Package Modules

Table of Contents

See the packages promises documentation for a comprehensive reference on the body types and attributes used here.

Package Module API

This section describes how to create a package module for the CFEngine package promise. Package modules are backends that enable the package promise to work with different types of platform package managers.

The CFEngine side

CFEngine never calls any package manager commands, it only ever calls the package module. The information that CFEngine deals in is:

  • Which packages are currently installed:
    • Name
    • Version
    • Architecture
  • Which of the installed packages have updates available:
    • Name
    • Version
    • Architecture

These two lists are everything that CFEngine needs to know to decide whether its package promises are fulfilled or not. In addition to this it will carry out package operations by asking the package module to do certain tasks, but in the end, the result of the operation is always determined by comparing the promise against those two lists. Therefore it is very important that the package module has a robust and deterministic way of returning these lists.

To take one example: When a package is installed, CFEngine does not really care what the return code of the operation is, the reason being that not all package managers can be trusted to return the correct return code (yum has a tendency to always report success, for instance). Instead, it ignores the return code, and instead asks for the list of currently installed packages after the installation, and if the package we tried to install has not appeared, it knows that the promise failed.

In the same fashion, if we try to install an update, that update is expected to disappear from the list of available updates after the operation.

The package module

The package module itself is simply an executable, and can be in any executable format, whether than is Python, Perl, Shell script or native binary.

The package module resides in the /var/cfengine/modules/packages both on the hub and the clients, and out of the box CFEngine is set up to synchronize this directory among its clients. For this reason it is advised to avoid native binary formats for package modules, to reduce the burden of distribution to different platforms, but the API does not forbid it, and it may be useful in some circumstances.

The API

The API consists of commands which are passed in the module arguments, combined with a simple text protocol that will be fed on the module's standard input, and replies are expected on its standard output.

In the examples below we use simple "Here Documents" to show how standard input can be passed to a package module in order to produce the intented output or effect. If you don't know about Here Documents, the man page for bash is a good place to read about them. During actual execution, the input will come from CFEngine itself.

The API commands are listed roughly in the order that they should be implemented, in order to facilitate a nice debugging cycle during development.

options attribute

All the API commands listed below, except supports-api-version, support the options attribute. This attribute will contain the contents from the options attribute in the promise, or the default_options in the package module body, if the former is unspecified. It may appear more than once if the options list has more than one element.

This attribute has no inherent meaning to CFEngine, and will be passed verbatim. It is meant as a mechanism to communicate special attributes to the package module that are not covered by the main API. For example, for certain package modules it may be used to pass a repository URL.

The options attribute will not be explicitly listed in the examples below, but it is valid in all of them except supports-api-version, even when the description reads "no input".

supports-api-version

The very first command that any package module must implement is supports-api-version. This command takes no input, and is expected to print a single digit followed by a newline. This is simply a way for CFEngine and the package module to agree on which version of the protocol to use. For now there is only one such version, and the expected output is simply "1".

This is the only command which does not support the options attribute.

Example:

$ ./package-module supports-api-version < /dev/null
1
$

get-package-data

CFEngine uses this command to determine what kind of promise has been made. Currently two types are supported: "file" and "repo".

The input is expected to be a triplet of File/Version/Architecture, where File is the promiser string from the promise, and Version and Architecture contain the strings from the corresponding promise attributes, if they were specified. This implies that either one of Version and Architecture may not be included, so some entries may only contain File or File with one more attribute.

What the module should do is figure out whether the string passed in File is referring to a file based or a repository based package. Exactly what identifies each one is up to the package module, but generally it means that file based packages should refer to actual files on the file system, whereas repository based packages should refer to package names that a "smart" package manager can resolve, such as for instance "apt". There are exceptions to this rule however, for example if a string is a URL referring to a downloadable package file, the type of package would still be file based, since it refers to a single package file which is not part of a repository.

The module should start by returning one attribute PackageType, which should be either file or repo. Next, it should return the proper name of the package in a Name attribute. Proper name means the name that will be displayed in package listings, so for example, /home/johndoe/zip-3.0-4.el5.x86_64.rpm would resolve to simply zip. For repository based package name, in most cases the returned Name will be the same as what what passed in, but this may not be the case for all package managers.

Next, for file based package name it should return Version and Architecture if it is able to determine these, but it is allowed to omit them if the module doesn't know (if the resource is remote, for instance).

For repository based package names the module should not return Version and Architecture, since they are often ambiguous in repository situations, and any discrepancies will be handled at the install stage instead.

Example 1:

$ ./package-module get-package-data <<EOF
File=zip
Version=3.0-4
Architecture=amd64
EOF
PackageType=repo
Name=zip
$

Example 2:

$ ./package-module get-package-data <<EOF
File=zip
EOF
PackageType=repo
Name=zip
$

Example 3:

$ ./package-module get-package-data <<EOF
File=/home/johndoe/zip-3.0-4.el5.x86_64.rpm
Version=3.0-4
Architecture=amd64
EOF
PackageType=file
Name=zip
Version=3.0-4
Architecture=amd64
$

Example 4:

$ ./package-module get-package-data <<EOF
File=/home/johndoe/zip-3.0-4.el5.x86_64.rpm
EOF
PackageType=file
Name=zip
Version=3.0-4
Architecture=amd64
$

list-installed

This command is expected to return a list of all currently installed packages on the system. It takes no input, and the output is expected to be a list of triplets of Name/Version/Architecture.

