Trait PageCache 

pub trait PageCache {
    // Required methods
    fn cache_size(&mut self, cache_size: usize);
    fn page_count(&mut self) -> usize;
    fn fetch(
        &mut self,
        key: usize,
        create_flag: CreateFlag,
    ) -> Option<&mut PageWithMetadata>;
    fn unpin(&mut self, key: usize, discard: DiscardStrategy);
    fn rekey(&mut self, old_key: usize, new_key: usize);
    fn truncate(&mut self, limit: usize);
    fn destroy(&mut self);
    fn shrink(&mut self);
}

Required Methods

fn cache_size(&mut self, cache_size: usize)

The cache_size 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. It is advisory only.

fn page_count(&mut self) -> usize

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

fn fetch( &mut self, key: usize, create_flag: CreateFlag, ) -> Option<&mut PageWithMetadata>

The fetch method locates a page in the cache or None (see CreateFlag for detail on cache miss). The page to be fetched is determined by the key. The minimum key value is 1. After it has been retrieved using fetch, the page is considered to be “pinned”.

SQLite will normally invoke fetch with a createFlag of NoAllocation or AllocateIfConvenient. SQLite will only use a createFlag of Allocate after a prior call with a createFlag of AllocateIfConvenient failed. In between the fetch 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.

fn unpin(&mut self, key: usize, discard: DiscardStrategy)

unpin is called by SQLite with a pointer to a currently pinned page. The page cache implementation may choose to evict unpinned pages at any time.

fn rekey(&mut self, old_key: usize, new_key: usize)

The rekey 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 new_key, it must be discarded. Any prior cache entry associated with new_key is guaranteed not to be pinned.

fn truncate(&mut self, limit: usize)

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

fn destroy(&mut self)

The destroy method is used to delete a cache allocated by create. All resources associated with the specified cache should be freed.

fn shrink(&mut self)

SQLite invokes the shrink 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.

Implementors