New Method Spec
Here is the first just mostly finished draft of the new method interface..
APT Method Interface
---------------------
Jason Gunthorpe <jgg@debian.org>
$Id: method.sgml,v 1.1 1998/10/04 04:49:17 jgg Exp $
0.1 Abstract
------------
This document describes the interface that APT uses to the archive
access methods.
0.2 Contents
------------
1. Introduction
1.1. General
1.2. Terms
2. Specification
2.1. Overview
2.2. Message Overview
2.3. Header Fields
2.4. Examples
0.3 Copyright Notice
--------------------
Copyright © Jason Gunthorpe, 1998.
"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.
For more details, on Debian GNU/Linux systems, see the file
/usr/doc/copyright/GPL for the full license.
-------------------------------------------------------------------------------
1. Introduction
---------------
1.1. General
-------------
The APT method interface allows APT to aquire archive files (.deb),
index files (Packages, Revision, 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.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/
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
aquire
The act of bring a URI into the local pathname space. This may
simply be verifiying the existance of the URI or actually
downloading it from a remote site.
-------------------------------------------------------------------------------
2. Specification
----------------
2.1. Overview
--------------
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 corrispond to the well known
unix FD's, stdin, stdout and stderr.
The basic startup sequence depends on how the method is invoked. If
any command line arguments are passed then the method should start in
automatic mode. This facility is provided soley to make the methods
easier to test and perhaps use outside of APT. Upon startup the method
will print out a header describing its capabilities and requirements.
After that it either begins processing the command line arugments and
exits when done or waits for commands to be fed to it.
Throught 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.
In some instances APT may pre-invoke a method to allow things like
file URI's to determine how many files are available locally.
2.2. Message Overview
----------------------
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 successfull 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.
* 100 Capabilities - Method capabilities
* 101 Log - General Logging
* 102 Status - Inter-URI status reporting (login progress)
* 200 URI Start - URI is starting aquire
* 201 URI Done - URI is finished aquire
* 400 URI Failure - URI has failed to aquire
* 401 General Failure - Method did not like something sent to it
* 402 Authorization Required - Method requires authorization to
access the URI. Authorization is User/Pass
* 403 Media Failure - Method requires a media change
* 600 URI Aquire - Request a URI be aquired
* 601 Configuration - Sends the configuration space
* 602 Authorization Credentials - Response to the 402 message
* 603 Media Changed - Response to the 403 message
* 605 Shutdown - Exit
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 recieved. This means the method must
be capable of handling an unlimited number of 600 messages.
The flow of messages starts with the method sending out a *100
Capabilities* and APT sending out a *601 Configuration*. After that
APT begins sending *600 URI Aquire* and the method sends out *200 URI
Start*, *201 URI Done* or *400 URI Failure*. No syncronization is
performed, it is expected that APT will send *600 URI Aquire* 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
neccessary for the method to be constantly ready to recieve them.
2.3. Header Fields
-------------------
The following is a short index of the header fields that are supported
URI
URI being described by the message
Filename
Location in the filesystem
Last-Modified
A time stamp in RFC1123 notation for use by IMS checks
Size
Size of the file in bytes
Resume-Point
Location that transfer was started
MD5-Hash
Computed MD5 hash for the file
Message
String indicating some displayable message
Media
String indicating the media name required
Site
String indicating the site authorization is required for
User
Username for authorization
Password
Password for authorization
Config-Item
A string of the form <item>=<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.
Single-Instance
Requires that only one instance of the method be run This is a
yes/no value.
Pre-Scan
Method can detect if archives are already available. This is a
yes/no value.
Version
Version string for the method
This is a list of which headers each status code can use
100 Capabilities
Displays the capabilities of the method. Fields: Version,
Single-Interface, Pre-Scan
101 Log
A log message may be printed to the screen if debugging is
enabled. This is only for debugging the method. Fields: Message
102 Status
Message gives a progress indication for the method. It can be
used to show pre-transfer status for internet type methods.
Fields: Message
200 URI Start
Indicates the URI is starting to be transfered. The URI is
specified along with stats about the file itself. Fields: URI,
Size, Last-Modified, Resume-Point
201 URI Done
Indicates that a URI has completed being transfered. It is
possible to specify a *201 URI Done* without a *URI Start* which
would mean no data was transfered 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. Fields: URI, Size,
Last-Modified, Filename, MD5-Hash
400 URI Failure
Indicates a fatal URI failure. The URI is not retrievable from
this source. As with *201 URI Done* *200 URI Start* is not
required to preceed this message Fields: URI, Message
401 General Failure
Indicates that some unspecific failure has occured 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
402 Authorization Required
The method requires a Username and Password pair to continue.
After sending this message the method will expect APT to send a
*602 Authorization Credentials* message with the required
information. It is possible for a method to send this multiple
times. Fields: Site
403 Media Failure
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
600 URI Aquire
APT is requesting that a new URI be added to the aquire list.
Last-Modified has the time stamp of the currently cache file if
applicable. Filename is the name of the file that the aquired URI
should be written to. Fields: URI, Filename Last-Modified
601 Configuration
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.
602 Authorization Credentials
This is sent in response to a *402 Authorization Required*
message. It contains the entered username and password. Fields:
Site, User, Password
603 Media Changed
This is sent in response to a *403 Media Failure* message. It
indicates that the user has changed media and it is safe to
proceed. Fields: Media
605 Shutdown
APT sends this to signal the shutdown of the method. The method
should terminate immidiately. Fields: None
2.4. Examples
--------------
-------------------------------------------------------------------------------
APT Method Interface
Jason Gunthorpe <jgg@debian.org> - $Id: method.sgml,v 1.1 1998/10/04
04:49:17 jgg Exp $
Reply to: