Zookeeper C API 指南八(Zookeeper C API 应用示例)

前面七讲我们基本上介绍完了 Zookeeper C API 的所有内容，本文将结合一个小例子讲讲如何在你的实际项目中使用 Zookeeper 服务。

设想如下场景：

假设程序 A 需要 7* 24 小时在线对外提供服务，但是 A 程序在生产环境下总是不稳定，时常崩溃，不过幸运的是解决方案很简单，在 A 程序崩溃以后只需要重启它就可以了。当然如此简单的问题你可以提出多种解决方案，比方说自己实现一个服务程序，每隔一定时间去轮询 A 的状态，如果发现 A 崩溃了，立即重启它，并向管理人员报告问题。不过我们并不打算这么做，毕竟本文主题是讲 Zookeeper C API 的应用，所以我们采用 Zookeeper 服务来解决该问题。

若采用 Zookeeper 服务可以按照如下方案解决问题，程序 A 在启动时创建一个临时(ZOO_EPHEMERAL) znode 节点 /A，然后按照正常流程对外提供服务。另外监控程序对 /A 节点设置监视，当 /A 节点消失(说明 A 程序已经崩溃)时，重启 A 程序。假设 A 的名称是 QueryServer，即对外提供查询服务的程序，具体提供什么查询服务由应用自身决定，我们这里只是简单地模拟一下。QueryServer 在启动时创建一个 /QueryServer 的临时节点(ZOO_EPHEMERAL)，然后，程序 QueryServerd 监控 /QueryServer 节点，当 /QueryServer 节点消失(说明 A 程序已经崩溃)时，重启 QueryServer 程序。

下面是 QueryServer 的实现代码：

/*
 * =============================================================================
 *
 *       Filename:  QueryServer.c
 *
 *    Description:  QueryServer
 *
 *        Created:  02/15/2013 08:48:49 PM
 *
 *         Author:  Fu Haiping (forhappy), haipingf@gmail.com
 *        Company:  ICT ( Institute Of Computing Technology, CAS )
 *
 * =============================================================================
 */
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <zookeeper/zookeeper.h>
#include <zookeeper/zookeeper_log.h>

void QueryServer_watcher_g(zhandle_t* zh, int type, int state,
        const char* path, void* watcherCtx)
{
    if (type == ZOO_SESSION_EVENT) {
        if (state == ZOO_CONNECTED_STATE) {
            printf("[[[QueryServer]]] Connected to zookeeper service successfully!\n");
        } else if (state == ZOO_EXPIRED_SESSION_STATE) { 
            printf("Zookeeper session expired!\n");
        }
    }  
}

void QueryServer_string_completion(int rc, const char *name, const void *data)
{
    fprintf(stderr, "[%s]: rc = %d\n", (char*)(data==0?"null":data), rc);
    if (!rc) {
        fprintf(stderr, "\tname = %s\n", name);
    }
}

void QueryServer_accept_query()
{
    printf("QueryServer is running...\n");
}

int main(int argc, const char *argv[])
{
    const char* host = "127.0.0.1:2181,127.0.0.1:2182,"
        "127.0.0.1:2183,127.0.0.1:2184,127.0.0.1:2185";
    int timeout = 30000;
    
    zoo_set_debug_level(ZOO_LOG_LEVEL_WARN);
    zhandle_t* zkhandle = zookeeper_init(host,
            QueryServer_watcher_g, timeout, 0, "hello zookeeper.", 0);
    if (zkhandle == NULL) {
        fprintf(stderr, "Error when connecting to zookeeper servers...\n");
        exit(EXIT_FAILURE);
    }

