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
       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@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.


SEE ALSO
       depot(3), curia(3), relic(3), hovel(3), cabin(3), villa(3), odeum(3),
       ndbm(3), gdbm(3)



Man Page                          2004-04-22                           QDBM(3)
