org.apache.lucene.facet.taxonomy.writercache

Interface TaxonomyWriterCache

All Known Implementing Classes:

Cl2oTaxonomyWriterCache, LruTaxonomyWriterCache

public interface TaxonomyWriterCache

TaxonomyWriterCache is a relatively simple interface for a cache of category->ordinal mappings, used in TaxonomyWriter implementations (such as DirectoryTaxonomyWriter).

It basically has put() methods for adding a mapping, and get() for looking a mapping up the cache. The cache does not guarantee to hold everything that has been put into it, and might in fact selectively delete some of the mappings (e.g., the ones least recently used). This means that if get() returns a negative response, it does not necessarily mean that the category doesn't exist - just that it is not in the cache. The caller can only infer that the category doesn't exist if it knows the cache to be complete (because all the categories were loaded into the cache, and since then no put() returned true).

However, if it does so, it should clear out large parts of the cache at once, because the user will typically need to work hard to recover from every cache cleanup (see put(CategoryPath, int)'s return value).

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

Method Summary

Methods

Modifier and Type	Method and Description
void	close() Let go of whatever resources the cache is holding.
int	get(CategoryPath categoryPath) Lookup a category in the cache, returning its ordinal, or a negative number if the category is not in the cache.
int	get(CategoryPath categoryPath, int length) Like get(CategoryPath), but for a given prefix of the category path.
boolean	hasRoom(int numberOfEntries) Sometimes the cache is either unlimited in size, or limited by a very big size, and in that case when we add a lot of categories it might make sense to pre-load the cache with all the existing categories.
boolean	put(CategoryPath categoryPath, int ordinal) Add a category to the cache, with the given ordinal as the value.
boolean	put(CategoryPath categoryPath, int prefixLen, int ordinal) Like put(CategoryPath, int), but for a given prefix of the category path.

Method Detail

close

void close()

Let go of whatever resources the cache is holding. After a close(), this object can no longer be used.

get

int get(CategoryPath categoryPath)

Lookup a category in the cache, returning its ordinal, or a negative number if the category is not in the cache.

It is up to the caller to remember what a negative response means: If the caller knows the cache is complete (it was initially fed with all the categories, and since then put() never returned true) it means the category does not exist. Otherwise, the category might still exist, but just be missing from the cache.

get

int get(CategoryPath categoryPath,
      int length)

Like get(CategoryPath), but for a given prefix of the category path.

If the given length is negative or bigger than the path's actual length, the full path is taken.

put

boolean put(CategoryPath categoryPath,
          int ordinal)

Add a category to the cache, with the given ordinal as the value.

If the implementation keeps only a partial cache (e.g., an LRU cache) and finds that its cache is full, it should clear up part of the cache and return true. Otherwise, it should return false.

The reason why the caller needs to know if part of the cache was cleared is that in that case it will have to commit its on-disk index (so that all the latest category additions can be searched on disk, if we can't rely on the cache to contain them).

Ordinals should be non-negative. Currently there is no defined way to specify that a cache should remember a category does NOT exist. It doesn't really matter, because normally the next thing we do after finding that a category does not exist is to add it.

put

boolean put(CategoryPath categoryPath,
          int prefixLen,
          int ordinal)

Like put(CategoryPath, int), but for a given prefix of the category path.

If the given length is negative or bigger than the path's actual length, the full path is taken.

hasRoom

boolean hasRoom(int numberOfEntries)

Sometimes the cache is either unlimited in size, or limited by a very big size, and in that case when we add a lot of categories it might make sense to pre-load the cache with all the existing categories. However, this pre-load does not make sense when the allowed cache size is small. The hasRoom() method allows to differentiate between these cases.

After hasRoom(n) returned true, the following n put() should return false (meaning that the cache was not cleared).