# Standard BEFORE and AFTER Filters

    /*
    **	(c) COPYRIGHT MIT 1995.
    **	Please first read the full copyright statement in the file COPYRIGH.
    */

This module provides a set of default BEFORE and AFTER filters that can be registered by the Net manager to be called before and after a request. All filters can be registered either to be called globally (all requests) or locally (pr request basis).

This module is implemented by HTFilter.c, and it is a part of the W3C Reference Library.

    #ifndef HTFILTER_H
    #define HTFILTER_H
    #include "WWWLib.h"

## BEFORE Filters

This is a standard set of BEFORE filters which the application may initialize. This can be done in an easy way using the function `HTBeforeInit()` in the Initialization interface.

### Proxy and Gateway BEFORE filter

Checks for registerd proxy servers or gateways and sees whether this request should be redirected to a proxy or a gateway. Proxies have higher priority than gateways so we look for them first! For HTTP/1.0 and HTTP/1.1 we may only send a full URL (including the host portion) to proxy servers. Therefore, we tell the Library whether to use the full URL or the traditional HTTP one without the host part.

    extern HTNetCallback HTProxyFilter;

### Rule Translation BEFORE Filter

If we have a set of rules loaded (see the Rule manager) then check before each request whether how that should be translated. The trick is that a parent anchor has a "address" which is the part from the URL we used when we created the anchor. However, it also have a "physical address" which is the place we are actually going to look for the resource. Hence this filter translates from the address to the physical address (if any translations are found)

    extern HTNetCallback HTRuleFilter;

### Memory Cache (History List) Validation BEFORE Filter

Check if document is already loaded. The user can define whether the history list should follow normal expiration or work as a traditional history list where expired documents are not updated. We don't check for anything but existence proof of a document associated with the anchor as the definition is left to the application.

    extern HTNetCallback HTMemoryCacheFilter;

### Persistent Cache Validation BEFORE Filter

Check the cache mode to see if we can use an already loaded version of this document. If so and our copy is valid then we don't have to go out and get it unless we are forced to

    extern HTNetCallback HTCacheFilter;

### Client side authentication BEFORE filter

The filter generates the credentials required to access a document Getting the credentials may involve asking the user in which case we use the methods registered by the HTAlert module

    extern HTNetCallback HTCredentialsFilter;

## AFTER Filters

Like BEFORE filters we provide a default set of typical AFTER filters that may be initialized by the application. Again, an easy way of doing this is to call the `HTAfterInit()` function in the Initialization interface.

### Persistent Cache Update AFTER filter

On our way out we catch the metainformation and stores it in our persistent store. If we have a cache validation (a 304 response then we use the new metainformation and merges it with the existing information already captured in the cache.

    extern HTNetCallback HTCacheUpdateFilter;

### Error and Information filter

It checks the status code from a request and generates an error/information message if required.

    extern HTNetCallback HTInfoFilter;

### Redirection filter

The redirection handler only handles redirections on the `GET` or `HEAD` method (or any other safe method)

    extern HTNetCallback HTRedirectFilter;

### Proxy Redirection filter

This filter handles a "`305 Use Proxy"` response and retries the request through the proxy

    extern HTNetCallback HTUseProxyFilter;

### Client side authentication filter

The client side authentication filter uses the user dialog messages registered in the HTAlert module. By default these are the ones used by the line mode browser but you can just register something else.

    extern HTNetCallback HTAuthFilter;

### Request Logging filter

Default Logging filter using the log manager provided by the Log Manager.

    extern HTNetCallback HTLogFilter;

    #endif /* HTFILTER_H */