Introducing JavaScript::Dependency::Manager

Nearly a year ago my grandfather passed away. He had some form of dementia for a long time and I personally wasn’t hit very hard by it, but as is the custom I went home to visit my family when it happened. On the drive down I listened to Childhood’s End and Rendezvous with Rama. At work I’d been tackling the problem of users with custom dashboards and possibly even the ability to have gadgets that we sell separately. The whole drive down I had trouble focusing on the audiobooks and instead was thinking about how to deal with this problem of loading the right JavaScript for the right users.

Of course it’s not a difficult problem and really once you realize what your problem is it’s a Simple Matter of Programming. So when I stopped for gas, energy drinks, and gas at the Tallulah Travel Center in Louisiana I went ahead and implemented my solution. I didn’t write docs, but I wrote tests and the basic API that hasn’t changed and has served me well so far. So here it is:

Introducing JavaScript::Dependency::Manager

In this modern age we have more and more JavaScript to deal with. The project I worked on before my current one was actually 5050 JavaScript and Perl. If you are ok with the number of requests required when using client side dependency management, check out RequireJS. Personally though I’d rather bundle, minify, and cache all my JavaScript on the server side.

Using JSDM is easy

All that you need to do to use JSDM is annotate the requirements and provisions in your JavaScript files and instantiate and use a JSDM object:

// provides: oldYeller
// requires: underscore
var oldYeller = _.throttle(function(voice) { alert(voice + "!!") }, 1000);

use JavaScript::Dependency::Manager;

my $mgr = JavaScript::Dependency::Manager->new(
  lib_dir => ['root/js/lib'],
  provisions => {
    underscore => ['root/js/lib/underscore/underscore.js'],
  },
);

my @files = $mgr->file_list_for_provisions(['oldYeller']);

The return value from file_list_for_provisions is an ordered list of files that provide the requested provisions, as well as all of the provisions’ dependencies, recursively. Basically it gives you a list of files you can load on the page and make it work.

There are a couple missing features I’d like to implement at some point. First off is cycle detection. At work we actually have a legitimate cycle and the best way to fix it was to just take out the requirement that makes it a cycle. Although this may not be the solution for everyone, at the very least I’d rather JSDM say “cycle detected” or something. The other thing is that sometimes JavaScript needs CSS to be loaded as well, so I might make a way to plug into JSDM and load other required resources.

This may seem like overkill compared to, say, a manifest of JS files to load, but once you use it it’s so much nicer due to automatically handling of load order and whatnot.

Posted Mon, May 28, 2012

Introducing DBIx::Class::UnicornLogger

More than a 1.5 years ago we added color coded, formatted SQL output to DBIx::Class. Since then I’ve tried adding various configurable logging facilities to the core, but I haven’t had much luck getting the API for that whipped into shape. So I’m giving up on getting it into the core for now and releasing it separately. It’s pretty rough around the edges, but it’s a logger, so it’s not like you could depend on it working a certain way and get into any kind of trouble with it (yet.)

Meet DBIx::Class::UnicornLogger

(Every time someone gets upset about a module called Dad I name a module named something actually ridiculous.) First off, here’s some of the output (after color has been stripped)

[2012-05-18 23:06:43]\[/home/frew/perl5/perlbrew/perls/perl-5.14.1/lib/site_perl/5.14.1/DBIx/Class/Storage/DBI.pm:1489]
 ** SELECT "me"."id", "me"."kind_of_id", "me"."materialized_path", "me"."name", "me"."description"
 --   FROM "ingredients" "me"
 --   LIMIT '3'

The above was generated by the following configuration:

my $pp = DBIx::Class::UnicornLogger->new({
   tree => { profile => 'console' },
   profile => 'console',
   format => '[%d]\[%F:%L]%n ** %m',
   multiline_format => ' -- %m',
});

First off, clearly I need to set up a good default “clan.” The code supports it, but for some reason I never got around to making a good default. Next, there’s a bug somewhere that makes the location logging (specifically the subroutine part) not work. That’s a hassle and desperately needs to be fixed. Anyway, the idea is to make a neat logger that gives more information than the default trace tool. Eventually I plan on including total time for SQL to execute as well.

If you’re interested in trying this out yourself put the following snippet in your schema:

use DBIx::Class::UnicornLogger;
my $pp = DBIx::Class::UnicornLogger->new({
   tree => { profile => 'console' },
   profile => 'console',
   format => '[%d]\[%F:%L]%n ** %m',
   multiline_format => ' -- %m',
});

sub connection {
   my $self = shift;

   my $ret = $self->next::method(@_);

   $self->storage->debugobj($pp);

   $ret
};

So please, try it out, send me patches and ideas, and we can realize our hopes and dreams of unicorn logging!

Posted Fri, May 25, 2012

Introducing Catalyst::ActionRole::DetachOnDie

In my last post I introduced Catalyst::Controller::Accessors, which is mostly aimed at users who do a lot of chaining. This module is similarly targeted for chaining users. Anyone who has used chaining for more than a few weeks will know that exceptions in chains are stupid; an exception will not stop the chain, but merely end the current part of the chain, add to $c->errors, and run the next part of the chain. I would understand this if it were something that you could choose to turn on in a per-chain basis or something, but as a default it’s horrible.

