berkeleydb.pod

关于Berkelay数据库的共享源码

=item * compact_pages_examine
=item * compact_pages_truncated

=back

You need to be running Berkeley DB 4.4 or better if you wan to make use of
C<compact>.

=head1 CURSORS

A cursor is used whenever you want to access the contents of a database
in sequential order.
A cursor object is created with the C<db_cursor>

A cursor object has the following methods available:

=head2 $newcursor = $cursor->c_dup($flags)

Creates a duplicate of C<$cursor>. This method needs Berkeley DB 3.0.x or better.

The C<$flags> parameter is optional and can take the following value:

=over 5

=item DB_POSITION

When present this flag will position the new cursor at the same place as the
existing cursor.

=back

=head2 $status = $cursor->c_get($key, $value, $flags)

Reads a key/value pair from the database, returning the data in C<$key>
and C<$value>. The key/value pair actually read is controlled by the
C<$flags> parameter, which can take B<one> of the following values:

=over 5

=item B<DB_FIRST>

Set the cursor to point to the first key/value pair in the
database. Return the key/value pair in C<$key> and C<$value>.

=item B<DB_LAST>

Set the cursor to point to the last key/value pair in the database. Return
the key/value pair in C<$key> and C<$value>.

=item B<DB_NEXT>

If the cursor is already pointing to a key/value pair, it will be
incremented to point to the next key/value pair and return its contents.

If the cursor isn't initialised, B<DB_NEXT> works just like B<DB_FIRST>.

If the cursor is already positioned at the last key/value pair, B<c_get>
will return B<DB_NOTFOUND>.

=item B<DB_NEXT_DUP>

This flag is only valid when duplicate keys have been enabled in
a database.
If the cursor is already pointing to a key/value pair and the key of
the next key/value pair is identical, the cursor will be incremented to
point to it and their contents returned.

=item B<DB_PREV>

If the cursor is already pointing to a key/value pair, it will be
decremented to point to the previous key/value pair and return its
contents.

If the cursor isn't initialised, B<DB_PREV> works just like B<DB_LAST>.

If the cursor is already positioned at the first key/value pair, B<c_get>
will return B<DB_NOTFOUND>.

=item B<DB_CURRENT>

If the cursor has been set to point to a key/value pair, return their
contents.
If the key/value pair referenced by the cursor has been deleted, B<c_get>
will return B<DB_KEYEMPTY>.

=item B<DB_SET>

Set the cursor to point to the key/value pair referenced by B<$key>
and return the value in B<$value>.

=item B<DB_SET_RANGE>

This flag is a variation on the B<DB_SET> flag. As well as returning
the value, it also returns the key, via B<$key>.
When used with a B<BerkeleyDB::Btree> database the key matched by B<c_get>
will be the shortest key (in length) which is greater than or equal to
the key supplied, via B<$key>. This allows partial key searches.
See ??? for an example of how to use this flag.

=item B<DB_GET_BOTH>

Another variation on B<DB_SET>. This one returns both the key and
the value.

=item B<DB_SET_RECNO>

TODO.

=item B<DB_GET_RECNO>

TODO.

=back

In addition, the following value may be set by bitwise OR'ing it into
the B<$flags> parameter:

=over 5

=item B<DB_RMW>

TODO.

=back

=head2  $status = $cursor->c_put($key, $value, $flags)

Stores the key/value pair in the database. The position that the data is
stored in the database is controlled by the C<$flags> parameter, which
must take B<one> of the following values:

=over 5

=item B<DB_AFTER>

When used with a Btree or Hash database, a duplicate of the key referenced
by the current cursor position will be created and the contents of
B<$value> will be associated with it - B<$key> is ignored.
The new key/value pair will be stored immediately after the current
cursor position.
Obviously the database has to have been opened with B<DB_DUP>.

When used with a Recno ... TODO


=item B<DB_BEFORE>

When used with a Btree or Hash database, a duplicate of the key referenced
by the current cursor position will be created and the contents of
B<$value> will be associated with it - B<$key> is ignored.
The new key/value pair will be stored immediately before the current
cursor position.
Obviously the database has to have been opened with B<DB_DUP>.

When used with a Recno ... TODO

=item B<DB_CURRENT>

If the cursor has been initialised, replace the value of the key/value
pair stored in the database with the contents of B<$value>.

=item B<DB_KEYFIRST>

Only valid with a Btree or Hash database. This flag is only really
used when duplicates are enabled in the database and sorted duplicates
haven't been specified.
In this case the key/value pair will be inserted as the first entry in
the duplicates for the particular key.

