- * A TaxonomyReader holds a list of categories. Each category has a serial - * number which we call an "ordinal", and a hierarchical "path" name: - *
- * An implementation must allow multiple readers to be active concurrently - * with a single writer. Readers follow so-called "point in time" semantics, - * i.e., a TaxonomyReader object will only see taxonomy entries which were - * available at the time it was created. What the writer writes is only - * available to (new) readers after the writer's commit() is called. - *
- * In faceted search, two separate indices are used: the main Lucene index, - * and the taxonomy. Because the main index refers to the categories listed - * in the taxonomy, it is important to open the taxonomy *after* opening the - * main index, and it is also necessary to reopen() the taxonomy after - * reopen()ing the main index. - *
- * This order is important, otherwise it would be possible for the main index - * to refer to a category which is not yet visible in the old snapshot of - * the taxonomy. Note that it is indeed fine for the the taxonomy to be opened - * after the main index - even a long time after. The reason is that once - * a category is added to the taxonomy, it can never be changed or deleted, - * so there is no danger that a "too new" taxonomy not being consistent with - * an older index. - * - * @lucene.experimental - */ -public interface TaxonomyReader extends Closeable { - - /** - * The root category (the category with the empty path) always has the - * ordinal 0, to which we give a name ROOT_ORDINAL. - * getOrdinal() of an empty path will always return ROOT_ORDINAL, and - * getCategory(ROOT_ORDINAL) will return the empty path. - */ - public final static int ROOT_ORDINAL = 0; - - /** - * Ordinals are always non-negative, so a negative ordinal can be used to - * signify an error. Methods here return INVALID_ORDINAL (-1) in this case. - */ - public final static int INVALID_ORDINAL = -1; - - /** - * getOrdinal() returns the ordinal of the category given as a path. - * The ordinal is the category's serial number, an integer which starts - * with 0 and grows as more categories are added (note that once a category - * is added, it can never be deleted). - *
- * If the given category wasn't found in the taxonomy, INVALID_ORDINAL is - * returned. - */ - public int getOrdinal(CategoryPath categoryPath) throws IOException; - - /** - * getPath() returns the path name of the category with the given - * ordinal. The path is returned as a new CategoryPath object - to - * reuse an existing object, use {@link #getPath(int, CategoryPath)}. - *
- * A null is returned if a category with the given ordinal does not exist. - */ - public CategoryPath getPath(int ordinal) throws IOException; - - /** - * getPath() returns the path name of the category with the given - * ordinal. The path is written to the given CategoryPath object (which - * is cleared first). - *
- * If a category with the given ordinal does not exist, the given
- * CategoryPath object is not modified, and the method returns
- * false. Otherwise, the method returns true.
- */
- public boolean getPath(int ordinal, CategoryPath result) throws IOException;
-
- /**
- * refresh() re-reads the taxonomy information if there were any changes to
- * the taxonomy since this instance was opened or last refreshed. Calling
- * refresh() is more efficient than close()ing the old instance and opening a
- * new one.