This module solves that problem. It just detaches the chain and sets $c->errors when an exception is thrown. To use it you just need to do the following in your controllers (base controller anyone?):

package MyApp::Controller::Foo;
use Moose;

BEGIN { extends 'Catalyst::Controller::ActionRole' }

__PACKAGE__->config(
   action_roles => ['DetachOnDie'],
);

...;

If for some reason you can’t use the excellent Catalyst::Controller::ActionRole you can use the ActionClass version as follows:

package MyApp::Controller::Foo;
use Moose;

BEGIN { extends 'Catalyst::Controller' }

__PACKAGE__->config(
   action => {
      '*' => { ActionClass => 'DetachOnDie' },
   },
);

...;
Posted Wed, May 23, 2012

Introducing Catalyst::Controller::Accessors

Ugh, I first released this eight months ago, but I fell off the blogging wagon pretty badly. It’s so hard to write when I could be writing code, docs, and tests! So anyway, I’m trying to get caught up on the eight announcements that need to be made as well as a few DBIx::Class::DeploymentHandler related PSA’s. I’ll schedule them to get auto posted with at least a few days between so I don’t melt your feed reader or bore you too much.

Do you use Catalyst chaining? I do and for the most part I really enjoy the structure it brings my applications. Here is a typical example of a chain based controller of mine, structure stolen and mutated from the inimitable t0m:

package Lynx::SMS::Controller::Accounts;

use Moose;
use namespace::autoclean;

use syntax 'method';

BEGIN { extends 'Lynx::SMS::RESTController' };

with 'Catalyst::TraitFor::Controller::DBIC::DoesPaging',
     'Catalyst::TraitFor::Controller::DoesExtPaging';

sub base : Chained('/') PathPart('accounts') CaptureArgs(0) {
   my ($self, $c) = @_;
   $c->stash->{+__PACKAGE__}{rs} = $c->model('DB::Account');
}

sub item : Chained('base') PathPart('') CaptureArgs(1) {
   my ($self, $c, $id) = @_;
   $c->stash->{+__PACKAGE__}{id} = $id;
   $c->stash->{+__PACKAGE__}{thing} =
      $c->stash->{+__PACKAGE__}{rs}->find($id);
}

sub accounts :Chained('base') PathPart('') Args(0) ActionClass('REST') {}

method accounts_POST($c) : RequiresRole('write') {
   my $params = $c->request->data->{data};

   my $foo = $c->stash->{+__PACKAGE__}{rs}->create($params);

   $c->stash->{rest} = { success => 1, data => $foo };
}

method accounts_GET($c) : RequiresRole('read') {
   $c->stash->{rest} = $self->ext_paginate(
      $self->search($c,
         $self->paginate($c,
            $self->sort($c, $c->stash->{+__PACKAGE__}{rs})
         )
      )
   );
}

sub account :Chained('item') PathPart('') Args(0) ActionClass('REST') {}

method account_GET($c) : RequiresRole('read') {
   $c->stash->{rest} = {
      success => 1,
      data => $c->stash->{+__PACKAGE__}{thing},
   };
}

method account_PUT($c) : RequiresRole('write') {
   my $foo = $c->stash->{+__PACKAGE__}{thing};
   my $params = $c->request->data->{data};
   $foo->update($params);

   $c->stash->{rest} = { success => 1, data => $foo };
}

method account_DELETE($c) : RequiresRole('delete') {
   $c->stash->{+__PACKAGE__}{rs}->search({
      id => $c->stash->{+__PACKAGE__}{id},
   })->delete;
   $c->stash->{rest} = { success => 1 };
}

1;

So the above works great and given the little idiom up there you get a safely namespaced stash. That’s all good, but we can do better.

Introducing Catalyst::Controller::Accessors

Catalyst::Controler::Accessors is a module to abstract the above idiom into actual controller methods. The great thing is that when you use actual methods not only is the result much more clear code, but you can change the method if you need to and have a much smaller ripple effect of changes. Without CCA, if you change where you store something in the stash you need to audit every single action that chains off the thing you chained. With CCA such audits should not be needed at all (with one caveat; I’ll get to that.)

Catalyst::Controller::Accessors gives you a cat_has export that works very similar to the has export from Moose. Here is the above example rewritten with CCA:

package Lynx::SMS::Controller::Accounts;

use Moose;
use Catalyst::Controller::Accessors;
use namespace::autoclean;

use syntax 'method';

BEGIN { extends 'Lynx::SMS::RESTController' };

with 'Catalyst::TraitFor::Controller::DBIC::DoesPaging',
     'Catalyst::TraitFor::Controller::DoesExtPaging';

cat_has rs => (
   is => 'rw',
);

cat_has id => (
   is => 'rw',
);

cat_has thing => (
   is => 'rw',
);

sub base : Chained('/') PathPart('accounts') CaptureArgs(0) {
   my ($self, $c) = @_;
   $self->rs($c, $c->model('DB::Account'));
}

sub item : Chained('base') PathPart('') CaptureArgs(1) {
   my ($self, $c, $id) = @_;
   $self->id($c, $id);
   $self->thing($c, $self->rs($c)->find($id));
}

sub accounts :Chained('base') PathPart('') Args(0) ActionClass('REST') {}

method accounts_POST($c) : RequiresRole('write') {
   my $params = $c->request->data->{data};

   my $foo = $self->rs($c)->create($params);

   $c->stash->{rest} = { success => 1, data => $foo };
}

method accounts_GET($c) : RequiresRole('read') {
   $c->stash->{rest} = $self->ext_paginate(
      $self->search($c,
         $self->paginate($c,
            $self->sort($c, $self->rs($c))
         )
      )
   );
}

sub account :Chained('item') PathPart('') Args(0) ActionClass('REST') {}

method account_GET($c) : RequiresRole('read') {
   $c->stash->{rest} = {
      success => 1,
      data => $self->thing($c),
   };
}

method account_PUT($c) : RequiresRole('write') {
   my $foo = $self->thing($c);
   my $params = $c->request->data->{data};
   $foo->update($params);

   $c->stash->{rest} = { success => 1, data => $foo };
}

method account_DELETE($c) : RequiresRole('delete') {
   $self->rs($c)->search({
      id => $self->id($c),
   })->delete;
   $c->stash->{rest} = { success => 1 };
}

1;

You still have to pass around $c, as the stash is still being used under the hood, but your access is now hidden and you are free to change that method later if you need to.

