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authorGuillem Jover <guillem@debian.org>2014-07-02 02:22:32 +0200
committerMichael Vogt <mvo@debian.org>2014-07-08 13:15:27 +0200
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-<!-- -*- mode: sgml; mode: fold -*- -->
-<!doctype debiandoc PUBLIC "-//DebianDoc//DTD DebianDoc//EN">
-<book>
-<title>APT Method Interface </title>
-
-<author>Jason Gunthorpe <email>jgg@debian.org</email></author>
-<version>$Id: method.sgml,v 1.10 2003/02/12 15:05:46 doogie Exp $</version>
-
-<abstract>
-This document describes the interface that APT uses to the archive
-access methods.
-</abstract>
-
-<copyright>
-Copyright &copy; Jason Gunthorpe, 1998.
-<p>
-"APT" and this document are free software; you can redistribute them and/or
-modify them under the terms of the GNU General Public License as published
-by the Free Software Foundation; either version 2 of the License, or (at your
-option) any later version.
-
-<p>
-For more details, on Debian systems, see the file
-/usr/share/common-licenses/GPL for the full license.
-</copyright>
-
-<toc sect>
-
-<chapt>Introduction
-<!-- General {{{ -->
-<!-- ===================================================================== -->
-<sect>General
-
-<p>
-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:
-
-<enumlist>
-<item>Remote methods that download files from a distant site
-<item>Resume of aborted downloads
-<item>Progress reporting
-<item>If-Modified-Since (IMS) checking for index files
-<item>In-Line MD5 generation
-<item>No-copy in-filesystem methods
-<item>Multi-media methods (like CD's)
-<item>Dynamic source selection for failure recovery
-<item>User interaction for user/password requests and media swaps
-<item>Global configuration
-</enumlist>
-
-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.
-</sect>
- <!-- }}} -->
-<!-- Terms {{{ -->
-<!-- ===================================================================== -->
-<sect>Terms
-
-<p>
-Several terms are used through out the document, they have specific
-meanings which may not be immediately evident. To clarify they are summarized
-here.
-
-<taglist>
-<tag>source<item>
-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.
-
-<tag>archive file<item>
-Refers to a binary package archive (.deb, .rpm, etc).
-
-<tag>source file<item>
-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.
-
-<tag>URI<item>
-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
-&lt;access&gt;:&lt;location&gt;. Network addresses are given with the form
-&lt;access&gt;://[&lt;user&gt;[:&lt;pas&gt;]@]hostname[:port]/&lt;location&gt;.
-Some examples:
-<example>
-file:/var/mirrors/debian/
-ftp://ftp.debian.org/debian
-ftp://jgg:MooCow@localhost:21/debian
-nfs://bigred/var/mirrors/debian
-rsync://debian.midco.net/debian
-cdrom:Debian 2.0r1 Disk 1/
-</example>
-
-<tag>method<item>
-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.
-
-<tag>method instance<item>
-A specific running method. There can be more than one instance of each method
-as APT is capable of concurrent method handling.
-
-<tag>message<item>
-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
-
-<tag>acquire<item>
-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.
-
-</taglist>
-
-</sect>
- <!-- }}} -->
-<chapt>Specification
-<!-- Overview {{{ -->
-<!-- ===================================================================== -->
-<sect>Overview
-
-<p>
-All methods operate as a sub process of a main controlling parent. 3 FD's
-are opened for use by the method allowing two way communication and
-emergency error reporting. The FD's correspond to the well known unix FD's,
-stdin, stdout and stderr.
-
-<p>
-Through operation of the method communication is done via http
-style plain text. Specifically RFC-822 (like the Package file) fields
-are used to describe items and a numeric-like header is used to indicate
-what is happening. Each of these distinct communication messages should be
-sent quickly and without pause.
-
-<p>
-In some instances APT may pre-invoke a method to allow things like file
-URI's to determine how many files are available locally.
-</sect>
- <!-- }}} -->
-<!-- Message Overview {{{ -->
-<!-- ===================================================================== -->
-<sect>Message Overview
-
-<p>
-The first line of each message is called the message header. The first
-3 digits (called the Status Code) have the usual meaning found in the
-http protocol. 1xx is informational, 2xx is successful and 4xx is failure.
