NAME
    DBIx::MySQLSequence - Proper and correct (emulated) sequence support for
    MySQL

SYNOPSIS
      # Get a handle to a new or existing sequence
      $dbh      = DBI->connect( 'dbi:mysql:db:host', 'user', 'pass' );
      $sequence = DBIx::MySQLSequence->new( $dbh, 'sequence_name' );
  
      # Does the sequence already exist?
      if ( $sequence->exists ) {
            die "Sequence already exists";
      }
  
      # Create the sequence
      unless ( $sequence->create ) {
            die "Failed to create sequence";
      }
  
      # Get the next value off the sequence
      $id = $sequence->nextval;
  
      # Drop the sequence
      unless ( $sequence->drop ) {
            die "Failed to drop sequence";
      }
  
      # Remove sequence emulation support entirely
      DBIx::MySQLSequence->remove_sequence_support( $dbh );

STATUS
    "DBIx::MySQLSequence" is complete and has been used to real application,
    but does not have paranoidly thorough unit testing (yet).

    Please report any issues you encounter.

DESCRIPTION
    The "DBIx::MySQLSequence" package implements an emulation layer that
    provides "real" sequences on MySQL. The module works by creating a
    "sequence table", a single table where each record represents a single
    sequence, and performing some "magic" MySQL specific SQL to ensure the
    sequences will work correctly.

  What is a sequence?
    A sequence is a source of guarenteed unique numbers within a particular
    context. These may or may not be in order, and in fact in typical
    database systems they are rarely perfectly incremental. It is much more
    preferrable that they are strictly unique than that they are perfectly
    in order. In any case, DBIx::MySQLSequence does actually return sequence
    values in order, but this will probably change once caching is
    implemented.

    In short, this is AUTO_INCREMENT done right. Oracle, PostgreSQL and
    practically all other major database support sequences. MySQL does not.

  Why do I need sequences? Isn't AUTO_INCREMENT enough?
    MySQL provides its own AUTO_INCREMENT extention to SQL92 to implement
    incrementing values for primary keys.

    However, this is not a very nice way to do them. I won't get into the
    reasoning in depth here, but primarily there are huge advantages to be
    had by knowing the value you are going to use BEFORE you insert the
    record into the database. Additionally, if records with the highest
    value for the AUTO_INCREMENT are deleted, their values will (in some
    versions of MySQL) be re-used for the next record. This is very very
    bad.

  DBIx::MySQLSequence Feature Summary
      - Sequence names are case insensitive.
      - Sequence names can be any string 1 to 32 chars in length.
      - Sequence names can include spaces and other control characters.
      - Sequence values use BIGINT fields, so the start, increment
        and current values can be any integer between 
        -9223372036854775808 and 9223372036854775807.
      - The module is safe for multiple database users or connections.
      - The module is not transaction friendly. ( See below )
      - The module is probably NOT thread safe.

  Transaction Safety
    Because the sequences are emulated through tables, they will have
    problems with transactions, if used inside the same database connection
    as your normal code. This is not normally a problem, since MySQL
    databases are not historically used for transaction based database work.

    If you are using transactions in MySQL, you can and should ensure have a
    seperate connection open to do additional statements outside the scope
    of the task the transaction is being used for.

    You should use that connection to get the sequence values.

    Any "DBIx::MySQLSequence" methods called on a handle that isn't in an
    autocommit state will cause a fatal error.

    It is highly recommended that if you need to do transactions, you should
    consider looking at something ore robust that supports suequences
    properly. Most people running up against the limits and idiosyncracies
    of MySQL tend to be much more relaxed once they discover PostgreSQL.

  MySQL Permissions
    At the time the first sequence is created, you will need "CREATE"
    permissions in the database. After this, you will need "INSERT",
    "UPDATE" and "DELETE" on the sequence table. Should you want to remove
    sequence support completely, the "DROP" permission will also be needed.

    The default name for the sequence table is contained in the variable
    $DBIx::MySQLSequence::MYSQL_SEQUENCE_TABLE.

INTERFACE
    The interface for "DBIx::MySQLSequence" is very flexible, and largely
    inspired by the interface to "DBIx::OracleSequence". It is somewhat
    simpler though, as we don't need or aren't capable of everything Oracle
    does.

    To quickly summarise the main methods.

      exists  - Does a sequence exist
      create  - Create a sequence
      drop    - Drop a sequence
      reset   - Resets the current value to the start value
      currval - Get the current value
      nextval - Get the next value
      errstr  - Retrieve an error message should one occur
      remove_sequence_support - Removes the sequence table completely

  Hybrid Interface
    Most of the methods in "DBIx::MySQLSequence" will act in a hybrid
    manner, allowing you to interact with an object or directly with the
    class (statically).

    For example, the following two code fragments are equivalent.

      # Instantiation and Object Method
      $sequence = DBIx::MySQLSequence->new( $dbh, 'sequence_name' );
      $sequence->create( $start_value );
  
      # Static Method
      DBIx::MySQLSequence->create( $dbh, 'sequence_name', $start_value );

    As demonstated here, when calling a method statically, you should
    prepend a DBI database handle and sequence name to the method's
    arguments.