Catalyst::Controller::Accessors also has a few other handy features that. Due to the confusing nature of catalyst chaining I actually think that having validation on these accessors is much more helpful than in typical Moose objects, so type constraints are supported:

use Check::ISA;
cat_has resultset => (
   is => 'rw',
   isa => sub {
     die 'resultset needs to be a DBIx::Class::ResultSet, but you passed "$_[0]"'
        unless obj($_[0], 'DBIx::Class::ResultSet')
   }
);

The isa checks are Moo style, so you can use MooX::Types to generate your type subs.

Also note, when you’ve chained into another controller you probably want readonly access to the values from said controller. Here’s how that’s done:

cat_has other_user => (
  is => 'ro',
  namespace => 'MyApp::Controller::Users',
  slot => 'user',
);

Note that if you change what your stuff is chaining off of you’ll obviously need to change this as well.

Posted Mon, May 21, 2012

Using search.cpan.org AND metacpan

I appreciate the effort and openness of metacpan, but their search is still pretty bad. To be clear, compare the results of the search for DBIx:Class::Source on SCO and metacpan. That’s why I made the following greasemonkey/dotjs script:

$('a').each(function(i,x){
   var obj = $(this);
   var href = obj.attr('href');
   var re = new RegExp('^/~([^/]+)/(.*)$');
   this.href = href.replace(re, 'https://metacpan.org/module/$1/$2');
})

Put this in ~/.js/search.cpan.org.js to install it with dotjs. Feel free to extend it to work for more than just modules.

Posted Wed, May 16, 2012

The Rise and Fall of Event Loops (in one very small place of my code)

In the spirit of one of my other posts I’ve decided to chronicle my path with at least a couple event loops.

More than eighteen months ago I documented my decision to start using an event loop as it would handle things I may not have considered, the example mentioned specifically in that post being exceptions. Things went well! I used the code I documented in that post for a long time with no issues until recently. It turns out that the event loop I was using didn’t actually handle exceptions at all, thus completely nullifying my reason to use it.

So I looked elsewhere. I looked at the grandfather of event loops, POE. I like a lot of the components that have been written on top of POE, but POE itself is frustratingly low level. That’s a topic for another post though (yes I looked at Reflex.)

After my last post and speaking with Rocco Caputo, auther of our venerable POE, I came up with the following runner role:

package Lynx::SMS::DoesRun;

use Moose::Role;
use POE;

# this merely uses our logger etc
with 'Lynx::SMS::HandlesDieForPOE';

requires 'single_run';

has period => (
   is => 'ro',
   required => 1,
);

has schema => (
   is => 'ro',
);

sub run {
   my $self = shift;

   POE::Session->create(
      inline_states => {
         _start => sub {
            $_[KERNEL]->sig( DIE => 'sig_DIE' );
            $_[KERNEL]->yield('loop');
         },
         sig_DIE => \&die_handler,
         loop => sub {
            $_[KERNEL]->delay( loop => $self->period );
            $self->single_run;
         },
      },
   );

   POE::Kernel->run;
}

no Moose::Role;

1;

This works fine. It’s (to me) a little ugly, but I imagine that I’d get used to it if I were to write much more POE. But then Rocco pointed out that maybe I’m just wasting my time with event loops for this use case. Ultimately using POE as a glorified Try::Tiny is stupid and really not even the goal. So finally I’ve ended up just a few steps beyond where I started:

package Lynx::SMS::DoesRun;

use Moose::Role;
use Try::Tiny;
use Log::Contextual qw(:log :dlog);

requires 'single_run';

has period => (
   is => 'ro',
   required => 1,
);

has schema => (
   is => 'ro',
);

sub run {
   my $self = shift;

   while (1) {
      try {
         $self->single_run;
      } catch {
         my $error = $_;
         log_error { $error }
      };
      sleep($self->period)
   }
}

no Moose::Role;

1;

The observant reader will notice that despite me mentioning the above use case, which is really the only important one for me given that our actual server will run all of our services in separate processes, there is still the benefit of Event Loops mentioned in the first post for development purposes (starting all services in a single program.) I have indeed converted that to POE, but that probably doesn’t matter. I run my unified service script maybe once or twice a year at this point. Here it is if anyone is interested:

package Lynx::SMS::Runner;

use Moose;
use POE;

with 'Lynx::SMS::HandlesDieForPOE';

has tasks => (
   is => 'ro',
   default => sub { [] },
);

sub run {
   my $self = shift;

   POE::Session->create(
      inline_states => {
         _start => sub {
            $_[KERNEL]->sig( DIE => 'sig_DIE' );
            $self->create_children_sessions,
         },
         sig_DIE => \&die_handler,
      },
   );

   POE::Kernel->run;
}

sub create_children_sessions {
   my $self = shift;
   my $x = 0;
   my @tasks = @{$self->tasks};
   for my $task (@tasks) {
      POE::Session->create(
         inline_states => {
            _start => sub {
               $_[KERNEL]->delay(loop => ($x++ / @tasks ));
            },
            loop => sub {
               $_[KERNEL]->delay( loop => $task->period );
               $task->single_run;
            },
         },
      );
   }
}

no Moose;

__PACKAGE__->meta->make_immutable;

1;

I look forward to using POE for actual heavy-lifting in another one of our projects, and will post about the experience when I get there.

