com.bigdata.htree.data.IDirectoryData Maven / Gradle / Ivy
/**
Copyright (C) SYSTAP, LLC DBA Blazegraph 2006-2016. All rights reserved.
Contact:
SYSTAP, LLC DBA Blazegraph
2501 Calvert ST NW #106
Washington, DC 20008
[email protected]
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; version 2 of the License.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
*/
/*
* Created on Dec 2, 2010
*/
package com.bigdata.htree.data;
import com.bigdata.btree.data.ITreeNodeData;
import com.bigdata.rawstore.IRawStore;
/**
* Interface for the data record of a hash directory. A hash directory provides
* an address space. In a hash table, all of the children are buckets. In a hash
* tree, children may be either buckets or directories. If the hash tree is
* balanced, then all children on the same level are of the same type (either
* buckets or directories). If the hash tree is unbalanced, then children may be
* of either type on a given level.
*
* The hash directory provides an index which maps a subset of the bits in a
* hash value onto a directory entry. The directory entry provides the storage
* address of the child page to which a lookup with that hash value would be
* directed. The directory entry also indicates whether the child is a bucket or
* another directory. This requires 1-bit per directory entry, which amounts to
* an overhead of 3% when compared to a record which manages to encode the
* bucket / directory distinction into the storage address.
*
* The number of entries in a hash directory is a function of the globalDepth of
* that directory: entryCount := 2^globalDepth. The globalDepth of
* a child directory is its localDepth in the parent directory. While the
* localDepth of a persistent child may be computed by scanning and
* counting the #of references to that child, the copy-on-write policy used to
* support MVCC for the hash tree requires that the storage address of a dirty
* child is undefined. Therefore, the localDepth MUST be explicitly stored in
* the directory record. Assuming 32-bit hash codes, this is a cost of 4 bits
* per directory entry which amounts to a 11% overhead when compared to a record
* which manages to encode that information using a scan of the directory
* entries.
*
* By far the largest storage cost associated with a directory page are the
* addresses of the child pages. Bigdata uses long addresses for
* the {@link IRawStore} interface. However, it is possible to get by with int32
* addresses when using the RWStore.
*
* Finally, bigdata uses checksums on all data records. Therefore the maximum
* space available on a 4k page is actually 4096-4 := 4094 bytes. [Yikes! This
* means that we can not store power of 2 addresses efficiently. That means that
* we really need to use a compressed dictionary representation in order to have
* efficient storage utilization with good fan out.]
*/
public interface IDirectoryData extends ITreeNodeData {
/**
* true iff this is an overflow directory page. An overflow
* directory page is created when a bucket page overflows as the parent of
* that bucket page. The children of the overflow directory page may be
* other overflow directory pages or bucket pages. All bucket pages below an
* overflow directory page will have the same key. That key is recorded once
* in each overflow bucket page.
*/
public boolean isOverflowDirectory();
/**
* If this is an overflow directory, then there is a single key for which
* the directory will reference multiple BucketPages storing the associated
* values. The key is used to constrain insertions to the Directory, adding
* extra levels to discriminate as necessary.
*/
public byte[] getOverflowKey();
}