1.基本介绍

按照ZooKeeper典型应用场景一览里的说法，分布式队列有两种，一种是常规的先进先出队列，另一种是要等到队列成员聚齐之后的才统一按序执行。

第二种队列可以先建立一个/queue，赋值为n，表达队列的大小。然后每个队列成员加入时，就判断是否达到队列要求的大小，如果是可以进行下一步动作，否则继续等待队列成员的加入。比较典型的情况是，当一个大的任务可能需要很多的子任务完成才能开始进行。

比如汇总账单的时候，就必须先将用户的消费数据，积分数据等都统计完成后才能开始。汇总账单的程序建立一个队列/Queue,赋值为2，然后分别统计消费数据和积分数据的程序当完成任务时就往/Queue下创建一个临时节点。而汇总账单程序监测到/Queue的子节点个数为2时，就可以开始执行任务了。

实际上，我们也可以先建立一个数目为2的子节点。当一个子任务完成的时候，就删除一个子节点，当所有子节点都被删除的时候，主任务就可以开始执行了。这个过程可以形象的理解为拆除屏障。因此这种队列还有一个专门的词语描述，叫做屏障（barrier）。

2.场景分析

讲了那么多的关于屏障的认识，但是并不打算就去实现它，并且Zookeeper的官方文档也有相关的知识。这次的主要目标是常规的FIFO队列。我将实现队列的两个主要操作：push和pop。

1). int push(zhandle_t *zkhandle,const char *path,char *element)

• zkhandle为zookeeper_init初始化后的句柄
• path为队列的路径
• element为要压入队列的内容

2). int pop(zhandle_t *zkhandle,const char *path,char *element_buffer,int *buffer_len)

• zkhandle为zookeeper_init初始化后的句柄
• path为队列的路径
• element_buffer为要弹出的缓冲区
• buffer_len为指向缓冲区的大小的指针

简单来说，假设队列的路径为/Queue,push就是就是创建一个临时有序的/Queue/queue-节点。pop就是取出/Queue/下序列号最小的节点。
我们知道在C++中stl里有一个queue的类，实现了push,pop等操作，然而它是非线程安全的，即多个线程同时push/pop的时候可能会出现错误。而由于ZooKeeper保证了创建节点和删除节点的一致性，因此可以说利用Zookeeper实现的队列是进程安全的。

3. 场景实践

来看push和pop的具体实现。push的实现很简单，就是在{path}下创建一个有序的{path}/queue-子节点.

int push(zhandle_t *zkhandle,const char *path,char *element)
{
    char child_path[512] = {0};
    char path_buffer[512] = {0};
    int bufferlen = sizeof(path_buffer);

    sprintf(child_path,"%s/queue-",path);
    int ret = zoo_create(zkhandle,child_path,element,strlen(element),  
                     &ZOO_OPEN_ACL_UNSAFE,ZOO_SEQUENCE,  
                     path_buffer,bufferlen);  
    if(ret != ZOK){
        fprintf(stderr,"failed to create the path %s!\n",path);
    }else{
        printf("create path %s successfully!\n",path);
    }

    return ret;
}

pop的功能则是取出{path}下序号最小的子节点，如果没有子节点，则返回-1.

int pop(zhandle_t *zkhandle,const char *path,char *element,int *len)
{
    int i = 0;
    struct String_vector children;
    int ret = zoo_get_children(zkhandle,path,0,&children);


    if(ret != ZOK){
        fprintf(stderr,"failed to create the path %s!\n",path);
    }else if (children.count == 0){
        strcpy(element,"");
        *len = 0;
        ret = -1;
    }else{
        char *min = children.data[0];
        for(i = 0; i < children.count; ++i){
            printf("%s:%s\n",min,children.data[i]);
            if(strcmp(min,children.data[i]) > 0){
                min = children.data[i];
            }
        }
        if(min != NULL){
            char child_path[512]={0};
            sprintf(child_path,"%s/%s",path,min);
            ret = zoo_get(zkhandle,child_path,0,element,len,NULL);

            if(ret != ZOK){
                fprintf(stderr,"failed to get data of the path %s!\n",child_path);
            }else{
                ret = zoo_delete(zkhandle,child_path, -1);

                if(ret != ZOK){
                    fprintf(stderr,"failed to delete the path %s!\n",child_path);
                }
            }
        }
    }

    for(i = 0; i < children.count; ++i){
        free(children.data[i]);
        children.data[i] = NULL;
    }


    return ret;
}

最后，再来看看模拟队列操作的程序。和其他程序类似，它的选项有

• -p:指定队列的路径
• -m:指定操作是push还是pop
• -v:只在push时有用，用与指定要push的元素的值
• -s:指定Zookeeper的服务器的ip：port.

