interface CacheTagsChecksumInterface

interface CacheTagsChecksumInterface {
    
    /**
     * The invalid checksum returned if a database transaction is in progress.
     *
     * Every cache backend SHOULD detect this and not write cache items that have
     * this checksum. Not detecting this would not yield incorrect cache reads,
     * but would be a useless write.
     *
     * While a database transaction is progress, cache tag invalidations are
     * delayed to occur just before the commit, to allow:
     * - deadlock potential to be minimized, since semaphores to avoid concurrent
     *   writes can be acquired for the shortest period possible
     * - Non-database-based implementations of this service can delay tag
     *   invalidations until the transaction is committed to avoid
     *   race conditions.
     */
    const INVALID_CHECKSUM_WHILE_IN_TRANSACTION = -1;
    
    /**
     * Returns the sum total of validations for a given set of tags.
     *
     * Called by a backend when storing a cache item.
     *
     * @param string[] $tags
     *   Array of cache tags.
     *
     * @return string
     *   Cache tag invalidations checksum.
     */
    public function getCurrentChecksum(array $tags);
    
    /**
     * Returns whether the checksum is valid for the given cache tags.
     *
     * Used when retrieving a cache item in a cache backend, to verify that no
     * cache tag based invalidation happened.
     *
     * @param int $checksum
     *   The checksum that was stored together with the cache item.
     * @param string[] $tags
     *   The cache tags that were stored together with the cache item.
     *
     * @return bool
     *   FALSE if cache tag invalidations happened for the passed in tags since
     *   the cache item was stored, TRUE otherwise.
     */
    public function isValid($checksum, array $tags);
    
    /**
     * Reset statically cached tags.
     *
     * This is only used by tests.
     */
    public function reset();

}