PHP. Руководство по PHP. 2005

Database (dbm-style) Abstraction Layer Functions

XXI. Database (dbm-style) Abstraction Layer Functions

Введение

These functions build the foundation for accessing Berkeley DB style databases.

This is a general abstraction layer for several file-based databases. As such, functionality is limited to a common subset of features supported by modern databases such as Sleepycat Software's DB2. (This is not to be confused with IBM's DB2 software, which is supported through the ODBC functions.)

Требования

The behaviour of various aspects depends on the implementation of the underlying database. Functions such as dba_optimize() and dba_sync() will do what they promise for one database and will do nothing for others. You have to download and install supported dba-Handlers.

Таблица 1. List of DBA handlers

Handler	Notes
dbm	- Dbm is the oldest (original) type of Berkeley DB style databases. You should avoid it, if possible. We do not support the compatibility functions built into DB2 and gdbm, because they are only compatible on the source code level, but cannot handle the original dbm format.
ndbm	- Ndbm is a newer type and more flexible than dbm. It still has most of the arbitrary limits of dbm (therefore it is deprecated).
gdbm	- Gdbm is the GNU database manager.
db2	- DB2 is Sleepycat Software's DB2. It is described as "a programmatic toolkit that provides high-performance built-in database support for both standalone and client/server applications.
db3	- DB3 is Sleepycat Software's DB3.
db4	- DB4 is Sleepycat Software's DB4. This is available since PHP4.3.2.
cdb	- Cdb is "a fast, reliable, lightweight package for creating and reading constant databases." It is from the author of qmail and can be found at http://cr.yp.to/cdb.html. Since it is constant, we support only reading operations. And since PHP4.3.0 we support writing (not updating) through the internal cdb library.
cdb_make	- Since PHP4.3.0 we support creation (not updating) of cdb files when the bundled cdb library is used.
flatfile	- This is available since PHP4.3.0 for compatibility with the deprecated dbm extension only and should be avoided. However you may use this where files were created in this format. That happens when configure could not find any external library.
inifile	- This is available since PHP4.3.3 to be able to modify php.ini files from within PHP scripts. When working with ini files you can pass arrays of the form array(0=>group,1=>value_name) or strings of the form "[group]value_name" where group is optional. As the functions dba_firstkey() and dba_nextkey() return string representations of the key there is a new function dba_key_split() available since PHP5 which allows to convert the string keys into array keys without loosing FALSE.
qdbm	- This is available since PHP5.0.0. The qdbm library can be loaded from http://qdbm.sourceforge.net.

When invoking the dba_open() or dba_popen() functions, one of the handler names must be supplied as an argument. The actually available list of handlers is displayed by invoking phpinfo() or dba_handlers().

Установка

By using the --enable-dba=shared configuration option you can build a dynamic loadable module to enable PHP for basic support of dbm-style databases. You also have to add support for at least one of the following handlers by specifying the --with-XXXX configure switch to your PHP configure line.

Внимание

- After configuring and compiling PHP you must execute the following test from commandline: php run-tests.php ext/dba. This shows whether your combination of handlers works. Most problematic are dbm and ndbm which conflict with many installations. The reason for this is that on several systems these libraries are part of more than one other library. The configuration test only prevents you from configuring malfaunctioning single handlers but not combinations.

Таблица 2. Supported DBA handlers

Handler	Configure Switch
dbm	- To enable support for dbm add --with-dbm[=DIR]. Замечание: dbm normally is a wrapper which often results in failures. This means you should only use dbm if you are sure it works and if you really need this format.
ndbm	- To enable support for ndbm add --with-ndbm[=DIR]. Замечание: ndbm normally is a wrapper which often results in failures. This means you should only use ndbm if you are sure it works and if you really need this format.
gdbm	- To enable support for gdbm add --with-gdbm[=DIR].
db2	- To enable support for db2 add --with-db2[=DIR]. Замечание: db2 conflicts with db3 and db4.
db3	- To enable support for db3 add --with-db3[=DIR]. Замечание: db3 conflicts with db2 and db4.
db4	- To enable support for db4 add --with-db4[=DIR]. Замечание: db4 conflicts with db2 and db3. Замечание: This was added in PHP4.3.2. In earlier versions of PHP you need to use --with-db3=DIR with DIR being the path to db4 library. It is not possible to use db versions starting from 4.1 with PHP prior to version 4.3.0. Also, the db libraries with versions 4.1 through 4.1.24 cannot be used in any PHP version.
cdb	- To enable support for cdb add --with-cdb[=DIR]. Замечание: Since PHP4.3.0 you can omit DIR to use the bundled cdb library that adds the cdb_make handler which allows creation of cdb files and allows to access cdb files on the network using PHP's streams.
flatfile	- To enable support for flatfile add --with-flatfile. Замечание: This was added in PHP4.3.0 to add compatibility with deprecated dbm extension. Use this handler only when you cannot install one of the libraries required by the other handlers and when you cannot use bundled cdb handler.
inifile	- To enable support for inifile add --with-inifile. Замечание: This was added in PHP5.0.0 and allows to read and set microsoft style .ini files (like the php.ini file).
qdbm	- To enable support for qdbm add --with-qdbm[=DIR]. Замечание: qdbm conflicts with dbm and gdbm. Замечание: This was added in PHP5.0.0. The qdbm library can be loaded from http://qdbm.sourceforge.net.

Замечание: Up to PHP4.3.0 you are able to add both db2 and db3 handler but only one of them can be used internally. That means that you cannot have both file formats. Starting with PHP5.0.0 there is a configuration check avoid such misconfigurations.

Настройка во время выполнения

Данное расширение не определяет никакие директивы конфигурации в php.ini.

Типы ресурсов

The functions dba_open() and dba_popen() return a handle to the specified database file to access which is used by all other dba-function calls.

Предопределенные константы

Данное расширение не определяет никакие константы.

Примеры

Пример 1. DBA example


<?
$id = dba_open("/tmp/test.db", "n", "db2");
if (!$id){
echo "dba_open failed\n";
exit;
}
dba_replace("key", "This is an example!", $id);
if (dba_exists("key", $id)){
echo dba_fetch("key", $id);
dba_delete("key", $id);
}
dba_close($id);

?>

DBA is binary safe and does not have any arbitrary limits. However, it inherits all limits set by the underlying database implementation.

All file-based databases must provide a way of setting the file mode of a new created database, if that is possible at all. The file mode is commonly passed as the fourth argument to dba_open() or dba_popen().

You can access all entries of a database in a linear way by using the dba_firstkey() and dba_nextkey() functions. You may not change the database while traversing it.

Пример 2. Traversing a database


<?
//...open database...
$key = dba_firstkey($id);
while ($key != false){
if (true){ //remember the key to perform some action later
$handle_later[] = $key;
}
$key = dba_nextkey($id);
}
foreach ($handle_later as $val){
dba_delete($val, $id);
}

?>

Содержание

dba_close -- Close a DBA database

dba_delete -- Delete DBA entry specified by key

dba_exists -- Check whether key exists

dba_fetch -- Fetch data specified by key

dba_firstkey -- Fetch first key

dba_handlers -- List all the handlers available

dba_insert -- Insert entry

dba_key_split -- Splits a key in string representation into array representation

dba_list -- List all open database files

dba_nextkey -- Fetch next key