concept

filter is a function that can do something before or after handle http request. it is the same as jsp filter, .NET http module, nodejs and django middleware.

define

you should register you filter in www/app_initialized.lua like below:

------------------------------------------------------------------
--  这里注册web应用的各种过滤器
------------------------------------------------------------------
nwf.registerFilter(function(ctx, doNext)
    local req = ctx.request;
    local res = ctx.response;
    doSomething();
    doNext();
    doSomething();
end);

parameter ctx is the same as controller.
parameter doNext is a function, you should invoke it anyway while your work was done to avoid terminate process http request. invoke doNext will call the next filter or handle request by controller.