Example:

$ ./package-module list-installed < /dev/null
Name=zip
Version=3.0-4
Architecture=amd64
Name=libc6
Version=2.15
Architecture=amd64
Name=libc6
Version=2.15
Architecture=i386
...
$

list-updates

This command is expected to return a list of all the available updates for currently installed updates. The command takes no input, and the output is expected to be a list of triplets of Name/Version/Architecture.

It is not an error to include updates to packages that are not installed, but this information will not be used, and it is therefore recommended to omit it for performance purposes.

If the available updates come from an external source, such as an online repository service or a remote file server, this command is expected to fetch the information from there. CFEngine will make sure that this command is not called too often, so there is no need to try to limit the online resource usage in this command. See more about caching and list-updates-local below.

Example:

$ ./package-module list-updates < /dev/null
Name=zip
Version=3.0-4
Architecture=amd64
Name=libc6
Version=2.15
Architecture=amd64
Name=libc6
Version=2.15
Architecture=i386
...
$

list-updates-local

This command is expected to return a list of all the available updates for currently installed updates. The command takes no input, and the output is expected to be a list of triplets of Name/Version/Architecture.

It is not an error to include updates to packages that are not installed, but this information will not be used, and it is therefore recommended to omit it for performance purposes.

Unlike list-updates, this command is not expected to use the network to fetch information from external sources, but should fetch all the information from local storage. This command exists precisely to limit such expensive operations.

Example:

$ ./package-module list-updates-local < /dev/null
Name=zip
Version=3.0-4
Architecture=amd64
Name=libc6
Version=2.15
Architecture=amd64
Name=libc6
Version=2.15
Architecture=i386
...
$

repo-install

This command is used by CFEngine to ask the package module to install packages from the package repository. Note that CFEngine itself has no notion of which package repository it should come from. This is up to the package module, and may either be a platform configured default, such as is the case for for example yum, or a specific repository which is passed in via the options attribute. The command will be called for promises where get-package-data returned PackageType=repo.

The command takes a list of triplets, Name, Version and Architecture, where the last two may be omitted. In this case the module is expected to provide some default, which is usually the latest version and the native platform architecture.

No output is expected.

Example:

$ ./package-module repo-install <<EOF
Name=zip
Name=libc6
Version=2.15
Architecture=amd64
Name=libc6
Version=2.15
Architecture=i386
EOF
$

file-install

This command is used by CFEngine to ask the package module to install a specific package file. The command will be called for promises where get-package-data returned PackageType=file.

The command takes a list of triplets, File, Version and Architecture, where the last two may be omitted. For package files that can contain more than one package, the last two attributes may be used to select the correct one. The command should never be called with attributes that are not present in the package, since this will already have been detected after querying get-package-data.

No output is expected.

Example:

$ ./package-module file-install <<EOF
File=/mnt/storage/zip-3.0-4.el5.x86_64.rpm
File=/mnt/storage/libc6-2.15.el5.i386.rpm
Version=2.15
Architecture=i386
EOF
$

remove

To remove packages, CFEngine will call the package module with the remove command.

The command takes a list of triplets, Name, Version and Architecture, where the last two may be omitted. If so, the module is expected to remove all packages matching the other attribute(s). Note that Name is the basename of the package, the same format that get-package-data returns in Name.

No output is expected.

Example:

$ ./package-module remove <<EOF
Name=zip
Name=libc6
Version=2.15
Architecture=amd64
Name=libc6
Version=2.15
Architecture=i386
EOF
$

Error messages

All of the package module commands except supports-api-version have the option of returning error messages. The error messages are simply an attribute ErrorMessage with a string, which may optionally be preceded by whatever Name or File triplet was given to the command initially, in order to tie it to a specific promise.

Example:

$ ./package-module file-install <<EOF
File=/mnt/storage/zip-3.0-4.el5.x86_64.rpm
File=/mnt/storage/libc6-2.15.el5.i386.rpm
Version=2.15
Architecture=i386
EOF
File=/mnt/storage/zip-3.0-4.el5.x86_64.rpm
ErrorMessage=File not found
File=/mnt/storage/libc6-2.15.el5.i386.rpm
Version=2.15
Architecture=x86_64
ErrorMessage=Doesn't contain architecture 'x86_64'
$

Other output

CFEngine does not expect any other output on the package module's standard output, so the module should make sure it silences the output from its sub commands. Alternatively, it may redirect their output to standard error instead, but this will not be formatted using CFEngine's normal log formatting and is not recommended.

Caching

For performance reasons, CFEngine will cache the list of packages returned from list-packages and the list of updates from either of list-updates or list-updates-local. The exact circumstances where each is called is:

Whenever one is called its result is cached by CFEngine and will be used internally. It is a good idea to set the two policy attributes, query_installed_ifelapsed and query_updates_ifelapsed to zero during module development to avoid any issues with caching during debugging, but they should be set back when deploying in production.