-The 6xx series is used to specify things sent to the method. After the
-status code is an informational string provided for visual debugging.
-
-<list>
-<item>100 Capabilities - Method capabilities
-<item>101 Log - General Logging
-<item>102 Status - Inter-URI status reporting (login progress)
-<item>200 URI Start - URI is starting acquire
-<item>201 URI Done - URI is finished acquire
-<item>400 URI Failure - URI has failed to acquire
-<item>401 General Failure - Method did not like something sent to it
-<item>402 Authorization Required - Method requires authorization
- to access the URI. Authorization is User/Pass
-<item>403 Media Failure - Method requires a media change
-<item>600 URI Acquire - Request a URI be acquired
-<item>601 Configuration - Sends the configuration space
-<item>602 Authorization Credentials - Response to the 402 message
-<item>603 Media Changed - Response to the 403 message
-</list>
-
-Only the 6xx series of status codes is sent TO the method. Furthermore
-the method may not emit status codes in the 6xx range. The Codes 402
-and 403 require that the method continue reading all other 6xx codes
-until the proper 602/603 code is received. This means the method must be
-capable of handling an unlimited number of 600 messages.
-
-<p>
-The flow of messages starts with the method sending out a
-<em>100 Capabilities</> and APT sending out a <em>601 Configuration</>.
-After that APT begins sending <em>600 URI Acquire</> and the method
-sends out <em>200 URI Start</>, <em>201 URI Done</> or
-<em>400 URI Failure</>. No synchronization is performed, it is expected
-that APT will send <em>600 URI Acquire</> messages at -any- time and
-that the method should queue the messages. This allows methods like http
-to pipeline requests to the remote server. It should be noted however
-that APT will buffer messages so it is not necessary for the method
-to be constantly ready to receive them.
-</sect>
- <!-- }}} -->
-<!-- Header Fields {{{ -->
-<!-- ===================================================================== -->
-<sect>Header Fields
-
-<p>
-The following is a short index of the header fields that are supported
-
-<taglist>
-<tag>URI<item>URI being described by the message
-<tag>Filename<item>Location in the filesystem
-<tag>Last-Modified<item>A time stamp in RFC1123 notation for use by IMS checks
-<tag>IMS-Hit<item>The already existing item is valid
-<tag>Size<item>Size of the file in bytes
-<tag>Resume-Point<item>Location that transfer was started
-<tag>MD5-Hash<item>Computed MD5 hash for the file
-<tag>Message<item>String indicating some displayable message
-<tag>Media<item>String indicating the media name required
-<tag>Site<item>String indicating the site authorization is required for
-<tag>User<item>Username for authorization
-<tag>Password<item>Password for authorization
-<tag>Fail<item>Operation failed
-<tag>Drive<item>Drive the media should be placed in
-<tag>Config-Item<item>
-A string of the form <var>item</>=<var>value</> derived from the APT
-configuration space. These may include method specific values and general
-values not related to the method. It is up to the method to filter out
-the ones it wants.
-<tag>Single-Instance<item>Requires that only one instance of the method be run
- This is a yes/no value.
-<tag>Pipeline<item>The method is capable of pipelining.
-<tag>Local<item>The method only returns Filename: fields.
-<tag>Send-Config<item>Send configuration to the method.
-<tag>Needs-Cleanup<item>The process is kept around while the files it returned
-are being used. This is primarily intended for CD-ROM and File URIs that need
-to unmount filesystems.
-<tag>Version<item>Version string for the method
-</taglist>
-
-This is a list of which headers each status code can use
-
-<taglist>
-<tag>100 Capabilities<item>
-Displays the capabilities of the method. Methods should set the
-pipeline bit if their underlying protocol supports pipelining. The
-only known method that does support pipelining is http.
-Fields: Version, Single-Instance, Pre-Scan, Pipeline, Send-Config,
-Needs-Cleanup
-
-<tag>101 Log<item>
-A log message may be printed to the screen if debugging is enabled. This
-is only for debugging the method.
-Fields: Message
-
-<tag>102 Status<item>
-Message gives a progress indication for the method. It can be used to show
-pre-transfer status for Internet type methods.
