QDBM(3) Quick Database Manager QDBM(3)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 data‐
base, so it is impossible to store two or more records with a key over‐
laps. The following access methods are provided to the database: stor‐
ing 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 pro‐
vided 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. Accord‐
ing 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 stor‐
ing 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)'.
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, prepara‐
tion 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 data‐
base, other readers can be connect, but writers can not. According to
this mechanism, data consistency is guaranteed with simultaneous con‐
nections 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 over‐
writing 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 ineffi‐
cient. However, QDBM deal with this problem by alignment. If incre‐
ment 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 the‐
ory. 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 appli‐
cation is crushed while the transaction, the database file is not bro‐
ken.
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 com‐
pressed and stored in a file. Because each record in a page has simi‐
lar patterns, high efficiency of compression is expected due to 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 direc‐
tory. 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 pro‐
vided. 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 process‐
ing 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 use‐
ful for prototyping, testing, debugging, and so on. The C++ API encap‐
sulates 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 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
-lqdbm
AUTHOR
QDBM is written by Mikio Hirabayashi. You can contact the author by
e-mail to <mikio@fallabs.com>. Any suggestion or bug report is welcome
to the author.
COPYRIGHTCopyright(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 FIT‐
NESS 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.
SEE ALSOdepot(3), curia(3), relic(3), hovel(3), cabin(3), villa(3), odeum(3),
ndbm(3), gdbm(3)Man Page 2004-04-22 QDBM(3)
