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The idea of middleware is to wrap the JSGI application (or any middleware already wrapping the application) to run some pre- or post-request code. Since middleware exposes the same JSGI interface to the outside world, it can be nested or "stacked" at will.

Usually, JSGI middleware comes as a function that takes a JSGI application as argument, and returns a middleware function wrapping that app. For example, a simple middleware that adds logging to each request might be implemented as follows:

exports.Logger = function(app) {
    return function(env) {
        log(env.SCRIPT_NAME + env.PATH_INFO);
        return app(env);

The middleware that comes with Helma NG follows this pattern. Additionally, the Helma NG JSGI implementation offers a convenience feature to automatically instantiate and wrap middleware so you don't have to. All you need to do is export your middleware factory functions as an array called middleware in your config module. If your middleware factory function is called middleware or handleRequest it is even enough to just export the module name:

exports.middleware = [

The above list shows some of the middleware that comes with Helma NG. Have a look at the modules/helma/middlware directory for more middleware.

Of course, the great thing about JSGI middleware is that it's easy to write your own, or use middleware from another package.