Chapter 2. Distribution of content in RHEL 9

The following sections provide an overview of software distribution in Red Hat Enterprise Linux 9:

2.1. Repositories

RHEL 9 content is distributed through the two main repositories: BaseOS and AppStream. Both the BaseOS and AppStream content sets are required for a basic RHEL installation and are available with all RHEL subscriptions. For installation instructions, see the Performing a standard RHEL installation document.

BaseOS
Content in the BaseOS repository is intended to provide the core set of the underlying OS functionality that provides the foundation for all installations. This content is available in the RPM format and is subject to support terms similar to those in previous releases of Red Hat Enterprise Linux.
AppStream
Content in the AppStream repository includes additional user-space applications, runtime languages, and databases in support of the varied workloads and use cases.
CodeReady Linux Builder
The CodeReady Linux Builder repository is available with all RHEL subscriptions. It provides additional packages for use by developers. Packages included in the CodeReady Linux Builder repository are unsupported.

2.2. Application Streams

Multiple versions of user-space components are delivered as Application Streams and updated more frequently than the core operating system packages. This provides greater flexibility to customize RHEL without impacting the underlying stability of the platform or specific deployments.

Each Application Stream component has a given lifecycle, either the same as RHEL 9 or shorter, more suitable to the particular application.

Application Streams are available in the familiar RPM format, as an extension to the RPM format called modules, as Software Collections, or as Flatpaks.

RHEL 9 improves Application Streams experience by providing initial Application Stream versions that can be simply installed as RPM packages by using the traditional yum install command.

Some additional Application Stream versions will be distributed as modules with a shorter lifecycle in future minor RHEL 9 releases.

2.3. Modules

A module is a set of RPM packages that represent a component and are usually installed together. A typical module contains packages with an application, packages with the application-specific dependency libraries, packages with documentation for the application, and packages with helper utilities.

2.4. Module streams

Module streams are filters that can be imagined as virtual repositories in the AppStream physical repository. Module streams represent versions of the AppStream components. Each of the streams receives updates independently.

Module streams can be active or inactive. Active streams give the system access to the RPM packages within the particular module stream, allowing installation of the respective component version. Streams are active if they are explicitly enabled by a user action.

Only one stream of a particular module can be active at a given point in time. Thus only one version of a component can be installed on a system. Different versions can be used in separate containers.

Each module can have a default stream. Default streams make it easy to consume RHEL packages the usual way without the need to learn about modules. The default stream is active, unless the whole module has been disabled or another stream of that module enabled.

Certain module streams depend on other module streams. For example, the perl-App-cpanminus, perl-DBD-MySQL, perl-DBD-Pg, perl-DBD-SQLite, perl-DBI, perl-YAML, and freeradius module streams depend on certain perl module streams.

To select a particular stream for a runtime user application or a developer application, consider the following:

  • Required functionality and which component versions support that functionality
  • Compatibility

For per-component changes, see the Release Notes.

2.5. Module profiles

A profile is a list of recommended packages to be installed together for a particular use case such as for a server, client, development, minimal install, or other. These package lists can contain packages outside the module stream, usually from the BaseOS repository or the dependencies of the stream.

Installing packages by using a profile is a one-time action provided for the user’s convenience. It does not prevent installing or uninstalling any of the packages provided by the module. It is also possible to install packages by using multiple profiles of the same module stream without any further preparatory steps.

Each module stream can have any number of profiles, including none. For any given module stream, some of its profiles can be marked as default and are then used for profile installation actions when no profile is explicitly specified. However, existence of a default profile for a module stream is not required.