launchpad development
Components
Components are used to separate discrete parts of a distribution archive. The most common reason for doing this is licensing and support. Ubuntu has the following components:
main |
The core supported packages in Ubuntu. All packages are provided with an acknowledged open source licence. |
restricted |
Packages that are supported but have a non-free licence. |
universe |
Packages that are only supported by the community. They have an open source licence. |
multiverse |
Packages that may or may not be supported and have a non-free licence. |
Component dependencies
Boundaries between the components are strictly enforced at build time. As ordered in the table, a package cannot depend on another package that is in a lower component.
The Soyuz publishing tables ([Source|Binary]PackagePublishingHistory) record in which component a package is published. It may be different to the originally intended component when the package was uploaded, as archive admins can override it either when it's waiting in the distroseries queues, or after it's been published.
A source may not be published in more than one component, so if it is overridden to a different component after being already published the new publication supersedes the old one.
Component selection at the end PC
Every Ubuntu machine has a /etc/apt/sources.list file. The contents of this file determine which components that routine updates will be received from.
Components and PPAs
PPAs do not have components. But because you need at least one component in the archive repository due to the directory layout conditions, the publisher uses main. Therefore the sources.list entries for PPAs will always refer to the main component even though there are no others.
