NAME¶
Rinci::Undo - (DEPRECATED) Protocol for undo operations in functions
VERSION¶
This document describes version 1.1.52 of Rinci::Undo (from Perl distribution
Rinci), released on 2014-06-29.
SPECIFICATION VERSION¶
1.1
STATUS¶
This protocol (riundo for short) is now deprecated in favor of
Rinci::Transaction (ritx for short) for several reasons:
- •
- riundo is inherently unreliable
Undo information is returned by function after the function has
performed the action. If function dies in the middle of action, client
does not have the information to undo the (partially completed) action.
That is why in ritx, the TM asks the function first for undo information
before asking the function to perform its action.
- •
- ritx does not limit using the same function for undo
In riundo, we must call the same function (passing the previously obtained
undo data from the that function) to undo the information. This is
sometimes slightly cumbersome. The undo action might be provided by other
functions, but we still have to go through the same function first.
- •
- ritx can also implement undo/redo
So there is no need for maintaining two specifications.
SPECIFICATION¶
This document describes the Rinci undo protocol. This protocol must be followed
by functions that claim that they support undo (have their "undo"
"feature" set to true). Such functions are from here on called
undoable function (or just function, unless when ambiguous).
The protocol is basically the non-OO version of the command pattern, a design
pattern most commonly used to implement undo/redo functionality. In this case,
each function behaves like a command object. You pass a special argument
"-undo_action" with the value of "do" and "undo"
to execute or undo a command, respectively. For "do" and
"undo", the same set of arguments are passed.
Requirements¶
Function MUST check special argument "-undo_action" before it checks
other arguments. Function MUST at least support the following undo action:
"do", "undo". On unsupported/unknown undo action, function
MUST return status 400, with message like "Unsupported undo action".
If "-undo_action" is not set, it means caller does not care about
undo. Undoable function should execute as any normal function.
To indicate that we need undo, we call function by passing special argument
"-undo_action" with the value of "do". Function should
perform its operation and save undo data along the way. If
"-undo_action" is not passed or false/undef, function should assume
that caller does not need undo later, so function need not save any undo data.
After completing operation successfully, function should return status 200,
the result, and undo data. Undo data is returned in the result metadata (the
fourth element of result envelope), example:
[200, "OK", $result, {undo_data=>$undo_data}]
Undo data should be serializable so it is easy to be made persistent if
necessary (e.g. by some undo/transaction manager).
To perform an undo, caller must call the function again with the same previous
arguments, except "-undo_action" should be set to "undo"
and "-undo_data" set to undo data previously given by the function.
Function should perform the undo operation using the undo data. Upon success,
it must return status 200, the result, and an undo data (in other words, redo
data, since it can be used to undo the undo operation).
To perform redo, caller can call the function again with <-undo_action>
set to "undo" and "-undo_data" set to the redo data given
in the undo step. Or, alternatively, caller can just perform a normal do (see
above).
An example:
$SPEC{setenv} = {
v => 1.1,
summary => 'Set environment variable',
args => {
name => {req=>1, schema=>'str*'},
value => {req=>1, schema=>'str*'},
},
features => {undo=>1},
};
sub setenv {
my %args = @_;
my $name = $args{name};
my $value = $args{value};
my $undo_action = $args{-undo_action} // '';
my $undo_data = $args{-undo_data};
my $old;
if ($undo_action) {
# save original value and existence state
$old = [exists($ENV{$name}), $ENV{$name}];
}
if ($undo_action eq 'undo') {
if ($undo_data->[0]) {
$ENV{$name} = $undo_data->[1];
} else {
delete $ENV{$name};
}
} else {
$ENV{$name} = $value;
}
[200, "OK", undef, $undo_action ? {undo_data=>$old} : {}];
}
The above example declares an undoable command "setenv" to set an
environment variable (%ENV).
To perform command:
my $res = setenv(name=>"DEBUG", value=>1, -undo_action=>"do");
die "Failed: $res->[0] - $res->[1]" unless $res->[0] == 200;
my $undo_data = $res->[3]{undo_data};
To perform undo:
$res = setenv(name=>"DEBUG", value=>1,
-undo_action="undo", -undo_data=>$undo_data);
die "Can't undo: $res->[0] - $res->[1]" unless $res->[0] == 200;
After this undo, DEBUG environment variable will be set to original value. If it
did not exist previously, it will be deleted.
To perform redo:
my $redo_data = $res->[3]{undo_data};
$res = setenv(name=>"DEBUG", value=>1,
-undo_action="undo", -undo_data=>$redo_data);
or you can just do:
$res = setenv(name=>"DEBUG", value=>1, -undo_action="do");
Saving undo data in external storage¶
Although the complete undo data can be returned by the function in the
"undo_data" result metadata property, sometimes it is more efficient
to just return a pointer to said undo data, while saving the actual undo data
in some external storage.
For example, if a function deletes a big file and wants to save undo data, it is
more efficient to move the file to trash directory and return its path as the
undo data, instead of reading the whole file content and its metadata to
memory and return it in "undo_data" result metadata.
Functions which require undo trash directory should specify this in its
metadata, through the "undo_trash_dir" dependency clause. For
example:
deps => {
...
trash_dir => 1,
}
When calling function, caller needs to provide path to undo trash directory via
special argument "-trash_dir", for example:
-trash_dir => "/home/.trash/2fe2f4ad-a494-0044-b2e0-94b2b338056e"
What about non-undoable actions?¶
Like in real life, not all actions are undoable. Examples of
undoable/irreversible actions include wiping a file/directory (more generally
speaking, any action to permanently delete/destroy something, without backing
up the data first), sending an email (more generally speaking, any action that
is sent to an external entity beyond our control, unless that external entity
provides a way to undo the action).
An undoable function MUST NOT mix undoable and non-undoable actions. For
example:
safe_delete(file=>'/path/to/file'); # puts file into Trash, undoable action
safe_delete(file=>'/path/to/file', permanent=>1); # deletes file, non-undoable
The "safe_delete" function above mixes undoable action (putting a file
into Trash directory) and non-undoable action (permanently deleting a file
without putting it in Trash). Without domain knowledge of the function, a
caller cannot know whether a call will be undoable or not. This will also
prevent the function from participating in a transaction, because transaction
requires function call to always be undoable, for rollback purpose.
The solution is to separate non-undoable action in another function, for
example:
trash(file=>'/path/to/file'); # undoable, can execute inside transaction
delete(file=>'/path/to/file'); # non-undoable, executes outside transaction
empty_trash(); # non-undoable, executes outside transaction
The non-undoable function is also non-transactional (it operates outside the
scope of a transaction). But it can still be idempotent. And it can manipulate
the transactions if it needs too. In the example, the
empty_trash()
function instructs the transaction manager to discard the
trash()
transactions, since after the trash is emptied, the
trash()
transactions cannot be undone anyway.
SEE ALSO¶
Related specifications: Rinci::Transaction
HOMEPAGE¶
Please visit the project's homepage at
<
https://metacpan.org/release/Rinci>.
SOURCE¶
Source repository is at <
https://github.com/sharyanto/perl-Rinci>.
BUGS¶
Please report any bugs or feature requests on the bugtracker website
<
https://rt.cpan.org/Public/Dist/Display.html?Name=Rinci>
When submitting a bug or request, please include a test-file or a patch to an
existing test-file that illustrates the bug or desired feature.
AUTHOR¶
Steven Haryanto <stevenharyanto@gmail.com>
COPYRIGHT AND LICENSE¶
This software is copyright (c) 2014 by Steven Haryanto.
This is free software; you can redistribute it and/or modify it under the same
terms as the Perl 5 programming language system itself.