A Practical Introduction to Docker Container Terminology

Red Hat Software Collections (RHSCL) 2.0 brings Perl 5.20 as a Docker image. This allows you to deploy Perl applications easily.

The basic idea is to combine your application code from Git tree and Red Hat's rhscl/perl-520-rhel7 base image into an application image that will run your application in mod_perl environment. Your application can either be a simple Common Gateway Interface (CGI) script or a full-fledged Perl Web Server Gateway Interface (PSGI) application.

Following this step-by-step procedure will show you how to deploy a simple pastebin-like web service implemented as a PSGI application.

The Base Image

First we install docker package and start the docker service:

# yum install docker
# systemctl start docker

Then we download the rhscl/perl-520-rhel7 Docker image:

# docker pull rhscl/perl-520-rhel7

Now you can verify that the base image really contains Perl 5.20:

# docker run --rm -ti rhscl/perl-520-rhel7 perl -v

This is perl 5, version 20, subversion 1 (v5.20.1) built for x86_64-linux-thread-multi

The Application Source

Back to the application. We will create a simple pastebin-like application using Web::Paste::Simple Perl module. We will need git tool, so we will install it:

# yum install git

We will begin with an empty Git repository:

$ mkdir application
$ cd application
$ git init-db .
Initialized empty Git repository in /tmp/application/.git/

Now we will write our application:

$ cat >application.psgi
use Web::Paste::Simple;

If you had locally installed some PSGI server, for example Plack from CPAN, you could test the application locally:

$ plackup application.psgi
HTTP::Server::PSGI: Accepting connections at http://0:5000/

But let's assume our code is correct and continue with building the application image.

As I wrote, the application uses Web::Paste::Simple CPAN module. Thus we need to declare this dependency. We will use a cpanfile for that:

$ cat >cpanfile
requires 'Web::Paste::Simple';

If you haven't yet configured your Git identity, do it now:

$ git config --global user.email "developer@example.com"
$ git config --global user.name "Developer"

And then commit the sources into Git tree:

$ git add application.psgi cpanfile
$ git commit -m 'Simple application'
[master (root-commit) 78d64e4] Simple application
 2 files changed, 3 insertions(+)
 create mode 100644 application.psgi
 create mode 100644 cpanfile

The Application Image

Now we are ready to build our application image. We will use s2i tool for that. The tool is provided by source-to-image package, so install it:

# yum install source-to-image

The magic command to build an application from a base Docker image and a Git tree is:

# s2i build file://$PWD rhscl/perl-520-rhel7 application
I1109 15:39:58.995684 06268 sti.go:406] ---> Installing application source
I1109 15:39:59.031843 06268 sti.go:406] ---> PSGI application found in
I1109 15:39:59.053222 06268 sti.go:406] ---> Installing cpanminus 1.7102 ...
I1109 15:41:57.491068 06268 sti.go:406] <== Installed dependencies for ..
I1109 15:41:57.491118 06268 sti.go:406] 44 distributions installed

The second argument is a URL to the Git tree with your application. We used current working directory in this example. But you can use any remote location. For example a repository on the GitHub. The third argument is base Docker image identifier. We used the Red Hat's Perl 5.20 RHSCL image. The last argument is a tag for the resulting application image.

If you forget this incantation, don't worry. If you run the base image without specifying a command (docker run --rm -ti rhscl/perl-520-rhel7), you will get nice help. You can also see Using Red Hat Software Collections Container Images for more details.

The Service

As you can see, the application image was built successfully. And we can verify it in Docker images listing:

# docker images
REPOSITORY              TAG     IMAGE ID        CREATED         VIRTUAL SIZE
application             latest  eed01e5e8b24    8 minutes ago   459.4 MB
rhscl/perl-520-rhel7    latest  f3ed77ae8490    7 weeks ago     502.7 MB

So we did it. Now we can run the application:

# docker run --rm -ti application
[Mon Nov 09 09:53:58.308695 2015] [so:warn] [pid 1] AH01574: module perl_module is already loaded, skipping
AH00558: httpd: Could not reliably determine the server's fully qualified domain name, using Set the 'ServerName' directive globally to suppress this message

And the application will be available on the reported IP address and TCP port 8080. You can use docker's option -p to publish the container's port 8080 on different external port number.

Finally, we can use a web browser to connect to the application:

Empty Wep::Paste::Simple form

And play with it:

Submitted Wep::Paste::Simple form

Last updated: November 1, 2023