# Persistent Cache Manager

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

The cache contains details of persistent files which contain the contents of remote documents. The existing cache manager is somewhat naive - especially in its garbage collection but it is just an example of how it can be done.However, it is a fully HTTP/1.1 compliant cache manager. More advanced implementations are welcome!

This module is implemented by HTCache.c, and it is a part of the W3C Sample Code Library.

    #ifndef HTCACHE_H
    #define HTCACHE_H

    #include "WWWLib.h"

## Initialize and Terminate the Persistent Cache

If `cache_root' is `NULL` then use `HT_CACHE_ROOT` which by default is set to "`/tmp/w3c-lib`". The `cache_root` location does not have to exist, it will be created automatically if not. An empty string will make '/' as cache root. The size is the total size in MBytes - the default size is 20M. The cache can not be less than 5M. We can only enable the cache if we are in secure mode where we can not access the local file system. This is for example the case if using an application as a telnet shell.

    extern BOOL HTCacheInit (const char * cache_root, int size);

After the cache has been terminated it can not be used anymore unless you do another `HTCacheInit()` call.

    extern BOOL HTCacheTerminate (void);

## Cache Mode Parameters

The persistent cache has a set of overall parameters that you can adust

### Enable and Disable the Cache

The cache can be temporarily suspended by using the enable/disable flag. This does not prevent the cache from being enabled/disable at a later point in time.

    extern void HTCacheMode_setEnabled (BOOL mode);
    extern BOOL HTCacheMode_enabled (void);

### What is the current Cache Root?

Return the value of the cache root. The cache root can only be set through the `HTCacheInit()` function

    extern const char * HTCacheMode_getRoot	(void);

### Total Cache Size

We set the default cache size to 20M. We set the minimum size to 5M in order not to get into weird problems while writing the cache. The size is indicated in Mega bytes. The size is given in MBytes and is also returned in MBytes. We don't consider the metainformation as part of the total cache size which is the the reason for why the min cache size should not be less than 5M.

    extern BOOL HTCacheMode_setMaxSize (int size);
    extern int  HTCacheMode_maxSize    (void);

### How do we handle Expiration of Cached Objects?

There are various ways of handling `Expires` header when met in a history list. Either it can be ignored all together, the user can be notified with a warning, or the document can be reloaded automatically. This flag decides what action to be taken. The default action is `HT_EXPIRES_IGNORE`. In `HT_EXPIRES_NOTIFY` mode , we push a message on to the Error stack which is presented to the user.

    typedef enum _HTExpiresMode {
        HT_EXPIRES_IGNORE = 0,
        HT_EXPIRES_NOTIFY,
        HT_EXPIRES_AUTO
    } HTExpiresMode;

    extern void HTCacheMode_setExpires (HTExpiresMode mode);
    extern HTExpiresMode HTCacheMode_expires (void);

### Disconnected Operation

The cache can be set to handle disconnected operation where it does not use the network to validate entries and do not attempt to load new versions. All requests that can not be fulfilled by the cache will be returned with a `"504 Gateway Timeout"` response. There are two modes of how the cache can operate in disconnected mode: it can use diconnected mode on its own persistent cache or it can forward the disconnected request to a proxy cache, for example. The latter mode only really makes sense when you are using a proxy, of course.

    typedef enum _HTDisconnectedMode {
        HT_DISCONNECT_NONE     = 0,
        HT_DISCONNECT_NORMAL   = 1,
        HT_DISCONNECT_EXTERNAL = 2
    } HTDisconnectedMode;

    extern void HTCacheMode_setDisconnected (HTDisconnectedMode mode);
    extern HTDisconnectedMode HTCacheMode_disconnected (void);
    extern BOOL HTCacheMode_isDisconnected (HTReload mode);

## The Cache Index

The persistent cache keeps an index of its current entries so that garbage collection and lookup becomes more efficient. This index is stored automatically at regular intervals so that we don't get out of sync. Also, it is automatically loaded at startup and saved at closedown of the cache.

