基本含义

• $bytes_sent（1.3.8, 1.2.5）：nginx返回给客户端的字节数，包括响应头和响应体。
• $body_bytes_sent：nginx返回给客户端的响应体的字节数（即不含响应头）。
• $content_length：“Content-Length”请求头的值，等同于$http_content_length。即，nginx从客户端收到的请求头中Content-Length字段的值，不是nginx返回给客户端响应中的Content-Length字段（$sent_http_content_length）的值。
• $request_length（1.3.12, 1.2.7）：请求的字节数（包括请求行、请求头和请求体）。注意，由于$request_length是请求解析过程中不断累加的，如果解析请求时出现异常或提前完成，则$request_length只是已经累加部分的长度，并不是nginx从客户端收到的完整请求的总字节数（包括请求行、请求头、请求体）。例如，向nginx的静态文件的URL POST数据，则POST的数据（即请求体）不会计算在内。
• $upstream_response_length（0.7.27）：保存从upstream服务器获得的响应体的字节数，不包括响应头，1.11.4中增加的$upstream_bytes_received变量是包括响应头的。如果有多个响应长度的话使用逗号和冒号分隔，就像$upstream_addr中的地址那样。

代码分析

注：代码版本是nginx-1.10.2

$bytes_sent

变量处理代码：

static ngx_int_t
ngx_http_variable_bytes_sent(ngx_http_request_t *r,
    ngx_http_variable_value_t *v, uintptr_t data)
{
    u_char  *p;

    p = ngx_pnalloc(r->pool, NGX_OFF_T_LEN);
    if (p == NULL) {
        return NGX_ERROR;
    }

    v->len = ngx_sprintf(p, "%O", r->connection->sent) - p;
    v->valid = 1;
    v->no_cacheable = 0;
    v->not_found = 0;
    v->data = p;

    return NGX_OK;
}

从代码中可以看出$bytes_sent就是r->connection->sent，真正发送给客户端的全部的字节数。

$body_bytes_sent

变量处理代码：

static ngx_int_t
ngx_http_variable_body_bytes_sent(ngx_http_request_t *r,
    ngx_http_variable_value_t *v, uintptr_t data)
{
    off_t    sent;
    u_char  *p;

    sent = r->connection->sent - r->header_size;

    if (sent < 0) {
        sent = 0;
    }

    p = ngx_pnalloc(r->pool, NGX_OFF_T_LEN);
    if (p == NULL) {
        return NGX_ERROR;
    }

    v->len = ngx_sprintf(p, "%O", sent) - p;
    v->valid = 1;
    v->no_cacheable = 0;
    v->not_found = 0;
    v->data = p;

    return NGX_OK;
}

从代码中可以看出$body_bytes_sent就是r->connection->sent减去r->header_size（响应头），即响应体的字节数。

$content_length

变量处理代码：

static ngx_int_t
ngx_http_variable_content_length(ngx_http_request_t *r,
    ngx_http_variable_value_t *v, uintptr_t data)
{
    u_char  *p;

    if (r->headers_in.content_length) {
        v->len = r->headers_in.content_length->value.len;
        v->data = r->headers_in.content_length->value.data;
        v->valid = 1;
        v->no_cacheable = 0;
        v->not_found = 0;

    } else if (r->reading_body) {
        v->not_found = 1;
        v->no_cacheable = 1;

    } else if (r->headers_in.content_length_n >= 0) {
        p = ngx_pnalloc(r->pool, NGX_OFF_T_LEN);
        if (p == NULL) {
            return NGX_ERROR;
        }

        v->len = ngx_sprintf(p, "%O", r->headers_in.content_length_n) - p;
        v->data = p;
        v->valid = 1;
        v->no_cacheable = 0;
        v->not_found = 0;

    } else {
        v->not_found = 1;
    }

    return NGX_OK;
}

$content_length是r->headers_in中的content_length或content_length_n的值，不是r->headers_out中的content_length，所以是接收的请求的Content-Length。

$request_length

变量处理代码：

static ngx_int_t
ngx_http_variable_request_length(ngx_http_request_t *r,
    ngx_http_variable_value_t *v, uintptr_t data)
{
    u_char  *p;

    p = ngx_pnalloc(r->pool, NGX_OFF_T_LEN);
    if (p == NULL) {
        return NGX_ERROR;
    }

    v->len = ngx_sprintf(p, "%O", r->request_length) - p;
    v->valid = 1;
    v->no_cacheable = 0;
    v->not_found = 0;
    v->data = p;

    return NGX_OK;
}

$request_length就是r->request_length的值
解析请求行代码：

        rc = ngx_http_parse_request_line(r, r->header_in);

        if (rc == NGX_OK) {

            /* the request line has been parsed successfully */

            r->request_line.len = r->request_end - r->request_start;
            r->request_line.data = r->request_start;
            r->request_length = r->header_in->pos - r->request_start;

            ngx_log_debug1(NGX_LOG_DEBUG_HTTP, c->log, 0,
                           "http request line: \"%V\"", &r->request_line);

            r->method_name.len = r->method_end - r->request_start + 1;
            r->method_name.data = r->request_line.data;
            ......
        }

r->request_length初始化为请求行的长度。
处理请求头的代码：

    for ( ;; ) {
        ......

        if (rc == NGX_OK) {

            r->request_length += r->header_in->pos - r->header_name_start;

            ......

