Linux下使用ZMQ实践之与libevent结合

本文主要是介绍Linux下使用ZMQ实践之与libevent结合，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！

1. 前言

最近考虑到一个问题，项目中有同时处理socket、zeromq的逻辑需求，想通过libevent（I/O服用）一块将zmq-socket的事件也放一个线程中处理。
网上了解了一些实现，大部分都是通过将zmq的sockfd拿到，加入libevent_dispatch中一并处理，但是存在问题是实时性不够，写法不对导致丢事件的情况。
上述的根本原因是：zeromq是通过底层独立线程完成socket层面的收发，拷贝到内存队列供给上层业务使用。强行监听socket事件，但实际数据获取还是从内存队列拿取，肯定是存在时效性的问题。
所以，本文尝试一种方法，服务端这块直接剖析zeromq协议zmtp，不要zmq子线程、内存队列，直接从socket层面收取数据；客户端仍然兼容libzmq常规的写法。

2. 相关知识

根据 zmtp官方说明的解读:
协议流程主要为：打招呼greeting、握手handshake、通信traffic；
通信内容主要分为命令command、消息message；

2.1 Greeting格式

Greeting 又可以细分为：签名signature、版本号version、机器码mechanism、类型as-server、扩展filter；主要是zmq考虑到了多种协议版本、多种协议类型，考虑的字段信息就多了。

;   The greeting announces the protocol detailsgreeting = signature version mechanism as-server fillersignature = %xFF padding %x7F
padding = 8OCTET        ; Not significantversion = version-major version-minor
version-major = %x03
version-minor = %x00;   The mechanism is a null padded string
mechanism = 20mechanism-char
mechanism-char = "A"-"Z" | DIGIT| "-" | "_" | "." | "+" | %x0;   Is the peer acting as server?
as-server = %x00 | %x01;   The filler extends the greeting to 64 octets
filler = 31%x00             ; 31 zero octets

2.1 Handshake格式

Handshake相比Greeting简单多了，一个command搞定

;   The handshake consists of at least one command
;   The actual grammar depends on the security mechanism
handshake = 1*command

2.2 Command格式

Command提供了长命令、短命令进行选择；与Message不同的是，Command数据域body还拆分出命令名称Command-name和命令数据Command-data，这块主要用到的地方是包含在数据帧中，配合Message进行信息控制交互；

;   A command is a single long or short frame
command = command-size command-body
command-size = %x04 short-size | %x06 long-size
short-size = OCTET          ; Body is 0 to 255 octets
long-size = 8OCTET          ; Body is 0 to 2^63-1 octets
command-body = command-name command-data
command-name = OCTET 1*255command-name-char
command-name-char = ALPHA
command-data = *OCTET

2.3 Message格式

Message内部其实提供了两种类型（长消息、短消息）、两种标记（独立报文、组合报文）进行灵活选择

;   A message is one or more frames
message = *message-more message-last
message-more = ( %x01 short-size | %x03 long-size ) message-body
message-last = ( %x00 short-size | %x02 long-size ) message-body
message-body = *OCTET

3. 实践

通过zmtp协议的指导，我们选取一个方向来实践一下：push-pull 协议、libevent::buffer_event；

3.1 连接握手

这次先看一下主函数，有个大概的印象：socket连接__do_connect，zmq握手__do_handshake，回调函数拉数据on_recv。

int main(int argc, char *argv[])
{int res = -1;struct bufferevent *bev = NULL;struct event_base *base = NULL;struct msg_ctx mctx = {0};mctx.evbuf = evbuffer_new();__do_reset(&mctx);if (argc < 2) {printf("%s <address>\n", argv[0], argv[1]);exit(EXIT_FAILURE);}printf("Program start...\n");base = event_base_new();assert(base);bev = __do_connect(base, argv[1]);res = __do_handshark(bev);assert(res == 0);bufferevent_setwatermark(bev, EV_READ, 0, 0);bufferevent_setcb(bev, on_recv, NULL, NULL, &mctx);bufferevent_enable(bev, EV_READ);event_base_dispatch(base);printf("Program quit...\n");sleep(5);return EXIT_SUCCESS;
}

本端实现的是pull的功能，需要连接到push，然后进行拉取数据；
socket建立连接使用的常规的bufferevent_socket_connect的方法

static struct bufferevent *__do_connect(struct event_base *base, const char *paddr)
{int res = -1;int dstlen = sizeof(struct sockaddr_storage);struct bufferevent *bev = NULL;struct sockaddr_storage dst;res = evutil_parse_sockaddr_port(paddr, (struct sockaddr *)&dst, &dstlen);assert(0 == res);bev = bufferevent_socket_new(base, -1, BEV_OPT_CLOSE_ON_FREE);assert(bev);res = bufferevent_socket_connect(bev, (struct sockaddr *)&dst, dstlen);assert(0 == res);return bev;
}

