Zookeeper C API 指南四(C API 概览)

上一节《Zookeeper C API 指南三(回调函数)》重点讲了 Zookeeper C API 中各种回调函数的原型，本节将切入正题，正式讲解 Zookeeper C API。相信大家读完本文后应该对 Zookeeper C API 的使用有一个比较清晰的认识。

Zookeeper C API 概览

Zookeeper C API 很规范，接口很容易记忆，大部分接口均以 zoo_ 开头，只有少量接口以 zookeeper_ 开头，所有的 API 汇总如下：

void zoo_create_op_init(zoo_op_t * op, const char *path, const char *value,
                        int valuelen, const struct ACL_vector *acl,
                        int flags, char *path_buffer, int path_buffer_len);

void zoo_delete_op_init(zoo_op_t * op, const char *path, int version);

void zoo_set_op_init(zoo_op_t * op, const char *path,
                     const char *buffer, int buflen, int version,
                     struct Stat *stat);

void zoo_check_op_init(zoo_op_t * op, const char *path, int version);

ZOOAPI zhandle_t *zookeeper_init(const char *host, watcher_fn fn,
                                 int recv_timeout,
                                 const clientid_t * clientid,
                                 void *context, int flags);

ZOOAPI int zookeeper_close(zhandle_t * zh);

ZOOAPI const clientid_t *zoo_client_id(zhandle_t * zh);

ZOOAPI int zoo_recv_timeout(zhandle_t * zh);

ZOOAPI const void *zoo_get_context(zhandle_t * zh);

ZOOAPI void zoo_set_context(zhandle_t * zh, void *context);

ZOOAPI watcher_fn zoo_set_watcher(zhandle_t * zh, watcher_fn newFn);

ZOOAPI struct sockaddr *zookeeper_get_connected_host(zhandle_t * zh, struct sockaddr
                                                     *addr,
                                                     socklen_t * addr_len);

ZOOAPI int zookeeper_interest(zhandle_t * zh, int *fd, int *interest,
                              struct timeval *tv);

ZOOAPI int zookeeper_process(zhandle_t * zh, int events);

ZOOAPI int zoo_state(zhandle_t * zh);

ZOOAPI int zoo_acreate(zhandle_t * zh, const char *path,
                       const char *value, int valuelen,
                       const struct ACL_vector *acl, int flags,
                       string_completion_t completion, const void *data);

ZOOAPI int zoo_adelete(zhandle_t * zh, const char *path, int version,
                       void_completion_t completion, const void *data);

ZOOAPI int zoo_aexists(zhandle_t * zh, const char *path, int watch,
                       stat_completion_t completion, const void *data);

ZOOAPI int zoo_awexists(zhandle_t * zh, const char *path,
                        watcher_fn watcher, void *watcherCtx,
                        stat_completion_t completion, const void *data);

ZOOAPI int zoo_aget(zhandle_t * zh, const char *path, int watch,
                    data_completion_t completion, const void *data);

ZOOAPI int zoo_awget(zhandle_t * zh, const char *path,
                     watcher_fn watcher, void *watcherCtx,
                     data_completion_t completion, const void *data);

ZOOAPI int zoo_aset(zhandle_t * zh, const char *path,
                    const char *buffer, int buflen, int version,
                    stat_completion_t completion, const void *data);

ZOOAPI int zoo_aget_children(zhandle_t * zh, const char *path,
                             int watch,
                             strings_completion_t completion,
                             const void *data);

ZOOAPI int zoo_awget_children(zhandle_t * zh, const char *path,
                              watcher_fn watcher, void *watcherCtx,
                              strings_completion_t completion,
                              const void *data);

ZOOAPI int zoo_aget_children2(zhandle_t * zh, const char *path,
                              int watch,
                              strings_stat_completion_t completion,
                              const void *data);

ZOOAPI int zoo_awget_children2(zhandle_t * zh, const char *path,
                               watcher_fn watcher, void *watcherCtx,
                               strings_stat_completion_t completion,
                               const void *data);

ZOOAPI int zoo_async(zhandle_t * zh, const char *path,
                     string_completion_t completion, const void *data);

ZOOAPI int zoo_aget_acl(zhandle_t * zh, const char *path,
                        acl_completion_t completion, const void *data);

ZOOAPI int zoo_aset_acl(zhandle_t * zh, const char *path, int version,
                        struct ACL_vector *acl, void_completion_t,
                        const void *data);