如：

向队列/Queue中压人一个元素，元素的值为"Hello":

>myqueue -s 172.17.0.36:2181 -p /Queue -m push -v Hello

将队列/Queue弹出一个元素

>myqueue -s 172.17.0.36:2181 -p /Queue -m pop

最后附上完整的源代码：

#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]= "/Queue";
char g_value[512]="msg";
enum MODE{PUSH_MODE,POP_MODE} g_mode;

void print_usage();
void get_option(int argc,const char* argv[]);

/**********unitl*********************/  
void print_usage()
{
    printf("Usage : [myqueue] [-h] [-m mode] [-p path ] [-v value][-s ip:port] \n");
    printf("        -h Show help\n");
    printf("        -p Queue path\n");
    printf("        -m mode:push or pop\n");
    printf("        -v the value you want to push\n");
    printf("        -s zookeeper server ip:port\n");
    printf("For example:\n");
    printf("    push the message \"Hello\" into the queue Queue:\n");
    printf("        >myqueue -s172.17.0.36:2181 -p /Queue -m push -v Hello\n");
    printf("    pop one message from the queue Queue:\n");
    printf("        >myqueue -s172.17.0.36:2181 -p /Queue -m pop\n");
}

void get_option(int argc,const char* argv[])
{
    extern char    *optarg;
    int            optch;
    int            dem = 1;
    const char    optstring[] = "hv:m:p:s:";


    g_mode = PUSH_MODE;
    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':
            if(strcasecmp(optarg,"push")==0){
                g_mode = PUSH_MODE;
            }else{
                g_mode = POP_MODE;
            }
            break;
        case 's':
            strncpy(g_host,optarg,sizeof(g_host));
            break;
        case 'p':
            strncpy(g_path,optarg,sizeof(g_path));
            break;
        case 'v':
            strncpy(g_value,optarg,sizeof(g_value));
            break;
        default:
            break;
        }
    }
} 

int push(zhandle_t *zkhandle,const char *path,char *element)
{
    char child_path[512] = {0};
    char path_buffer[512] = {0};
    int bufferlen = sizeof(path_buffer);

    sprintf(child_path,"%s/queue-",path);
    int ret = zoo_create(zkhandle,child_path,element,strlen(element),  
                     &ZOO_OPEN_ACL_UNSAFE,ZOO_SEQUENCE,  
                     path_buffer,bufferlen);  
    if(ret != ZOK){
        fprintf(stderr,"failed to create the path %s!\n",path);
    }else{
        printf("create path %s successfully!\n",path);
    }

    return ret;
}

int pop(zhandle_t *zkhandle,const char *path,char *element,int *len)
{
    int i = 0;
    struct String_vector children;
    int ret = zoo_get_children(zkhandle,path,0,&children);


    if(ret != ZOK){
        fprintf(stderr,"failed to create the path %s!\n",path);
    }else if (children.count == 0){
        strcpy(element,"");
        *len = 0;
        ret = -1;
    }else{
        char *min = children.data[0];
        for(i = 0; i < children.count; ++i){
            printf("%s:%s\n",min,children.data[i]);
            if(strcmp(min,children.data[i]) > 0){
                min = children.data[i];
            }
        }
        if(min != NULL){
            char child_path[512]={0};
            sprintf(child_path,"%s/%s",path,min);
            ret = zoo_get(zkhandle,child_path,0,element,len,NULL);

            if(ret != ZOK){
                fprintf(stderr,"failed to get data of the path %s!\n",child_path);
            }else{
                ret = zoo_delete(zkhandle,child_path, -1);

                if(ret != ZOK){
                    fprintf(stderr,"failed to delete the path %s!\n",child_path);
                }
            }
        }
    }

    for(i = 0; i < children.count; ++i){
        free(children.data[i]);
        children.data[i] = NULL;
    }


    return ret;
}

int front(zhandle_t *zkhandle,char *path,char *element,int *len)
{
    int i = 0;
    struct String_vector children;
    int ret = zoo_get_children(zkhandle,path,0,&children);

    if(ret != ZOK){
        fprintf(stderr,"failed to create the path %s!\n",path);
    }else if(children.count == 0){
        strcpy(element,"");
        *len = 0;
        ret = -1;
    }else{
        char *min = NULL;
        for(i = 0; i < children.count; ++i){
            if(strcmp(min,children.data[i]) > 0){
                min = children.data[i];
            }
        }
        if(min != NULL){
            char child_path[512]={0};
            sprintf(child_path,"%s/%s",path,min);
            ret = zoo_get(zkhandle,child_path,0,element,len,NULL);

            if(ret != ZOK){
                fprintf(stderr,"failed to get data of the path %s!\n",child_path);
            }
        }
    }

    for(i = 0; i < children.count; ++i){
        free(children.data[i]);
        children.data[i] = NULL;
    }

    return ret;

}


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(g_mode == PUSH_MODE){
        push(zkhandle,g_path,g_value); 
        printf("push:%s\n",g_value);
    }else{
        int len = sizeof(g_value);
        ret = pop(zkhandle,g_path,g_value,&len) ;

        if(ret == ZOK){
            printf("pop:%s\n",g_value);
        }else if( ret == -1){
            printf("queue is empty\n");
        }
    }



    zookeeper_close(zkhandle); 

    return 0;
}