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Modules¶
Salt modules are the functions called by the salt command.
See also
Salt ships with many modules that cover a wide variety of tasks.
Easy Modules to write¶
Salt modules are amazingly simple to write. Just write a regular Python module
or a regular Cython module and place it in the salt/modules directory. You
can also place them in a directory called _modules/ in your state directory.
Since Salt modules are just Python/Cython modules, there are no restraints on
what you can put inside of a Salt module. If a Salt module has errors and
cannot be imported, the Salt minion will continue to load without issue and the
module with errors will simply be omitted.
If adding a Cython module the file must be named <modulename>.pyx so that
the loader knows that the module needs to be imported as a Cython module. The
compilation of the Cython module is automatic and happens when the minion
starts, so only the *.pyx file is required.
Cross Calling Modules¶
All of the Salt modules are available to each other, and can be “cross called”.
This means that, when creating a module, functions in modules that already exist
can be called.
The variable __salt__ is packed into the modules after they are loaded into
the Salt minion. This variable is a Python dictionary of all of the Salt
functions, laid out in the same way that they are made available to the Salt
command.
Salt modules can be cross called by accessing the value in the __salt__
dict:
This code will call the Salt cmd module’s run function and pass the argument
bar.
Preloaded Modules Data¶
When interacting with modules often it is nice to be able to read information
dynamically about the minion, or load in configuration parameters for a module.
Salt allows for different types of data to be loaded into the modules by the
minion, as of this writing Salt loads information gathered from the Salt Grains
system and from the minion configuration file.
Grains Data¶
The Salt minion detects information about the system when started. This allows
for modules to be written dynamically with respect to the underlying hardware
and operating system. This information is referred to as Salt Grains, or
“grains of salt”. The Grains system was introduced to replace Facter, since
relying on a Ruby application from a Python application was both slow and
inefficient. Grains support replaces Facter in all Salt releases after 0.8
The values detected by the Salt Grains on the minion are available in a dict
named __grains__ and can be accessed from within callable objects in
the Python modules.
To see the contents of the grains dict for a given system in your deployment
run the grains.items() function:
To use the __grains__ dict simply call it as a Python dict from within your
code, an excellent example is available in the Grains module:
salt.modules.grains.
Module Configuration¶
Since parameters for configuring a module may be desired, Salt allows for
configuration information stored in the main minion config file to be passed to
the modules.
Since the minion configuration file is a YAML document, arbitrary configuration
data can be passed in the minion config that is read by the modules. It is
strongly recommended that the values passed in the configuration file match
the module. This means that a value intended for the test module should be
named test.<value>.
Configuration also requires that default configuration parameters need to be
loaded as well. This can be done simply by adding the __opts__ dict to the
top level of the module.
The test module contains usage of the module configuration, and the default
configuration file for the minion contains the information and format used to
pass data to the modules. salt.modules.test, conf/minion.
Printout Configuration¶
Since module functions can return different data, and the way the data is
printed can greatly change the presentation, Salt has a printout
configuration.
When writing a module the __outputter__ dict can be declared in the module.
The __outputter__ dict contains a mapping of function name to Salt
Outputter.
This will ensure that the text outputter is used.
Virtual Modules¶
Sometimes a module should be presented in a generic way. A good example of this
can be found in the package manager modules. The package manager changes from
one operating system to another, but the Salt module that interfaces with the
package manager can be presented in a generic way.
The Salt modules for package managers all contain a __virtual__ function
which is called to define what systems the module should be loaded on.
The __virtual__ function is used to return either a string or False. If
False is returned then the module is not loaded, if a string is returned then
the module is loaded with the name of the string.
This means that the package manager modules can be presented as the pkg module
regardless of what the actual module is named.
The package manager modules are the best example of using the __virtual__
function:
https://github.com/saltstack/salt/blob/v0.10.1/salt/modules/pacman.py
https://github.com/saltstack/salt/blob/v0.10.1/salt/modules/yumpkg.py
https://github.com/saltstack/salt/blob/v0.10.1/salt/modules/apt.py
Documentation¶
Salt modules are self documenting, the sys.doc() function will return the
documentation for all available Facter modules:
This function simple prints out the docstrings found in the modules, when
writing Salt modules, please follow the formating conventions for docstrings as
they appear in the other modules.
Adding Documentation to Salt Modules¶
Since life is much better with documentation, it is strongly suggested that
all Salt modules have documentation added. Any Salt modules submitted for
inclusion in the main distribution of Salt will be required to have
documentation.
Documenting Salt modules is easy! Just add a Python docstring to the function.
Now when the sys.doc call is executed the docstring will be cleanly returned
to the calling terminal.
How Functions are Read¶
In Salt, Python callable objects contained within a module are made available to
the Salt minion for use. The only exception to this rule is a callable object
with a name starting with an underscore _.
Objects Loaded Into the Salt Minion¶
Objects NOT Loaded into the Salt Minion¶
Examples of Salt Modules¶
The existing Salt modules should be fairly easy to read and understand, the
goal of the main distribution’s Salt modules is not only to build a set of
functions for Salt, but to stand as examples for building out more Salt
modules.
The existing modules can be found here:
https://github.com/saltstack/salt/blob/v0.10.1/salt/modules
The most simple module is the test module, it contains the simplest Salt
function, test.ping:
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