ZOOAPI int zoo_amulti(zhandle_t * zh, int count, const zoo_op_t * ops,
                      zoo_op_result_t * results, void_completion_t,
                      const void *data);

ZOOAPI const char *zerror(int c);

ZOOAPI int zoo_add_auth(zhandle_t * zh, const char *scheme,
                        const char *cert, int certLen,
                        void_completion_t completion, const void *data);

ZOOAPI int is_unrecoverable(zhandle_t * zh);

ZOOAPI void zoo_set_debug_level(ZooLogLevel logLevel);

ZOOAPI void zoo_set_log_stream(FILE * logStream);

ZOOAPI void zoo_deterministic_conn_order(int yesOrNo);

ZOOAPI int zoo_create(zhandle_t * zh, const char *path,
                      const char *value, int valuelen,
                      const struct ACL_vector *acl, int flags,
                      char *path_buffer, int path_buffer_len);

ZOOAPI int zoo_delete(zhandle_t * zh, const char *path, int version);

ZOOAPI int zoo_exists(zhandle_t * zh, const char *path, int watch,
                      struct Stat *stat);

ZOOAPI int zoo_wexists(zhandle_t * zh, const char *path,
                       watcher_fn watcher, void *watcherCtx,
                       struct Stat *stat);

ZOOAPI int zoo_get(zhandle_t * zh, const char *path, int watch,
                   char *buffer, int *buffer_len, struct Stat *stat);

ZOOAPI int zoo_wget(zhandle_t * zh, const char *path,
                    watcher_fn watcher, void *watcherCtx,
                    char *buffer, int *buffer_len, struct Stat *stat);

ZOOAPI int zoo_set(zhandle_t * zh, const char *path,
                   const char *buffer, int buflen, int version);

ZOOAPI int zoo_set2(zhandle_t * zh, const char *path,
                    const char *buffer, int buflen, int version,
                    struct Stat *stat);

ZOOAPI int zoo_get_children(zhandle_t * zh, const char *path,
                            int watch, struct String_vector *strings);

ZOOAPI int zoo_wget_children(zhandle_t * zh, const char *path,
                             watcher_fn watcher, void *watcherCtx,
                             struct String_vector *strings);

ZOOAPI int zoo_get_children2(zhandle_t * zh, const char *path,
                             int watch, struct String_vector *strings,
                             struct Stat *stat);

ZOOAPI int zoo_wget_children2(zhandle_t * zh, const char *path,
                              watcher_fn watcher, void *watcherCtx,
                              struct String_vector *strings,
                              struct Stat *stat);

ZOOAPI int zoo_get_acl(zhandle_t * zh, const char *path,
                       struct ACL_vector *acl, struct Stat *stat);

ZOOAPI int zoo_set_acl(zhandle_t * zh, const char *path, int version,
                       const struct ACL_vector *acl);

ZOOAPI int zoo_multi(zhandle_t * zh, int count, const zoo_op_t * ops,
                     zoo_op_result_t * results);

除了基本的初始化、销毁 Zookeeper 句柄(zhandle)，设置日志等级、日志流以及一些具有辅助功能 API(zerror(), zoo_state()等) 外，Zookeeper C API 大部分接口可以根据同步和异步特性分为两类，同步接口以 zoo_* 开头，异步接口以则以 zoo_a* 开头。并且除了 zookeeper_init() 以及与 zoo_multi() 或 zoo_amulti() 批量操作相关的 zoo_op_t 初始化外，其他的 API 的第一个参数均为 zhandle_t * zh, 即 Zookeeper 句柄的指针。

Zookeeper C API 分类

• 初始化、销毁 Zookeeper 句柄

ZOOAPI zhandle_t *zookeeper_init(const char *host, watcher_fn fn,
                                 int recv_timeout,
                                 const clientid_t * clientid,
                                 void *context, int flags);

ZOOAPI int zookeeper_close(zhandle_t * zh);

• 辅助函数

// 设置日志等级、日志流
ZOOAPI void zoo_set_debug_level(ZooLogLevel logLevel);
ZOOAPI void zoo_set_log_stream(FILE * logStream);

ZOOAPI const clientid_t *zoo_client_id(zhandle_t * zh);

ZOOAPI int zoo_recv_timeout(zhandle_t * zh);

ZOOAPI const void *zoo_get_context(zhandle_t * zh);

ZOOAPI void zoo_set_context(zhandle_t * zh, void *context);

ZOOAPI watcher_fn zoo_set_watcher(zhandle_t * zh, watcher_fn newFn);

