local thread = require "ngx.thread"

local result

function query_mysql1()
    -- use lua-resty-mysql as usual
    -- to access mysql server A
    result = ...
end

function query_mysql2()
    -- use lua-resty-mysql as usual
    -- to access mysql server B
    result = ...
end

local t1 = thread.new(query_mysql1)
local t2 = thread.new(query_mysql2)

local t = thread.waitany({t1, t2})
if t == t1 then
    t2:abort()
else
    t1:abort()
end

ngx.say(result)