    // struct ACL ALL_ACL[] = {{ZOO_PERM_ALL, ZOO_ANYONE_ID_UNSAFE}};
    // struct ACL_vector ALL_PERMS = {1, ALL_ACL};
    int ret = zoo_acreate(zkhandle, "/QueryServer", "alive", 5,
           &ZOO_OPEN_ACL_UNSAFE, ZOO_EPHEMERAL,
           QueryServer_string_completion, "zoo_acreate");
    if (ret) {
        fprintf(stderr, "Error %d for %s\n", ret, "acreate");
        exit(EXIT_FAILURE);
    }

    do {
        // 模拟 QueryServer 对外提供服务.
        // 为了简单起见, 我们在此调用一个简单的函数来模拟 QueryServer.
        // 然后休眠 5 秒，程序主动退出(即假设此时已经崩溃).
        QueryServer_accept_query();
        sleep(5);
    } while(false);

    zookeeper_close(zkhandle);
}

Makefile如下：

all:QueryServer

QueryServer:QueryServer.o
    gcc -L/usr/local/lib/ -lzookeeper_mt -o $@ $^ 

QueryServer.o:QueryServer.c
    gcc -DTHREADED -I/usr/local/include/zookeeper -o $@ -c $^

.PHONY:clean

clean:
    rm QueryServer.o QueryServer

QueryServerd 代码如下：

/*
 * =============================================================================
 *
 *       Filename:  QueryServerd.c
 *
 *    Description:  QueryServer daemon using zookeeper. 
 *
 *        Created:  02/15/2013 08:48:49 PM
 *
 *         Author:  Fu Haiping (forhappy), haipingf@gmail.com
 *        Company:  ICT ( Institute Of Computing Technology, CAS )
 *
 * =============================================================================
 */
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <zookeeper/zookeeper.h>
#include <zookeeper/zookeeper_log.h>

void QueryServerd_watcher_global(zhandle_t * zh, int type, int state,
                            const char *path, void *watcherCtx);
static void QueryServerd_dump_stat(const struct Stat *stat);
void QueryServerd_stat_completion(int rc, const struct Stat *stat,
                             const void *data);
void QueryServerd_watcher_awexists(zhandle_t *zh, int type, int state,
                              const char *path, void *watcherCtx);
static void QueryServerd_awexists(zhandle_t *zh);

void
QueryServerd_watcher_global(zhandle_t * zh, int type, int state,
                            const char *path, void *watcherCtx)
{
    if (type == ZOO_SESSION_EVENT) {
        if (state == ZOO_CONNECTED_STATE) {
            printf("Connected to zookeeper service successfully!\n");
        } else if (state == ZOO_EXPIRED_SESSION_STATE) { 
            printf("Zookeeper session expired!\n");
        }
    }
}

static void
QueryServerd_dump_stat(const struct Stat *stat)
{
    char tctimes[40];
    char tmtimes[40];
    time_t tctime;
    time_t tmtime;

    if (!stat) {
        fprintf(stderr, "null\n");
        return;
    }
    tctime = stat->ctime / 1000;
    tmtime = stat->mtime / 1000;

    ctime_r(&tmtime, tmtimes);
    ctime_r(&tctime, tctimes);

    fprintf(stderr, "\tctime = %s\tczxid=%llx\n"
            "\tmtime=%s\tmzxid=%llx\n"
            "\tversion=%x\taversion=%x\n"
            "\tephemeralOwner = %llx\n",
            tctimes, stat->czxid,
            tmtimes, stat->mzxid,
            (unsigned int) stat->version, (unsigned int) stat->aversion,
            stat->ephemeralOwner);
}

void
QueryServerd_stat_completion(int rc, const struct Stat *stat,
                             const void *data)
{
    // fprintf(stderr, "%s: rc = %d Stat:\n", (char *) data, rc);
    // QueryServerd_dump_stat(stat);
}

void
QueryServerd_watcher_awexists(zhandle_t *zh, int type, int state,
                              const char *path, void *watcherCtx)
{
    if (state == ZOO_CONNECTED_STATE) {
        if (type == ZOO_DELETED_EVENT) {
            printf("QueryServer gone away, restart now...\n");
            // re-exists and set watch on /QueryServer again.
            QueryServerd_awexists(zh);
            pid_t pid = fork();
            if (pid < 0) {
                fprintf(stderr, "Error when doing fork.\n");
                exit(EXIT_FAILURE);
            }
            if (pid == 0) { /* child process */
                // 重启 QueryServer 服务.
                execl("/tmp/QueryServer/QueryServer", "QueryServer", NULL);
                exit(EXIT_SUCCESS);
            }
            sleep(1); /* sleep 1 second for purpose. */
        } else if (type == ZOO_CREATED_EVENT) {
            printf("QueryServer started...\n");
        }
    }

