How to Enable Software Collections (SCL) on RHEL

Package is the basic unit used for the distribution, management and update of software customized for a Linux system.  Collections of packages stored in different software repositories (e.g., YUM Repositories) can be accessed locally or over a network connection.  Metadata are combined with packages themselves to determine (and resolve, if possible) dependencies among the packages.

In this article, we will try to learn the following entities/concepts in more details:

• RPM 
• May refers to the .rpm file format, files in this format, software packaged in such files, and the package manager itself
• RPM's dependency processing is based on knowing what capabilities are provided by a package and what capabilities a package requires.
• It does automatically determine what shared libraries a package requires
• YUM 
• One of the several front-ends to RPM, which ease the process of obtaining and installing RPMs from repositories and help in resolving their dependencies.
• YUM Repository
• YUM repository metatadata is structured as a series of XML files to organize packages and their associated package groups for installation.
• Need to setup Apache, nginx, or some other web server and point it at the base directory of the repository to make it available
• Red Hat Software Collections
• Is a prescribed set of content intended for use in Red Hat Enterprise Linux production environments

RPM

The RPM Package Manager (RPM) is a package management system that

• Facilitates the distribution, management and update of software customized for a Linux system
• Works with standard package management tools (e.g., Yum or PackageKit) to install, reinstall, remove, upgrade and verify RPM packages
• Provides metadata to describe packages, installation instructions, and so on
• Each RPM package includes metadata that describes the package's components, version, release, size, project URL, installation instructions, and so on
• Separates source and binary packages
• In source packages, you have the pristine sources along with any patches that were used, plus complete build instructions.
• Allows you to use the database of installed packages to query and verify packages
• Allows you to add your package to a Yum repository
• Allows you to Digitally sign your packages (e.g., using a GPG signing key)

Red Hat Software Collections

Red Hat Software Collections is a prescribed set of content intended for use in Red Hat Enterprise Linux production environments. Through Red Hat Software Collections, you can choose the runtime versions best suited for your projects, preserve application stability, and deploy your applications with confidence.

Software Collections Functionality

Software collections functionality (from hereafter, we will refer it simply as "Software Collections")—not to be confused with Red Hat Software Collections—has been available in earlier Red Hat Enterprise Linux distributions. Software collections provides a structural definition, independent of the operating system, for applications or tools.

With Software Collections, you can build and concurrently install multiple versions of the same software components on your system. Software Collections have no impact on the system versions of the packages installed by any of the conventional RPM package management utilities.

To summarize, Software Collections have the following characteristics:

• Do not overwrite system files
• Are designed to avoid conflicts with system files
• Software Collections make use of a special file system hierarchy to avoid possible conflicts between a single Software Collection and the base system installation.
• Require no changes to the RPM package manager
• Need only minor changes to the spec file
• To convert a conventional package to a single Software Collection, you only need to make minor changes to the package spec file.
• Allow you to build a conventional package and a Software Collection package with a single spec file
• Uniquely name all included packages
• Do not conflict with updated packages
• Software Collection's namespace ensures that updating packages on your system causes no conflicts. 
• Can depend on other Software Collections 
• Because one Software Collection can depend on another, you can define multiple levels of dependencies.

Enabling and Building Software Collections

To enable support for Software Collections on your system so that you can enable and build Software Collections, you need to have installed the following packages:

• scl-utils 
• Provides the scl tool (an utility for running software packaged as a Software Collection) that lets you enable Software Collections on your system
• scl-utils-build
• Provides macros that are essential for building Software Collections.

References

1. Software Collections Guide (Redhat)
2. 20 Linux YUM (Yellowdog Updater, Modified) Commands for Package Management
3. PackageKit
4. Red Hat Enterprise Linux 6 Deployment Guide 
5. Red Hat Enterprise Linux 5 Deployment Guide.
6. Learn Linux, 101: RPM and YUM package management
• Yum allows automatic updates, package and dependency management, on RPM-based distributions.
7. Creating a Local Yum Repository Using an ISO Image
• Yum works with software repositories (collections of packages), which can be accessed locally or over a network connection.
8. Configuring Yum and Yum Repositories (important)
9. Converting a Conventional Spec File
10. HOWTO: GPG sign and verify RPM packages and yum repositories
11. The Software Collections (SCL) Repository (CentOS)
12. Red Hat will provide PHP 5.4 for RHEL-6