RHCSA Rapid Track

Objectives

After completing this section, you should be able to:

Introduction to Application Stream

Red Hat Enterprise Linux 8.0 introduces the concept of Application Streams. Multiple versions of user space components shipped with the distribution are now delivered at the same time. They may be updated more frequently than the core operating system packages. This provides you with greater flexibility to customize Red Hat Enterprise Linux without impacting the underlying stability of the platform or specific deployments.

Traditionally, managing alternate versions of an application's software package and its related packages meant maintaining different repositories for each different version. For developers who wanted the latest version of an application and administrators who wanted the most stable version of the application, this created a situation that was tedious to manage. This process is simplified in Red Hat Enterprise Linux 8 using a new technology called Modularity. Modularity allows a single repository to host multiple versions of an application's package and its dependencies.

Red Hat Enterprise Linux 8 content is distributed through two main software repositories: BaseOS and Application Stream (AppStream).

BaseOS

The BaseOS repository provides the core operating system content for Red Hat Enterprise Linux as RPM packages. BaseOS components have a life cycle identical to that of content in previous Red Hat Enterprise Linux releases.

Application Stream

The Application Stream repository provides content with varying life cycles as both modules and traditional packages. Application Stream contains necessary parts of the system, as well as a wide range of applications previously available as a part of Red Hat Software Collections and other products and programs.

The Application Stream repository contains two types of content: Modules and traditional RPM packages. A module describes a set of RPM packages that belong together. Modules can contain several streams to make multiple versions of applications available for installation. Enabling a module stream gives the system access to the RPM packages within that module stream.

Modules

A module is a set of RPM packages that are a consistent set that belong together. Typically, this is organized around a specific version of a software application or programming language. A typical module can contain packages with an application, packages with the application’s specific dependency libraries, packages with documentation for the application, and packages with helper utilities.

Module Streams

Each module can have one or more module streams, which hold different versions of the content. Each of the streams receives updates independently. Think of the module stream as a virtual repository in the Application Stream physical repository.

For each module, only one of its streams can be enabled and provide its packages.

Module Profiles

Each module can have one or more profiles. A profile is a list of certain packages to be installed together for a particular use-case such as for a server, client, development, minimal install, or other.

Installing a particular module profile simply installs a particular set of packages from the module stream. You can subsequently install or uninstall packages normally. If you do not specify a profile, the module will install its default profile.

Managing modules using Yum

Yum version 4, new in Red Hat Enterprise Linux 8, adds support for the new modular features of Application Stream.

For handling the modular content, the yum module command has been added. Otherwise, yum works with modules much like does with regular packages.

Listing Modules

To display a list of available modules, use yum module list:

[user@host ~]$ yum module list
Red Hat Enterprise Linux 8 for x86_64 - AppStream (RPMs)
Name                    Stream        Profiles   Summary
389-ds                  1.4           default    389 Directory Server (base)
ant                     1.10 [d]      common [d] Java build tool
container-tools         1.0 [d]       common [d] Common tools and dependencies for container runtimes
...output omitted...
   Hint: [d]efault, [e]nabled, [x]disabled, [i]nstalled
   

To list the module streams for a specific module and retrieve their status:

[user@host ~]$ yum module list perl
Red Hat Enterprise Linux 8 for x86_64 - AppStream (RPMs)
Name  Stream       Profiles             Summary
perl  5.24         common [d], minimal  Practical Extraction and Report Language
perl  5.26 [d]     common [d], minimal  Practical Extraction and Report Language
   

To display details of a module:

[user@host ~]$ yum module info perl
Name             : perl
Stream           : 5.24
Version          : 820190207164249
Context          : ee766497
Profiles         : common [d], minimal
Default profiles : common
Repo             : rhel-8-for-x86_64-appstream-rpms
Summary          : Practical Extraction and Report Language
...output omitted...
Artifacts        : perl-4:5.24.4-403.module+el8+2770+c759b41a.x86_64
                 : perl-Algorithm-Diff-0:1.1903-9.module+el8+2464+d274aed1.noarch
                 : perl-Archive-Tar-0:2.30-1.module+el8+2464+d274aed1.noarch
...output omitted...

[user@host ~]$ yum module info --profile perl:5.24

Enabling Module Streams and Installing Modules

Module streams must be enabled in order to install their module. To simplify this process, when a module is installed it enables its module stream if necessary. Module streams can be enabled manually using yum module enable and providing the name of the module stream.

