NAME
    Object::Transaction - Virtual base class for transactions on files
    containing serialized hash objects

SYNOPSIS
            package Pkg;

            @ISA = qw(Object::Transaction);

            use Object::Transaction;

            $obj = sub new { ... }
            sub file($ref,$id) { ... }

            $obj = load Pkg $id;
            $obj->savelater();
            $obj->save();
            $obj->removelater();
            $obj->remove();
            $obj->commit();
            $obj->uncache();
            $obj->abandon();

            $id = sub id { ... }
            @passby = sub presave($old) { ... }
            sub postsave($old,@passby) { ... }
            $newid = sub preload($id) { .... }
            sub postload() { ... }
            sub preremove() { ... }
            sub postremove() { ... }

DESCRIPTION
    Object::Transaction provides transaction support for hash-based objects that
    are stored one-per-file using Storable. Multiuser access is supported. In
    the future, serializing methods other than Storable will be supported.

    Object::Transaction is a virtual base class. In order to use it, you must
    inherit from it and override the `new' method and the `file' method.

    Optomistic locking is used: it is possible that a transaction will fail
    because the data that is is based upon has changed out from under it.

EXAMPLE
            package User;

            @ISA = qw(Object::Transaction);

            use Object::Transaction;

            my $top = "/some/path";

            sub new { 
                    my ($package, $login) = @_;
                    die unless getpwnam($login);
                    return bless { 'UID' => getpwnam($login) };
            }

            sub file { 
                    my ($ref, $id) = @_;
                    $id = $ref->id() unless $id;
                    return "$top/users/$id/data.storable";
            }

            sub id {
                    my ($this) = @_;
                    return $this->{'UID'};
            }

            sub preload
            {
                    my ($id) = @_;
                    return if getpwuid($id);
                    return getpwnam($id) if getpwnam($id);
                    die;
            }

            sub postload
            {
                    my ($this) = @_;
                    my ($name,$passwd,$uid,$gid,$quota,$comment,$gcos,$dir,
                            $shell,$expire) = getpwuid($this->{'UID'});
                    $this->{'SHELL'} = $shell;
            }

            sub presave
            {
                    my ($this, $old) = @_;
                    my $id = $this->{'UID'};
                    mkdir("$top/users/$id", 0700);
                    delete $this->{'SHELL'};
            }

            sub postsave
            {
                    goto &postload;
            }

            sub postremove
            {
                    delete from pw file...
            }

            my $joe = new User "joe";
            $joe->savelater();

            my $fred = new User "fred";
            $fred->savelater();

            $joe->commit();

METHODS PROVIDED
    Object::Transaction provides the following methods.

    `load($id)'    `load' is the way to bring an object into memory. It is usually
                   invoked as `my $obj = load MyObject $id'.

                   There are two opporunities to customize the behavior of
                   `load': `preload' for things that should happen before
                   loading and `postload' for things that should happen after
                   loading.

                   Object::Transaction caches objects that are loaded. This is
                   done both for performance reasons and to make sure that only
                   one copy of an object is in memory at a time. If caching is
                   not desired, the `uncache' method must be invoked after
                   loading.


    `savelater()'  `savelater' is the usual method of saving an object. The object
                   is not saved at the time that `savelater' is invoked. It is
                   actually saved when `commit' is invoked.

                   There are two opporunities to customize the behavior of
                   `savelater': `presave' for things that should happen before
                   saving and `postsave' for things that should happen after
                   saving. These are invoked when the object is actually being
                   saved.

    `save()'       Simply `savelater' combined with a `commit'.


    `removelater()'
                   `removelater' is the usual method of removing an object. The
                   object is not removed at the time that `removelater' is
                   invoked. It is actually removed when `commit' is invoked.

                   There are two opporunities to customize the behavior of
                   `removelater': `preremove' for things that should happen
                   before removing and `postremove' for things that should
                   happen after removing. These are invoked when the object is
                   actually being removed.

    `remove()'     Simply `removelater' combined with a `commit'


    `commit()'     `commit' writes all pending changes to disk. Either all changes
                   will be saved or none of them will. Deadlocks are avoided by
                   locking files in order.

                   Object::Transaction uses opportunistic locking. Commit can
                   fail. If it fails, it will `die' with a message that begins
                   `DATACHANGE: file'. It is advisable to wrap your entire
                   transaction inside an eval so that it can be re-tried in the
                   event that the data on disk changed between the time is was
                   loaded and commited.

                   In the event of a commit failure, the object cache will be
                   reset. Do not keep any old references to objects after such a
                   failure.

    `transaction($funcref,@args)'
                   `transaction()' is a wrapper for a complete transaction.
                   Transactions that fail due to opportunistic locking problems
                   will be re-run automatically. Beware side-effects!

                   The first parameter is a reference to a function. Any
                   additional parameters will be passed as parameters to that
                   function. The return value of `transaction()' is the return
                   value of `&$funcref()'.

