ip_pid_len = sizeof(ip_pid);
printf("--------------\n");
printf("ip\tpid\n");
for(i = 0; i < procs.count; ++i){
sprintf(child_path,"%s/%s",path,procs.data[i]);
//printf("%s\n",child_path);
ret = zoo_get(zkhandle,child_path,0,ip_pid,&ip_pid_len,NULL);
if(ret != ZOK){
fprintf(stderr,"failed to get the data of path %s!\n",child_path);
}else if(strcmp(ip_pid,localhost)==0){
printf("%s(Master)\n",ip_pid);
}else{
printf("%s\n",ip_pid);
}
}
}
for(i = 0; i < procs.count; ++i){
free(procs.data[i]);
procs.data[i] = NULL;
}
}
监控函数如下，当发现path的子节点发生变化，就会尝试重新选主，如果当前进程被选为主，就立即执行show_list，打印path下的所有子节点对应的ip_pid.
void zktest_watcher_g(zhandle_t* zh, int type, int state, const char* path, void* watcherCtx)
{
/*
printf("watcher event\n");
printf("type: %d\n", type);
printf("state: %d\n", state);
printf("path: %s\n", path);
printf("watcherCtx: %s\n", (char *)watcherCtx);
*/
if(type == ZOO_CHILD_EVENT &&
state == ZOO_CONNECTED_STATE ){
choose_mater(zh,path);
if(g_mode == MODE_MONITOR){
show_list(zh,path);
}
}
}
完整代码如下：
1.monitor.c
#include<stdio.h>
#include<string.h>
#include<unistd.h>
#include <netinet/in.h>
#include <netdb.h>
#include <arpa/inet.h>
#include"zookeeper.h"
#include"zookeeper_log.h"
enum WORK_MODE{MODE_MONITOR,MODE_WORKER} g_mode;
char g_host[512]= "172.17.0.36:2181";
//watch function when child list changed
void zktest_watcher_g(zhandle_t* zh, int type, int state, const char* path, void* watcherCtx);
//show all process ip:pid
void show_list(zhandle_t *zkhandle,const char *path);
//if success,the g_mode will become MODE_MONITOR
void choose_mater(zhandle_t *zkhandle,const char *path);
//get localhost ip:pid
void getlocalhost(char *ip_pid,int len);
void print_usage();
void get_option(int argc,const char* argv[]);
/**********unitl*********************/
void print_usage()
{
printf("Usage : [monitor] [-h] [-m] [-s ip:port] \n");
printf(" -h Show help\n");
printf(" -m set monitor mode\n");
printf(" -s zookeeper server ip:port\n");
printf("For example:\n");
printf("monitor -m -s172.17.0.36:2181 \n");
}
void get_option(int argc,const char* argv[])
{
extern char *optarg;
int optch;
int dem = 1;
const char optstring[] = "hms:";
//default
g_mode = MODE_WORKER;
while((optch = getopt(argc , (char * const *)argv , optstring)) != -1 )
{
switch( optch )
{
case ‘h‘:
print_usage();
exit(-1);
case ‘?‘:
print_usage();
printf("unknown parameter: %c\n", optopt);
exit(-1);
case ‘:‘:
print_usage();
printf("need parameter: %c\n", optopt);
exit(-1);
case ‘m‘:
g_mode = MODE_MONITOR;
break;
case ‘s‘:
strncpy(g_host,optarg,sizeof(g_host));
break;
default:
break;
}
}
}
void zktest_watcher_g(zhandle_t* zh, int type, int state, const char* path, void* watcherCtx)
{
/*
printf("watcher event\n");
printf("type: %d\n", type);
printf("state: %d\n", state);
printf("path: %s\n", path);
printf("watcherCtx: %s\n", (char *)watcherCtx);
*/
if(type == ZOO_CHILD_EVENT &&
state == ZOO_CONNECTED_STATE ){
choose_mater(zh,path);
if(g_mode == MODE_MONITOR){
show_list(zh,path);
}
}
}
void getlocalhost(char *ip_pid,int len)
{
char hostname[64] = {0};
struct hostent *hent ;
gethostname(hostname,sizeof(hostname));
hent = gethostbyname(hostname);
char * localhost = inet_ntoa(*((struct in_addr*)(hent->h_addr_list[0])));
snprintf(ip_pid,len,"%s:%lld",localhost,getpid());
}
void choose_mater(zhandle_t *zkhandle,const char *path)
{
struct String_vector procs;
int i = 0;
int ret = zoo_get_children(zkhandle,path,1,&procs);
if(ret != ZOK || procs.count == 0){