Posted Wed, Mar 7, 2012

Perl Event Loop

I have some extremely basic code using AnyEvent but I recently found out that I was doing it wrong. That is, the entire reason I am using an event loop is to catch errors, log them, and keep going. That’s one of the great benefits that Catalyst gives me; I override one thing and I get universal error logging. The problem is that AnyEvent specifically does not handle this use case.

I have a working solution, but as I am planning on rewriting our services in evented code this prohibition makes me really worried. The problem is that you can’t just know your code won’t die. Exceptions happen and as a developer of a language that’s not Java or C# I don’t know where they come from. My current solution is ok, but I don’t think it’s really viable long term. Here’s my current code:

#!/usr/bin/env perl

use strict;
use warnings;

use AnyEvent;
use Try::Tiny;

sub event {
   print "looped\n";
   die "lol" if rand() < .5;
}

sub NEVER_DIE {
   my $code = shift;
   return sub {
      try \&$code, catch { warn $_ } # <-- this should be logging, you get the idea
   }
}

my $cv = AE::cv;
my $w = AE::timer 0, 1, NEVER_DIE(\&event);
$cv->recv;

This works for simple cases, but if I chose to go down this route in the long term I’d have to wrap every single code ref in NEVER_DIE, which is pretty lame.

I looked at POE as it may support my use case better but as far as I can tell it’s support is WORSE. Here’s what I came up with:

#!/usr/bin/perl

use strict;
use warnings;

use POE;
use Try::Tiny;

sub handler_start {
  my ($kernel, $heap, $session) = @_[KERNEL, HEAP, SESSION];
  $kernel->yield('event');
}

sub NEVER_DIE {
   my $code = shift;
   return sub {
      try \&$code, catch { warn $_ } # <-- this should be logging, you get the idea
   }
}

sub event {
   print "looped\n";
   die "lol" if rand() < .5;
   $_[KERNEL]->delay_add('event', 1);
}

POE::Session->create(
 inline_states => {
   _start    => \&handler_start,
   event     => NEVER_DIE(\&event),
   _stop     => sub{},
 }
);

POE::Kernel->run();
exit;

So I still have to use NEVER_DIE, so that’s a lose, and worse, if event dies before the call to delay_add we end anyway. Sure, I could put delay_add at the beginning of event, but that brings me to another thing that really bothers me about the “POE Way” (my own terminology, I may just not be getting it), for my AnyEvent code I can add a bunch of things and they don’t have to know about each other. The loop handles calling the events. With POE it seems like I have to manually tell it “call this, now call this.” That seems to defeat the entire purpose! What am I missing here?

If anyone knows an event loop I should consider (MUST RUN WELL ON WINDOWS) or maybe some setting in POE and some kind of POE timer thing, or some way of safely overriding how AE calls it’s events, please, comment and let me know.

Posted Sun, Mar 4, 2012

Using Catalyst::Plugin::Authentication with an old setup

Recently I took it upon myself to make Catalyst::Plugin::Authentication know users had logged in after users had logged in in a completely non-Catalyst part of our app. After LOTS of frustration, code spelunking, and bugging a couple people in #catalyst (hobbs and t0m) I got it working.

Basically what I did was have the session plugin look at a different cookie and load information from our own strange brew of session table. It’s not perfect, but I’m much happier with it than I was before. Here’s the code:

First, you need to create your own Session Store, our app is called Lynx, so the namespace reflects that:

package Lynx::Session::Store;

use strict;
use warnings;

use base qw/Catalyst::Plugin::Session::Store/;

use DateTime::Format::MSSQL;
use Catalyst::Authentication::Store::DBIx::Class::User;
sub get_session_data {
   my ($c, $key) = @_;

   my ($k, $v) = split /:/, $key;

   if ($k eq 'session') {
      if (my $login = $c->model('DB::Login')->single({ access_num => $v })) {
         return {
            __user_realm => 'default',
            __user       => {
               # this must be the primary key
               user => $login->userid,
            },
         }
      }
   } elsif ($k eq 'expires') {
      if (my $cookie = $c->request->cookie('Access_Num')) {
         if (my $login = $c->model('DB::Login')->single({ access_num => $v })) {
            my $ex = DateTime::Format::MSSQL->parse_datetime($login->last_accessed)->epoch + 720 * 60 - DateTime->now(time_zone => 'local')->offset;
            return $ex;
         }
      }
   }
}

sub store_session_data { }
sub delete_session_data { }
sub delete_expired_sessions { }

1;

We have stub methods for the session stuff that we don’t support. Eventually I may fill those out, but what’s more likely is that we remove this code entirely and just use what’s provided by CPA.

Next is get_session_data, which gets arguments like session:1234 and expires:1234. They are meant to return the session data and the expiry time (seconds since epoch) respectively. Clearly I had to do a lot of really weird stuff with datetime to get that expiration date from our database, but it works, so that’s cool. You may store your expiration directly. Who knows.

So far, so weird. Then I had to figure out how to “inflate” the session. The keys __user_realm and __user are hardcoded in CPA, and I kinda think they should change to just current_user_realm and current_user, or maybe catalyst-plugin-authentication-user. Whatever. But the fact is they are what they are. The value for __user_realm is which realm is currently selected. I imagine the vast majority of people should have that set to default, as they typically only have a single realm (we actually have two, but I didn’t realize till this code broke in a special way.) The value for __user is not a user object, but instead what get’s passed to the auth store’s from_session method. Note that the DBIx::Class store actually will only use the primary key, so if you change the primary key of your user class this thing will break. I am mostly sure about that, but it’s a pretty deep stack trace at that point.

Next up I made a Session subclass:

package Lynx::Session;

use strict;
use warnings;

use base qw/Catalyst::Plugin::Session/;

sub sessionid {
   my $c = shift;

   my $access_num;
   if (my $cookie = $c->request->cookie('Access_Num')) {
      $access_num = $cookie->value;
   }

   return $access_num;
}

1

This is clearly pretty basic. I just overrode sessionid to look at our cookie to get the sessionid.