    // re-exists and set watch on /QueryServer again.
    QueryServerd_awexists(zh);
}

static void
QueryServerd_awexists(zhandle_t *zh)
{
    int ret =
        zoo_awexists(zh, "/QueryServer",
                     QueryServerd_watcher_awexists,
                     "QueryServerd_awexists.",
                     QueryServerd_stat_completion,
                     "zoo_awexists");
    if (ret) {
        fprintf(stderr, "Error %d for %s\n", ret, "aexists");
        exit(EXIT_FAILURE);
    }
}

int
main(int argc, const char *argv[])
{
    const char *host = "127.0.0.1:2181,127.0.0.1:2182,"
        "127.0.0.1:2183,127.0.0.1:2184,127.0.0.1:2185";
    int timeout = 30000;

    zoo_set_debug_level(ZOO_LOG_LEVEL_WARN);
    zhandle_t *zkhandle = zookeeper_init(host,
                                         QueryServerd_watcher_global,
                                         timeout,
                                         0, "QueryServerd", 0);
    if (zkhandle == NULL) {
        fprintf(stderr, "Error when connecting to zookeeper servers...\n");
        exit(EXIT_FAILURE);
    }

    QueryServerd_awexists(zkhandle);
    // Wait for asynchronous zookeeper call done.
    getchar();

    zookeeper_close(zkhandle);

    return 0;
}

Makefile 如下：

all:QueryServerd

QueryServerd:QueryServerd.o
    gcc -L/usr/local/lib/ -lzookeeper_mt -o $@ $^ 

QueryServerd.o:QueryServerd.c
    gcc -g -DTHREADED -I/usr/local/include/zookeeper -o $@ -c $^

.PHONY:clean

clean:
    rm QueryServerd.o QueryServerd

首先执行 QueryServerd，

forhappy@haiping-ict:/tmp/QueryServerd$ ./QueryServerd 
Connected to zookeeper service successfully!

然后执行 QueryServer，

forhappy@haiping-ict:/tmp/QueryServer$ ./QueryServer 
QueryServer is running...
[[[QueryServer]]] Connected to zookeeper service successfully!
[zoo_acreate]: rc = 0
    name = /QueryServer

可见 Queryerver 创建了 /QueryServer 节点，5 秒后 QueryServer 模拟程序崩溃而退出，那么此时在 QueryServerd 端输出如下：

Connected to zookeeper service successfully!
QueryServer started... # QueryServerd 感知到 QueryServer 已正常启动.
QueryServer gone away, restart now... # 5 秒钟后，QueryServer 崩溃，QueryServerd 准备重启 QueryServer.
QueryServer is running... # QueryServer 正在运行，以下 3 行是 QueryServer 输出结果。
[[[QueryServer]]] Connected to zookeeper service successfully!
[zoo_acreate]: rc = 0
    name = /QueryServer
QueryServer started... # QueryServerd 感知到 QueryServer 已正常启动.
QueryServer gone away, restart now...# 又过了 5 秒钟后，QueryServer 崩溃，QueryServerd 准备重启 QueryServer.
QueryServer is running... # QueryServer 再次运行，以下 3 行是 QueryServer 输出结果。
[[[QueryServer]]] Connected to zookeeper service successfully!
[zoo_acreate]: rc = 0
    name = /QueryServer
QueryServer started... # QueryServerd 再次感知到 QueryServer 已正常启动，如此反复.
QueryServer gone away, restart now...
QueryServer is running...
[[[QueryServer]]] Connected to zookeeper service successfully!
[zoo_acreate]: rc = 0
    name = /QueryServer
QueryServer started...

即 QueryServer 每 5 秒钟崩溃一次，然后又被 QueryServerd 重启，模拟了上面的应用场景。

好了 Zookeeper C API 的应用小示例讲完了，可能应用场景选取的不好，不过大致可以一些说明问题吧，如果你想看 Zookeeper 更贴近现实的应用场景，可以参考淘宝的一篇文章《ZooKeeper典型应用场景一览》和 IBM developerWorks 的一篇博文《分布式服务框架 Zookeeper -- 管理分布式环境中的数据》。