-Fields: Message
-
-<tag>200 URI Start<item>
-Indicates the URI is starting to be transferred. The URI is specified
-along with stats about the file itself.
-Fields: URI, Size, Last-Modified, Resume-Point
-
-<tag>201 URI Done<item>
-Indicates that a URI has completed being transferred. It is possible
-to specify a <em>201 URI Done</> without a <em>URI Start</> which would
-mean no data was transferred but the file is now available. A Filename
-field is specified when the URI is directly available in the local
-pathname space. APT will either directly use that file or copy it into
-another location. It is possible to return Alt-* fields to indicate that
-another possibility for the URI has been found in the local pathname space.
-This is done if a decompressed version of a .gz file is found.
-Fields: URI, Size, Last-Modified, Filename, MD5-Hash
-
-<tag>400 URI Failure<item>
-Indicates a fatal URI failure. The URI is not retrievable from this source.
-As with <em>201 URI Done</> <em>200 URI Start</> is not required to precede
-this message
-Fields: URI, Message
-
-<tag>401 General Failure<item>
-Indicates that some unspecific failure has occurred and the method is unable
-to continue. The method should terminate after sending this message. It
-is intended to check for invalid configuration options or other severe
-conditions.
-Fields: Message
-
-<tag>402 Authorization Required<item>
-The method requires a Username and Password pair to continue. After sending
-this message the method will expect APT to send a <em>602 Authorization
-Credentials</> message with the required information. It is possible for
-a method to send this multiple times.
-Fields: Site
-
-<tag>403 Media Failure<item>
-A method that deals with multiple media requires that a new media be inserted.
-The Media field contains the name of the media to be inserted.
-Fields: Media, Drive
-
-<tag>600 URI Acquire<item>
-APT is requesting that a new URI be added to the acquire list. Last-Modified
-has the time stamp of the currently cache file if applicable. Filename
-is the name of the file that the acquired URI should be written to.
-Fields: URI, Filename Last-Modified
-
-<tag>601 Configuration<item>
-APT is sending the configuration space to the method. A series of
-Config-Item fields will be part of this message, each containing an entry
-from the configuration space.
-Fields: Config-Item.
-
-<tag>602 Authorization Credentials<item>
-This is sent in response to a <em>402 Authorization Required</> message.
-It contains the entered username and password.
-Fields: Site, User, Password
-
-<tag>603 Media Changed<item>
-This is sent in response to a <em>403 Media Failure</> message. It
-indicates that the user has changed media and it is safe to proceed.
-Fields: Media, Fail
-</taglist>
-
-</sect>
- <!-- }}} -->
-<!-- Method Notes {{{ -->
-<!-- ===================================================================== -->
-<sect>Notes
-
-<p>
-The methods supplied by the stock apt are:
-<enumlist>
-<item>cdrom - For Multi-Disc CD-ROMs
-<item>copy - (internal) For copying files around the filesystem
-<item>file - For local files
-<item>gzip - (internal) For decompression
-<item>http - For HTTP servers
-</enumlist>
-
-<p>
-The two internal methods, copy and gzip, are used by the acquire code to
-parallize and simplify the automatic decompression of package files as well
-as copying package files around the file system. Both methods can be seen to
-act the same except that one decompresses on the fly. APT uses them by
-generating a copy URI that is formed identically to a file URI. The destination
-file is send as normal. The method then takes the file specified by the
-URI and writes it to the destination file. A typical set of operations may
-be:
-<example>
-http://foo.com/Packages.gz -> /bar/Packages.gz
-gzip:/bar/Packages.gz -> /bar/Packages.decomp
-rename Packages.decomp to /final/Packages
-</example>
-
-<p>
-The http method implements a fully featured HTTP/1.1 client that supports
-deep pipelining and reget. It works best when coupled with an apache 1.3
-server. The file method simply generates failures or success responses with
-the filename field set to the proper location. The cdrom method acts the same
-except that it checks that the mount point has a valid cdrom in it. It does
-this by (effectively) computing a md5 hash of 'ls -l' on the mountpoint.
-
-</sect>
- <!-- }}} -->
-
-</book>