    Note: "remove_sequence_support" can ONLY be called as a static method.

METHODS
  new $dbh, $name
    The "new" constructor creates a handle to a new or existing sequence. It
    is passed as arguments a valid autocommit state MySQL DBI handle, and
    the name of the sequence. Returns a new DBIx::MySQLSequence object, or
    undef on error.

  dbh
    The "dbh" object method returns the DBI handle of the database the
    object is using.

  name
    The "name" object method returns the sequence name for the handle

  exists
    Static Syntax: "DBIx::MySQLSequence-"exists( $dbh, $name );>

    Examines the database to determine if a sequence exists in the database.
    Returns true if the sequence exists. Returns false if the sequence does
    not exists, or sequence support has not been created in the database.

  create [ $start ][, $increment ]
    Static Syntax: "DBIx::MySQLSequence-"create( $dbh, $name [, $start ][,
    $increment ] );>

    Creates a sequence in the database. The create method takes optional
    arguments of the value you want to sequence to start at, and the amount
    you want the value to increment ( or decrement ) by.

    For example

    "$sequence-"create( 10, 5 )>

    The above would create a new sequence whose value starts at 10, and
    increments by 5 each time a value is returned. If not passed, the
    default is a starting value of 1, and an increment of 1. These are the
    defaults typically used by databases internally.

    If called as an object method, returns a true if the sequence is
    created, or undef if an error occurs, or the sequence already exists.

    If called as a static method, it will return a new handle to the created
    sequence, or undef if an error occurs, or the sequence already exists.
    You can use this as a sort of alternate constructor.

    "my $sequence = DBIx::MySQLSequence-"create( $dbh, $name, 5 );>

    DBIx::MySQLSequence will work quite happily without the sequence table
    existing. It will be automatically created for you the first time that
    you create a sequence. Please note that this will mean that you need
    CREATE and INSERT permissions when you create the first sequence.

    Once the first sequence is created, you will only need INSERT
    permissions.

    DBIx::MySQLSequence will not check for permissions for you, as the MySQL
    process for checking permissions is a bit too involved, so you will most
    likely only find out about this when the SQL statement fails. You should
    check that you have CREATE permissions before you start using the
    database.

  drop
    Static Syntax: "DBIx::MySQLSequence-"drop( $dbh, $name );>

    The "drop" method will drop a sequence from the database. It returns
    true on success, or undef on error.

    Please note that when the last sequence is removed, the module will NOT
    remove the sequence table. This is done in case you are operating on a
    database, and do not have CREATE permissions. In this situation, the
    module would not be able to re-create the sequence table should it need
    to.

    To remove the sequence table completely, see the
    "remove_sequence_support" method.

  reset
    Static Syntax: "DBIx::MySQLSequence-"reset( $dbh, $name );>

    The "reset" method will return the sequence to the state it was in when
    it was originally created. Unlike Oracle, we do not need to drop and
    re-create the sequence in order to do this. Returns true on success, or
    undef on error.

  currval
    Static Syntax: "DBIx::MySQLSequence-"currval( $dbh, $name );>

    The "currval" method retrieves the current value of a sequence from the
    database. The value that this returns is currently unreliable, but
    SHOULD match the last value returned from the sequence. Returns the
    sequence value, or undef on error.

  nextval
    Static Syntax: "DBIx::MySQLSequence-"nextval( $dbh, $name );>

    The "nextval" method retrieves the next value of a sequence from the
    database. Returns the next value, or undef on error.

  remove_sequence_support
    The "remove_sequence_support" method is a static only method that is
    used to remove sequence support completely from a database, should you
    no longer need it. Effectively, this just deletes the sequence table.
    Once you have removed sequence support, any existing sequence object
    will most likely throw errors should you try to use them.

  errstr
    Static Syntax: "DBIx::MySQLSequence-"errstr;>

    When an error occurs ( usually indicated by a method return value of
    "undef" ), the "errstr" method is used to retrieve any error message
    that may be available. Any error message specific to a object method
    will be available from that object using.

    "$sequence-"errstr;>

    If you use a static method, or one of the above object method in its
    static form, you should retrieve the error message from the class
    statically, using

    "DBIx::MySQLSequence-"errstr;>

TO DO
    - More testing, but then there's ALWAYS more testing to do

    In Oracle, sequence values are cached server side. We can emulate this
    by creating a DBIx::MySQLSequence::Cache object to do caching client
    side, for when people want to get a lot of sequence values without
    having to go back to the server all the time.

    This would be a good thing. It would make things MUCH faster.

AUTHORS
    Adam Kennedy <adamk@cpan.org>

    Patches are welcome

SEE ALSO
    DBIx::OracleSequence

COPYRIGHT
    Copyright 2002, 2007 Adam Kennedy.

    This program is free software; you can redistribute it and/or modify it
    under the same terms as Perl itself.

    The full text of the license can be found in the LICENSE file included
    with this module.