- *
- * If there were no changes since this instance was opened or last refreshed, - * then this call does nothing. Note, however, that this is still a relatively - * slow method (as it needs to verify whether there have been any changes on - * disk to the taxonomy), so it should not be called too often needlessly. In - * faceted search, the taxonomy reader's refresh() should be called only after - * a reopen() of the main index. - *
- * It should be noted that refresh() is similar in purpose to - * IndexReader.reopen(), but the two methods behave differently. refresh() - * refreshes the existing TaxonomyReader object, rather than opening a new one - * in addition to the old one as reopen() does. The reason is that in a - * taxonomy, one can only add new categories and cannot modify or delete - * existing categories; Therefore, there is no reason to keep an old snapshot - * of the taxonomy open - refreshing the taxonomy to the newest data and using - * this new snapshots in all threads (whether new or old) is fine. This saves - * us needing to keep multiple copies of the taxonomy open in memory. - */ - public void refresh() throws IOException; - - /** - * getParent() returns the ordinal of the parent category of the category - * with the given ordinal. - *
- * When a category is specified as a path name, finding the path of its - * parent is as trivial as dropping the last component of the path. - * getParent() is functionally equivalent to calling getPath() on the - * given ordinal, dropping the last component of the path, and then calling - * getOrdinal() to get an ordinal back. However, implementations are - * expected to provide a much more efficient implementation: - *
- * getParent() should be a very quick method, as it is used during the - * facet aggregation process in faceted search. Implementations will most - * likely want to serve replies to this method from a pre-filled cache. - *
- * If the given ordinal is the ROOT_ORDINAL, an INVALID_ORDINAL is returned. - * If the given ordinal is a top-level category, the ROOT_ORDINAL is returned. - * If an invalid ordinal is given (negative or beyond the last available - * ordinal), an ArrayIndexOutOfBoundsException is thrown. However, it is - * expected that getParent will only be called for ordinals which are - * already known to be in the taxonomy. - */ - public int getParent(int ordinal) throws IOException; - - /** - * getParentArray() returns an int array of size getSize() listing the - * ordinal of the parent category of each category in the taxonomy. - *
- * The caller can hold on to the array it got indefinitely - it is - * guaranteed that no-one else will modify it. The other side of the - * same coin is that the caller must treat the array it got as read-only - * and not modify it, because other callers might have gotten the - * same array too (and getParent() calls might be answered from the - * same array). - *
- * If you use getParentArray() instead of getParent(), remember that - * the array you got is (naturally) not modified after a refresh(), - * so you should always call getParentArray() again after a refresh(). - *
- * This method's function is similar to allocating an array of size
- * getSize() and filling it with getParent() calls, but implementations
- * are encouraged to implement it much more efficiently, with O(1)
- * complexity. This can be done, for example, by the implementation
- * already keeping the parents in an array, and just returning this
- * array (without any allocation or copying) when requested.
- */
- public int[] getParentArray() throws IOException;
-
- /**
- * Equivalent representations of the taxonomy's parent info,
- * used internally for efficient computation of facet results:
- * "youngest child" and "oldest sibling"
- */
- public static interface ChildrenArrays {
- /**
- * getYoungestChildArray() returns an int array of size getSize()
- * listing the ordinal of the youngest (highest numbered) child
- * category of each category in the taxonomy. The value for a leaf
- * category (a category without children) is
- * INVALID_ORDINAL.
- */
- public int[] getYoungestChildArray();
- /**
- * getOlderSiblingArray() returns an int array of size getSize()
- * listing for each category the ordinal of its immediate older
- * sibling (the sibling in the taxonomy tree with the highest ordinal
- * below that of the given ordinal). The value for a category with no
- * older sibling is INVALID_ORDINAL.
- */
- public int[] getOlderSiblingArray();
- }
-
- /**
- * getChildrenArrays() returns a {@link ChildrenArrays} object which can
- * be used together to efficiently enumerate the children of any category.
- *
- * The caller can hold on to the object it got indefinitely - it is - * guaranteed that no-one else will modify it. The other side of the - * same coin is that the caller must treat the object which it got (and - * the arrays it contains) as read-only and not modify it, because - * other callers might have gotten the same object too. - *
- * Implementations should have O(getSize()) time for the first call or - * after a refresh(), but O(1) time for further calls. In neither case - * there should be a need to read new data from disk. These guarantees - * are most likely achieved by calculating this object (based on the - * getParentArray()) when first needed, and later (if the taxonomy was not - * refreshed) returning the same object (without any allocation or copying) - * when requested. - *
- * The reason we have one method returning one object, rather than two
- * methods returning two arrays, is to avoid race conditions in a multi-
- * threaded application: We want to avoid the possibility of returning one
- * new array and one old array, as those could not be used together.
- */
- public ChildrenArrays getChildrenArrays();
-
- /**
- * Retrieve user committed data.
- * @see TaxonomyWriter#commit(Map)
- */
- public Map
- * Because categories are numbered consecutively starting with 0, it
- * means the taxonomy contains ordinals 0 through getSize()-1.
- *
- * Note that the number returned by getSize() is often slightly higher
- * than the number of categories inserted into the taxonomy; This is
- * because when a category is added to the taxonomy, its ancestors
- * are also added automatically (including the root, which always get
- * ordinal 0).
- */
- public int getSize();
-
-}