### Reading the Cache Index

Read the saved set of cached entries from disk. we only allow the index ro be read when there is no entries in memory. That way we can ensure consistancy.

    extern BOOL HTCacheIndex_read (const char * cache_root);

### Write the Cache Index

Walk through the list of cached objects and save them to disk. We override any existing version but that is normally OK as we have already read its contents.

    extern BOOL HTCacheIndex_write (const char * cache_root);

## The HTCache Object

The cache object is what we store about a cached objet in memory.

    typedef struct _HTCache HTCache;

### Create and Update a Cache Object

Filling the cache is done as all other transportation of bulk data in libwww using streams. The cache object creater is a stream which in many cases sits on a T stream so that we get the original feed and at the same time can parse the contents.

In some situations, we want to append data to an already exiting cache entry. This is the case when a use has interrupted a download and we are stuck with a subpart of the document. If the user later on whishes to download the object again we can issue a range request and continue from where we were. This will in many situations save a lot of bandwidth.

    extern HTConverter HTCacheWriter, HTCacheAppend;

This function writes the metainformation along with the data object stored by the HTCacheWriter stream above. If no headers are available then the meta file is empty

    extern BOOL HTCache_writeMeta (HTCache * cache, HTRequest * request,
                                   HTResponse * response);

In case we received a "`304 Not Modified`" response then we do not have to tough the body but must merge the metainformation with the previous version. Therefore we need a special metainformation update function.

    extern BOOL HTCache_updateMeta (HTCache * cache, HTRequest * request,
                                    HTResponse * response);

### Check Cached Entry

After we get a response back, we should check whether we can still cache an entry and/or we should add an entry for a resource that has just been created so that we can remember the etag and other things. The latter allows us to guarantee that we don't loose data due to the lost update problem.

    extern HTCache * HTCache_touch (HTRequest * request, HTResponse * response,
                                    HTParentAnchor * anchor);

### Load a Cached Object

Loading a cached object is also done as all other loads in libwww by using a protocol load module. For the moment, this load function handles the persistent cache as if it was on local file but in fact it could be anywhere.

    extern HTProtCallback HTLoadCache;

### Delete a Cache Object

Remove a HTCache object from memory and from disk. You must explicitly remove a lock before this operation can succeed

    extern BOOL HTCache_remove (HTCache * cache);

### Delete All Cache Objects in Memory

Destroys all cache entried in memory but does not write anything to disk. Use the index methods above for doing that. We do not delete the disk contents.

    extern BOOL HTCache_deleteAll (void);

### Delete all Cache Object and File Entries

Destroys all cache entried in memory and on disk. This call basically resets the cache to the inital state but it does not terminate the cache. That is, you don't have to reinitialize the cache before you can use it again.

    extern BOOL HTCache_flushAll (void);

### Find a Cached Object

Verifies if a cache object exists for this URL and if so returns a URL for the cached object. It does not verify whether the object is valid or not, for example it might have expired. Use the cache validation methods for checking this.

    extern HTCache * HTCache_find (HTParentAnchor * anchor);

### Verify if an Object is Fresh

This function checks whether a document has expired or not. The check is based on the metainformation passed in the anchor object The function returns the level of validation needed for getting a fresh version. We also check the cache control directives in the request to see if they change the freshness discission.

    extern HTReload HTCache_isFresh (HTCache * me, HTRequest * request);

### Register a Cache Hit

As a cache hit may occur several places, we have a public function where we can declare a download to be a true cache hit. The number of hits a cache object has affects its status when we are doing garbage collection.

    extern BOOL HTCache_addHit (HTCache * cache);

### Find the Location of a Cached Object

Is we have a valid entry in the cache then we also need a location where we can get it. Hopefully, we may be able to access it thourgh one of our protocol modules, for example the local file module. The name returned is in URL syntax and must be freed by the caller

    extern char * HTCache_name (HTCache * cache);

### Locking a Cache Object

While we are creating a new cache object or while we are validating an existing one, we must have a lock on the entry so that not other requests can get to it in the mean while. A lock can be broken if the same request tries to create the cache entry again. This means that we have tried to validate the cache entry but we got a new shipment of bytes back from the origin server or an intermediary proxy.

    extern BOOL HTCache_getLock     (HTCache * cache, HTRequest * request);
    extern BOOL HTCache_breakLock   (HTCache * cache, HTRequest * request);
    extern BOOL HTCache_hasLock     (HTCache * cache);
    extern BOOL HTCache_releaseLock (HTCache * cache);

    #endif