After that I just loaded the plugins I needed and configured CPA:

...
use Catalyst qw(
   Authentication
   +Lynx::Session
   Session::State::Cookie
   +Lynx::Session::Store
);
...
   'Plugin::Authentication' => {
      default => {
         credential => {
            class => 'Password',
            password_field => 'password',
            password_type => 'clear'
         },
         store => {
            class => 'DBIx::Class',
            user_model => 'DB::User',
         },
      },
   },
  ...

Note that the credential is unused in my use case as catalyst doesn’t do the actual authentication at all.

Hope this helps someone!

Posted Wed, Jan 18, 2012

Cloning Objects in Perl

Recently I needed to do some deep cloning of some objects at work. I think I ended up looking at all of the major ways to do it, and I figure I might as well discuss them here.

What is deep cloning?

Nearly everyone should be able to answer this, but it doesn’t hurt to define it anyway. Deep cloning means you clone other things the current object is related to, recursively. So while a shallow clone of a hashref (in Perl) would be merely:

my $clone = { %{ $other_hash_ref } };

That doesn’t do if the things in the hash get mutated and are also references, because in that case you’ll be modifying parts of the other hash, possibly surprisingly.

Isn’t this solved?

Well yes. If it’s something as basic as a simple data structure you can just use Storable. The code for above would become:

use Storable 'dclone';
my $clone = dclone($other_hash_ref);

Storable has been core enough for long enough that if it’s not core you need to upgrade ;-)

What’s your problem?

Sadly just default Storable isn’t good enough. I needed to deeply clone the objects, but not clone any related schemata. That is, the objects had a DBIx::Class::Schema object attached to them and for various reasons I do not want to clone that at all. The correct way to deal with such an issue is to define the two Storable hooks as follows:

my @stack;
sub STORABLE_freeze {
   my ($self, $cloning) = @_;

   die q(you can't freeze this thing silly!) unless $cloning;

   my %ret = %$self;

   my %frame;
   $frame{schema} = delete $ret{schema};
   push @stack, \%frame;

   return \%ret
}

sub STORABLE_thaw {
   my ($self, $cloning, $ice) = @_;

   die q(you can't thaw this thing silly!) unless $cloning;
   my %frame = %{pop @stack};
   my $new = $self->new({
      %$self,
      map {
         $_ => $frame{$_}
      } keys %frame,
   });

   %$self = %$new;
}

This is a little more generic than you probably need, and came from my prototype module, Clone::Hooker, but I gave up on that as well as Storable.

Why did you give up on Storable?

Two reasons; first, defining the hooks above might be a bad thing. Storable is something that someone other than me may use, and by defining the hooks above I am changing the relatively generic interface of Storable for my module. Second, there’s a better alternative that I ended up using.

WHAT DID YOU DO?!

I ended up settling on the handy MooseX::Clone. Obviously it is for Moose modules only, but all of my modules are Moose objects in this case. It’s very simple to use, here’s how it works for me:

   package Dashboard;

   use Moose;

   with 'MooseX::Clone';

   has gadgets => (
      is => 'rw',
      isa => 'ArrayRef',
      traits => [qw(Clone)],
   );

   1;

   package Gadget;

   use Moose;

   with 'MooseX::Clone';

   has schema => (
      is => 'ro',
   );

   1;

   my $d = Dashboard->new(
      gadgets => [
         Gadget->new(
            schema => $schema,
         )
      ]
   );

   my $cloned_d = $d->clone;

This avoids the “global” nature of changing the interface of Storable, is fairly unobtrusive in my code, and works well.

Posted Tue, Sep 20, 2011

Shortcut Constructor Method & Conversion

I left my book and notes at work yesterday, hence the late post.

Shortcut Constructor Method

What is the external interface for creating a new object when a Constructor Method is too wordy?

Sometimes creating an object is exorbitantly wordy. The example that the author gives (in javascript) is the following:

var p = new Point({ x: 1, y: 2 })

Add methods to a lower level object that can construct your objects. Take care to only do this rarely.

This can’t be done with the example given in javascript, but the idea is to do something like the following:

var p = ( 1 x 2 )

Personally, I’m very wary of this idea. I see the value, but even operator overloading, which is a step HIGHER level than this, is usually viewed skeptically. I do think it’s a good idea to make shortcut methods to instantiate related objects, but that’s a far sight better than creating a method on all integers. If you do monkey-patch something like integer, it would be best if it were done dynamically, so only the code in your own project sees it.

Conversion

How do you convert an object’s format to another object’s format?

This is (at least to me) quite obvious. Some would think that they should add methods to every object to convert to other formats. So one might monkey-patch the DOM stuff to return a jquery DOM thing with the asJQDom method or something like that. Of course doing that means you’re going to end up with a ton of random conversion methods.

Convert objects by merely instantiating the second object type

This just seems so obvious I almost feel bad even writing it…

Posted Wed, Sep 7, 2011

Creating a pseudo attribute with DBIx::Class

I’m surprised I haven’t actually blogged this before. I had to do it recently for the first time in a long time and I figured I’d share the secret sauce.

