Database (dbm-style) abstraction layer functions

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# XVII. Database (dbm-style) abstraction layer functions

# Introduction

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.)

# Requirements

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.

Table 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.
`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 here. Since it is constant, we support only reading operations.

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().

# Installation

By using the `--enable-dba=shared` configuration option you can build a dynamic loadable modul 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.

Table 2. Supported DBA handlers

Handler | Configure Switch
--- | ---
`dbm` | To enable support for dbm add `--with-dbm[=DIR]`.
`ndbm` | To enable support for ndbm add `--with-ndbm[=DIR]`.
`gdbm` | To enable support for gdbm add `--with-gdbm[=DIR]`.
`db2` | To enable support for db2 add `--with-db2[=DIR]`.
`db3` | To enable support for db3 add `--with-db3[=DIR]`.
`cdb` | To enable support for cdb add `--with-cdb[=DIR]`.

# Runtime Configuration

This extension has no configuration directives defined in `php.ini`.

# Resource Types

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.

# Predefined Constants

This extension has no constants defined.

# Examples

Example 1. DBA example
    <?php

    $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.

Example 2. Traversing a database
    <?php

    // ...open database...

    $key = dba_firstkey ($id);

    while ($key != false) {
        if (...) {          // remember the key to perform some action later
            $handle_later[] = $key;
        }
        $key = dba_nextkey ($id);
    }

    for ($i = 0; $i < count($handle_later); $i++)
        dba_delete ($handle_later[$i], $id);

    ?>

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