            /* a header line has been parsed successfully */

            h = ngx_list_push(&r->headers_in.headers);
            ......

            continue;
        }

        if (rc == NGX_HTTP_PARSE_HEADER_DONE) {

            /* a whole header has been parsed successfully */

            ngx_log_debug0(NGX_LOG_DEBUG_HTTP, r->connection->log, 0,
                           "http header done");

            r->request_length += r->header_in->pos - r->header_name_start;

            r->http_state = NGX_HTTP_PROCESS_REQUEST_STATE;

            rc = ngx_http_process_request_header(r);

            if (rc != NGX_OK) {
                return;
            }

            ngx_http_process_request(r);

            return;
        }

        ......

        ngx_http_finalize_request(r, NGX_HTTP_BAD_REQUEST);
        return;
    }

循环解析请求头时，不断累加r->request_length。
处理pre-read请求体代码：

    preread = r->header_in->last - r->header_in->pos;

    if (preread) {

        /* there is the pre-read part of the request body */

        ngx_log_debug1(NGX_LOG_DEBUG_HTTP, r->connection->log, 0,
                       "http client request body preread %uz", preread);

        out.buf = r->header_in;
        out.next = NULL;

        rc = ngx_http_request_body_filter(r, &out);

        if (rc != NGX_OK) {
            goto done;
        }

        r->request_length += preread - (r->header_in->last - r->header_in->pos);
        ......
    }

r->request_length累加上pre-read请求体长度。
处理请求体代码：

static ngx_int_t
ngx_http_do_read_client_request_body(ngx_http_request_t *r)
{
    ......
    for ( ;; ) {
        for ( ;; ) {
            ......
            n = c->recv(c, rb->buf->last, size);

            ngx_log_debug1(NGX_LOG_DEBUG_HTTP, c->log, 0,
                           "http client request body recv %z", n);

            if (n == NGX_AGAIN) {
                break;
            }

            if (n == 0) {
                ngx_log_error(NGX_LOG_INFO, c->log, 0,
                              "client prematurely closed connection");
            }

            if (n == 0 || n == NGX_ERROR) {
                c->error = 1;
                return NGX_HTTP_BAD_REQUEST;
            }

            rb->buf->last += n;
            r->request_length += n;
            ......
        }
        ......
    }
    ......
}

r->request_length累加读取的请求体长度。

$upstream_response_length

变量处理代码：

static ngx_int_t
ngx_http_upstream_response_length_variable(ngx_http_request_t *r,
    ngx_http_variable_value_t *v, uintptr_t data)
{
    u_char                     *p;
    size_t                      len;
    ngx_uint_t                  i;
    ngx_http_upstream_state_t  *state;

    v->valid = 1;
    v->no_cacheable = 0;
    v->not_found = 0;

    if (r->upstream_states == NULL || r->upstream_states->nelts == 0) {
        v->not_found = 1;
        return NGX_OK;
    }

    len = r->upstream_states->nelts * (NGX_OFF_T_LEN + 2);

    p = ngx_pnalloc(r->pool, len);
    if (p == NULL) {
        return NGX_ERROR;
    }

    v->data = p;

    i = 0;
    state = r->upstream_states->elts;

    for ( ;; ) {
        p = ngx_sprintf(p, "%O", state[i].response_length);

        if (++i == r->upstream_states->nelts) {
            break;
        }

        if (state[i].peer) {
            *p++ = ',';
            *p++ = ' ';

        } else {
            *p++ = ' ';
            *p++ = ':';
            *p++ = ' ';

            if (++i == r->upstream_states->nelts) {
                break;
            }

            continue;
        }
    }

    v->len = p - v->data;

    return NGX_OK;
}

ngx_http_upstream_state_t结构体代码：

typedef struct {
    ngx_msec_t                       bl_time;
    ngx_uint_t                       bl_state;

    ngx_uint_t                       status;
    ngx_msec_t                       response_time;
    ngx_msec_t                       connect_time;
    ngx_msec_t                       header_time;
    off_t                            response_length;

    ngx_str_t                       *peer;
} ngx_http_upstream_state_t;

$upstream_response_length就是遍历r->upstream_states（nginx数组），取出ngx_http_upstream_state_t中的response_length。upstream处理过程中，将后端响应信息存入r->upstream->state（ngx_http_upstream_state_t类型，实际存储在r->upstream_states数组中）中。如果请求了多次后端server，后端响应信息顺序保存在r->upstream_states数组中，数组中的最后一个元素，也就是r->upstream->state，保存着最后一次请求后端server的响应信息。