At work we just added a complete permission system on top of our existing user system, but we didn’t want to make the UI as flexible as the underlying code. We ended up making a single role (which has all permissions) called “Full Control”. Without that role all you get is the stuff configured directly for your user; that is, your user gets a dashboard. So instead of making a grid of roles etc etc we just made a single checkbox on the user edit form. Of course I could have put in controller code to handle this special case, but I’m trying to get better at factoring code correctly. (As an aside: two years ago I would have also put all of this in the model; the frustrating thing is that Fat Model Skinny Controller only really works for relatively small apps. I’ll try to do a blog post on why I think that at another point later :-) )

Anyway, first off, here’s the full_control accessor I made:

sub full_control {
   my $self = shift;

   if (exists $_[0]) {
      my $full_control = $_[0];
      if ($full_control) {
         $self->set_roles({ name => FULL_CONTROL });
      } else {
         $self->user_roles->delete;
      }
      return $full_control
   } else {
      $self->roles->search({ name => FULL_CONTROL })->count
   }
}

Not a whole lot going on. If an argument is passed we set the user’s roles based on the truthiness of the argument. Because the system is currently just the one role we delete all roles for clearing it. Later on if we make the system more full featured we’ll have to change this up a bit of course. If no argument is passed we just return the count of full control roles, as that approximates truthiness just fine.

Next up are the “insert” and update wrappers. I quote insert because I actually override new:

sub update {
   my ($self, $args, @rest) = @_;

   my $full_control = delete $args->{full_control};

   my $ret = $self->next::method($args, @rest);

   $ret->full_control($full_control);

   return $ret
}

sub new {
   my ($self, $args, @rest) = @_;

   my $full_control = delete $args->{full_control};

   $args->{user_roles} = [ { role => { name => 'Full Control' } } ] if $full_control;

   my $ret = $self->next::method($args, @rest);

   return $ret
}

The code for update should be abundantly clear. We just update the object, calling our accessor afterwards. The new code is a little bit more messy. Basically, instead of trying to use the accessor on new (which is wrong as new doesn’t actually imply an insert) we just leverage the excellent MultiCreate which DBIx::Class provides for us.

And that’s it! I hope this helps you get your job done that much faster/better :-)

Posted Sun, Sep 4, 2011

Smalltalk Best Practice Patterns - Constructor Parameter Method

How do you set instance variables from a constructor method?

The fundamental issue here is that often validation is bypassed at construction time, for whatever reason. So one’s accessor may look something like this:

sub x {
   my $self = shift;

   if ($self->constructing) {
     if (exists $_[0]) {
       $self->{x} = $_[0];
     } else {
       return $self->{x}
     }
   } else {
     if (exists $_[0]) {
       die 'too high!' if $_[0] > 100;
       die 'too low!'  if $_[0] < 0;
       $self->{x} = $_[0];
     } else {
       return $self->{x}
     }
   }
}

Clearly this method is just doing to much. To solve this we make special set methods that are entirely to be used during construction. So in Perl this might look like the following:

sub _set_x {
  my ($self, $x) = @_;
  $self->{x} = $x;
}

Interestingly, with Moose we happily side-step this issue, as the default constructor doesn’t go through the accessors and already sets the raw values.


Ok, so I think I may start trying to apply this stuff to JavaScript instead of Perl. I almost feel like the fact that I have Moose in Perl is cheating. I know that there is Joose in JavaScript, but I’ve yet to use that in production, and I find that I have a harder time making well factored code in JavaScript than Perl. Part of that is that the underlying libraries I use in JS (ExtJS 3) are not really well factored either, but I still struggle with overall structure.

Posted Sat, Sep 3, 2011

Smalltalk Best Practice Patterns: Constructor Method

Sadly reading is going slower than expected due to being so busy with various things in life. Oh well, just a single pattern today.

Constructor Method

How do you represent instantiation?

In addition to a vanilla constructor, add methods for common cases to instantiate typical objects. For strange cases allow the use of accessors.

Using Perl (with Moose) an example might be:

package Point;

use Moose;

has x => (is => 'ro');
has y => (is => 'ro');

sub r_theta {
  my ($class, $r, $theta) = @_;

  $class->new(
    x => $r * cos($theta),
    y => $r * sin($theta),
  );
}

1;

So now both of the following work:

my $p = Point->new(5, 6);
my $v = Point->r_theta(10, 1.4);
Posted Thu, Sep 1, 2011

Smalltalk Best Practice Patterns - Chapter 3 - Behavior - Methods

Today I had to spend time taking care of passport stuff for my upcoming honeymoon, so I only got to read a handful of pages. I’ll post my notes nonetheless.

Methods are more important that state because, correctly factored, methods paper over any changes in state over time. Most of us who took OO classes in college had this hammered into our brains :-)

Methods should be written to get something done, but should also be written to communicate with the reader. Method names like “task_1”, “task_2”, etc are completely useless for a regular person, and should be named as to what they actually do.

Small methods are expensive in that they cost more CPU cycles and typically cause the novice trouble in following the structure of a program. On the other hand, more methods means more human readable names, easier maintenance (pinpointing changes,) and method overrideability is much more feasible with small methods.

Composed Method

How do you split your program into methods?

As already mentioned, large methods are faster and easier for the reader to follow, but small methods with good names work well in the long run. A seasoned programmer is able to see a method and assume what it does without needing to read the code for it. On top of that, small methods with good names allow you to communicate the structure of your code to the reader. Also, small methods are a must for inheritance.

Split your program into methods that do a single identifiable task.

A Perl example might be something like:

sub run_app {
  my $self = shift;

  $self->intialize_app;
  $self->app_loop;
  $self->shutdown_app;
}

The Composed Method patter can be used in a top down fashion, that is, write your higher level methods in an almost pseudo-code fashion, and then fill in the details of the lower level methods as you work. You may also opt to use the bottom up approach of writing a larger method and splitting it into smaller methods as you notice repetition or other reusable structures. Or lastly (and I think the most new idea to me) you can use this to find holes in your API. So if an object is calling more than one method on another object, the second object probably needs to implement a method that will encapsulate the multiple calls.

