<chapter id="gencache">
<chapterinfo>
	<author>
		<firstname>Rafal</firstname><surname>Szczesniak</surname>
	</author>
	<pubdate>April 2003</pubdate>
</chapterinfo>

<title>General cache mechanism and API</title>

<sect1>
<title>Abstract</title>
<para>
General cache (gencache) was designed to combine various kinds of caching
mechanisms into one, defined by a simple API. This way, anyone can use it
to create their own caching layer on top of gencache. An example of
such approach is the netbios name cache.
</para>
</sect1>

<sect1>
<title>The mechanism</title>
<para>
Gencache utilises <emphasise>tdb</emphasise> database, like many other
parts of Samba. As its origins are in Berkeley DB implementation, it
uses key/value pairs stored in binary file. The values gencache
operates on are string-based, however. This makes very easy to use it
in command line environment eg. to quickly take a look at what's in
the cache or set some value.
</para>

<para>
All the data is stored in <filename>gencache.tdb</filename>
file. Records put there are in key/value format as mentioned below,
but as it's a cache, the timeout plays also important role and has a
special place in the key/value pair, as well as API.
</para>
</sect1>


<sect1>
<title>The data structure</title>
<para>
The record stored in <filename>gencache.tdb</filename> file consists
of the key, the value and the expiration timeout. While the first part
is stored completely independent from the others, the last two are
kept together. The form the record has is:
</para>

<para><programlisting>
key:     &lt;string&bt;
value:   &lt;12-digit timeout&bt;/&lt;string&gt;
</programlisting></para>

<para>The timeout part is the ASCII representation of
<emphasis>time_t</emphasis> value of the time when the cache entry
expires. Obviously the API, the programmer is provided with, hides this detail,
so that you don't have to care about checking it. Simply watch
carefully the return status of the function.
</para>
</sect1>

<sect1>
<title>The API</title>

<para><programlisting>
BOOL gencache_init()
</programlisting></para>

<para>This is used to initialise to whole caching mechanism. It means
opening the file or creating it if non-existing. If it's already been
opened earlier, then the routine just does nothing and returns
<constant>true</constant>. If something goes wrong, say the user
doesn't have necessary rights, the function returns
<constant>false</constant>.</para>

<para><programlisting>
BOOL gencache_shutdown()
</programlisting></para>

<para>This is the proper way to close the cache file. It simply
returns <constant>true</constant> after successful closing file and
<constant>false</constant> upon a failure.</para>

<para><programlisting>
BOOL gencache_set(const char* keystr, const char* value, time_t timeout)
</programlisting></para>

<para>This is one of the most basic functions. What it allows you to
do is to set some particular cache entry. If the entry haven't
existed yet, the function will act just as it was "gencache_add"
function. If it's already been in the cache, the entry will be set to
the new value. In either case, the cache entry will be set with given
key, value and timeout. Thus it is comfortable way to just set the
entry and not care about the details.</para>

<para><programlisting>
BOOL gencache_set_only(const char* keystr, const char* value, time_t timeout)
</programlisting></para>

<para><programlisting>
BOOL gencache_del(const char* keystr)
</programlisting></para>

<para><programlisting>
BOOL gencache_get(const char* keystr, char** valstr, time_t* timeout)
</programlisting></para>

<para><programlisting>
void gencache_iterate(void (*fn)(const char* key, const char *value, time_t timeout, void* dptr),
                      void* data, const char* keystr_pattern)

</programlisting></para>

<sect1>
<title>Writing your own caching layer</title>
</sect1>

</chapter>
