Adam Kennedy > DBIx-MySQLSequence-1.04 > DBIx::MySQLSequence

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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.

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