QDBM(3) Man Page QDBM(3)
Quick Database Manager Quick Database Manager
2004-04-22
NAME
QDBM - quick database manager
OVERVIEW
QDBM is a library of routines for managing a database. The database
is a simple data file containing records, each is a pair of a key and
a value. Every key and value is serial bytes with variable length.
Both binary data and character string can be used as a key and a
value. There is neither concept of data tables nor data types.
Records are organized in hash table or B+ tree.
As for database of hash table, each key must be unique within a
database, so it is impossible to store two or more records with a key
overlaps. The following access methods are provided to the database:
storing a record with a key and a value, deleting a record by a key,
retrieving a record by a key. Moreover, traversal access to every key
are provided, although the order is arbitrary. These access methods
are similar to ones of DBM (or its followers: NDBM and GDBM) library
defined in the UNIX standard. QDBM is an alternative for DBM because
of its higher performance.
As for database of B+ tree, records whose keys are duplicated can be
stored. Access methods of storing, deleting, and retrieving are
provided as with the database of hash table. Records are stored in
order by a comparing function assigned by a user. It is possible to
access each record with the cursor in ascending or descending order.
According to this mechanism, forward matching search for strings and
range search for integers are realized. Moreover, transaction is
available in database of B+ tree.
EFFECTIVE IMPLEMENTATION OF HASH DATABASE
QDBM is developed referring to GDBM for the purpose of the following
three points: higher processing speed, smaller size of a database
file, and simpler API. They have been achieved. Moreover, the
following three restrictions of traditional DBM: a process can handle
only one database, the size of a key and a value is bounded, a
database file is sparse, are cleared.
QDBM uses hash algorithm to retrieve records. If a bucket array has
sufficient number of elements, the time complexity of retrieval is
`O(1)'. That is, time required for retrieving a record is constant,
regardless of the scale of a database. It is also the same about
storing and deleting. Collision of hash values is managed by separate
chaining. Data structure of the chains is binary search tree. Even
if a bucket array has unusually scarce elements, the time complexity
of retrieval is `O(log n)'.
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Quick Database Manager Quick Database Manager
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QDBM attains improvement in retrieval by loading RAM with the whole of
a bucket array. If a bucket array is on RAM, it is possible to access
a region of a target record by about one path of file operations. A
bucket array saved in a file is not read into RAM with the `read' call
but directly mapped to RAM with the `mmap' call. Therefore,
preparation time on connecting to a database is very short, and two or
more processes can share the same memory map.
If the number of elements of a bucket array is about half of records
stored within a database, although it depends on characteristic of the
input, the probability of collision of hash values is about 56.7%
(36.8% if the same, 21.3% if twice, 11.5% if four times, 6.0% if eight
times). In such case, it is possible to retrieve a record by two or
less paths of file operations. If it is made into a performance
index, in order to handle a database containing one million of
records, a bucket array with half a million of elements is needed.
The size of each element is 4 bytes. That is, if 2M bytes of RAM is
available, a database containing one million records can be handled.
QDBM provides two modes to connect to a database: `reader' and
`writer'. A reader can perform retrieving but neither storing nor
deleting. A writer can perform all access methods. Exclusion control
between processes is performed when connecting to a database by file
locking. While a writer is connected to a database, neither readers
nor writers can be connected. While a reader is connected to a
database, other readers can be connect, but writers can not.
According to this mechanism, data consistency is guaranteed with
simultaneous connections in multitasking environment.
Traditional DBM provides two modes of the storing operations: `insert'
and `replace'. In the case a key overlaps an existing record, the
insert mode keeps the existing value, while the replace mode
transposes it to the specified value. In addition to the two modes,
QDBM provides `concatenate' mode. In the mode, the specified value is
concatenated at the end of the existing value and stored. This
feature is useful when adding a element to a value as an array.
Moreover, although DBM has a method to fetch out a value from a
database only by reading the whole of a region of a record, QDBM has a
method to fetch out a part of a region of a value. When a value is
treated as an array, this feature is also useful.
Generally speaking, while succession of updating, fragmentation of
available regions occurs, and the size of a database grows rapidly.
QDBM deal with this problem by coalescence of dispensable regions and
reuse of them, and featuring of optimization of a database. When
overwriting a record with a value whose size is greater than the
existing one, it is necessary to remove the region to another position
of the file. Because the time complexity of the operation depends on
the size of the region of a record, extending values successively is
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Quick Database Manager Quick Database Manager
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inefficient. However, QDBM deal with this problem by alignment. If
increment can be put in padding, it is not necessary to remove the
region.
As for many file systems, it is impossible to handle a file whose size
is more than 2GB. To deal with this problem, QDBM provides a
directory database containing multiple database files. Due to this
feature, it is possible to handle a database whose total size is up to
1TB in theory. Moreover, because database files can be deployed on
multiple disks, the speed of updating operations can be improved as
with RAID-0 (striping). It is also possible for the database files to
deploy on multiple file servers using NFS and so on.
USEFUL IMPLEMENTATION OF B+ TREE DATABASE
Although B+ tree database is slower than hash database, it features
ordering access to each record. The order can be assigned by users.
