Wednesday, 26 February 2014

Habemus Strehler!

Well, i worked a lot about it behind the scene, so i'm now a little excited to talk about Strehler, my most complex Perl work.

Link to github first:

As the README says, Strehler is a light-weight, nerdy, smart CMS in perl based on Perl Dancer2 framework. The story of his creation is very simple: I had to create a website (well, more than one) and after I decided to use Dancer2 I also decided to write the backend interface for it once for all, developing a tool easy to install on any type of webapp you need and with a lot of customizations available out of the box.

Strehler implements just the standard CRUD needed for a standard site. It can create articles, upload images, organize things  with tags and categories. I said more about it in the github wiki.

In my opinion the software reached enough stability to deserve version 1.0.0 tag. My hope now is for people to download it, try it and tell me where I'm wrong thinking this. At this point of development Strehler needs to be seen by fresh eyes, this is the reason i'm announcing that with so much emphasis.

You can try Strehler, you can use it, you can just find something you don't like and tell me. If you like collaborative work or if you think that Strehler can be what you need with some modifications, just work on it. Code is open and the git repository is there to be used. If you need some advice about how to improve the tool, try the TODO page. It's there for me as for everyone ho like to develope using Perl.

Strehler is ready! Cheers!

Saturday, 22 February 2014

Entering the time zone

It' a lot of time I don't write here, i'm working on something that will make me write a lot (I hope) but i'm keeping silence until it is finished. However, today I had to understand timezones more that how much I did before so I want to throw all the knowledge I acquired in this blog, to make it remeber it instead of me.

Problem of the day is common, but not simple. You have a database with a timestamp field, you want to print it on the screen. It seems easy, a timestamp is just a date and an hour, what could go wrong? Easy, the hour you print IS NOT the hour you expected.

We are a lot of people who lives in a lot of places. Sun can't shine on all these places at the same time. But everyone wants to wake up in the morning and go to bed in the evening so: timezones are born.

We assume that you have a MySQL database, that you will read the timestamp column using DBIx::Class and then that a Dancer app will make it visibile on the screen. Every step has its timezone issue.

Timezone Steps


MySQL knows its timezone and every time it gives you a timestamp the timestamp is considered in that timezone.
MySQL timezone is the default_time_zone option in my.cnf. If no timezone is provided, the server will use the one of the machine where it it hosted.


DBIx::Class has a lot of tools to manage timestamps. Using InflateColumn::DateTime is nearly a standard. Problem is DBIx::Class can't ask MySQL what is its timezone so you have to configure it when you write the model. Your timestamp field will be:

      data_type => "timestamp",
      datetime_undef_if_invalid => 1,
      default_value => \"current_timestamp",
      timezone => "Europe/Rome",
      is_nullable => 0,


Dancer is not really involved in this question. Every time you want to print a timestamp you'll go through this code. But this could be used when Dancer has to give you an output on the screen. Here you can decide once more what is the timezone to use. 

Few lines:

It's a trap!

What could go wrong? 
If timezone is not configured in the DBIx::Class you will have, as output, the hour in the timezone of MySQL, but with a "floating" timezone. This means that conversion will be... well... erratic.
If timezone configured in DBIx::Class is different from the timezone in the MySQL, MySQL timezone will be ignored although the time you receive is made from it. DBIx::Class timezone will be attached to timestamp instead. No automatic conversion will be triggered and you'll have to work just with wrong data.

How to deal with this

Problem I see with this situation is that MySQL is part of the environment while the rest is part of your code. You write the code, but the environment can change. I think that a good idea is always configure the timezone when you output the data ("Dancer" level) so you have a good point to change it if it's needed, without touching the structure below. Than be always sure of your MySQL configuration when you write DBIx::Classes.