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The APT project design document
Chapter 2 Requirements

1. APT should be a replacement for dselect. Therefore it should have all the functionality that dselect has currently. This is the primary means of interaction between the user and the package management system, and it should be able to handle all tasks involved in installing, upgrading, and routine management without having the users take recourse to the underlying management system.
2. It should be easier to use and less confusing for novice users. The primary stimulus for the creation of APT was the perceived intractability, complexity, and non-intuitive behavior of the existing user interface, and as such, human factors must be a primary mandate of APT.
3. It should be able to group packages more flexibly, and possibly allow operations based on a group. One should be able to select, or deselect, a coherent group of related packages simultaneously, allowing one to add, remove, or upgrade functionality to a machine as one step.
4. This would allow APT to handle standard installations, namely, one could then install a set of packages to enable a machine to fulfill specific tasks. Define a few standard installations, and which packages are included therein. The packages should be internally consistent.
5. Make use of a keywords field in package headers; provide a standard list of keywords for people to use. This could be the underpinning to allow the previous two requirements to work (though the developers are not constrained to implement the previous requirements using keywords)
6. Use dependencies, conflicts, and reverse dependencies to properly order packages for installation and removal. This has been a complaint in the past that the installation methods do not really understand dependencies, causing the upgrade process to break, or allowing the removal of packages that left the system in an untenable state by breaking the dependencies on packages that were dependent on the package being removed. A special emphasis is placed on handling pre-dependencies correctly; the target of a predependency has to be fully configured before attempting to install the pre-dependent package. Also, configure immediately requests mentioned below should be handled.
7. Handle replacement of a package providing a virtual package with another (for example, it has been very difficult replacing sendmail with smail, or vice versa), making sure that the dependencies are still satisfied.
8. Handle source lists for updates from multiple sources. APT should also be able to handle diverse methods of acquiring new packages; local filesystem, mountable CD-ROM drives, FTP accessible repositories are some of the methods that come to mind. Also, the source lists can be separated into categories, such as main, contrib, non-us, non-local, non-free, my-very-own, etc. APT should be set up to retrieve the Packages files from these multiple source lists, as well as retrieving the packages themselves.
9. Handle base of source and acquire all Packages files underneath. (possibly select based on architecture), this should be a simple extension of the previous requirement.
10. Handle remote installation (to be implemented maybe in a future version, it still needs to be designed). This would ease the burden of maintaining multiple Debian machines on a site. In the authors opinion this is a killer difference for the distribution, though it may be too hard a problem to be implemented with the initial version of APT. However, some thought must be given to this to enable APT to retain hooks for future functionality, or at least to refrain from methods that may preclude remote activity. It is desirable that adding remote installation not require a redesign of APT from the ground up.
11. Be scalable. Dselect worked a lot better with 400 packages, but at last count the number of packages was around twelve hundred and climbing. This also requires APT to pay attention to the needs of small machines which are low on memory (though this requirement shall diminish as we move towards bigger machines, it would still be nice if Debian worked on all old machines where Linux itself would work).
12. Handle install immediately requests. Some packages, like watchdog, are required to be working for the stability of the machine itself. There are others which may be required for the correct functioning of a production machine, or which are mission critical applications. APT should, in these cases, upgrade the packages with minimal downtime; allowing these packages to be one of potentially hundreds of packages being upgraded concurrently may not satisfy the requirements of the package or the site. (Watchdog, for example, if not restarted quickly, may cause the machine to reboot in the midst of installation, which may cause havoc on the machine)