ZOOAPI struct sockaddr *zookeeper_get_connected_host(zhandle_t * zh, struct sockaddr
                                                     *addr,
                                                     socklen_t * addr_len);

ZOOAPI int zookeeper_interest(zhandle_t * zh, int *fd, int *interest,
                              struct timeval *tv);

ZOOAPI int zookeeper_process(zhandle_t * zh, int events);

ZOOAPI int zoo_state(zhandle_t * zh);

ZOOAPI const char *zerror(int c);

ZOOAPI int is_unrecoverable(zhandle_t * zh);

ZOOAPI void zoo_deterministic_conn_order(int yesOrNo);

• 与 zoo_multi() 和 zoo_amulti() 批量操作相关的 zoo_op_t 初始化

void zoo_create_op_init(zoo_op_t * op, const char *path, const char *value,
                        int valuelen, const struct ACL_vector *acl,
                        int flags, char *path_buffer, int path_buffer_len);

void zoo_delete_op_init(zoo_op_t * op, const char *path, int version);

void zoo_set_op_init(zoo_op_t * op, const char *path,
                     const char *buffer, int buflen, int version,
                     struct Stat *stat);

void zoo_check_op_init(zoo_op_t * op, const char *path, int version);

• Zookeeper C API 同步接口

ZOOAPI int zoo_add_auth(zhandle_t * zh, const char *scheme,
                        const char *cert, int certLen,
                        void_completion_t completion, const void *data);


ZOOAPI int zoo_create(zhandle_t * zh, const char *path,
                      const char *value, int valuelen,
                      const struct ACL_vector *acl, int flags,
                      char *path_buffer, int path_buffer_len);

ZOOAPI int zoo_delete(zhandle_t * zh, const char *path, int version);

ZOOAPI int zoo_exists(zhandle_t * zh, const char *path, int watch,
                      struct Stat *stat);

ZOOAPI int zoo_wexists(zhandle_t * zh, const char *path,
                       watcher_fn watcher, void *watcherCtx,
                       struct Stat *stat);

ZOOAPI int zoo_get(zhandle_t * zh, const char *path, int watch,
                   char *buffer, int *buffer_len, struct Stat *stat);

ZOOAPI int zoo_wget(zhandle_t * zh, const char *path,
                    watcher_fn watcher, void *watcherCtx,
                    char *buffer, int *buffer_len, struct Stat *stat);

ZOOAPI int zoo_set(zhandle_t * zh, const char *path,
                   const char *buffer, int buflen, int version);

ZOOAPI int zoo_set2(zhandle_t * zh, const char *path,
                    const char *buffer, int buflen, int version,
                    struct Stat *stat);

ZOOAPI int zoo_get_children(zhandle_t * zh, const char *path,
                            int watch, struct String_vector *strings);

ZOOAPI int zoo_wget_children(zhandle_t * zh, const char *path,
                             watcher_fn watcher, void *watcherCtx,
                             struct String_vector *strings);

ZOOAPI int zoo_get_children2(zhandle_t * zh, const char *path,
                             int watch, struct String_vector *strings,
                             struct Stat *stat);

ZOOAPI int zoo_wget_children2(zhandle_t * zh, const char *path,
                              watcher_fn watcher, void *watcherCtx,
                              struct String_vector *strings,
                              struct Stat *stat);

ZOOAPI int zoo_get_acl(zhandle_t * zh, const char *path,
                       struct ACL_vector *acl, struct Stat *stat);

ZOOAPI int zoo_set_acl(zhandle_t * zh, const char *path, int version,
                       const struct ACL_vector *acl);

ZOOAPI int zoo_multi(zhandle_t * zh, int count, const zoo_op_t * ops,
                     zoo_op_result_t * results);

• Zookeeper C API 异步接口

ZOOAPI int zoo_acreate(zhandle_t * zh, const char *path,
                       const char *value, int valuelen,
                       const struct ACL_vector *acl, int flags,
                       string_completion_t completion, const void *data);

ZOOAPI int zoo_adelete(zhandle_t * zh, const char *path, int version,
                       void_completion_t completion, const void *data);

ZOOAPI int zoo_aexists(zhandle_t * zh, const char *path, int watch,
                       stat_completion_t completion, const void *data);

ZOOAPI int zoo_awexists(zhandle_t * zh, const char *path,
                        watcher_fn watcher, void *watcherCtx,
                        stat_completion_t completion, const void *data);

ZOOAPI int zoo_aget(zhandle_t * zh, const char *path, int watch,
                    data_completion_t completion, const void *data);

