以前用tengine自带了session_sticky，现在换成openresty，没有现成的，nginx-sticky-module 太老， 编译有点问题，于是自己写了一个，废话不多说，直接看代码

lua/balancer.lua

local balancer = require "ngx.balancer"
local upstream = require "ngx.upstream"local upstream_name = 'backend'local srvs = upstream.get_servers(upstream_name)function get_server()local cache = ngx.shared.cachelocal key = "req_index"local index = cache:get(key)if index == nil or index > #srvs thenindex = 1cache:set(key, index)endcache:incr(key, 1)return index
endfunction is_down(server)local down = falselocal perrs = upstream.get_primary_peers(upstream_name)for i = 1, #perrs dolocal peer = perrs[i]if server == peer.name and peer.down == true thendown = trueendendreturn down
end----------------------------local route = ngx.var.cookie_routelocal serverif route thenfor k, v in pairs(srvs) doif ngx.md5(v.name) == route thenserver = v.addrendendif is_down(server) thenroute = nilend
endif not route thenfor i = 1, #srvs doif not server or is_down(server) thenserver = srvs[get_server()].addrendendngx.header["Set-Cookie"] = 'route=' .. ngx.md5(server) .. '; path=/;'
endlocal index = string.find(server, ':')
local host = string.sub(server, 1, index - 1)
local port = string.sub(server, index + 1)
balancer.set_current_peer(host, tonumber(port))

nginx.conf

lua_shared_dict cache 1m;
upstream backend {server 192.168.0.2:8080;server 192.168.0.3:8080; balancer_by_lua_file lua/balancer.lua;
} server {listen 80;server_name    localhost;location / {proxy_pass http://backend;...}}# 健康检查
lua_shared_dict healthcheck 1m;
lua_socket_log_errors off;
init_worker_by_lua_block {local hc = require "resty.upstream.healthcheck"local ok, err = hc.spawn_checker{shm = "healthcheck",  -- defined by "lua_shared_dict"upstream = "backend", -- defined by "upstream"type = "http",http_req = "GET / HTTP/1.0\r\nHost: localhost\r\n\r\n",-- raw HTTP request for checkinginterval = 2000,  -- run the check cycle every 2 sectimeout = 1000,   -- 1 sec is the timeout for network operationsfall = 3,  -- # of successive failures before turning a peer downrise = 2,  -- # of successive successes before turning a peer upvalid_statuses = {200, 302},  -- a list valid HTTP status codeconcurrency = 10,  -- concurrency level for test requests}if not ok thenngx.log(ngx.ERR, "failed to spawn health checker: ", err)returnend}

主要是利用ngx.upstream、ngx.balancer 这两个模块，动态获取upstream，以及设置返回的上游名单，然后写到cookie里面，这里有判断后端是否down掉，如果down掉的话，则获取下一个，后端的状态通过resty.upstream.healthcheck模块健康检查来实现，以此为模型可以写更复杂的负载均衡逻辑

我这里比较简单，嫌丑了