上文那种实现管家模式的方法比较简单，client还是简单海盗模式中的，仅仅是用API重写了一下。我在测试机上运行了程序，处理10万条请求大约需要14秒的时间，这和代码也有一些关系，因为复制消息帧的时间浪费了CPU处理时间。但真正的问题在于，我们总是逐个循环进行处理（round-trip），即发送-接收-发送-接收……ZMQ内部禁用了TCP发包优化算法（Nagle's algorithm），但逐个处理循环还是比较浪费。
理论归理论，还是需要由实践来检验。我们用一个简单的测试程序来看看逐个处理循环是否真的耗时。这个测试程序会发送一组消息，第一次它发一条收一条，第二次则一起发送再一起接收。两次结果应该是一样的，但速度截然不同。
tripping: Round-trip demonstrator in C
//
// Round-trip 模拟
//
// 本示例程序使用多线程的方式启动client、worker、以及代理，
// 当client处理完毕时会发送信号给主程序。
//
#include "czmq.h"
static void
client_task (void *args, zctx_t *ctx, void *pipe)
{
void *client = zsocket_new (ctx, ZMQ_DEALER);
zmq_setsockopt (client, ZMQ_IDENTITY, "C", 1);
zsocket_connect (client, "tcp://localhost:5555");
printf ("开始测试...\n");
zclock_sleep (100);
int requests;
int64_t start;
printf ("同步 round-trip 测试...\n");
start = zclock_time ();
for (requests = 0; requests < 10000; requests++) {
zstr_send (client, "hello");
char *reply = zstr_recv (client);
free (reply);
}
printf (" %d 次/秒\n",
(1000 * 10000) / (int) (zclock_time () - start));
printf ("异步 round-trip 测试...\n");
start = zclock_time ();
for (requests = 0; requests < 100000; requests++)
zstr_send (client, "hello");
for (requests = 0; requests < 100000; requests++) {
char *reply = zstr_recv (client);
free (reply);
}
printf (" %d 次/秒\n",
(1000 * 100000) / (int) (zclock_time () - start));
zstr_send (pipe, "完成");
}
static void *
worker_task (void *args)
{
zctx_t *ctx = zctx_new ();
void *worker = zsocket_new (ctx, ZMQ_DEALER);
zmq_setsockopt (worker, ZMQ_IDENTITY, "W", 1);
zsocket_connect (worker, "tcp://localhost:5556");
while (1) {
zmsg_t *msg = zmsg_recv (worker);
zmsg_send (&msg, worker);
}
zctx_destroy (&ctx);
return NULL;
}
static void *
broker_task (void *args)
{
// 准备上下文和套接字
zctx_t *ctx = zctx_new ();
void *frontend = zsocket_new (ctx, ZMQ_ROUTER);
void *backend = zsocket_new (ctx, ZMQ_ROUTER);
zsocket_bind (frontend, "tcp://*:5555");
zsocket_bind (backend, "tcp://*:5556");
// 初始化轮询对象
zmq_pollitem_t items [] = {
{ frontend, 0, ZMQ_POLLIN, 0 },
{ backend, 0, ZMQ_POLLIN, 0 }
};
while (1) {
int rc = zmq_poll (items, 2, -1);
if (rc == -1)
break; // 中断
if (items [0].revents & ZMQ_POLLIN) {
zmsg_t *msg = zmsg_recv (frontend);
zframe_t *address = zmsg_pop (msg);
zframe_destroy (&address);
zmsg_pushstr (msg, "W");
zmsg_send (&msg, backend);
}
if (items [1].revents & ZMQ_POLLIN) {
zmsg_t *msg = zmsg_recv (backend);
zframe_t *address = zmsg_pop (msg);
zframe_destroy (&address);
zmsg_pushstr (msg, "C");
zmsg_send (&msg, frontend);
}
}
zctx_destroy (&ctx);
return NULL;
}
int main (void)
{
// 创建线程
zctx_t *ctx = zctx_new ();
void *client = zthread_fork (ctx, client_task, NULL);
zthread_new (ctx, worker_task, NULL);
zthread_new (ctx, broker_task, NULL);
// 等待client端管道的信号
char *signal = zstr_recv (client);
free (signal);
zctx_destroy (&ctx);
return 0;
}
在我的开发环境中运行结果如下：
Setting up test...