=item B<DB_KEYLAST>

Only valid with a Btree or Hash database. This flag is only really
used when duplicates are enabled in the database and sorted duplicates
haven't been specified.
In this case the key/value pair will be inserted as the last entry in
the duplicates for the particular key.

=back

=head2  $status = $cursor->c_del([$flags])

This method deletes the key/value pair associated with the current cursor
position. The cursor position will not be changed by this operation, so
any subsequent cursor operation must first initialise the cursor to
point to a valid key/value pair.

If the key/value pair associated with the cursor have already been
deleted, B<c_del> will return B<DB_KEYEMPTY>.

The B<$flags> parameter is not used at present.

=head2 $status = $cursor->c_count($cnt [, $flags])

Stores the number of duplicates at the current cursor position in B<$cnt>.

The B<$flags> parameter is not used at present. This method needs 
Berkeley DB 3.1 or better.

=head2  $status = $cursor->status()

Returns the status of the last cursor method as a dual type.

=head2  $status = $cursor->c_pget() ;

TODO

=head2  $status = $cursor->c_close()

Closes the cursor B<$cursor>.

=head2 Cursor Examples

TODO

Iterating from first to last, then in reverse.

examples of each of the flags.

=head1 JOIN

Join support for BerkeleyDB is in progress. Watch this space.

TODO

=head1 TRANSACTIONS

TODO.

=head1 CDS Mode

The Berkeley Db Concurrent Data Store is a lightweight locking mechanism
that is useful in scenarios where transactions are overkill. See the
accompanying document .. for details of using this module in CDS mode.

=head1 DBM Filters

A DBM Filter is a piece of code that is be used when you I<always>
want to make the same transformation to all keys and/or values in a DBM
database. All of the database classes (BerkeleyDB::Hash,
BerkeleyDB::Btree and BerkeleyDB::Recno) support DBM Filters.

There are four methods associated with DBM Filters. All work
identically, and each is used to install (or uninstall) a single DBM
Filter. Each expects a single parameter, namely a reference to a sub.
The only difference between them is the place that the filter is
installed.

To summarise:

=over 5

=item B<filter_store_key>

If a filter has been installed with this method, it will be invoked
every time you write a key to a DBM database.

=item B<filter_store_value>

If a filter has been installed with this method, it will be invoked
every time you write a value to a DBM database.


=item B<filter_fetch_key>

If a filter has been installed with this method, it will be invoked
every time you read a key from a DBM database.

=item B<filter_fetch_value>

If a filter has been installed with this method, it will be invoked
every time you read a value from a DBM database.

=back

You can use any combination of the methods, from none, to all four.

All filter methods return the existing filter, if present, or C<undef>
in not.

To delete a filter pass C<undef> to it.

=head2 The Filter

When each filter is called by Perl, a local copy of C<$_> will contain
the key or value to be filtered. Filtering is achieved by modifying
the contents of C<$_>. The return code from the filter is ignored.

=head2 An Example -- the NULL termination problem.

Consider the following scenario. You have a DBM database that you need
to share with a third-party C application. The C application assumes
that I<all> keys and values are NULL terminated. Unfortunately when
Perl writes to DBM databases it doesn't use NULL termination, so your
Perl application will have to manage NULL termination itself. When you
write to the database you will have to use something like this:

    $hash{"$key\0"} = "$value\0" ;

Similarly the NULL needs to be taken into account when you are considering
the length of existing keys/values.

It would be much better if you could ignore the NULL terminations issue
in the main application code and have a mechanism that automatically
added the terminating NULL to all keys and values whenever you write to
the database and have them removed when you read from the database. As I'm
sure you have already guessed, this is a problem that DBM Filters can
fix very easily.

    use strict ;
    use BerkeleyDB ;

    my %hash ;
    my $filename = "filt.db" ;
    unlink $filename ;

    my $db = tie %hash, 'BerkeleyDB::Hash',
    		-Filename   => $filename,
	        -Flags      => DB_CREATE
      or die "Cannot open $filename: $!\n" ;

    # Install DBM Filters
    $db->filter_fetch_key  ( sub { s/\0$//    } ) ;
    $db->filter_store_key  ( sub { $_ .= "\0" } ) ;
    $db->filter_fetch_value( sub { s/\0$//    } ) ;
    $db->filter_store_value( sub { $_ .= "\0" } ) ;

    $hash{"abc"} = "def" ;
    my $a = $hash{"ABC"} ;
    # ...
    undef $db ;
    untie %hash ;

Hopefully the contents of each of the filters should be
self-explanatory. Both "fetch" filters remove the terminating NULL,
and both "store" filters add a terminating NULL.


=head2 Another Example -- Key is a C int.