下来准备看握手过程的实现，先看一下结构体的定义是怎么跟zmtp结合的；这里有个地方注意一下，zmtp3.0与2.0协议格式还不一致，所以先制定约束：使用v3协议，不做v2的兼容。

/** ZMTP 3.0* connection = greeting handshake traffic** This is the 3.0 greeting (64 bytes)* greeting = signature version mechanism as-server filler*/
struct zmtp_greeting {char signature [10];    // %xFF padding %x7Fchar version [2];        // version-major %x03, version-minor %x00char mechanism [20];    // The mechanism is a null padded stringchar as_server [1];        // Is the peer acting as serverchar filler [31];        // The filler extends the greeting to 64 octets
};

从第2小节可以看出，Message和Command是类似的，这里我们全抽象为：flags、size、data（柔性数组）

/** A message is one or more frames** message = *message-more message-last* message-more = ( %x01 short-size | %x03 long-size ) message-body* message-last = ( %x00 short-size | %x02 long-size ) message-body* message-body = *OCTET** short-size = OCTET          ; Body is 0 to 255 octets* long-size = 8OCTET          ; Body is 0 to 2^63-1 octets**/
struct zmtp_msg_shdr {u8 flags;             //  Must be zerou8 size;              //  Size, 0 to 255 bytesu8 data [0];          //  Message data
};struct zmtp_msg_lhdr {u8 flags;             //  Must be zerou64 size;             //  Size, 0 to 255 bytesu8 data [0];          //  Message data
};

结构体准备好了，我们看一个握手的快速实现，这里说的快速是指使用阻塞同步的形式完成；另外一种常规的做法是使用状态机+事件回调的方式完成。

static int __do_handshark(struct bufferevent *bev)
{int res = -1;int fd = bufferevent_getfd(bev);struct zmtp_greeting gt = ZMTP_GREETING_INIT;char buffer[SIZE_LINE_NORMAL];struct zmtp_msg_shdr *phead = (struct zmtp_msg_shdr *)buffer;char pull_data[] = ZMTP_HANDSHAKE_PULL;struct zmtp_msg_shdr pull_head = {.flags = ZMTP_FLAGS_SCMD,.size = sizeof(pull_data) - 1,};/* Greeting */res = sdk_tcp_send_nbytes(fd, &gt, sizeof(struct zmtp_greeting), TIMEO);assert(0 == res);res = sdk_tcp_recv_nbytes(fd, &gt, sizeof(struct zmtp_greeting), TIMEO);assert(0 == res);printf("Greeting done...\n");/* Handshark */res = sdk_tcp_recv_nbytes(fd, phead, sizeof(struct zmtp_msg_shdr), TIMEO);assert(0 == res);assert(0x04 == phead->flags);res = sdk_tcp_recv_nbytes(fd, phead->data, phead->size, TIMEO);assert(0 == res);printf("Handsharke size: %d\n\n", phead->size);DSP_TOTAL(phead->data, phead->size);res = sdk_tcp_send_nbytes(fd, &pull_head, sizeof(struct zmtp_msg_shdr), TIMEO);assert(0 == res);res = sdk_tcp_send_nbytes(fd, &pull_data, pull_head.size, TIMEO);assert(0 == res);printf("Handsharke done...\n");return 0;
}

上述其实可以看到，报文填充非常暴力，通过宏直接将握手信息全部填充进去了。

#define ZMTP_GREETING_INIT { \{ 0xFF, 0, 0, 0, 0, 0, 0, 0, 1, 0x7F }, \{ 3, 0 }, \{ 'N', 'U', 'L', 'L', 0 }, \{ 0 }, \{ 0 } \
}#define ZMTP_HANDSHAKE_PULL { \0x05, \'R', 'E', 'A', 'D', 'Y', \0x0b, \'S', 'o', 'c', 'k', 'e', 't', '-', 'T', 'y', 'p', 'e', \0x00, 0x00, 0x00, 0x04, \'P', 'U', 'L', 'L', '0'\
}

3.2 数据拉取

数据拉取过程，由于Message数据域是不定长的，所以我们通过状态机的形式剖报文

void on_recv(struct bufferevent *bev, void *args)
{int res = 0;struct msg_ctx *mctx = (struct msg_ctx *)args;/* Et mode */while (1) {size_t length = evbuffer_get_length(bufferevent_get_input(bev));if (length == 0) {break;}printf("Evbuffer length: %u\n", length);switch (mctx->status) {case STATUS_MSG_HEAD:if (0 == __do_head_parser(bev, mctx)) {/* state transition */mctx->status = STATUS_MSG_BODY;printf("Head done, data: %u\n", mctx->size);}break;case STATUS_MSG_BODY:if (0 == __do_body_parser(bev, mctx)) {/* state transition */mctx->status = STATUS_MSG_HEAD;printf("Body done, data: %u\n", mctx->size);}break;default:assert(0);break;}}return;
}