Synchronous round-trip test...
9057 calls/second
Asynchronous round-trip test...
173010 calls/second
需要注意的是client在运行开始会暂停一段时间，这是因为在向ROUTER套接字发送消息时，若指定标识的套接字没有连接，那么ROUTER会直接丢弃该消息。这个示例中我们没有使用LRU算法，所以当worker连接速度稍慢时就有可能丢失数据，影响测试结果。
我们可以看到，逐个处理循环比异步处理要慢将近20倍，让我们把它应用到管家模式中去。
首先，让我们修改client的API，添加独立的发送和接收方法：
mdcli_t *mdcli_new (char *broker);
void mdcli_destroy (mdcli_t **self_p);
int mdcli_send (mdcli_t *self, char *service, zmsg_t **request_p);
zmsg_t *mdcli_recv (mdcli_t *self);
然后花很短的时间就能将同步的client API改造成异步的API：
mdcliapi2: Majordomo asynchronous client API in C
/* =====================================================================
mdcliapi2.c
Majordomo Protocol Client API (async version)
Implements the MDP/Worker spec at http://rfc.zeromq.org/spec:7.
---------------------------------------------------------------------
Copyright (c) 1991-2011 iMatix Corporation <www.imatix.com>
Copyright other contributors as noted in the AUTHORS file.
This file is part of the ZeroMQ Guide: http://zguide.zeromq.org
This is free software; you can redistribute it and/or modify it under
the terms of the GNU Lesser General Public License as published by
the Free Software Foundation; either version 3 of the License, or (at
your option) any later version.
This software is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with this program. If not, see
<http://www.gnu.org/licenses/>.
=====================================================================
*/
#include "mdcliapi2.h"
// 类结构
// 使用成员函数访问属性
struct _mdcli_t {
zctx_t *ctx; // 上下文
char *broker;
void *client; // 连接至代理的套接字
int verbose; // 在标准输出打印运行状态
int timeout; // 请求超时时间
};
// ---------------------------------------------------------------------
// 连接或重连代理
void s_mdcli_connect_to_broker (mdcli_t *self)
{
if (self->client)
zsocket_destroy (self->ctx, self->client);
self->client = zsocket_new (self->ctx, ZMQ_DEALER);
zmq_connect (self->client, self->broker);
if (self->verbose)
zclock_log ("I: 正在连接代理 %s...", self->broker);
}
// ---------------------------------------------------------------------
// 构造函数
mdcli_t *
mdcli_new (char *broker, int verbose)
{
assert (broker);
mdcli_t *self = (mdcli_t *) zmalloc (sizeof (mdcli_t));
self->ctx = zctx_new ();
self->broker = strdup (broker);
self->verbose = verbose;
self->timeout = 2500; // 毫秒
s_mdcli_connect_to_broker (self);
return self;
}
// ---------------------------------------------------------------------
// 析构函数
void
mdcli_destroy (mdcli_t **self_p)
{
assert (self_p);
if (*self_p) {
mdcli_t *self = *self_p;
zctx_destroy (&self->ctx);
free (self->broker);
free (self);
*self_p = NULL;
}
}
// ---------------------------------------------------------------------
// 设置请求超时时间
void
mdcli_set_timeout (mdcli_t *self, int timeout)
{
assert (self);
self->timeout = timeout;
}
// ---------------------------------------------------------------------
// 发送请求给代理
// 取得请求消息的所有权，发送后销毁
int
mdcli_send (mdcli_t *self, char *service, zmsg_t **request_p)
{
