1.基本介绍
分布式锁是控制分布式系统之间同步访问共享资源的一种方式,需要互斥来防止彼此干扰来保证一致性。利用Zookeeper的强一致性可以完成锁服务。Zookeeper的官方文档是列举了两种锁，独占锁和共享锁。独占锁保证任何时候都只有一个进程能或者资源的读写权限。共享锁可以同时有多个读，但是同一时刻最多只能有一个写，读和写是互斥的。
2.场景分析
我们准备来实现互斥的锁，按照官网的思路，给定一个锁的路径，如/Lock,所有要申请这个锁的进程都在/Lock目录下创建一个/Lock/lock-的临时序列节点，并监控/Lock的子节点变化事件。当子节点发送变化时用get_children()获取子节点的列表，如果发现进程发现自己拥有最小的一个序号，则获得锁。处理业务完毕后需要释放锁，此时只需要删除该临时节点即可。简单来说就是永远是拥有最小序号的进程获得锁。
3.场景实践
使用锁有两个基本的函数，就是lock或unlock.定义为
-
Lock *lock(zhandle_t *zkhandle,const char *path)
lock函数有两个参数，一个是zookeeper_init返回的句柄zkhandle,另一个是锁的路径,如果成功则返回一个Lock的结构体指针，并同时获得锁，否则返回NULL。 -
int unlock(zhandle_t *zkhandle,Lock * *lock)
unlock函数也有两个参数，一个是zookeeper_init返回的句柄zkhandle,另一个是lock函数返回的结构体指针的指针。
接下来在看具体的实现。
Lock *lock(zhandle_t *zkhandle,const char *path)
{
Lock *lock = create_lock(zkhandle,path);
if(lock != NULL){
while(try_lock(zkhandle,lock) == 0){
sleep(1);
}
}else{
fprintf(stderr,"error when create lock %s.\n",path);
}
return lock;
}
-
create_lock：负责锁的初始化，主要功能是负责创建{path}的节点已经{path}/lock-的临时序列节点。{path}如果存在则不再创建。 -
try_lock：尝试加锁，这个函数不会等待，失败和成功都立即返回。其主要功能是获取{path}的子节点列表，并查看自己是否是拥有最小序列号的节点，如果是则返回1，否则返回0;
lock函数初始化锁后，会持续的尝试加锁，直到成功。虽然我是这样实现的，但是过于简单粗暴（哈哈）。如果拿不到锁的话，持续就会阻塞在lock函数。
int unlock(zhandle_t *zkhandle,Lock * *lock)
{
if(*lock){
int ret = zoo_delete(zkhandle,(*lock)->selfpath,-1);
if(ret != ZOK){
fprintf(stderr,"error when release lock %s.\n",(*lock)->selfpath);
}
free(*lock);
*lock = NULL;
return ret;
}
return ZOK;
}
unlock函数就非常简单了，就是将create_lock中创建的临时序列节点删除就可以了。
接下来在看下模拟程序的功能。
> ./mylock -h
Usage : [mylock] [-h] [-p path][-s ip:port]
-h Show help
-p lock path
-s zookeeper server ip:port
For example:
mylock -s 172.17.0.36:2181 -p /Lock
模拟程序有3个选项。其中-s：为Zookeeper的服务器的ip：port.-p: 为锁的路径。
分别同时运行多个mylock程序，就可以看到各个程序之间是如何获取锁的了。
最后是完整的代码：
#include<stdio.h>
#include<string.h>
#include<unistd.h>
#include"zookeeper.h"
#include"zookeeper_log.h"
char g_host[512]= "172.17.0.36:2181";
char g_path[512]= "/Lock";
typedef struct Lock
{
char lockpath[1024];
char selfpath[1024];
}Lock;
void print_usage();
void get_option(int argc,const char* argv[]);
/**********unitl*********************/
void print_usage()
{
printf("Usage : [mylock] [-h] [-p path][-s ip:port] \n");
printf(" -h Show help\n");
printf(" -p lock path\n");
printf(" -s zookeeper server ip:port\n");
printf("For example:\n");
printf(" mylock -s172.17.0.36:2181 -p /Lock\n");
}
void get_option(int argc,const char* argv[])
{
extern char *optarg;
int optch;
int dem = 1;
const char optstring[] = "hp:s:";
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 's':
strncpy(g_host,optarg,sizeof(g_host));
break;
case 'p':
strncpy(g_path,optarg,sizeof(g_path));
break;
default:
break;
}
}
}
Lock *create_lock(zhandle_t *zkhandle,const char *path)
{
char path_buffer[512]={0};
int bufferlen = sizeof(path_buffer);
Lock * lock = NULL;
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){
char child_path[512];
sprintf(child_path,"%s/lock-",path);
ret = zoo_create(zkhandle,child_path,"1.0",strlen("1.0"),
&ZOO_OPEN_ACL_UNSAFE,ZOO_SEQUENCE|ZOO_EPHEMERAL,
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){
lock = (Lock *)malloc(sizeof(Lock));
strcpy(lock->lockpath,path);
strcpy(lock->selfpath,path_buffer);
}
return lock;
}
int try_lock(zhandle_t *zkhandle,Lock *lock)
{
struct String_vector children;
int i = 0;
int ret = zoo_get_children(zkhandle,lock->lockpath,0,&children);
if(ret != ZOK){
fprintf(stderr,"error when get children of path %s\n",lock->lockpath);
ret = -1;
}else{
char *myseq = rindex(lock->selfpath,'/');
if (myseq != NULL) myseq += 1;
ret = 1;
for(i = 0; i < children.count; ++i){
if(strcmp(children.data[i],myseq) < 0){
ret = 0;
break;
}
}
for(i = 0; i < children.count; ++i){
free(children.data[i]);
children.data[i] = NULL;
}
}
return ret;
}
Lock *lock(zhandle_t *zkhandle,const char *path)
{
Lock *lock = create_lock(zkhandle,path);
if(lock != NULL){
while(try_lock(zkhandle,lock) == 0){
sleep(1);
}
}else{
fprintf(stderr,"error when create lock %s.\n",path);
}
return lock;
}
int unlock(zhandle_t *zkhandle,Lock * *lock)
{
if(*lock){
int ret = zoo_delete(zkhandle,(*lock)->selfpath,-1);
if(ret != ZOK){
fprintf(stderr,"error when release lock %s.\n",(*lock)->selfpath);
}
free(*lock);
*lock = NULL;
return ret;
}
return ZOK;
}
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,NULL, timeout, 0, (char *)"lock Test", 0);
if (zkhandle ==NULL)
{
fprintf(stderr, "Error when connecting to zookeeper servers...\n");
exit(EXIT_FAILURE);
}
int ret = zoo_exists(zkhandle,g_path,0,NULL);
if(ret != ZOK){
ret = zoo_create(zkhandle,g_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",g_path);
}else{
printf("create path %s successfully!\n",g_path);
}
}
if(ret == ZOK ){
Lock *mylock = lock(zkhandle,g_path);
if(mylock){
printf("get lock of %s.\n",g_path);
printf("self path is %s.\n",mylock->selfpath);
printf("do something....\n");
getchar();
unlock(zkhandle,&mylock);
}
}
zookeeper_close(zkhandle);
return 0;
}