Install a module using the default stream and profiles:

[user@host ~]$ sudo yum module install perl
Dependencies resolved.
================================================================================
 Package         Arch   Version      Repository                            Size
================================================================================
Installing group/module packages:
 perl            x86_64 4:5.26.3-416.el8
                                     rhel-8-for-x86_64-appstream-htb-rpms  72 k
Installing dependencies:
...output omitted...
Running transaction
  Preparing        :                                                        1/1
  Installing       : perl-Exporter-5.72-396.el8.noarch                    1/155
  Installing       : perl-Carp-1.42-396.el8.noarch                        2/155
...output omitted...
Installed:
  perl-4:5.26.3-416.el8.x86_64
  perl-Encode-Locale-1.05-9.el8.noarch
...output omitted...
Complete!

To verify the status of the module stream and the installed profile:

[user@host ~]$ yum module list perl
Red Hat Enterprise Linux 8 for x86_64 - AppStream (RPMs)
Name  Stream       Profiles             Summary
perl  5.24         common, minimal      Practical Extraction and Report Language
perl  5.26 [d][e]  common [i], minimal  Practical Extraction and Report Language

Hint: [d]efault, [e]nabled, [x]disabled, [i]nstalled

Removing Modules and Disabling Module Streams

Removing a module removes all of the packages installed by profiles of the currently enabled module stream, and any further packages and modules that depend on these. Packages installed from this module stream not listed in any of its profiles remain installed on the system and can be removed manually.

To remove an installed module:

[user@host ~]$ sudo yum module remove perl
Dependencies resolved.
================================================================================
 Package                        ArchVersion            Repository                            Size
================================================================================
Removing:
 perl                           x86_644:5.26.3-416.el8   @rhel-8.0-for-x86_64-appstream-rpms 0
Removing unused dependencies:
...output omitted...
Running transaction
Preparing        :                                                        1/1
Erasing          : perl-4:5.26.3-416.el8.x86_64                         1/155
Erasing          : perl-CPAN-2.18-397.el8.noarch                        2/155
...output omitted...
Removed:
perl-4:5.26.3-416.el8.x86_64
dwz-0.12-9.el8.x86_64
...output omitted...
Complete!

After the module is removed, the module stream is still enabled. To verify the module stream is still enabled:

[user@host ~]$ yum module list perl
Red Hat Enterprise Linux 8 for x86_64 - AppStream (RPMs)
Name  Stream       Profiles             Summary
perl  5.24         common [d], minimal  Practical Extraction and Report Language
perl  5.26 [d][e]   common [d], minimal  Practical Extraction and Report Language

Hint: [d]efault, [e]nabled, [x]disabled, [i]nstalled
  

To disable the module stream:

[user@host ~]$ sudo yum module disable perl
  ...output omitted...
Dependencies resolved.
=================================================================================
 Package           Arch             Version              Repository         Size
=================================================================================
Disabling module streams:
 perl                               5.26
Is this ok [y/N]: y
Complete!
  

Switching Module Streams

Switching module streams generally requires upgrading or downgrading the content to a different version.

To ensure a clean switch, you should remove the modules provided by the module stream first. That will remove all the packages installed by the profiles of the module, and any modules and packages that those packages have dependencies on.

To list the packages installed from the module, in the example below the postgresql:9.6 module is installed:

[user@host ~]$ sudo yum module info postgresql | grep module+el8 | \
sed 's/.*: //g;s/\n/ /g' | xargs yum list installed
Installed Packages
postgresql.x86_64          9.6.10-1.module+el8+2470+d1bafa0e   @rhel-8.0-for-x86_64-appstream-rpms
postgresql-server.x86_64   9.6.10-1.module+el8+2470+d1bafa0e   @rhel-8.0-for-x86_64-appstream-rpms

Remove the packages listed from the previous command. Mark the module profiles to be uninstalled.

[user@host ~]$ sudo yum module remove postgresql
...output omitted...
Is this ok [y/N]: y
...output omitted...
Removed:
  postgresql-server-9.6.10-1.module+el8+2470+d1bafa0e.x86_64   libpq-10.5-1.el8.x86_64  postgresql-9.6.10-1.module+el8+2470+d1bafa0e.x86_64
Complete

After removing the module profiles reset the module stream. Use the yum module reset command to reset the module stream.

[user@host ~]$ sudo yum module reset postgresql
=================================================================
 Package       Arch             Version     Repository      Size
=================================================================
Resetting module streams:
postgresql                      9.6

Transaction Summary
=================================================================

Is this ok [y/N]: y
Complete!

To enable a different module stream and install the module:

[user@host ~]$ sudo yum module install postgresql:10

The new module stream will be enabled and the current stream disabled. It may be necessary to update or downgrade packages from the previous module stream that are not listed in the new profile. Use the yum distro-sync to perform this task if required. There may also be packages that remain installed from the previous module stream. Remove those using yum remove.