--- /dev/null
+<?php
+
+namespace Drupal\book;
+
+/**
+ * Defines a common interface for book outline storage classes.
+ */
+interface BookOutlineStorageInterface {
+
+ /**
+ * Gets books (the highest positioned book links).
+ *
+ * @return array
+ * An array of book IDs.
+ */
+ public function getBooks();
+
+ /**
+ * Checks if there are any books.
+ *
+ * @return bool
+ * TRUE if there are books, FALSE if not.
+ */
+ public function hasBooks();
+
+ /**
+ * Loads books.
+ *
+ * Each book entry consists of the following keys:
+ * - bid: The node ID of the main book.
+ * - nid: The node ID of the book entry itself.
+ * - pid: The parent node ID of the book.
+ * - has_children: A boolean to indicate whether the book has children.
+ * - weight: The weight of the book entry to order siblings.
+ * - depth: The depth in the menu hierarchy the entry is placed into.
+ *
+ * @param array $nids
+ * An array of node IDs.
+ * @param bool $access
+ * Whether access checking should be taken into account.
+ *
+ * @return array
+ * Array of loaded book items.
+ */
+ public function loadMultiple($nids, $access = TRUE);
+
+ /**
+ * Gets child relative depth.
+ *
+ * @param array $book_link
+ * The book link.
+ * @param int $max_depth
+ * The maximum supported depth of the book tree.
+ *
+ * @return int
+ * The depth of the searched book.
+ */
+ public function getChildRelativeDepth($book_link, $max_depth);
+
+ /**
+ * Deletes a book entry.
+ *
+ * @param int $nid
+ * Deletes a book entry.
+ *
+ * @return mixed
+ * Number of deleted book entries.
+ */
+ public function delete($nid);
+
+ /**
+ * Loads book's children using it's parent ID.
+ *
+ * @param int $pid
+ * The book's parent ID.
+ *
+ * @return array
+ * Array of loaded book items.
+ */
+ public function loadBookChildren($pid);
+
+ /**
+ * Builds tree data used for the menu tree.
+ *
+ * @param int $bid
+ * The ID of the book that we are building the tree for.
+ * @param array $parameters
+ * An associative array of build parameters. For info about individual
+ * parameters see BookManager::bookTreeBuild().
+ * @param int $min_depth
+ * The minimum depth of book links in the resulting tree.
+ * @param int $max_depth
+ * The maximum supported depth of the book tree.
+ *
+ * @return array
+ * Array of loaded book links.
+ */
+ public function getBookMenuTree($bid, $parameters, $min_depth, $max_depth);
+
+ /**
+ * Inserts a book link.
+ *
+ * @param array $link
+ * The link array to be inserted in the database.
+ * @param array $parents
+ * The array of parent ids for the link to be inserted.
+ *
+ * @return mixed
+ * The last insert ID of the query, if one exists.
+ */
+ public function insert($link, $parents);
+
+
+ /**
+ * Updates book reference for links that were moved between books.
+ *
+ * @param int $nid
+ * The nid of the book entry to be updated.
+ * @param array $fields
+ * The array of fields to be updated.
+ *
+ * @return mixed
+ * The number of rows matched by the update query.
+ */
+ public function update($nid, $fields);
+
+ /**
+ * Update the book ID of the book link that it's being moved.
+ *
+ * @param int $bid
+ * The ID of the book whose children we move.
+ * @param array $original
+ * The original parent of the book link.
+ * @param array $expressions
+ * Array of expressions to be added to the query.
+ * @param int $shift
+ * The difference in depth between the old and the new position of the
+ * element being moved.
+ *
+ * @return mixed
+ * The number of rows matched by the update query.
+ */
+ public function updateMovedChildren($bid, $original, $expressions, $shift);
+
+ /**
+ * Count the number of original link children.
+ *
+ * @param array $original
+ * The book link array.
+ *
+ * @return int
+ * Number of children.
+ */
+ public function countOriginalLinkChildren($original);
+
+ /**
+ * Get book subtree.
+ *
+ * @param array $link
+ * A fully loaded book link.
+ * @param int $max_depth
+ * The maximum supported depth of the book tree.
+ *
+ * @return array
+ * Array of unordered subtree book items.
+ */
+ public function getBookSubtree($link, $max_depth);
+
+}