fprintf(stderr,"failed to get the children of path %s!\n",path);
}else{
char master_path[512] ={0};
char ip_pid[64] = {0};
int ip_pid_len = sizeof(ip_pid);
char master[512]={0};
char localhost[512]={0};
getlocalhost(localhost,sizeof(localhost));
strcpy(master,procs.data[0]);
for(i = 1; i < procs.count; ++i){
if(strcmp(master,procs.data[i])>0){
strcpy(master,procs.data[i]);
}
}
sprintf(master_path,"%s/%s",path,master);
ret = zoo_get(zkhandle,master_path,0,ip_pid,&ip_pid_len,NULL);
if(ret != ZOK){
fprintf(stderr,"failed to get the data of path %s!\n",master_path);
}else if(strcmp(ip_pid,localhost)==0){
g_mode = MODE_MONITOR;
}
}
for(i = 0; i < procs.count; ++i){
free(procs.data[i]);
procs.data[i] = NULL;
}
}
void show_list(zhandle_t *zkhandle,const char *path)
{
struct String_vector procs;
int i = 0;
char localhost[512]={0};
getlocalhost(localhost,sizeof(localhost));
int ret = zoo_get_children(zkhandle,path,1,&procs);
if(ret != ZOK){
fprintf(stderr,"failed to get the children of path %s!\n",path);
}else{
char child_path[512] ={0};
char ip_pid[64] = {0};
int ip_pid_len = sizeof(ip_pid);
printf("--------------\n");
printf("ip\tpid\n");
for(i = 0; i < procs.count; ++i){
sprintf(child_path,"%s/%s",path,procs.data[i]);
//printf("%s\n",child_path);
ret = zoo_get(zkhandle,child_path,0,ip_pid,&ip_pid_len,NULL);
if(ret != ZOK){
fprintf(stderr,"failed to get the data of path %s!\n",child_path);
}else if(strcmp(ip_pid,localhost)==0){
printf("%s(Master)\n",ip_pid);
}else{
printf("%s\n",ip_pid);
}
}
}
for(i = 0; i < procs.count; ++i){
free(procs.data[i]);
procs.data[i] = NULL;
}
}
int main(int argc, const char *argv[])
{
int timeout = 30000;
char path_buffer[512];
int bufferlen=sizeof(path_buffer);
zoo_set_debug_level(ZOO_LOG_LEVEL_WARN); //设置日志级别,避免出现一些其他信息
get_option(argc,argv);
zhandle_t* zkhandle = zookeeper_init(g_host,zktest_watcher_g, timeout, 0, (char *)"Monitor Test", 0);
if (zkhandle ==NULL)
{
fprintf(stderr, "Error when connecting to zookeeper servers...\n");
exit(EXIT_FAILURE);
}
char path[512]="/Monitor";
int ret = zoo_exists(zkhandle,path,0,NULL);
if(ret != ZOK){
ret = zoo_create(zkhandle,path,"1.0",strlen("1.0"),
&ZOO_OPEN_ACL_UNSAFE,0,
path_buffer,bufferlen);
if(ret != ZOK){
fprintf(stderr,"failed to create the path %s!\n",path);
}else{
printf("create path %s successfully!\n",path);
}
}
if(ret == ZOK && g_mode == MODE_WORKER){
char localhost[512]={0};
getlocalhost(localhost,sizeof(localhost));
char child_path[512];
sprintf(child_path,"%s/proc-",path);
ret = zoo_create(zkhandle,child_path,localhost,strlen(localhost),
&ZOO_OPEN_ACL_UNSAFE,ZOO_SEQUENCE|ZOO_EPHEMERAL,
path_buffer,bufferlen);
if(ret != ZOK){
fprintf(stderr,"failed to create the child_path %s,buffer:%s!\n",child_path,path_buffer);
}else{
printf("create child path %s successfully!\n",path_buffer);
}
choose_mater(zkhandle,path);
}
if(g_mode == MODE_MONITOR){
show_list(zkhandle,path);
}
getchar();
zookeeper_close(zkhandle);
return 0;
}
2.Makefile
CC=gcc
CFLAGS=-g
ZOOKEEPER_INSTALL=/usr/local
ZOOKEEPER_INC=-I${ZOOKEEPER_INSTALL}/include/zookeeper
ZOOKEEPER_LIB= -L${ZOOKEEPER_INSTALL}/lib -lzookeeper_mt
APP=monitor
all:
${CC} monitor.c -DTHREAD ${CFLAGS} ${ZOOKEEPER_INC} ${ZOOKEEPER_LIB} -o ${APP}
clean:
rm -f ${APP}
可以单机上重复启动程序，它们的进程号都是不同的，也可以在集群中启动程序。
参数-s表示Zookeeper的服务器的ip和端口，（注意不要理解成master的ip和端口哦）
参数-m表示该进程是一个独立的监控进程，注意，指定这个参数的进程是不参加选主的，因为它不会在/Monitor目录下创建路径。
运行示例：
monitor -s172.17.0.36:2181
ZooKeeper场景实践：（6）集群监控和Master选举
原文地址：http://blog.csdn.net/qq910894904/article/details/40834083