Posted Thu, Sep 1, 2011

Smalltalk Best Practice Patterns, Chapters 1 and 2

For work I decided I’d start reading some technical books, taking notes, and then trying to reiterate what I’ve learned. Yesterday I read the Preface and Chapter 1 and today I read Chapter 2, but sadly it’s all still introductory. I might as well discuss what I’ve read nonetheless.

First though, I should say that I am reading the book because Thomas Doran of Catalyst development recommended it, and it clearly applies to Perl with Moose. I am sure some of it won’t be applicable (the whitespace formatting for example) but hopefully it can make me a better programmer.

Preface

The main point of the book is to go over things that programmers deal with all the time, examples given are naming, where to split up methods, and how to make comprehensible code.

The overall structure of a pattern is:

  • Recurring daily programming problem
  • Tradeoffs affecting the solution
  • Recipe for solution

Patterns (especially the ones in this book) are good for pretty much everyone. Beginners, obviously, as they don’t have experience or habits yet and these are “blessed” habits that they can start using immediately. Professional programmers may get a fresh perspective or even more importantly a shared vocabulary for what they already do. Teachers can use patterns as stepping stones to teach students or even just ideas for material to teach. Managers, lastly, can use patterns for communication, so that they can actually understand what their programmers are doing.

Chapter 1 - Introduction

The title, Smalltalk Best Practice Patterns, can be broken into three parts. Smalltalk is the language used in the book. Best Practice is a legal term which basically means you did what a professional would do, without neglect. And Patterns are, according to the book, decisions that experts make over and over.

Interestingly, the distinction is made between coding and programming, which I always thought was largely archaic. The fundamental point that Beck makes remains though. There is a time for planning, and a time for implementation, and if you run into an issue during implementation change the plan.

When programs get huge, hard to understand, and generally unwieldy, the information you should take from this is that you need to fix whatever issue is causing these problems, don’t just assume that the plan is set in stone and leave it forever.

The patterns described in the book should help you decrease duplicate code, remove conditional logic (not really sure what he means by that yet,) simplify complex methods, and clean up “structural code,” that is, using objects as data structures.

Good programs balance speed of development, project mutability over the lifetime of the system, and “copy pasta,” which is my term for making code that should be ok to be cargo culted.

Keep in mind that some patterns on help some of the above items listed. That’s reality and part of becoming a great programmer is knowing which thing to optimize for when.

Another thing stressed in the book is that code should be written to be read. That doesn’t mean write lots of comments (this is my interpretation) but it means name variables well, name methods well, and don’t do confusing things in the code if possible.

The things that all patterns take into account are productivity, life cycle cost (not making something so unmaintainable that you can’t do other work too,) time to market, and risk. Again, these things need to be balanced.

According to Beck, good style is not repeating yourself, writing short methods and small objects, things should be modular and replaceable, low coupling which again leads to replaceability, and lastly, segmentation based on rates of change. The last bit I’d never really considered before. A perfect example is Catalyst vs CGI::Application. With Catalyst you have a “context” that changes for each action and gets passed to the controllers. The context has a request and a response. With CGI::Application, on the other hand, the request and response are part of the controller, so the controller gets mutated on each response at a higher rate of change than the controller really needs.

A number of issues aren’t discussed in the book. Exception handling is one, as it is confusing and the author didn’t have a signification amount of experience with it. Copying objects isn’t really discussed as it’s usually a performance suck. Become (which I’m guessing is a Smalltalk feature) isn’t discussed because it’s confusing. Performance isn’t discussed as it’s typically not worth considering. Overall design isn’t discussed; he points to the Gang of Four book for that. Modelling and Requirements are entire books in themselves and thus left out. And lastly UI design is left out, as it’s probably an entire field in itself.

The book is organized into 6 parts, philosophy of patterns, patterns for methods, patterns for state, patterns for collections, a brief overview of classes (in Smalltalk I’m guessing,) and formatting (which I might not even read.)

There are three methods for adoption of patterns listed in the book. First is the Bag of Tricks method, where you pick and choose the few that you’ll start to implement over time. Next is as a fairly large style guide, complete with justifications. You pick the patterns for your organization, have programmers read them, and then ensure that people use them. Last is the Whole Hog method, where you keep the book in your lap and every time you need to make a choice the book discusses, you look up the appropriate pattern and apply it. I’m hoping to try the latter method, but we’ll see if I can keep to that.

To learn a pattern you should focus on the context (patterns before and after the pattern.) You should read the example. Focus on the problem shown and the solution shown. If possible try to find examples of the problem in your own codebase. Lastly, write your own example. I fully intend on writing an example of each pattern in Perl.

Chapter 2 - Patterns

One interesting point is that patterns are supposed to work where code reuse doesn’t. Another interesting fact is that a lot of these patterns are so small that reuse would be meaningless, for example, how do you “reuse” the naming of a method?

One of the main goals for patterns is to increase communication. To increase communication you either need to say more, or have the speaker and listener share more ground so words mean more. The latter is what patterns try to achieve.

By knowing patterns you should be able to understand code that uses such patterns better. Patterns will also help you answer questions that come up constantly during development. Shared patterns also help reduce the friction involved with code review as you just request that a pattern be implemented in a give place. Patterns can also reduce documentation as it is more shared by the writer and reader. Lastly, when joining an existing project, rote application of a handful of patterns can often help general understanding of the project as well as some basic cleanup.

The format for all patterns is: a title/name for the pattern, optionally any patterns that should be read before the current pattern, the problem that the pattern solves or maybe the question it answers, the forces that constrain the pattern, the solution to the problem, discussion or practical application of the pattern, and lastly, any patterns that should be read after the current pattern.

