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package com.sleepycat.je;
/**
* A tag interface used to mark a BTree or duplicate comparator class as a
* binary equality comparator, that is, a comparator that considers
* two keys (byte arrays) to be equal if and only if they have the same
* length and they are equal byte-per-byte.
*
* If both the BTree and duplicate comparators used by a databse are
* binary-equality comparators, then certain internal optimizations can be
* enabled. Specifically, the "BIN-delta blind-puts" optimization described
* below is made possible.
*
* We say that a record operation (insertion, update, or deletion) is performed
* blindly in a BIN-delta when the delta does not contain a slot with the
* operation's key and we don't need to access the full BIN to check whether
* such a slot exists there or to extract any information from the full-BIN
* slot, if it exists. Performing a blind operation involves inserting the
* record in the BIN-delta, and in case of deletion, marking the BIN slot as
* deleted. When the delta and the full BIN are merged at a later time, the
* blind operation will be translated to an insertion, update, or delete
* depending on whether the full BIN contained the record or not.
*
* Normally, blind puts are not possible: we need to know whether the put
* is actually an update or an insertion, i.e., whether the key exists in
* the full BIN or not. Furthermore, in case of update we also need to
* know the location of the previous record version to make the current
* update abortable. However, it is possible to answer at least the key
* existence question by adding a small amount of extra information in
* the deltas. If we do so, puts that are actual insertions can be done
* blindly.
*
* To answer whether a key exists in a full BIN or not, each BIN-delta
* stores a bloom filter, which is a very compact, approximate
* representation of the set of keys in the full BIN. Bloom filters can
* answer set membership questions with no false negatives and very low
* probability of false positives. As a result, put operation that are
* actual insertions can almost always be performed blindly.
*
* Because bloom filters work by applying hash functions on keys (where each
* key byte participates in the hash computation), an additional requirement
* for blind puts is that a database uses "binary equality" comparators, that
* is, a comparator that considers two keys to be equal if and only if they
* have the same length and they are equal byte-per-byte. Inheriting from the
* BinaryEqualityComparator interface marks an actual comparator as having the
* "binary equality" property.
*
* Comparators are configured using
* {@link DatabaseConfig#setBtreeComparator(java.util.Comparator)} or
* {@link DatabaseConfig#setBtreeComparator(Class)}, and
* {@link DatabaseConfig#setDuplicateComparator(java.util.Comparator)} or
* {@link DatabaseConfig#setDuplicateComparator(Class)}.
*
* As described in the javadoc for these methods, comparators must be used
* with great caution, since a badly behaved comparator can cause B-tree
* corruption.
*/
public interface BinaryEqualityComparator {
}