Package Dependency Management

Status

• Created: 20/04/05 by MarkShuttleworthBR

• Priority: MediumPriorityBR

• People: ScottJamesRemnantLead, MichaelVogtSecondBR

• Contributors: MarkShuttleworthBR

• Interested: BR

• Status: BrainDump, UduBof, DistroSpecificationBR

• Branch: BR

• Malone Bug: BR

• Packages: BR

• Depends: BR

Introduction

This specification describes an enhancement to the information the system stores about packages that are installed locally. The goal is to improve the ability of systems to evolve over time to continue to reflect the current Ubuntu release.

Three primary improvements are envisaged:

1. The system needs to have some idea *why* a package was installed. Some
   • of the higher-level package management tools, such as Aptitude, already do something similar to this. They track, for example, whether or not a package was installed because of a specific request by the system admiinistrator, or to satisfy the dependencies of a package that was being installed. This allows them to offer to remove those dependency packages when the chosen package is removed later.
2. Over time, it's inevitable that the Ubuntu team may choose to change
   • the open source products preferred for specific functionality in an Ubuntu system. For example, the Ubuntu developers might choose to

     migrate from Postfix to Nullmailer, or from Esound to PolypAudio. For new installations that presents no problem, someone installing the new release will simply get the new apps. However, for someone upgrading, it will be necessary to identify whether or not the user has modified any configuration files for the relevant app, and if not, to migrate the system to the new app.

Rationale

Over time, a Linux system accumulates a substantial amount of cruft. Traditionally, installing a package required installing its dependencies. Later, when that package is upgraded, new dependencies might be introduced and older ones dropped, but in most systems the old package dependencies are never uninstalled. The end result is a plethora of unwanted and unneeded libraries and supporting packages which take up disk space and potentially also slow down the day to day operation of the computer.

These improvements will make it easier to keep Ubuntu systems lean-and-mean over a series of releases, leaving only those packages which the Ubuntu team recommends, the packages explicitly selected by users, and the necessary dependencies required to support those packages.

Scope and Use Cases

The following use cases illustrate the ideas in this spec:

1. Jimmy installed Breezy, and has now updated to the next release. During
   • the update, no matter which package management tool he uses, old dependencies will be removed and new ones installed. There should be no old libraries left on his system once the upgrade is complete.
2. Annabel has installed openldap, and all of its dependencies were
   • automatically installed on her system. Now she is going to remove it from this system using a different package manager. That package manager identifies packages that were installed purely in support of openldap, and offers to remove those too.
3. Jack is upgrading from Breezy to the next Ubuntu release. His system has
   • postfix installed, because that was the default mailserver in Breezy, but he has never modified its configuration and so when he does the update, postfix is removed and replaced with newmailer, the Ubuntu team's chosen replacement for postfix in the new release.

Implementation Plan

Data Preservation and Migration

Packages Affected

User Interface Requirements

Outstanding Issues