=head1 mod_perl and dbm files



=head1 Where and Why to use dbm files

Some of the earliest databases implemented on Unix were dbm files, and
many are still in use today.  As of this writing the Berkeley DB is
the most powerful dbm implementation (http://www.sleepycat.com).

If you need a light database, with an easy API, using simple key-value
pairs to store and manipulate a relatively small number of records,
this is a solution that should be amongst the first you consider.

With dbm, it is rare to read the whole database into memory.  Combine
this feature with the use of smart storage techniques, and dbm files
can be manipulated much faster than flat files.  Flat file databases
can be very slow on insert, update and delete operations, when the
number of records starts to grow into the thousands.  Sort algorithms
on flat files can be very time-consuming.

The maximum practical size of a dbm database depends on many
factors--your data, your hardware and the desired response times of
course included--but as a rough guide consider 5,000 to 10,000 records
to be reasonable.

We will talk mostly about the Berkley DB version 1.x, as it provides
the best functionality while having a good speed and almost no
limitations. Other implementations might be faster in some cases, but
they are either limited in the length of the maximum value or the
total number of records.

There is a number of Perl interfaces to the major dbm implementations,
to list a few: C<DB_File>, C<NDBM_File>, C<ODBM_File>, C<GDBM_File>,
and C<SDBM_File>.  The original Perl module for Berkeley DB was
DB_File, which was written to interface to Berkeley DB version
1.85. The newer Perl module for Berkeley DB is C<BerkeleyDB>, which
was written to interface to version 2.0 and subsequent
releases. Because Berkeley DB version 2.X has a compatibility API for
version 1.85, you can (and should!) build C<DB_File> using version 2.X
of Berkeley DB, although C<DB_File> will still only support the 1.85
functionality.

Several different indexing algorithms (known also as access methods)
can be used with dbm implementations:

=over

=item *

The C<HASH> access method gives an C<0(1)> complexity of search and
update, fast insert and delete, but a slow sort (which you have to
implement yourself). (Used by almost all dbm implementations)

=item *

The C<BTREE> access method allows arbitrary key/value pairs to be
stored in a sorted, balanced binary tree.  This allows us to get a
sorted sequence of data pairs in C<0(1)>, but at the expense of much
slower insert, update, delete operations than is the case with
C<HASH>.  (Available mostly in Berkeley DB)

=item *

The C<RECNO> access method is more complicated, and enables both
fixed-length and variable-length flat text files to be manipulated
using the same key/value pair interface as in C<HASH> and C<BTREE>.
In this case the key will consist of a record (line) number.
(Available mostly in Berkeley DB)

=item *

The C<QUEUE> access method stores fixed-length records with logical
record numbers as keys. It is designed for fast inserts at the tail
and has a special cursor consume operation that deletes and returns a
record from the head of the queue. The C<QUEUE> access method uses
record level locking.  (Available only in Berkeley DB version 3.x)

=back

Most often you will want to use the C<HASH> method, but there are many
considerations and your choice may be dictated by your application.

In recent years dbm databases have been extended to allow you to store
more complex values, including data structures.  The C<MLDBM> module
can store and restore the whole symbol table of your script, including
arrays and hashes.

It is important to note that you cannot simply switch a dbm file from
one storage algorithm to another.  The only way to change the
algorithm is to copy all the records one by one into a new dbm file,
which was initialized according to a desired access method.  You can
use a script like this:

  #!/usr/bin/perl -w
  
  #
  # This script takes as its parameters a list of Berkeley DB
  # file(s) which are stored with the DB_BTREE algorithm.  It
  # will back them up using the .bak extension and create
  # instead dbms with the same records but stored using the
  # DB_HASH algorithm
  #
  # Usage: btree2hash.pl filename(s)
  
  use strict;
  use DB_File;
  use Fcntl;
  
    # Do checks 
  die "Usage: btree2hash.pl filename(s))\n" unless @ARGV;
  
  foreach my $filename (@ARGV) {
  
    die "Can't find $filename: $!\n" 
      unless -e $filename and -r $filename;
  
