set the balancer in nginx.conf

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upstream default_doc_upstream {
	
	server 192.168.1.170:8081;
	# tomcat-8
	server 192.168.1.170:8082;
	# tomcat-9
	server 192.168.1.170:8083;

	balancer_by_lua_file	/opt/openresty/balancer.lua;
}

traffic control

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local math = require('math');
local string = require('string');
local table = require('table');
local cjson = require('cjson');
local balancer = require('ngx.balancer');
local p = require('peer');

if nil ~= string.find(ngx.var.request_uri, 'favicon.ico') then
    return;
end

local is_download_action = nil == string.find(ngx.var.request_uri, 'docs/');

local cache_request_history = ngx.shared.cache_request_history;
local remote_addr = ngx.var.remote_addr;
-- local http_host = ngx.var.http_host;
-- local http_user_agent = ngx.var.http_user_agent;

local host = '192.168.1.170';
local port = {8081, 8082, 8083};

local peers = { peer:new(nil, '192.168.1.170', 8081),
                peer:new(nil, '192.168.1.170', 8082),
                peer:new(nil, '192.168.1.170', 8083)};

local access;
local succ, err, forcible;
if cache_request_history:get(remote_addr) == nil then
    access = {remote_addr=remote_addr, mapping=nil, times=0, scheme=nil, server_addr=nil, request_uri=nil};
    succ, err, forcible = cache_request_history:set(remote_addr, cjson.encode(access));
end

access = cjson.decode( cache_request_history:get(remote_addr) );
-- force reset the current server_addr to previous server_addr if
local prev_remote_addr = access['remote_addr'];
local prev_server_addr = access['server_addr'];
local prev_request_uri = access['request_uri'];
if is_download_action and prev_remote_addr == ngx.var.remote_addr and prev_request_uri == ngx.var.request_uri then
    ngx.var.server_addr = prev_server_addr;
end
access['times'] = access['times'] + 1;
access['scheme'] = ngx.var.scheme;
access['server_addr'] = ngx.var.server_addr;
access['request_uri'] = ngx.var.request_uri;
access['mapping'] = ngx.var.request_uri;
succ, err, forcible = cache_request_history:set(remote_addr, cjson.encode(access));

local ok, err = balancer.set_current_peer(host, port[math.random(#port)]);
if not ok then
    ngx.log(ngx.ERR, '***********************failed to set the current peer: ', err);
    return ngx.exit(500);
end

ngx.log(ngx.DEBUG, '***********************current peer ',
    ' remote_addr='..ngx.var.remote_addr..
    ' upstream_addr='..ngx.var.upstream_addr..
    ' server_port='..ngx.var.server_port..
    ' scheme='..ngx.var.scheme..
    ' server_addr='..ngx.var.server_addr..
    ' request_uri='..ngx.var.request_uri..
    ' uri='..ngx.var.uri);

ngx.log(ngx.INFO, 'nn');