Application Defined Page Cache.

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## SQLite C Interface

## Application Defined Page Cache.

>     typedef struct sqlite3_pcache_methods2 sqlite3_pcache_methods2;
>     struct sqlite3_pcache_methods2 {
>       int iVersion;
>       void *pArg;
>       int (*xInit)(void*);
>       void (*xShutdown)(void*);
>       sqlite3_pcache *(*xCreate)(int szPage, int szExtra, int bPurgeable);
>       void (*xCachesize)(sqlite3_pcache*, int nCachesize);
>       int (*xPagecount)(sqlite3_pcache*);
>       sqlite3_pcache_page *(*xFetch)(sqlite3_pcache*, unsigned key, int createFlag);
>       void (*xUnpin)(sqlite3_pcache*, sqlite3_pcache_page*, int discard);
>       void (*xRekey)(sqlite3_pcache*, sqlite3_pcache_page*,
>           unsigned oldKey, unsigned newKey);
>       void (*xTruncate)(sqlite3_pcache*, unsigned iLimit);
>       void (*xDestroy)(sqlite3_pcache*);
>       void (*xShrink)(sqlite3_pcache*);
>     };

The sqlite3_config(SQLITE_CONFIG_PCACHE2, ...) interface can register an alternative page cache implementation by passing in an instance of the sqlite3_pcache_methods2 structure. In many applications, most of the heap memory allocated by SQLite is used for the page cache. By implementing a custom page cache using this API, an application can better control the amount of memory consumed by SQLite, the way in which that memory is allocated and released, and the policies used to determine exactly which parts of a database file are cached and for how long.

The alternative page cache mechanism is an extreme measure that is only needed by the most demanding applications. The built-in page cache is recommended for most uses.

The contents of the sqlite3_pcache_methods2 structure are copied to an internal buffer by SQLite within the call to sqlite3_config. Hence the application may discard the parameter after the call to sqlite3_config() returns.

The xInit() method is called once for each effective call to sqlite3_initialize() (usually only once during the lifetime of the process). The xInit() method is passed a copy of the sqlite3_pcache_methods2.pArg value. The intent of the xInit() method is to set up global data structures required by the custom page cache implementation. If the xInit() method is NULL, then the built-in default page cache is used instead of the application defined page cache.

The xShutdown() method is called by sqlite3_shutdown(). It can be used to clean up any outstanding resources before process shutdown, if required. The xShutdown() method may be NULL.

SQLite automatically serializes calls to the xInit method, so the xInit method need not be threadsafe. The xShutdown method is only called from sqlite3_shutdown() so it does not need to be threadsafe either. All other methods must be threadsafe in multithreaded applications.

SQLite will never invoke xInit() more than once without an intervening call to xShutdown().

SQLite invokes the xCreate() method to construct a new cache instance. SQLite will typically create one cache instance for each open database file, though this is not guaranteed. The first parameter, szPage, is the size in bytes of the pages that must be allocated by the cache. szPage will always a power of two. The second parameter szExtra is a number of bytes of extra storage associated with each page cache entry. The szExtra parameter will a number less than 250. SQLite will use the extra szExtra bytes on each page to store metadata about the underlying database page on disk. The value passed into szExtra depends on the SQLite version, the target platform, and how SQLite was compiled. The third argument to xCreate(), bPurgeable, is true if the cache being created will be used to cache database pages of a file stored on disk, or false if it is used for an in-memory database. The cache implementation does not have to do anything special based with the value of bPurgeable; it is purely advisory. On a cache where bPurgeable is false, SQLite will never invoke xUnpin() except to deliberately delete a page. In other words, calls to xUnpin() on a cache with bPurgeable set to false will always have the "discard" flag set to true. Hence, a cache created with bPurgeable false will never contain any unpinned pages.

The xCachesize() method may be called at any time by SQLite to set the suggested maximum cache-size (number of pages stored by) the cache instance passed as the first argument. This is the value configured using the SQLite "PRAGMA cache_size" command. As with the bPurgeable parameter, the implementation is not required to do anything with this value; it is advisory only.

The xPagecount() method must return the number of pages currently stored in the cache, both pinned and unpinned.

The xFetch() method locates a page in the cache and returns a pointer to an sqlite3_pcache_page object associated with that page, or a NULL pointer. The pBuf element of the returned sqlite3_pcache_page object will be a pointer to a buffer of szPage bytes used to store the content of a single database page. The pExtra element of sqlite3_pcache_page will be a pointer to the szExtra bytes of extra storage that SQLite has requested for each entry in the page cache.

The page to be fetched is determined by the key. The minimum key value is 1. After it has been retrieved using xFetch, the page is considered to be "pinned".

If the requested page is already in the page cache, then the page cache implementation must return a pointer to the page buffer with its content intact. If the requested page is not already in the cache, then the cache implementation should use the value of the createFlag parameter to help it determined what action to take:

createFlag | Behavior when page is not already in cache
--- | ---
0 | Do not allocate a new page. Return NULL.
1 | Allocate a new page if it easy and convenient to do so. Otherwise return NULL.
2 | Make every effort to allocate a new page. Only return NULL if allocating a new page is effectively impossible.

SQLite will normally invoke xFetch() with a createFlag of 0 or 1. SQLite will only use a createFlag of 2 after a prior call with a createFlag of 1 failed. In between the xFetch() calls, SQLite may attempt to unpin one or more cache pages by spilling the content of pinned pages to disk and synching the operating system disk cache.

xUnpin() is called by SQLite with a pointer to a currently pinned page as its second argument. If the third parameter, discard, is non-zero, then the page must be evicted from the cache. If the discard parameter is zero, then the page may be discarded or retained at the discretion of page cache implementation. The page cache implementation may choose to evict unpinned pages at any time.

The cache must not perform any reference counting. A single call to xUnpin() unpins the page regardless of the number of prior calls to xFetch().

The xRekey() method is used to change the key value associated with the page passed as the second argument. If the cache previously contains an entry associated with newKey, it must be discarded. Any prior cache entry associated with newKey is guaranteed not to be pinned.

When SQLite calls the xTruncate() method, the cache must discard all existing cache entries with page numbers (keys) greater than or equal to the value of the iLimit parameter passed to xTruncate(). If any of these pages are pinned, they are implicitly unpinned, meaning that they can be safely discarded.

The xDestroy() method is used to delete a cache allocated by xCreate(). All resources associated with the specified cache should be freed. After calling the xDestroy() method, SQLite considers the sqlite3_pcache* handle invalid, and will not use it with any other sqlite3_pcache_methods2 functions.

SQLite invokes the xShrink() method when it wants the page cache to free up as much of heap memory as possible. The page cache implementation is not obligated to free any memory, but well-behaved implementations should do their best.

See also lists of Objects, Constants, and Functions.