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($id)'
`postload' is invoked after the object has been loaded into
memory but before transaction completeness is checked.
The underlying file is locked at the time that `postload' is
invoked.
If a transaction rollback is required, `postload' will be
invoked again after the object has been reverted to its pre-
transaction state.
`presave($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>.