ZOOAPI int zoo_awget(zhandle_t * zh, const char *path,
                     watcher_fn watcher, void *watcherCtx,
                     data_completion_t completion, const void *data);

ZOOAPI int zoo_aset(zhandle_t * zh, const char *path,
                    const char *buffer, int buflen, int version,
                    stat_completion_t completion, const void *data);

ZOOAPI int zoo_aget_children(zhandle_t * zh, const char *path,
                             int watch,
                             strings_completion_t completion,
                             const void *data);

ZOOAPI int zoo_awget_children(zhandle_t * zh, const char *path,
                              watcher_fn watcher, void *watcherCtx,
                              strings_completion_t completion,
                              const void *data);

ZOOAPI int zoo_aget_children2(zhandle_t * zh, const char *path,
                              int watch,
                              strings_stat_completion_t completion,
                              const void *data);

ZOOAPI int zoo_awget_children2(zhandle_t * zh, const char *path,
                               watcher_fn watcher, void *watcherCtx,
                               strings_stat_completion_t completion,
                               const void *data);

ZOOAPI int zoo_async(zhandle_t * zh, const char *path,
                     string_completion_t completion, const void *data);

ZOOAPI int zoo_aget_acl(zhandle_t * zh, const char *path,
                        acl_completion_t completion, const void *data);

ZOOAPI int zoo_aset_acl(zhandle_t * zh, const char *path, int version,
                        struct ACL_vector *acl, void_completion_t,
                        const void *data);

ZOOAPI int zoo_amulti(zhandle_t * zh, int count, const zoo_op_t * ops,
                      zoo_op_result_t * results, void_completion_t,
                      const void *data);

 Zookeeper C API 初体验

有了上面的介绍，下面我们来看一看如何使简单地使用 Zookeeper C API 吧。

在使用 Zookeeper C API 时应注意：

• 头文件包含 #include <zookeeper/zookeeper.h>
• 如果你需要编译多线程版本客户端程序，请添加编译选项 -DTHREADED，同时链接时应链接 zookeeper_mt 库；如果你需要编译单线程客户端程序，请不要添加编译选项 -DTHREADED，同时链接时应链接 zookeeper_st 库。

一个基本的程序如下(更详细的例子可以参看 src/c/src/cli.c)：

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

void zktest_watcher_g(zhandle_t* zh, int type, int state,
        const char* path, void* watcherCtx)
{
    printf("Something happened.\n");
    printf("type: %d\n", type);
    printf("state: %d\n", state);
    printf("path: %s\n", path);
    printf("watcherCtx: %s\n", (char *)watcherCtx);
}

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

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

void zktest_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);
    }
}

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,
            zktest_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, "/xyz", "hello", 5,
           &ZOO_OPEN_ACL_UNSAFE, 0 /* ZOO_SEQUENCE */,
           zktest_string_completion, "acreate");
    if (ret) {
        fprintf(stderr, "Error %d for %s\n", ret, "acreate");
        exit(EXIT_FAILURE);
    }
    ret = 0;

    ret = zoo_aexists(zkhandle, "/xyz", 1, zktest_stat_completion, "aexists");
    if (ret) {
        fprintf(stderr, "Error %d for %s\n", ret, "aexists");
        exit(EXIT_FAILURE);
    }
    ret = 0;
    // Wait for asynchronous zookeeper call done.
    getchar();

    ret = zoo_adelete(zkhandle, "/xyz", -1, zktest_void_completion, "adelete");
    if (ret) {
        fprintf(stderr, "Error %d for %s\n", ret, "adelete");
        exit(EXIT_FAILURE);
    }
    // Wait for asynchronous zookeeper call done.
    getchar();

    zookeeper_close(zkhandle);
}

下面简单讲讲这个程序的结构：

• 首先声明 host, timeout 变量。

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;

其中 host 字符串格式为逗号隔开的 IP:PORT对，可以是 Zookeeper 集群中的全部或部分 Zookeeper 实例的 IP:PORT对，我们在第一讲《准备工作》中介绍了如何部署一个伪分布式的集群，上述的 host 就是这些 zookeeper 实例的IP:PORT对。

另外 timeout 是 Zookeeper 客户端连接服务器的超时时间，单位为毫秒，timeout = 30000 说明如果 30 秒内客户端没有连接上 Zookeeper 服务则表示连接超时。

• 设置日志等级。

zoo_set_debug_level(ZOO_LOG_LEVEL_WARN);

