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Retrievers
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----------
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Retrievers are modules that can download or otherwise retrieve files[1],
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including other modules and Packages files. It is expected that at least the
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following types will eventually be implemented:
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- cd retriever
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Ensures an install cd is mounted, and finds files on it. No real
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retrieving done.
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- floppy retriever
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May or may not need to deal with files split across multiple
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floppies. In any case, pulls files off floppies.
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- hard disk retriever
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Gets a file from a local hard disk (dos, linux, etc).
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- http retriever
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Downloads a file from a remote http mirror.
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- ftp retriever (needed?)
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Downloads a file from a remote ftp mirror.
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- nfs retriever
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Gets a file via nfs.
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A retriever must be able to do the following:
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- Provides: retriever
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- Set itself up so it can access the cdrom, mirror, or whatever.
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- Get a list of the modules that are available. This will be done by
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retrieving a standard Packages file.
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- Retrieve a file.
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A separate module will take care of parsing the Packages files, and
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checking md5sums of downloaded modules, and a separate program
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(udpkg) will handle actually installing modules.
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Since retrievers are debian packages, they can have a postinst that is
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run to set them up. During setup, they may need to prompt the user for
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information (using debconf). For example, a nfs retriever will probably
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need to prompt the user for a nfs server name.
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Retrievers may also need to depend on additional modules. For example the
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http/ftp/nfs retrievers will need to depend on a module to set up networking,
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while the cdrom retriever may need to depend on module(s) that include kernel
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modules for the iso9660 filesystem and cd drivers.
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Retrievers may also need to interact with the user when they are retrieving
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a file. For example, the floppy retriever will need to prompt for disks.
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This interaction will also be accomplished via debconf.
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Retriever interface
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-------------------
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All retrievers should support the following interface.
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[ This interface is preliminary and depends on the filesystem layout and
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ramdisk allocation scheme used by the installer, and so the locations may
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change. ]
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/usr/lib/debian-installer/retriever/<retriever name> is an executable (the
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name of the exectable must be the same as the name of the module that
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contains it). It is run with the following parameters:
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retriever [command] [args]
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Currently, there are four commands:
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retriever config
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Perform retriever-specific configuration. This may include creating
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a /var/cache/anna/include file that contains a list of udebs to be
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installed automatically.
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retriever retrieve [source-filename] [dest-filename]
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Download the specified file to the specified destination.
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The source-filename is the Filename field of a Packages file.
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retriever packages [dest-filename]
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Download a Packages file, or a Packages-like file. The fields used by
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anna are:
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- Package
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- Version
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- Filename
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Whether other fields are needed is currently undecided. A retriever
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now must support this command, because it is supposed to download and
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parse a Release file if appropriate. This means that the result of
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this command can (and often will be) a concatenation of several
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Packages files from the install medium.
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In the step the retreiver can also retreive udeb_includes and
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udeb_excludes files.
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retriever cleanup
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Clean up after a retriever round. This is used by e.g. the floppy
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retriever to unmount the floppy.
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If the destination file already exists, the retriever may optionally attempt
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to continue a download.
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Once run, retrievers are free to prompt the user, or do whatever else is
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necessary to retrieve the file[2].
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Note that this simple interface requires that retrievers know the location
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of the base directory of the archive. This will not be a problem for many
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-- the cdrom retriever simply uses whatever is available, in a hard-coded
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location. Other retrievers may require more configuration. For example, the
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http retriever will need to know what version of Debian is being installed,
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what site to install from, and probably what version of the debian-installer
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is being used as well. The user will likely have to be prompted for some of
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this data during retriever setup.
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Retriever controller (anna)
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---------------------------
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This is another module, that makes use of the retrievers. It doesn't
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actually depend on any though. What it does is, when it is picked from the
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menu, it builds a list of all available retrievers, and has the user pick
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one from the list. That retriever is then configured, and is used to
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retrieve whatever seems appropriate. Thus, the menu item can be picked
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multiple times, using a different retriever every time. And if a retriever
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downloads a second retriever, it will show up in future displays of the
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retrievers menu.
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The default retriever (if more than one is available, and it's the first
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time the question is asked) will be, in turn and if they exist:
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1. net-retriever (if you've got connectivity you probably want to use it)
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2. cdrom-retriever (using a CD is also very common)
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3. floppy-retriever (we usually don't have to use the floppy)
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(4. file-retriever? not currently used, but if resurrected, should
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probably have lower priority than the floppy
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retriever)
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As that is sometimes not flexible enough, it's also possible to create
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udebs that provide menu items that only use a specific retreiver. They do
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do by calling anna, passing it the name of the retreiver to use.
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Which modules are needed is an interesting question. Our current
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implementation takes all modules, except those of extra priority.
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Dependencies are taken into account, and there is a special hack to avoid
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loading mismatched kernel packages too. If the retreiver provides them in
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/var/cache/anna/, files udeb_include and udeb_exclude can be used to fine
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tune what packages are installed by default. All packages listed in the
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udebs_include will be installed, but none from udebs_exclude.
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The controller also needs to verify md5sums of downloaded files.
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Footnotes
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---------
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[1] Conceptually, rather similar to apt's methods, although the interface
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is a lot more simplistic and we don't need to support things like
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pipelining multiple downloads.
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[2] Some retrievers may want to show progress displays while downloading.
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Support for this may need to be added to debconf, via a new data type.
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It's rather hard to do in the context of debconf -- details to be
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determined.
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