                   It is not necessary to use the `transaction()' method. Just
                   beware that `commit()', `save()', and `remove()' can fail.
                   `transaction()' will keep trying until it suceeds or it
                   failes for a reason other than an opportunistic locking
                   problem.


    `abandon()'    As an alternative to `commit', all changes may be abandoned.
                   Calling `abandon()' does not undo changes made to the in-
                   memory copies of objects.


    `uncache()'    Object::Transaction caches all objects. To flush an object from
                   Object::Transaction's cache, invoke the `uncache' method on
                   the object.

                   Be careful when doing this -- it makes it possible to have
                   more than one copy of the same object in memory.

                   `uncache()' can be invoked as a class method rather than an
                   object method (`Object::Transaction-'uncache()>). When
                   invoked as a class method, the entire cache is flushed.

    `readlock()'   By default Object::Transaction does not lock objects unless they
                   are being modified.

                   The `readlock()' method insures that objects are properly
                   locked and unchanged during a transaction even if they are
                   not being modified. `savelater()' takes precedence over
                   `readlock()' so they can be combined freely.

                   Paranoid programmers should use `readlock()' on most objects.

REQUIRED METHODS TO OVERRIDE
    The following methods must be overriden.

    `new'          Object::Transaction does not provide a contructor. You must
                   provide one yourself.

    `file($ref,$id)'
                   You must provide a function that returns the filename that an
                   object is stored in. The `file' method can be invoked in two
                   ways: as an object method call without an `$id' parameter; or
                   as a class method call with an `$id' parameter.

OPTIONAL METHODS TO OVERRIDE
    The following methods may be overridden.


    `preload($id)' `preload()' is invoked as nearly the first step of `load'. It is
                   generally used to make sure that the `$id' is valid.
                   `preload()' is a class method rather than an object method.

                   The return value of `preload' is a replacement `$id'. For
                   example, it might be called as `preload("Joe")' to load the
                   user named Joe, but if users are numbered rather than named
                   it could return the number for Joe. A return value of undef
                   is ignored.

                   No lock on the underlying file is present at the time
                   `preload' is called.


    `postload()'   `postload' is invoked after the object has been loaded into
                   memory but before transaction completeness is checked.

                   The underlying file is locked at the time that `postload' is
                   invoked.

                   If a transaction rollback is required, `postload' will be
                   invoked again after the object has been reverted to its pre-
                   transaction state.


    `presave($old)'
                   `presave()' is invoked just before an object is written to
                   disk.

                   Objects are stored on disk in the file specified by the
                   `file' method. The directory in which that file resides must
                   exist by the time `presave()' finishes. `presave' should make
                   the directory if it isn't already made.

                   The underlying file may or may not be locked at the time
                   `presave' is invoked.

                   `presave' can be invoked as a side-effect of `load' if the
                   object must roll back to a previous version.

                   The parameter `$old' is a copy of the object as of the time
                   it was first loaded into memory.

                   Any return values from `presave' will be remembered and
                   passed to `postsave'.


    `postsave($old,@psv)'
                   `postsave' is invoked after an object has been written to
                   disk.

                   The underlying file is always locked at the time `postsave'
                   is invoked.

                   Invocations of `presave' and `postsave' are always paired.

                   The parameter `$old' is a copy of the object as of the time
                   it was first loaded into memory.

                   The parameter `@psv' is the return value from `presave'.


    `id()'         Object::Transaction expect to be able to find the unique
                   identifier (id) for each object as `$obj-'{'ID'}>. If that
                   isn't the case, you can override the `id' function to provide
                   an alternative.

PUBLIC MEMBER DATA
    The following data members are used by Object::Transaction.

    `ID'           Object::Transaction expect to find the id for an object in `$obj-
                   '{'ID'}>. This can be overridden by defining your own `id'
                   function.

    `OLD'          When an object is loaded into memory a copy is made. The copy can
                   be found at `$obj-'{'OLD'}>. The copy should not be modified.
                   The copy is explicitly passed in `presave' and `postsave'.

PRIVATE MEMBER DATA
    Object::Transaction ads a few data members to each object for its own
    internal use.

    These are:

            __frozen
            __transfollowers
            __transleader
            __rollback
            __removenow
            __toremove
            __transdata
            __readonly

    None of these should be touched.

BUGS
    A program or computer crash at just the wrong moment can allow an object
    that should be deleted to escape deletion. Any future attempt to access such
    an object will cause it to self-destruct.

    In some situations objects will be saved even if niether `save()' nor
    `savelater()' is invoked. This happens if `readlock()' is used and the
    transaction leader object (one per transaction) choosen turns out to be an
    object for which only `readlock()' was called.

AUTHOR
    David Muir Sharnoff <muir@idiom.com>

COPYRIGHT
    Copyright (C) 1999, Internet Journals Corporation <www.bepress.com>. All
    rights reserved. License hearby granted for anyone to use this module at
    their own risk. Please feed useful changes back to David Muir Sharnoff
    <muir@idiom.com>.