• 初始化 Zookeeper 句柄(zhandle_t)。

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

初始化 Zookeeper 句柄 zookeeper_init() 函数原型如下：

ZOOAPI zhandle_t *zookeeper_init(const char *host, watcher_fn fn,
                                 int recv_timeout,
                                 const clientid_t * clientid,
                                 void *context, int flags);

各个参数解释如下：

host	逗号隔开的 host:port 对, 每个代表一个 zk server, 例如: "127.0.0.1:3000,127.0.0.1:3001,127.0.0.1:3002"
fn	全局的监视器回调函数，当发生事件通知时，该函数会被调用。
clientid	客 户端尝试重连的先前会话的ID，如果不需要重连先前的会话，则设置为 0。客户端可以通过调用 zoo_client_id来访问一个已经连接上的并且有效的会话ID，如果clientid对应的会话超时，或者由于某种原因 clientid变为无效了，那么zookeeper_init 将返回一个非法的 zhandle_t， 通过 zhandle_t 的状态可以获知 zookeeper_init 调用失败的原因。 (通常为 ZOO_EXPIRED_SESSION_STATE).
context	与 zhandle_t 实例相关联的“上下文对象”（可以通过该参数为 zhandle_t 传入自定义类型的数据），应用程序可以通过 zoo_get_context 访问它（例如在监视器回调函数中），当然 zookeeper 内部没有用到该参数，所以 context 可以设置为 NULL。
flags	目前为保留参数，设置为 0。

• 创建一个 znode 节点。

    // struct ACL ALL_ACL[] = {{ZOO_PERM_ALL, ZOO_ANYONE_ID_UNSAFE}};
    // struct ACL_vector ALL_PERMS = {1, ALL_ACL};
    int ret = zoo_acreate(zkhandle, "/xyz", "hello", 5,
           &ZOO_OPEN_ACL_UNSAFE, 0 /* ZOO_SEQUENCE */,
           zktest_string_completion, "acreate");
    if (ret) {
        fprintf(stderr, "Error %d for %s\n", ret, "acreate");
        exit(EXIT_FAILURE);
    }

这里采用异步方式创建 znode 节点，zoo_acreate() 函数原型为：

ZOOAPI int zoo_acreate(zhandle_t * zh, const char *path,
                       const char *value, int valuelen,
                       const struct ACL_vector *acl, int flags,
                       string_completion_t completion, const void *data);

各个参数解释如下：

zh	zookeeper_init() 返回的 zookeeper 句柄。
path	节点路径。
value	该节点保存的数据。
valuelen	该节点保存数据的大小。
acl	该节点初始 ACL,ACL 不能为null 或空。
flags	该参数可以设置为 0，或者创建标识符 ZOO_EPHEMERAL, ZOO_SEQUENCE 的组合或(OR)。
completion	当创建节点请求完成时会调用该函数，该函数原型详见第三讲《回调函数》一节。同时传递给completion的 rc参数为: ZOK 操作完成；ZNONODE 父节点不存在；ZNODEEXISTS 节点已存在；ZNOAUTH 客户端没有权限创建节点。ZNOCHILDRENFOREPHEMERALS 临时节点不能创建子节点。
data	completino函数被调用时，传递给 completion 的数据。

• 调用 exists() 函数，设置监视器。

    ret = zoo_aexists(zkhandle, "/xyz", 1, zktest_stat_completion, "aexists");
    if (ret) {
        fprintf(stderr, "Error %d for %s\n", ret, "aexists");
        exit(EXIT_FAILURE);
    }

• 调用 delete 函数，删除 znode 节点。

    ret = zoo_adelete(zkhandle, "/xyz", -1, zktest_void_completion, "adelete");
    if (ret) {
        fprintf(stderr, "Error %d for %s\n", ret, "adelete");
        exit(EXIT_FAILURE);
    }

• 销毁 zookeeper 句柄

zookeeper_close(zkhandle);

好了，至此本文大致讲完了 Zookeeper C API 的分类和几个基本函数的用法。之所以为 Zookeeper C API 分类是方便记忆，开发者可以迅速找到自己需要的 API；另外，本文还讲了几个基本函数的使用方法，包括 zookeeper_init(),zoo_acreate(), zoo_aexists(), zoo_adelete() 以及 zookeeper_close()。相信大家对 Zookeeper C API 也有了一个大致的了解，第五讲我会给大家介绍 Zookeeper C API 中的同步调用的函数(即以 zoo_* 开头的函数)，然后第六讲给大家介绍 Zookeeper C API 中的异步调用的函数(即以 zoo_a* 开头的函数)。