      # First backup the file
    rename "$filename", "$filename.btree" 
      or die "can't rename $filename $filename.btree:$!\n";
  
      # tie both dbs (db_hash is a fresh one!)
    tie my %btree , 'DB_File',"$filename.btree", O_RDWR|O_CREAT, 
        0660, $DB_BTREE or die "Can't tie $filename.btree: $!";
    tie my %hash ,  'DB_File',"$filename" , O_RDWR|O_CREAT, 
        0660, $DB_HASH  or die "Can't tie $filename: $!";
  
      # copy DB
    %hash = %btree;
  
      # untie
    untie %btree ;
    untie %hash ;
  }

Note that some dbm implementations come with other conversion
utilities as well.

=head1 mod_perl and dbm

Where does mod_perl fit into the picture?

If you need to access a dbm file in your mod_perl code in the read
only mode the operation would be much faster if you keep the dbm file
open (tied) all the time and therefore ready to be used. This will
work with dynamic (read/write) databases accesses as well, but you
need to use locking and data flushing to avoid data corruption.

Although mod_perl and dbm can give huge performance gains compared to
the use of flat file databases you should be very careful.  In
addition to the need for locking, you need to consider the
consequences of C<die()> and unexpected process death.

If your locking mechanism cannot handle dropped locks, a stale lock
can deactivate your whole site.  You can enter a deadlock situation if
two processes simultaneously try to acquire locks on two separate
databases.  Each has locked only one of the databases, and cannot
continue without locking the second.  Yet this will never be freed
because it is locked by the other process.  If your processes all ask
for their DB files in the same order, this situation cannot occur.

If you modify the DB you should be make very sure that you flush the
data and synchronize it, especially when the process serving your
handler unexpectedly dies.  In general your application should be
tested very thoroughly before you put it into production to handle
important data.

=head1 Locking dbm Handlers and Write Lock Starvation Hazards

One has to deploy dbm file locking if there is chance that some
process will want to write to it. Note that once you need to do
locking you do it even when all you want is to read from the
file. Since if you don't, it's possible that someone writes to the
file at this very moment and you may read partly updated data.

Therefore we should distinguish between I<READ> and I<WRITE>
locks. Before doing an operation on the dbm file, we first issue
either a I<READ> or a I<WRITE> lock request, according to our needs.

If we are making a I<READ> lock request, it is granted as soon as the
I<WRITE> lock on the file is removed if any or if it is already
I<READ> locked.  The lock status becomes I<READ> on success.

If we make a I<WRITE> lock request, it is granted as soon as the file
becomes unlocked.  The lock status becomes I<WRITE> on success.

The treatment of the I<WRITE> lock request is most important.

If the DB is I<READ> locked, a process that makes a I<WRITE> request
will poll until there are no reading or writing processes left.  Lots
of processes can successfully read the file, since they do not block
each other.  This means that a process that wants to write to the file
may never get a chance to squeeze in, since it needs to obtain an
exclusive lock.

The following diagram represents a possible scenario where everybody
can read but no one can write (pX's represent different processes
running for different times and all acquiring the read lock on the dbm
file):

  [-p1-]                 [--p1--]
     [--p2--]                [--p2--]
   [---------p3---------] [--------p3----....
                 [------p4-----]

The result is a starving process, which will timeout the request, and
it will fail to update the DB.  Ken Williams solved the above problem
with his L<C<Tie::DB_Lock>|dbm/Tie__DB_Lock> module, which is
discussed in one of the following sections.

There are several locking wrappers for C<DB_File> in CPAN right now.
Each one implements locking differently and has different goals in
mind.  It is therefore worth knowing the difference, so that you can
pick the right one for your application.


=head1 Flawed Locking Methods Which Must Not Be Used

I<Caution>: The suggested locking methods in the Camel book and
C<DB_File> man page (before version 1.72, fixed in 1.73) are flawed.
If you use them in an environment where more than one process can
modify the dbm file, it can get corrupted!!!  The following is an
explanation of why this happens.

