Class TaxonomyReader

  • All Implemented Interfaces:
    Closeable, AutoCloseable
    Direct Known Subclasses:

    public abstract class TaxonomyReader
    extends Object
    implements Closeable
    TaxonomyReader is the read-only interface with which the faceted-search library uses the taxonomy during search time.

    A TaxonomyReader holds a list of categories. Each category has a serial number which we call an "ordinal", and a hierarchical "path" name:

    • The ordinal is an integer that starts at 0 for the first category (which is always the root category), and grows contiguously as more categories are added; Note that once a category is added, it can never be deleted.
    • The path is a CategoryPath object specifying the category's position in the hierarchy.
    Notes about concurrent access to the taxonomy:

    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.

    • Field Detail


        public static final int ROOT_ORDINAL
        The root category (the category with the empty path) always has the ordinal 0, to which we give a name ROOT_ORDINAL. getOrdinal(FacetLabel) of an empty path will always return ROOT_ORDINAL, and getPath(int) with ROOT_ORDINAL will return the empty path.
        See Also:
        Constant Field Values

        public static final int INVALID_ORDINAL
        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.
        See Also:
        Constant Field Values
    • Constructor Detail

      • TaxonomyReader

        public TaxonomyReader()
        Sole constructor.
    • Method Detail

      • openIfChanged

        public static <T extends TaxonomyReader> T openIfChanged​(T oldTaxoReader)
                                                          throws IOException
        If the taxonomy has changed since the provided reader was opened, open and return a new TaxonomyReader; else, return null. The new reader, if not null, will be the same type of reader as the one given to this method.

        This method is typically far less costly than opening a fully new TaxonomyReader as it shares resources with the provided TaxonomyReader, when possible.

      • decRef

        public final void decRef()
                          throws IOException
        Expert: decreases the refCount of this TaxonomyReader instance. If the refCount drops to 0 this taxonomy reader is closed.
      • getOrdinal

        public abstract int getOrdinal​(FacetLabel categoryPath)
                                throws IOException
        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).
        the category's ordinal or INVALID_ORDINAL if the category wasn't foun.
      • getPath

        public abstract FacetLabel getPath​(int ordinal)
                                    throws IOException
        Returns the path name of the category with the given ordinal.
      • getRefCount

        public final int getRefCount()
        Returns the current refCount for this taxonomy reader.
      • getSize

        public abstract int getSize()
        Returns the number of categories in the taxonomy. Note that the number of categories returned 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).
      • incRef

        public final void incRef()
        Expert: increments the refCount of this TaxonomyReader instance. RefCounts can be used to determine when a taxonomy reader can be closed safely, i.e. as soon as there are no more references. Be sure to always call a corresponding decRef(), in a finally clause; otherwise the reader may never be closed.
      • tryIncRef

        public final boolean tryIncRef()
        Expert: increments the refCount of this TaxonomyReader instance only if it has not been closed yet. Returns true on success.