Here is another real-life example. By default, whenever Perl writes to
a DBM database it always writes the key and value as strings. So when
you use this:

    $hash{12345} = "something" ;

the key 12345 will get stored in the DBM database as the 5 byte string
"12345". If you actually want the key to be stored in the DBM database
as a C int, you will have to use C<pack> when writing, and C<unpack>
when reading.

Here is a DBM Filter that does it:

    use strict ;
    use BerkeleyDB ;
    my %hash ;
    my $filename = "filt.db" ;
    unlink $filename ;


    my $db = tie %hash, 'BerkeleyDB::Btree',
    		-Filename   => $filename,
	        -Flags      => DB_CREATE
      or die "Cannot open $filename: $!\n" ;

    $db->filter_fetch_key  ( sub { $_ = unpack("i", $_) } ) ;
    $db->filter_store_key  ( sub { $_ = pack ("i", $_) } ) ;
    $hash{123} = "def" ;
    # ...
    undef $db ;
    untie %hash ;

This time only two filters have been used -- we only need to manipulate
the contents of the key, so it wasn't necessary to install any value
filters.

=head1 Using BerkeleyDB with MLDBM

Both BerkeleyDB::Hash and BerkeleyDB::Btree can be used with the MLDBM
module. The code fragment below shows how to open associate MLDBM with
BerkeleyDB::Btree. To use BerkeleyDB::Hash just replace
BerkeleyDB::Btree with BerkeleyDB::Hash.

    use strict ;
    use BerkeleyDB ;
    use MLDBM qw(BerkeleyDB::Btree) ;
    use Data::Dumper;
 
    my $filename = 'testmldbm' ;
    my %o ;
     
    unlink $filename ;
    tie %o, 'MLDBM', -Filename => $filename,
                     -Flags    => DB_CREATE
                    or die "Cannot open database '$filename: $!\n";
 
See the MLDBM documentation for information on how to use the module
and for details of its limitations.

=head1 EXAMPLES

TODO.

=head1 HINTS & TIPS

=head2 Sharing Databases With C Applications

There is no technical reason why a Berkeley DB database cannot be
shared by both a Perl and a C application.

The vast majority of problems that are reported in this area boil down
to the fact that C strings are NULL terminated, whilst Perl strings
are not. See L<An Example -- the NULL termination problem.> in the DBM
FILTERS section for a generic way to work around this problem.


=head2 The untie Gotcha

TODO

=head1 COMMON QUESTIONS

This section attempts to answer some of the more common questions that
I get asked.


=head2 Relationship with DB_File

Before Berkeley DB 2.x was written there was only one Perl module that
interfaced to Berkeley DB. That module is called B<DB_File>. Although
B<DB_File> can be build with Berkeley DB 1.x, 2.x, 3.x or 4.x, it only
provides an interface to the functionality available in Berkeley DB
1.x. That means that it doesn't support transactions, locking or any of
the other new features available in DB 2.x or better.

=head2 How do I store Perl data structures with BerkeleyDB?

See L<Using BerkeleyDB with MLDBM>.

=head1 HISTORY

See the Changes file.

=head1 AVAILABILITY

The most recent version of B<BerkeleyDB> can always be found
on CPAN (see L<perlmod/CPAN> for details), in the directory
F<modules/by-module/BerkeleyDB>.

The official web site for Berkeley DB is F<http://www.sleepycat.com>.

=head1 COPYRIGHT

Copyright (c) 1997-2004 Paul Marquess. All rights reserved. This program
is free software; you can redistribute it and/or modify it under the
same terms as Perl itself.

Although B<BerkeleyDB> is covered by the Perl license, the library it
makes use of, namely Berkeley DB, is not. Berkeley DB has its own
copyright and its own license. Please take the time to read it.

Here are few words taken from the Berkeley DB FAQ (at
F<http://www.sleepycat.com>) regarding the license:

    Do I have to license DB to use it in Perl scripts?

    No. The Berkeley DB license requires that software that uses
    Berkeley DB be freely redistributable. In the case of Perl, that
    software is Perl, and not your scripts. Any Perl scripts that you
    write are your property, including scripts that make use of Berkeley
    DB. Neither the Perl license nor the Berkeley DB license
    place any restriction on what you may do with them.

If you are in any doubt about the license situation, contact either the
Berkeley DB authors or the author of BerkeleyDB.
See L<"AUTHOR"> for details.


=head1 AUTHOR

Paul Marquess E<lt>[e-mail]<gt>.

Questions about Berkeley DB may be addressed to E<lt>[e-mail]<gt>.

=head1 SEE ALSO

perl(1), DB_File, Berkeley DB.

=cut