You may not use a tied file's filehandle for locking, since you get
the filehandle after the file has been already tied.  It's too late to
lock.  The problem is that the database file is locked I<after> it is
opened.  When the database is opened, the first 4k (in Berkley dbm
library) is read and then cached in memory.  Therefore, a process can
open the database file, cache the first 4k, and then block while
another process writes to the file.  If the second process modifies
the first 4k of the file, when the original process gets the lock is
now has an inconsistent view of the database.  If it writes using this
view it may easily corrupt the database on disk.

This problem can be difficult to trace because it does not cause
corruption every time a process has to wait for a lock.  One can do
quite a bit of writing to a database file without actually changing
the first 4k.  But once you suspect this problem you can easily
reproduce it by making your program modify the records in the first 4k
of the DB.

You better resort to using the standard modules for locking instead of
inventing your own.

If your dbm file is used only in the read-only mode generally there is
no need for locking at all. If you access the dbm file in read/write
mode, the safest method is to tie() the dbm file after acquiring an
external lock and untie() before the lock is released. So to access
the file in shared mode (FLOCK_SH) one should following this pseudo-code:

  flock FLOCK_SH <===== start critical section
  tie()
  read...
  untie()
  flock FLOCK_UN <===== end critical section

Similar for the exclusive (EX), write access:

  flock FLOCK_EX <===== start critical section
  tie()
  write...
  sync()
  untie()
  flock FLOCK_UN <===== end critical section

However you might want to save a few tie()/untie() calls if the same
request accesses the dbm file more than once. You should be careful
though. Based on the caching effect explained above, a process can
perform an atomic downgrade of an exclusive lock to a shared one
without re-tie()ing the file:

  flock FLOCK_EX <===== start critical section
  tie()
  write...
  sync()
                 <===== end critical section
  flock FLOCK_SH <===== start critical section
  read...
  untie()
  flock FLOCK_UN <===== end critical section

because it has the updated data in its cache. By atomic, we mean it's
ensured that the lock status gets changed, without any other process
getting an exclusive access in between.

If you can ensure that one process safely upgrades a shared lock with
an exclusive lock, one can save on tie()/untie(). But this operation
might lead to a dead-lock if two processes try to upgrade a shared
lock with exclusive at the same time. Remember that in order to
acquire an exclusive lock, all other processes need to release *all*
locks. If your OS locking implementation resolves this deadlock by
denying one of the upgrade requests, make sure your program handles
that appropriately. The process that were denied has to untie() the
dbm file and then ask for an exclusive lock.

A dbm file has always to be untie()'ed before the locking is released
(unless you do an atomic downgrade from exclusive to shared as we have
just explained). Remember that if at any given moment a process wants
to lock and access the dbm file it has to re-tie() this file, if it
was tied already. If this is not done, the integrity of the dbm file
is not ensured.

To conclude, the safest method of reading from dbm file is to lock the
file before tie()-ing it, untie() it before releasing the lock, and
in the case of write to call sync() before untie()-ing it.




=head1 Locking Wrappers Overview

Here are some of the correctly working dbm locking wrappers on (three
of them are available from CPAN):

=over

=item *

C<Tie::DB_Lock> -- C<DB_File> wrapper which creates copies of the dbm
file for read access, so that you have kind of a multiversioning
concurrent read system. However, updates are still serial. After each
update the read-only copies of the dbm file are recreated. Use this
wrapper in situations where reads may be very lengthy and therefore
write starvation problem may occur. On the other hand if you have big
dbm files, it may create a big load on the system if the updates are
quite frequent.  L<More information|dbm/Tie__DB_Lock>.

=item *

C<Tie::DB_FileLock> -- C<DB_File> wrapper that has the ability to lock
and unlock the database while it is being used.  Avoids the
tie-before-flock problem by simply re-tie-ing the database when you
get or drop a lock.  Because of the flexibility in dropping and
re-acquiring the lock in the middle of a session, this can be used in
a system that will work with long updates and/or reads.  Refer to the
C<Tie::DB_FileLock> manpage for more information.

=item *

C<DB_File::Lock> -- extremely lightweight C<DB_File> wrapper that
simply flocks an external lockfile before tie-ing the database and
drops the lock after untie.  Allows one to use the same lockfile for
multiple databases to avoid deadlock problems, if desired.  Use this
for databases where updates and reads are quick and simple flock
locking semantics are enough.  Refer to C<DB_File::Lock> manpage for
more information.