Message头部信息，我们主要需要区分是长消息、还是短消息；是独立消息、还是组合消息。

enum {ZMTP_FLAGS_SCMD = 0x04,ZMTP_FLAGS_LCMD = 0x06,ZMTP_FLAGS_SMSG_MORE = 0x01,ZMTP_FLAGS_LMSG_MORE = 0x03,ZMTP_FLAGS_SMSG_LAST = 0x00,ZMTP_FLAGS_LMSG_LAST = 0x02,
};static int __do_head_parser(struct bufferevent *bev, struct msg_ctx *mctx)
{if (mctx->flags == 0xFF) {bufferevent_read(bev, &mctx->flags, sizeof(u8));printf("Head flag: %x\n", mctx->flags);}if (mctx->size == 0x00) {size_t expect = 0;size_t length = evbuffer_get_length(bufferevent_get_input(bev));switch (mctx->flags) {case ZMTP_FLAGS_SMSG_MORE:case ZMTP_FLAGS_SMSG_LAST:expect = sizeof(u8);printf("Body short\n");break;case ZMTP_FLAGS_LMSG_MORE:case ZMTP_FLAGS_LMSG_LAST:expect = sizeof(u64);printf("Body long\n");break;default:assert(0);break;}if (length < expect) {printf("Retry\n");return RETRY;}size_t rlen = bufferevent_read(bev, &mctx->size, expect);assert(rlen == expect);if (expect == sizeof(u64)) {mctx->size = ntohll(mctx->size);}assert(mctx->size > 0);}return 0;
}

然后是数据域状态下的数据读取了，这个主要考虑缓冲消息到evbuffer中，消息收全了最后才调用__do_something和__do_reset函数

static int __do_body_parser(struct bufferevent *bev, struct msg_ctx *mctx)
{char buffer[SIZE_LINE_LONG];size_t rmax = _MIN(sizeof(buffer), mctx->size - evbuffer_get_length(mctx->evbuf));size_t rlen = bufferevent_read(bev, buffer, rmax);int res = evbuffer_add(mctx->evbuf, buffer, rlen);assert(res == 0);if (evbuffer_get_length(mctx->evbuf) >= mctx->size) {printf("Body done, length: %u/%u\n",evbuffer_get_length(mctx->evbuf), mctx->size);__do_something(mctx);__do_reset(mctx);return 0;}printf("Retry\n");return RETRY;
}static void __do_reset(struct msg_ctx *mctx)
{mctx->status = STATUS_MSG_HEAD;mctx->flags = 0xFF;mctx->size  = 0x00;evbuffer_drain(mctx->evbuf, evbuffer_get_length(mctx->evbuf));
}static void __do_something(struct msg_ctx *mctx)
{printf("-- 0x%x\n", mctx->flags);printf("-- %d\n", mctx->size);while (evbuffer_get_length(mctx->evbuf) > 0) {char buffer[SIZE_LINE_LONG];ssize_t rlen = evbuffer_remove(mctx->evbuf, buffer, sizeof(buffer));printf("-- %s\n", buffer);}
}

3.3 运行结果

短消息的拉取：

./pullx 127.0.0.1:5555Program start...
Greeting done...
Handsharke size: 260000  05 52 45 41 44 59 0b 53 - 6f 63 6b 65 74 2d 54 79         .READY.Socket-Ty
0010  70 65 00 00 00 04 50 55 - 53 48 ** ** ** ** ** **         pe....PUSH
Handsharke done...
Evbuffer length: 14
Head flag: 0
Body short
Head done, data: 12
Evbuffer length: 12
Body done, length: 12/12
-- 0x0
-- 12
-- Data-   -000
Body done, data: 0
Evbuffer length: 14
Head flag: 0
Body short
Head done, data: 12
Evbuffer length: 12
Body done, length: 12/12
-- 0x0
-- 12
-- Data-   -001
Body done, data: 0

长消息的拉取：

./pullx 127.0.0.1:5555
Program start...
Greeting done...
Handsharke size: 260000  05 52 45 41 44 59 0b 53 - 6f 63 6b 65 74 2d 54 79         .READY.Socket-Ty
0010  70 65 00 00 00 04 50 55 - 53 48 ** ** ** ** ** **         pe....PUSH
Handsharke done...
Evbuffer length: 1033
Head flag: 2
Body long
Head done, data: 1024
Evbuffer length: 1024
Body done, length: 1024/1024
-- 0x2
-- 1024
-- Data-   -000
Body done, data: 0
Evbuffer length: 1033
Head flag: 2
Body long
Head done, data: 1024
Evbuffer length: 1024
Body done, length: 1024/1024
-- 0x2
-- 1024
-- Data-   -001
Body done, data: 0
Evbuffer length: 1033
Head flag: 2
Body long
Head done, data: 1024
Evbuffer length: 1024
Body done, length: 1024/1024