How to implement Nginx reverse proxy to support long connections

前言

nginx upstream与后端的连接默认为短连接，通过http/1.0向后端发起连接，并把请求的"connection" header设为"close"。nginx与前端的连接默认为长连接，一个用户跟nginx建立连接之后，通过这个长连接发送多个请求。如果nginx只是作为reverse proxy的话，可能一个用户连接就需要多个向后端的短连接。如果后端的服务器（源站或是缓存服务器）处理并发连接能力不强的话，就可能导致瓶颈的出现。

nginx目前的upstream连接建立和获取的机制如下图。nginx会在一开始创建connection pool（进程间不共享，可以避免锁），提供给所有向前/后的连接。

如果要实现upstream长连接，则每个进程需要另外一个connection pool，里面都是长连接。一旦与后端服务器建立连接，则在当前请求连接结束之后不会立即关闭连接，而是把用完的连接保存在一个keepalive connection pool里面，以后每次需要建立向后连接的时候，只需要从这个连接池里面找，如果找到合适的连接的话，就可以直接来用这个连接，不需要重新创建socket或者发起connect()。这样既省下建立连接时三次握手的时间消耗，又可以避免tcp连接的slow start。如果在keepalive连接池找不到合适的连接，那就按照原来的步骤重新建立连接。假设连接查找时间可以忽略不计，那么这种方法肯定是有益而无害的（当然，需要少量额外的内存）。

具体如何来设计这个keepalive connection pool，不同人有不同的选择。比如nginx目前的第三方模块upstream keepalive（作者maxim dounin）使用了一个queue来做。因为upstream的服务器很可能是多个，所以可能当保持的连接数多的时候，查找的时间可能会较长。可以给每个upstream服务器都分配一个pool（queue），缩短查找时间。但是总体来说内存操作很快，影响不会很大。upstream keepalive模块目前只支持memcached，但是可以重用其代码来达到对http upstream的长连接。由于nginx作者之前没有考虑upstream的长连接，所以在设计上要把http upstream keepalive模块化可能比较难，只能通过手动修改代码来做到。

一个完整的让upstream支持长连接的配置示例如下：

#user nobody; 
worker_processes 1; 
 
#error_log logs/error.log; 
#error_log logs/error.log notice; 
#error_log logs/error.log info; 
 
#pid logs/nginx.pid; 
 
 
events { 
 worker_connections 1024; 
} 
 
 
http { 
 include mime.types; 
 default_type application/octet-stream; 
 
 #log_format main '$remote_addr - $remote_user [$time_local] "$request" ' 
 #   '$status $body_bytes_sent "$http_referer" ' 
 #   '"$http_user_agent" "$http_x_forwarded_for"'; 
 
 #access_log logs/access.log main; 
 
 client_max_body_size 20m; 
 client_header_buffer_size 32k; 
 large_client_header_buffers 4 32k; 
 
 sendfile on; 
 #tcp_nopush on; 
 
 #keepalive_timeout 0; 
 keepalive_timeout 65; 
 
 #gzip on; 
 
 proxy_buffer_size 64k; 
 proxy_buffers 32 32k; 
 proxy_busy_buffers_size 128k; 
 
 upstream aaucfg_backend { 
 server 127.0.0.1:97; 
 keepalive 16; 
 } 
 
 upstream hfc_backend { 
 server 127.0.0.1:8090; 
 keepalive 16; 
 } 
 
 upstream manager_backend { 
 server 127.0.0.1:8095; 
 keepalive 16; 
 } 
 
 server { 
 listen 80; 
 server_name localhost; 
 
 #charset koi8-r; 
 
 #access_log logs/host.access.log main; 
 
 root html/tools; 
 index index.html index.htm index.php; 
  
 proxy_http_version 1.1; 
 proxy_set_header connection ""; 
 proxy_set_header host $host; 
 proxy_set_header x-real_ip $remote_addr; 
 proxy_set_header x-forwarded-for $proxy_add_x_forwarded_for; 
  
 location / { 
  if (!-e $request_filename) { 
  #rewrite ^/(.*)$ /index.php/$1 last; 
  #break; 
  rewrite ^/(.*)$ /index.php/$1; 
  } 
 } 
  
 location ~* \.(ico|css|js|gif|jpe?g|png)(\?[0-9]+)?$ { 
  expires max; 
  log_not_found off; 
 } 
  
 location ^~ /aaucfg/ { 
  #proxy_pass http://$remote_addr:97$request_uri; 
  proxy_pass http://aaucfg_backend; 
 } 
  
 location ^~ /hfc/ { 
  #proxy_pass http://$remote_addr:8090$request_uri; 
  proxy_pass http://hfc_backend; 
 } 
  
 location ^~ /manager/ { 
  #proxy_pass http://$remote_addr:8095$request_uri; 
  proxy_pass http://manager_backend; 
 } 
  
 #error_page 404  /404.html; 
 
 # redirect server error pages to the static page /50x.html 
 # 
 error_page 500 502 503 504 /50x.html; 
 location = /50x.html { 
  root html; 
 } 
 
 # proxy the php scripts to apache listening on 127.0.0.1:80 
 # 
 #location ~ \.php$ { 
 # proxy_pass http://127.0.0.1; 
 #} 
 
 # pass the php scripts to fastcgi server listening on 127.0.0.1:9000 
 # 
 #location ~ \.php$ { 
 # fastcgi_pass 127.0.0.1:9000; 
 # fastcgi_index index.php; 
 # fastcgi_param script_filename $document_root$fastcgi_script_name; 
 # include fastcgi_params; 
 #} 
  
 location ~ .php 
 { 
  fastcgi_pass 127.0.0.1:9000; 
  fastcgi_index index.php; 
  fastcgi_param script_filename $document_root$fastcgi_script_name; 
  include fastcgi.conf; 
  include fastcgi_params; 
 
  #定义变量 $path_info ，用于存放pathinfo信息 
  set $path_info ""; 
  #定义变量 $real_script_name，用于存放真实地址 
  set $real_script_name $fastcgi_script_name; 
  #如果地址与引号内的正则表达式匹配 
  if ($fastcgi_script_name ~ "^(.+?\.php)(/.+)$") { 
   #将文件地址赋值给变量 $real_script_name 
   set $real_script_name $1; 
   #将文件地址后的参数赋值给变量 $path_info 
   set $path_info $2; 
  } 
  #配置fastcgi的一些参数 
  fastcgi_param script_filename $document_root$real_script_name; 
  fastcgi_param script_name $real_script_name; 
  fastcgi_param path_info $path_info; 
 } 
 
 # deny access to .htaccess files, if apache's document root 
 # concurs with nginx's one 
 # 
 #location ~ /\.ht { 
 # deny all; 
 #} 
 } 
 
 
 # another virtual host using mix of ip-, name-, and port-based configuration 
 # 
 #server { 
 # listen 8000; 
 # listen somename:8080; 
 # server_name somename alias another.alias; 
 
 # location / { 
 # root html; 
 # index index.html index.htm; 
 # } 
 #} 
 
 
 # https server 
 # 
 #server { 
 # listen 443 ssl; 
 # server_name localhost; 
 
 # ssl_certificate cert.pem; 
 # ssl_certificate_key cert.key; 
 
 # ssl_session_cache shared:ssl:1m; 
 # ssl_session_timeout 5m; 
 
 # ssl_ciphers high:!anull:!md5; 
 # ssl_prefer_server_ciphers on; 
 
 # location / { 
 # root html; 
 # index index.html index.htm; 
 # } 
 #} 
 
}

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