=item *

L<C<DB_File::Lock2>|dbm/DB_File__Lock2> -- does the same thing as
C<DB_File::Lock>, but has a slightly different implementation.  I
wrote it before David Harris released his C<DB_File::Lock> and I
didn't want to kill mine, so I'll keep it here for a while :).

=item *

On some Operating Systems (FreeBSD is one example) it is possible to
lock on tie:

  tie my %t, 'DB_File', $TOK_FILE, O_RDWR | O_EXLOCK, 0664;

and only release the lock by un-tie()-ing the file.  Check if the
C<O_EXLOCK> flag is available on your operating system before you try
to use this method!

=back


=head1 Tie::DB_Lock

C<Tie::DB_Lock> ties hashes to databases using shared and exclusive
locks.  This module, by Ken Williams, solves the problems raised in
the previous section.

The main difference from what I have described above is that
C<Tie::DB_Lock> copies a dbm file on read.  Reading processes do not
have to keep the file locked while they read it, and writing processes
can still access the file while others are reading.  This works best
when you have lots of long-duration reading, and a few short bursts of
writing.

The drawback of this module is the heavy IO performed when every
reader makes a fresh copy of the DB.  With big dbm files this can be
quite a disadvantage and can slow the server down considerably.

An alternative would be to have one copy of the dbm image shared by
all the reading processes.  This can cut the number of files that are
copied, and puts the responsibility of copying the read-only file on
the writer, not the reader.  It would need some care to make sure it
does not disturb readers when putting a new read-only copy into place.




=head1 DB_File::Lock2

=code DB_File-Lock2.pm Here is C<DB_File::Lock2> which does the
locking by using an external lockfile.

This allows you to gain the lock before the file is tied.  Note that
it's not yet on CPAN and so is linked from here in its entirety.  Note
also that this code still needs some testing, so I<be careful> if you
use it on a production machine.

You use it like this:

  use DB_File::Lock2 ();

A simple tie, READ lock and untie

  use DB_File::Lock2 ();
  my $dbfile = "/tmp/test";
  tie my %mydb, 'DB_File::Lock2', $dbfile, 'read';
  print $mydb{foo} if exists $mydb{foo};
  untie %mydb;

You can even skip the C<untie()> call.  When C<$mydb> goes out of
scope everything will be done automatically.  However it is better to
use the explicit call, to make sure the critical sections between lock
and unlock are as short as possible.  This is especially important
when requesting an exclusive (write) lock.

The following example shows how it might be convenient to skip the
explicit C<untie()>.  In this example, we don't need to save the
intermediate result, we just return and the cleanup is done
automatically.

  use DB_File::Lock2 ();
  my $dbfile = "/tmp/test";
  print user_exists("stas") ? "Yes" : "No";
  sub user_exists{
    my $username = shift || '';
  
    warn("No username passed\n"), return 0 unless $username;
  
    tie my %mydb, 'DB_File::Lock2', $dbfile, 'read';
  
    # if we match the username return 1, else 0
    return $mydb{$username} ? 1 : 0;
  
  } # end of sub user_exists

Now let's write all the upper case characters and their respective
ASCII values to a dbm file. Then read the file and print the contents
of the DB, unsorted.

  use DB_File::Lock2 ();
  my $dbfile = "/tmp/test";
  
    # write 
  tie my %mydb, 'DB_File::Lock2', $dbfile,'write';
  for (0..26) {
    $mydb{chr 65+$_} = $_;
  }
  untie %mydb;
  
    # now, read them and printout (unsorted) 
    # notice that 'read' is a default lock mode
  tie %mydb, 'DB_File::Lock2', $dbfile;
  while (my($k,$v) = each %mydb) {
    print "$k => $v\n";
  }
  untie %mydb;

If your CGI script is interrupted, the C<DESTROY> block will take care
of unlocking the dbm file and flush any changes.  So your DB will be
safe against possible corruption because of unclean program
termination.

=cut
