Creating and Using a Database with Perl

Databases are opened in Perl using the tie() function. This function is responsible for “joining” an associative array with a database package. Operations performed on the associative array are then translated by the database package into function calls that operate on the database file itself.

Here is an example of opening a database named “phone.db” using the DB_File database package:

tie (%phone_db, DB_File, "phone.db") ||
        die ("Cannot open phone.db");

This command binds the associative array named phone_db to the Berkeley DB database file named “phone.db”. In this example, the file must exist and must be readable by the Perl script.

Creating a database is nearly as simple as opening one. The following command will create a database named “phone.db” in the current directory with the file's permissions set to read-write for the owner and read-only for everyone else. The file will be created only if it does not already exist. If the database file exists in the current directory, the database file will simply be opened for read-write access by the Perl script.

tie (%phone_db, DB_File, "phone.db", O_CREAT|O_RDWR, 0644) ||
        die ("Cannot create or open phone.db");

The O_CREAT and O_RDWR flags are the same flags used as parameters to the Unix open() system call. They specify that the file should be created if it does not exist and opened with read-write access.

Reading from the database works exactly like reading data from an associative array. If the key is known, specific records can be read from the file with an expression like:

$record = $phone_db{"Bill Smith"};

All the records in the database file can be scanned (in a seemingly random order) with something like:

while (($name, $record) = each %phone_db) {
        [ commands to process data here ]
}

New data can be written into the database by creating a new key in the associative array and setting the key's value. This is done with a command similar to:

$phone_db{"Bill Smith"} = $data;

where $data is the information to be associated with the key “Bill Smith”. Any changes made to the associative array will be written into its corresponding database file.

Keys can be removed from the database in exactly the same way items are removed from an associative array in Perl—by using the delete() function. The following code removes the record in the database that refers to “Bill Smith”.

delete $phone_db{"Bill Smith"};

Changes to an associative array may not be immediately written out to the database file. To insure that changes are successfully written to the database file, the file must be closed.

Closing the database file involves un-binding the associative array from the database package. This is done with the untie() function in the following manner:

untie(%phone_db);

This closes the database file, making updates to the file if necessary. The associative array %phone_db can now no longer be used to access the records in the database.

All of the examples provided here use the default type of Berkeley DB database, the DB_HASH type. This form of database uses a hash table (like Perl does) to store the keys and their values in the database file. Two other types of databases are provided with the Berkeley DB package: DB_BTREE and DB_RECNO.

The DB_BTREE format uses a sorted, balanced binary tree to store the key and value pairs. This format allows data to be stored and read in a sorted order as opposed to the seemingly random order the DB_HASH format produces. The default comparison routine sorts the keys in the database file in lexical order (alphabetically). The DB_File man page discusses this format in more detail and shows how to replace the default comparison routine with one of your own.

The DB_RECNO format is designed to operate on flat text files. It is bound (with tie()) to normal Perl arrays, not associative arrays. Indexing this array with a number provides the text found on that line of the database file. This format is also discussed in more detail in the DB_File man page.

The desired format of database file is specified with an additional parameter for the tie() function.

tie (%phone_db, DB_File, "phone.db", O_RDONLY, 0644, $DB_BTREE) ||
                die ("Cannot open phone.db");

This command will open the DB_BTREE database named “phone.db” in read-only access mode. If the file does not exist, the command fails.