Chapter 1. Introduction
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# Chapter 1. Introduction

## 1.1. General

The APT method interface allows APT to acquire archive files (.deb), index files (Packages, Release, Mirrors) and source files (.tar.gz, .diff). It is a general, extensible system designed to satisfy all of these requirements:

1. Remote methods that download files from a distant site

2. Resume of aborted downloads

3. Progress reporting

4. If-Modified-Since (IMS) checking for index files

5. In-Line MD5 generation

6. No-copy in-filesystem methods

7. Multi-media methods (like CD's)

8. Dynamic source selection for failure recovery

9. User interaction for user/password requests and media swaps

10. Global configuration

Initial releases of APT (0.1.x) used a completely different method interface that only supported the first 6 items. This new interface deals with the remainder.

## 1.2. Terms

Several terms are used through out the document, they have specific meanings which may not be immediately evident. To clarify they are summarized here.

source

  Refers to an item in source list. More specifically it is the broken down item, that is each source maps to exactly one index file. Archive sources map to Package files and Source Code sources map to Source files.

archive file

  Refers to a binary package archive (.deb, .rpm, etc).

source file

  Refers to one of the files making up the source code of a package. In debian it is one of .diff.gz, .dsc. or .tar.gz.

URI

  Universal Resource Identifier (URI) is a super-set of the familiar URL syntax used by web browsers. It consists of an access specification followed by a specific location in that access space. The form is <access>:<location>. Network addresses are given with the form <access>://[<user>[:<pas>]@]hostname[:port]/<location>. Some examples:

      file:/var/mirrors/debian/
      ftp://ftp.example.org/debian
      ftp://jgg:MooCow@localhost:21/debian
      nfs://bigred/var/mirrors/debian
      rsync://debian.midco.net/debian
      cdrom:Debian 2.0r1 Disk 1/

method

  There is a one to one mapping of URI access specifiers to methods. A method is a program that knows how to handle a URI access type and operates according to the specifications in this file.

method instance

  A specific running method. There can be more than one instance of each method as APT is capable of concurrent method handling.

message

  A series of lines terminated by a blank line sent down one of the communication lines. The first line should have the form xxx TAG where xxx are digits forming the status code and TAG is an informational string

acquire

  The act of bring a URI into the local pathname space. This may simply be verifying the existence of the URI or actually downloading it from a remote site.

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