Learning AlmaLinux - Modules (AppStream) & Software Collections

#Introduction

In the previous episode, Episode 4, we got to know the BaseOS and AppStream repositories as package sources. Now we break down the most interesting part of AppStream: modules. A module is a feature that lets one repository offer several versions of the same application side by side — Python 3.9 alongside Python 3.12, Node.js 20 alongside Node.js 22, and so on.

This concept is the enterprise answer to the classic problem "one of my applications needs the old version, another needs the new version". In the RHEL 8 and 9 era it lives through module streams; in the older 8.x era there was also Software Collections (SCL). Let's understand both.

#The Module Stream Concept

A module is a group of packages managed as a single unit — for example the python, nodejs, or postgresql module. Each module can have several streams, which are the different versions users can choose. There are also profiles that determine which set of packages gets installed.

An analogy: a module is the application brand, a stream is the variant or major version, and a profile is the edition (for example minimal, server, or development).

dnf5 module list

The output shows a table of modules along with their available streams — this is the software version menu on your system. To view the details of one module:

dnf5 module list python

#Listing and Enabling Modules

To use a stream, you need to enable it first. Enabling determines which stream becomes the default when packages are installed from that module.

sudo dnf5 module enable python:3.12

Once the stream is active, the related packages can be installed as usual:

sudo dnf5 install python3

#Disabling a Module

sudo dnf5 module disable nodejs:22

Disabling a module makes its stream unavailable for new installations — useful for locking down an environment so no package can pull that version.

#Popular Module Examples

Here are some modules most commonly used in the real world:

dnf5 module list postgresql

You can see that some streams are marked with status [e] (enabled), [d] (default), or [x] (disabled) — understanding these three statuses is the key to module management.

#Switching Streams

Needs change, and streams can be switched. The process is two steps: reset the module state, then enable the new stream.

sudo dnf5 module reset python
sudo dnf5 module enable python:3.12
sudo dnf5 upgrade

#Software Collections (SCL): The 8.x Era Legacy

Before AppStream modules, the RHEL ecosystem used Software Collections (SCL) — the old way to provide several software versions side by side on RHEL 6 and 7, and the early 8 era.

SCL's trademark: software is installed to a separate prefix (/opt/rh/), and activated by hooking up the environment:

scl enable rh-python36 -- python3 --version

Popular collections from that era: gcc-toolset (modern compiler toolchain), rh-python36, and devtoolset. The output of the command above shows the Python version enabled only for the duration of that command.

#SCL vs AppStream

Key conclusion: on AlmaLinux 9 and 10, the correct way is AppStream modules — not SCL. SCL is largely retired on RHEL 9+, and you won't find scl promoted as a recommended practice. However, understanding SCL matters for managing 8.x servers still running in the field, especially because AlmaLinux 8 remains under maintenance until 2029.

#Common Pitfalls

1. Installing a package without enabling the stream. You may get an unexpected version — always enable the stream first.
2. Assuming every module has many streams. Some modules only have a single latest stream and don't need enabling.
3. Switching streams without testing. This is a runtime major version change — never do it in production without staging.
4. Using SCL on AlmaLinux 9/10. That's an old practice; use the AppStream modules that are the standard.
5. Forgetting dnf5 module reset. Without a reset, enabling a new stream often fails because the old state is still attached.

#Conclusion

In this episode 5 you've understood AppStream modules: the module, stream, and profile concepts, how to list, enable, disable, and switch streams, examples of popular modules like Python, Node.js, and PostgreSQL, and an honest comparison between modern modules and the Software Collections legacy of the 8.x era.

Key takeaways:

• Module = package group; stream = major version; profile = package set.
• Enable a stream with dnf5 module enable <name>:<stream>, then install its packages.
• Switch streams through the module reset → module enable → upgrade sequence.
• Common streams: python:3.12, nodejs:20, postgresql:16.
• SCL (for example gcc-toolset) is an 8.x legacy — use AppStream modules on 9/10.

Tidy software version management is a hallmark of a mature sysadmin. In the next episode, Episode 6, we'll cover Users, Permissions & Sudo — from creating users and groups, the /etc/passwd and /etc/shadow identity files, to sudo configuration, the wheel group, ACLs, and the SUID/SGID/sticky special bits. See you there!