Records of B+ tree are sorted and arranged in logical pages. Sparse
index organized in B tree that is multiway balanced tree are
maintained for each page. Thus, the time complexity of retrieval and
so on is `O(log n)'. Cursor is provided to access each record in
order. The cursor can jump to a position specified by a key and can
step forward or backward from the current position. Because each page
is arranged as double linked list, the time complexity of stepping
cursor is `O(1)'.
B+ tree database is implemented, based on above hash database.
Because each page of B+ tree is stored as each record of hash
database, B+ tree database inherits efficiency of storage management
of hash database. Because the header of each record is smaller and
alignment of each page is calculated statistically, in most cases, the
size of database file is cut by half compared to one of hash database.
Although operation of many pages are required to update B+ tree, QDBM
expedites the process by caching pages and reducing file operations.
In most cases, because whole of the sparse index is cached on memory,
it is possible to retrieve a record by one or less path of file
operations.
B+ tree database features transaction mechanism. It is possible to
commit a series of operations between the beginning and the end of the
transaction in a lump, or to abort the transaction and perform
rollback to the state before the transaction. Even if the process of
an application is crushed while the transaction, the database file is
not broken.
In case that QDBM is built with ZLIB, LZO, or BZIP2 enabled, a
lossless data-compression library, the content of each page of B+ tree
is compressed and stored in a file. Because each record in a page has
similar patterns, high efficiency of compression is expected due to
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the Lempel-Ziv algorithm and the like. In case handling text data,
the size of a database is reduced to about 25%. If the scale of a
database is large and disk I/O is the bottleneck, featuring
compression makes the processing speed improved to a large extent.
SIMPLE BUT VARIOUS INTERFACES
QDBM provides very simple APIs. You can perform database I/O as usual
file I/O with `FILE' pointer defined in ANSI C. In the basic API of
QDBM, entity of a database is recorded as one file. In the extended
API, entity of a database is recorded as several files in one
directory. Because the two APIs are very similar with each other,
porting an application from one to the other is easy.
APIs which are compatible with NDBM and GDBM are also provided. As
there are a lot of applications using NDBM or GDBM, it is easy to port
them onto QDBM. In most cases, it is completed only by replacement of
header including (#include) and re-compiling. However, QDBM can not
handle database files made by the original NDBM or GDBM.
In order to handle records on memory easily, the utility API is
provided. It implements memory allocating functions, sorting
functions, extensible datum, array list, hash map, and so on. Using
them, you can handle records in C language cheaply as in such script
languages as Perl or Ruby.
B+ tree database is used with the advanced API. The advanced API is
implemented using the basic API and the utility API. Because the
advanced API is also similar to the basic API and the extended API, it
is easy to learn how to use it.
In order to handle an inverted index which is used by full-text search
systems, the inverted API is provided. If it is easy to handle an
inverted index of documents, an application can focus on text
processing and natural language processing. Because this API does not
depend on character codes nor languages, it is possible to implement a
full-text search system which can respond to various requests from
users.
Along with APIs for C, QDBM provides APIs for C++, Java, Perl, and
Ruby. APIs for C are composed of seven kinds: the basic API, the
extended API, the NDBM-compatible API, the GDBM-compatible API, the
utility API, the advanced API, and the inverted API. Command line
interfaces corresponding to each API are also provided. They are
useful for prototyping, testing, debugging, and so on. The C++ API
encapsulates database handling functions of the basic API, the
extended API, and the advanced API with class mechanism of C++. The
Java API has native methods calling the basic API, the extended API,
and the advanced API with Java Native Interface. The Perl API has
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methods calling the basic API, the extended API, and the advanced API
with XS language. The Ruby API has method calling the basic API, the
extended API, and the advanced API as modules of Ruby. Moreover, CGI
scripts for administration of databases and full-text search are
provided.
WIDE PORTABILITY
QDBM is implemented being based on syntax of ANSI C (C89) and using
only APIs defined in ANSI C or POSIX. Thus, QDBM works on most UNIX
and its compatible OSs. As for C API, checking operations have been
done at least on Linux 2.2, Linux 2.4, FreeBSD 4.8, FreeBSD 5.0, SunOS
5.7, SunOS 5.8, SunOS 5.9, HP-UX 11.00, Cygwin 1.3.10, Mac OS X 10.2,
and RISC OS 5.03. Although a database file created by QDBM depends on
byte order of the processor, to do with it, utilities to dump data in
format which is independent to byte orders are provided.
BUILDING
For building a program using QDBM, the program should be linked with a
library file `libqdbm.a' or `libqdbm.so'. For example, the following
command is executed to build `sample' from `sample.c'.
gcc -I/usr/local/include -o sample sample.c -L/usr/local/lib
AUTHOR
QDBM is written by Mikio Hirabayashi. You can contact the author by
e-mail to <mikio@users.sourceforge.net>. Any suggestion or bug report
is welcome to the author.
COPYRIGHT
Copyright(c) 2000-2003 Mikio Hirabayashi
QDBM is free software; you can redistribute it and/or modify it under
the terms of the GNU Lesser General Public License as published by the
Free Software Foundation; either version 2 of the License, or any
later version.
QDBM 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 Lesser General Public
License for more details.
You should have received a copy of the GNU Lesser General Public
License along with QDBM; if not, write to the Free Software
Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307
USA.
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SEE ALSO
depot(3), curia(3), relic(3), hovel(3), cabin(3), villa(3), odeum(3),
ndbm(3), gdbm(3)
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