Saturday, 14 December 2013

Ballroom Blitz (2)

Silence in the library

When you write a perl program you don't need just the perl interpeter, but also a lot of libraries that someone else wrote. It's easy to fetch them while you're in your developement environment, but when you go to production you have to manage a lot of issues about this problem.

First of all, there's the scenario where you don't have root access for the system where you are so libraries installation appear to be an impossible task. 
A worse scenario is the one where you have root access and you install the libraries directly in the system.
The worst case scenario is the one where you're installing things in the system, the system die and someone behind you start hitting you with a club. No, the problem is not the one with the club, the problem is you. The club is the solution.

Never do installations at system level. System level is just for... the system, the libraries installed that are the libraries need by your OS, not the libraries you need for your site. In many years of developement and deployment  people found many ways to do things better.

The article I linked last time, for this task, uses perlbrew, one of the best practices for that. Perlbrew manages not only libraries, but also perl version, so it can also be used to ensure that the bizarre incompatibilty between 5.16.0 and the cutting edge 5.18.0 is not an issue because you will configure on your machine just the version that works.

But timtowtdi so in this article I'll speak about probably the oldest practice about managing libraries, the oldest but still useful, clean and sharp: local::lib.

local::lib is a library that you have to install at system level:

(as root) cpan -i local::lib

 after that you'll have the power of playing with perl libraries just moving directories. Remember the directory where we decided to place our site, /var/www/com/example/my/?
Create under this directory a directory env and then a file named perl_sandbox with these lines:
export PS1='\[\e[1;35m\]\u@\h:\w\$\[\e[0m\] '
eval $(perl -I$libdir -Mlocal::lib=$libdir)
Now source the file:

source perl_sandbox

This way your shell will become pink (the export PS1 line) and all the operations you'll do about perl libraries will have effect just on the env directory. However, with the perl_sandbox active, every perl command you'll use will see the libraries in the env directory.

Just a last tip: remember to install cpanm

cpan -i App::cpanminus

and use that for installations, with --notest option. Life is too short to let cpan to check all the repository every time you need an environment.

Up to the plack

Plack is a server used to run PSGI applications. PSGI is a interface that we can use to communicate with the webserver that will bring our site on the World Wide Web, so Plack is something we really need to make things work.

Talking about this issue, working with Dancer makes thing really easy because the standard bin/ script that run your webapp can be used as is as a PSGI startup file, as the Dancer2 Deployment page says.

bin/ script must run under a perl web server supporting PSGI. My favourite for this is starman (yes, I like David Bowie, but it's not just for that).
Copy&Paste from Dancer2 Deployment page how to run plackup and make your site works, however, is not enough for something that will need maintenance in the future.

The last piece of the puzzle is Server::Starter, a perl module that provide you the start_server script that will change you plackup server in a daemon (Argh! WItchcraft!), making easy for you stop/start procedures and monitoring.
After installing that module and after creating a /tmp directory under our site directory, you can try THIS copy&paste:

source perl_sandbox #as seen in the previous paragraph 
          cd $PROJECT
start_server \
--pid-file=$WORKINGDIR/tmp/ \
--status-file=$WORKINGDIR/tmp/starman.status \
-- \
plackup -E ec2 -s Starman --workers=2 -l $WORKINGDIR/tmp/plack.sock -a bin/ & 
as your start_server script.
And this as last command:

start_server \
--pid-file=$WORKINGDIR/tmp/ \
--status-file=$WORKINGDIR/tmp/starman.status \

For restart.

For the timtowtdi section, take a look at Daemon::Control for an alternative implementation of the daemon. For a PSGI webserver general purpouse, one of the most used technologies of these days is uWSGI.

Gentlemen, start your NGINX

Last step is the easiest one. NGINX is just a proxy from the internet to your plackup structure. NGINX and Plackup will communicate using the plack socket that we configured in the start script: /my/site/dir/tmp/plack.sock. Obviously we're talking about an environment where Plack and uWSGI are on the same machine, otherwise you'll have to sail for the TCP/IP, as everything on the network.
Let's see nginx configuration:

upstream siteplack {
  server unix:/your/site/directory/tmp/plack.sock;

server {

  access_log /your/site/directory/logs/;
  error_log /your/site/directory/logs/;

  root /your/site/directory/project/src/public;

  location / {
    try_files $uri $uri @proxy;
    access_log off;
    expires max;

  location @proxy {
    proxy_set_header Host $http_host;
    proxy_set_header X-Forwarded-Host $host;
    proxy_set_header X-Real-IP $remote_addr;
    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    proxy_pass http://siteplack;

Few things to say. The public directory of the Dancer2 project should be server directly, because DancerApp has nothing to do with static things like css and javascripts. All the others things are just to copy and paste.

Restarting NGINX, praying all your gods, if all you did was did well your site will rise.
If not probably it will be my fault writing something wrong in here, but I'll be too far from you to be blamed. Just solve your problems and tell me how to fix the article. Comments are there for this reason.