TaxonomyWriter (Lucene 3.6.2 API)
Keywords: org.apache.lucene.facet.taxonomy.TaxonomyWriter interface

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## org.apache.lucene.facet.taxonomy
Interface TaxonomyWriter

All Superinterfaces:
  Closeable, org.apache.lucene.util.TwoPhaseCommit

All Known Implementing Classes:
  DirectoryTaxonomyWriter

    public interface TaxonomyWriter

extends Closeable, org.apache.lucene.util.TwoPhaseCommit

TaxonomyWriter is the interface which the faceted-search library uses to dynamically build the taxonomy at indexing time.

Notes about concurrent access to the taxonomy:

An implementation must allow multiple readers and a single writer to be active concurrently. Readers follow so-called "point in time" semantics, i.e., a reader 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.

Faceted search keeps two indices - namely Lucene's main index, and this taxonomy index. When one or more readers are active concurrently with the writer, care must be taken to avoid an inconsistency between the state of these two indices: When writing to the indices, the taxonomy must always be committed to disk *before* the main index, because the main index refers to categories listed in the taxonomy. Such control can best be achieved by turning off the main index's "autocommit" feature, and explicitly calling commit() for both indices (first for the taxonomy, then for the main index). In old versions of Lucene (2.2 or earlier), when autocommit could not be turned off, a more complicated solution needs to be used. E.g., use some sort of (possibly inter-process) locking to ensure that a reader is being opened only right after both indices have been flushed (and before anything else is written to them).

WARNING: This API is experimental and might change in incompatible ways in the next release.

Method Summary
`int` | `addCategory(CategoryPath categoryPath)` addCategory() adds a category with a given path name to the taxonomy, and returns its ordinal.
`int` | `getParent(int ordinal)` getParent() returns the ordinal of the parent category of the category with the given ordinal.
`int` | `getSize()` getSize() returns the number of categories in the taxonomy.

Methods inherited from interface java.io.Closeable
`close`

Methods inherited from interface org.apache.lucene.util.TwoPhaseCommit
`commit, commit, prepareCommit, prepareCommit, rollback`

Method Detail

### addCategory

    int addCategory(CategoryPath categoryPath)
                    throws IOException

  addCategory() adds a category with a given path name to the taxonomy, and returns its ordinal. If the category was already present in the taxonomy, its existing ordinal is returned.

  Before adding a category, addCategory() makes sure that all its ancestor categories exist in the taxonomy as well. As result, the ordinal of a category is guaranteed to be smaller then the ordinal of any of its descendants.

  Throws:
    `IOException`

### getParent

    int getParent(int ordinal)
                  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.

  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. TODO (Facet): instead of a getParent(ordinal) method, consider having a

  getCategory(categorypath, prefixlen) which is similar to addCategory except it doesn't add new categories; This method can be used to get the ordinals of all prefixes of the given category, and it can use exactly the same code and cache used by addCategory() so it means less code.

  Throws:
    `IOException`

### getSize

    int getSize()

  getSize() returns the number of categories in the taxonomy.

  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).

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