/*

  					W3C Sample Code Library libwww Persistent Cache Manager


!
  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.&nbsp;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
.

The cache_root is the URI of the location of the persistent
cache. An example is "file:/tmp/w3c-lib". If
cache_root is NULL then determine a cache root
using the following algorithm:

  o 
	     Look for any environment variables (if supported) in the following order:
	     WWW_CACHE, TMP, and TEMP. If none
    are set then then fall back on "/tmp".
  o 
	     Append the folder name "w3c-cache" to the root identified above


The cache_root location does not have to exist, it will be created
automatically if not. An empty string will make '/' the 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 adjust
(
  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);

/*

The cache can be setup to whether cache password protected documents thru the
protected flag. By default this flag is turned off.
*/

extern void HTCacheMode_setProtected (BOOL mode);
extern BOOL HTCacheMode_protected (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. The string returned MUST be freed
by the caller
*/

extern 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);

/*
(
  Max Size of a Single Cache Entry
)

It is also possible to control the max size of a single cache entry so that
the cache doesn't get filled with a very few, very large cached entries.
The default max size for a single cached entry is 3M. The value indicated
must be in Mbytes, for example, a vaue of 3 would mean 3 MBytes.
*/

extern BOOL HTCacheMode_setMaxCacheEntrySize (int size);
extern int HTCacheMode_maxCacheEntrySize (void);

/*
(
 Default expiration time of cache entries
)

If a response does not arrive with an expiration time and does not
explicitly forbid its being cached, use the default expiration time. The
time is given in seconds (e.g., 3,600 is one hour).
*/

extern void HTCacheMode_setDefaultExpiration (const int exp_time);
extern int HTCacheMode_DefaultExpiration (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 documents.
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:

  
    No network activity at all
  
    Here is uses its own persistent cache
  
    Forward all disconnected requests to a proxy cache
  
    Here it uses the HTTP/1.1 cache-control to indicate that the proxy should
    operate in disconnected mode. This 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);

/*

Clear a cache entry
*/

extern BOOL HTCache_resetMeta (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
&nbsp;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, char * default_name);

/*
(
  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

/*

  

  @(#) $Id: HTCache.html,v 2.25 2001/08/30 13:41:03 kahan Exp $

*/
