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SQLITE3_PCACHE_METHODS2(3)                            Library Functions Manual

NAME
     sqlite3_pcache_methods2, sqlite3_pcache_methods2 - Application Defined
     Page Cache.

SYNOPSIS
     typedef struct sqlite3_pcache_methods2 sqlite3_pcache_methods2;
     struct sqlite3_pcache_methods2;

DESCRIPTION
     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:

     <table border=1 width=85% align=center> <tr><th> createFlag <th> Behavior
     when page is not already in cache <tr><td> 0 <td> Do not allocate a new
     page.  Return NULL.  <tr><td> 1 <td> Allocate a new page if it easy and
     convenient to do so.  Otherwise return NULL.  <tr><td> 2 <td> Make every
     effort to allocate a new page.  Only return NULL if allocating a new page
     is effectively impossible.  </table>

     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 to 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
     sqlite3_config(3), sqlite3_initialize(3), SQLITE_CONFIG_SINGLETHREAD(3)

NetBSD 8.99.34                 December 19, 2018                NetBSD 8.99.34