Concepts
What this document explains
In the training, we will use the Clear Linux OS Team vocabulary to describe processes, concepts, and data. Since this vocabulary has grown over time in the team, it may not be the most logical naming for people who start to learn about how Clear Linux OS is created. This chapter attempts to explain the idea behind the terminology.
Goal
The end goal of this training is that the student understands how to create and deliver a custom version of Clear Linux OS using the tooling explained in this training. The full picture of how that looks is graphically summarized in the following graph:.
The graph shows how content from the official Clear Linux OS and any custom content can be mixed to create both images, and an update content stream.
The update content stream will need to be made available as HTTP or HTTPS data on a network connection. It is the equivalent of a package repository.
The initial setup of machines needs to happen with bootable images that can either live boot, or install the base OS.
Updates
The concept of updates is central to the design of the Clear Linux OS method of delivering and maintaining the OS. In essence, every modification to the OS is considered an update. This includes installation, updates themselves, and the addition of optional components. In principle, even rolling back to an older version is an update.
The content originates from an "update server". This is implemented by a https enabled webserver where the files are served as static content. The Clear Linux OS periodically queries the data on the update server and determines whether updates are available, and in case the OS wants to install optional components, or even when a new installation is performed. In all those cases, the update content files provides all the data and metadata to perform all the needed actions.
More reading:
Software Updates in Clear Linux
Bundles
In Clear Linux OS, the choice was made to do away with packages as the smallest functional component size. One of the main reasons is that packages in a disproportionate sense are unusable to users by themselves, and require a large amount of packages to be installed before the functionality they offer can be used.
For example, the xorg-server package does not provide a function X server without the presence of about 30+ additional components. On top of that, in order to create these 30 or so components, an additional 200 or so packages are needed for various aspects of the creation of the X server binary.
In Clear Linux OS, bundles are the concept of the smallest usable collection of packages that provide a functional component, and traditional packages are not visible to the user. In some cases, it could mean that a bundle effectively contains a single package (e.g. the "curl" bundle), but in most cases a bundle contains several or even many packages.
More Reading:
Manifests
The Clear Linux OS software update content consists of data and metadata. The data is the files that end up in the OS. The metadata contains relevant information to properly provision the data to the OS file system, as well as update the system and add or remove additional content to the OS.
The Manifests are mostly long lists of hashes that describe content. Each bundle gets its own manifest file. There is a master manifest file that describes all manifests to tie it all together.
Fullfiles, packs, and delta packs
The data that an update provisions to a system can be obtained in three different ways. There are three different methods, and they exist to optimize the delivery of content and speed up updates.
Fullfiles are always generated for every file in every release. This allows any Clear Linux OS to obtain the exact copy of the content for each version directly. This would be used if the OS verification (swupd verify) needed to replace a single file, for instance.
Packs are available for some releases and combine many files to speed up the creation of installation media and large updates. Delta packs are an optimized version of packs that only contain updates (binary diffs) and cannot be used without having the original file content.
In most swupd update scenarios, the delta packs will be used as much as possible, since they deliver the update content in the smallest size possible.
In most swupd bundle-add scenarios, the packs will be used as much as possible, since they deliver the needed content in a single downloadable unit.
Upstream
An Open Source Project is by definition "upstream". The Clear Linux OS directly consumes project software from upstream as much as possible. In most cases, this is trivial and the upstream community creates proper source code releases, and addresses bugs and issues as needed appropriately.
In some cases, the Clear Linux OS team maintains some projects as an upstream project as well. Examples are obviously the mixer and swupd projects.
The term upstream describes a relationship where content moves from upstream to downstream in a fluid manner, and content is generally aimed to be submitted back to upstream but receive significant review. This concept applies also when people create a mixed Clear Linux OS version. In that case, the official Clear Linux OS is the upstream to the mixed version.
Packages vs. projects
In the Clear Linux OS terminology, we separate the source code that is in upstream repositories, and the distribution adaptation of the products of that project.
The definition of a project is the thing that is the upstream of a package. Projects are maintained by individual maintainers. The Clear Linux OS team does not modify the project unless they are the upstream maintainers. In the same way, people who are maintainers of projects are directly providing content to the Clear Linux OS, with all the responsibilities that go with it.
Packages describe how the project content is consumed. Many packages have optional dependencies and configuration items that may or may not be desired. On top of that, specifically targeted distributions like Clear Linux OS may want to further patch the software to configure the project to better work with the methods and rules that the Clear Linux OS dictates.
RPM
Internally, the Clear Linux OS uses the RPM package format to bridge the software source code and the binary software update content. The RPM format is an intermediate way of storing content.
Note: swupd does not use or recognize the RPM file format and the rpm program on a Clear Linux OS installation is not capable of using RPM files.
Within the build mechanisms that Clear Linux OS uses, the format is used to store the output of the compilation process and provide dependency relationships to the code that generates bundles. These bundles rely on the RPMs dependencies, and by extension, the source code dependency information. Using these dependencies, the mixer can create meaningful bundles that contain the needed components to make software functional without having to describe all dependencies in the bundle definition.
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