assert (self);
assert (request_p);
zmsg_t *request = *request_p;
// 在消息顶部加入协议规定的帧
// Frame 0: empty (模拟REQ套接字的行为)
// Frame 1: "MDPCxy" (6个字节, MDP/Client x.y)
// Frame 2: Service name (看打印字符串)
zmsg_pushstr (request, service);
zmsg_pushstr (request, MDPC_CLIENT);
zmsg_pushstr (request, "");
if (self->verbose) {
zclock_log ("I: 发送请求给 '%s' 服务:", service);
zmsg_dump (request);
}
zmsg_send (&request, self->client);
return 0;
}
// ---------------------------------------------------------------------
// 获取应答消息，若无则返回NULL；
// 该函数不会尝试从代理的崩溃中恢复，
// 因为我们没有记录那些未收到应答的请求，所以也无法重发。
zmsg_t *
mdcli_recv (mdcli_t *self)
{
assert (self);
// 轮询套接字以获取应答
zmq_pollitem_t items [] = { { self->client, 0, ZMQ_POLLIN, 0 } };
int rc = zmq_poll (items, 1, self->timeout * ZMQ_POLL_MSEC);
if (rc == -1)
return NULL; // 中断
// 收到应答后进行处理
if (items [0].revents & ZMQ_POLLIN) {
zmsg_t *msg = zmsg_recv (self->client);
if (self->verbose) {
zclock_log ("I: received reply:");
zmsg_dump (msg);
}
// 不要处理错误，直接报出
assert (zmsg_size (msg) >= 4);
zframe_t *empty = zmsg_pop (msg);
assert (zframe_streq (empty, ""));
zframe_destroy (&empty);
zframe_t *header = zmsg_pop (msg);
assert (zframe_streq (header, MDPC_CLIENT));
zframe_destroy (&header);
zframe_t *service = zmsg_pop (msg);
zframe_destroy (&service);
return msg; // Success
}
if (zctx_interrupted)
printf ("W: 收到中断消息，正在中止client...\n");
else
if (self->verbose)
zclock_log ("W: 严重错误，放弃请求");
return NULL;
}
下面是对应的测试代码：
mdclient2: Majordomo client application in C
//
// 异步管家模式 - client示例程序
// 使用mdcli API隐藏MDP协议的具体实现
//
// 直接编译源码，而不创建类库
#include "mdcliapi2.c"
int main (int argc, char *argv [])
{
int verbose = (argc > 1 && streq (argv [1], "-v"));
mdcli_t *session = mdcli_new ("tcp://localhost:5555", verbose);
int count;
for (count = 0; count < 100000; count++) {
zmsg_t *request = zmsg_new ();
zmsg_pushstr (request, "Hello world");
mdcli_send (session, "echo", &request);
}
for (count = 0; count < 100000; count++) {
zmsg_t *reply = mdcli_recv (session);
if (reply)
zmsg_destroy (&reply);
else
break; // 使用Ctrl-C中断
}
printf ("收到 %d 个应答\n", count);
mdcli_destroy (&session);
return 0;
}
代理和worker的代码没有变，因为我们并没有改变MDP协议。经过对client的改造，我们可以明显看到速度的提升。如以下是同步状况下处理10万条请求的时间：
$ time mdclient 100000 requests/replies processed
real 0m14.088s user 0m1.310s sys 0m2.670s
以下是异步请求的情况：
$ time mdclient2 100000 replies received
real 0m8.730s user 0m0.920s sys 0m1.550s
让我们建立10个worker，看看效果如何：
$ time mdclient2 100000 replies received
real 0m3.863s user 0m0.730s sys 0m0.470s
由于worker获得消息需要通过LRU队列机制，所以并不能做到完全的异步。但是，worker越多其效果也会越好。在我的测试机上，当worker的数量达到8个时，速度就不再提升了——四核处理器只能做这么多。但是，我们仍然获得了近四倍的速度提升，而改造过程只有几分钟而已。此外，代理其实还没有进行优化，它仍会复制消息，而没有实现零拷贝。不过，我们已经做到每秒处理2.5万次请求-应答，已经很不错了。
当然，异步的管家模式也并不完美，有一个显著的缺点：它无法从代理的崩溃中恢复。可以看到mdcliapi2的代码中并没有恢复连接的代码，重新连接需要有以下几点作为前提：
1、每个请求都做了编号，每次应答也含有相应的编号，这就需要修改协议，明确定义。
2、client的API需要保留并跟踪所有已发送、但仍未收到应答的请求。
3、如果代理发生崩溃，client会重发所有消息。
可以看到，高可靠性往往和复杂度成正比，值得在管家模式中应用这一机制吗？这就要看应用场景了。如果是一个名称查询服务，每次会话会调用一次，那不需要应用这一机制；如果是一个位于前端的网页服务，有数千个客户端相连，那可能就需要了。