Posted Tue, Aug 30, 2011

Weekly Status Report 4

This week I finished the following three git conversions:

  • Catalyst-Authentication-Store-DBIx-Class
  • Catalyst-Authentication-Credential-FBConnect
  • Catalyst-Authentication-Credential-OAuth

And I started on the following four git conversions (I think they are ready but I want t0m to sign off on them) :

  • Task-Catalyst
  • Catalyst-Plugin-Authentication
  • HTTP-Request-AsCGI
  • Catalyst-Action-RenderView
Posted Mon, Aug 29, 2011

Stop accidentally committing all with git

One of the things that annoys me a lot when using git is if I go through a lot of work to stage some changes, probably using git add -p to stage parts of files, and then from muscle memory I type git ci -am 'lolol I dummy'. If you didn’t know the -a says commit everything, so then of my painstaking staging is gone.

Well, on Thursday I finally fixed this problem. I wrote the following, very basic, git wrapper. All it does is:

  • Find all aliases for commit
  • Check if the current command is a commit or commit alias
  • Check if the current arguments have -a or –all
  • Check if there are staged modifications
  • And if all of those conditions are true, it prompts the user to ensure that they actually want to commit all.

I’m fairly happy with the alias detection; the only thing it should also do is introspect the arguments in the values of the alias as well as the current command. I don’t have any aliases like that, but if I wanted to make this a canned solution that would be a must.

The arguments detection is actually very dumb. It wouldn’t work if you did git ci -m 'Foo' -a. I’m ok with that because this is to battle my own muscle memory and I would never type that. But it is definitely a spot for improvement.

The staged checking I am very happy with. It only checks for staged modifications. So if you add a new file or delete a file and then do git ci -am "station" it will happily go on it’s way, which I like.

Anyway, here’s the script. To install it just put it somewhere in your path as wrap-git (I use ~/bin) and alias git=wrap-git

#!/usr/bin/env perl

use strict;
use warnings;

my %aliases = map { split(/\n/, $_, 2) }
   split /\0/,
   `git config -z --get-regexp alias\\.`;

my %commit_aliases = (( commit =&gt; 1 ),
   map { s/alias\.//; $_ =&gt; 1 }
   grep $aliases{$_} =~ /^commit\b/,
   keys %aliases);

my ($command, @args) = @ARGV;

if ($commit_aliases{$command} &amp;&amp; $args[0] =~ /^-a|^--all/) {
   my @staged = grep /^M/, split /\0/, `git status -z`;
   if (@staged) {
      print "There are staged changes, are you sure you want to commit all? (y/N) ";
      chomp(my $answer = <stdin>);
      if ($answer =~ /^y/i) {
         run_command()
      }
   } else {
      run_command()
   }
} else {
   run_command()
}

sub run_command {
   system 'git', $command, @args;
   exit $? &gt;&gt; 8;
}

One thing I think would be really cool would be to make a WrapGit.pm and wrap-git would just be coderefs passed to WrapGit.pm. I’d love to have full introspection of all git commands and arguments. It would let me do things like keep statistics about how you use git, maybe make a powerful achievement system, make more commands prompt the way this one does. Anyway, I’ll probably do that one of these days when I finish all the other stuff on my list :-)

Posted Mon, Aug 29, 2011

Refactoring Dispatch Tables into Objects

One of the cool ways of doing things in Perl is to use a dispatch table. The most obvious dispatch table is a hash of subroutines:

my $x;

my $table = {
   GET => sub { return $x  },
   PUT => sub { $x = $_[0] },
};

sub dispatch {
   my ($method, $data) = @_;

   if (my $fn = $table->{$method}) {
      $fn->($data)
   } else {
      die 'METHOD NOT ALLOWED!'
   }
}

This is a pretty cool thing to be able to do easily. But what’s even cooler is that we can refactor the dispatch table into a package, which allows us to make objects that can override bits of the dispatch table:

package Table {
   sub new { bless {}, $_[0] }
   sub GET { return $_[0]->{x}  }
   sub PUT { $_[0]->{x} = $_[1] }
}

package SubTable {
   use parent 'Table';
   sub DELETE { delete $_[0]->{x} }
}

my $table = SubTable->new;
sub dispatch {
   my ($method, $data) = @_;

   if (my $fn = $table->can($method)) {
      $table->$method($data)

      # the following would also work and would be
      # marginally faster
      # $table->$fn($data)
   } else {
      die 'METHOD NOT ALLOWED!'
   }
}

Note that one thing you might consider is prefixing the methods with “public_” or something like that; just in case your dispatcher object as private methods you don’t want web browsers executing. Generally though I’d just not put such methods in my dispatcher, but I haven’t yet made anything super complex using this pattern. I am using the pattern for a pluggable dashboard system at work, but the methods there are all called GET_foo or POST_bar, so users can’t run methods I didn’t specifically make for HTTP.

Posted Thu, Aug 25, 2011

New Stuff in Catalyst::ActionRole::PseudoCache 1.000001

I’m excited to announce a new version of Catalyst::ActionRole::PseudoCache.

New in the current release of Catalyst::ActionRole::PseudoCache is that it can now use Catalyst::Plugin::Cache as the underlying cache mechanism. The main reason was that the existing architecture didn’t work for multiple servers, which is how our system works. Plus this is just better overall.

In the long term I will be removing the old “Pseudo” cache. It might be a good idea to make a separate package with a better name at some point, but that will be for the next release. Enjoy!

Posted Tue, Aug 23, 2011

Weekly Status Report 3

I don’t feel great about this past week, but I was really busy with wedding planning stuff. I barely made either of my two main goals (2 blog posts and 2 patches/releases a week.)

Last week I:

I have high hopes for the coming week, that I can get more important releases and more interesting blog posts written. Stay tuned!

Posted Mon, Aug 22, 2011