Ubuntu Manpage: QDBM - quick database manager

Provided by: libqdbm-dev_1.8.78-12.1build2_amd64

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)'.

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 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 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 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.

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.