Writing Execution Modules¶
Salt execution modules are the functions called by the salt command.
Modules Are Easy to Write!¶
Writing Salt execution modules is straightforward.
A Salt execution module is a Python or Cython module placed in a directory
called _modules/ at the root of the Salt fileserver. When using the default
fileserver backend (i.e. roots), unless
environments are otherwise defined in the file_roots config
option, the _modules/ directory would be located in /srv/salt/_modules
on most systems.
Modules placed in _modules/ will be synced to the minions when any of the
following Salt functions are called:
state.highstate(orstate.applywith no state argument)
Modules placed in _modules/ will be synced to masters when any of the
following Salt runners are called:
Note that a module's default name is its filename
(i.e. foo.py becomes module foo), but that its name can be overridden
by using a __virtual__ function.
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.
Zip Archives as Modules¶
Python 2.3 and higher allows developers to directly import zip archives
containing Python code. By setting enable_zip_modules to
True in the minion config, the Salt loader will be able to import .zip
files in this fashion. This allows Salt module developers to package
dependencies with their modules for ease of deployment, isolation, etc.
For a user, Zip Archive modules behave just like other modules. When executing
a function from a module provided as the file my_module.zip, a user would
call a function within that module as my_module.<function>.
Creating a Zip Archive Module¶
A Zip Archive module is structured similarly to a simple Python package.
The .zip file contains a single directory with the same name as the module.
The module code traditionally in <module_name>.py goes in
<module_name>/__init__.py. The dependency packages are subdirectories of
<module_name>/.
Here is an example directory structure for the lumberjack module, which has
two library dependencies (sleep and work) to be included.
modules $ ls -R lumberjack
__init__.py sleep work
lumberjack/sleep:
__init__.py
lumberjack/work:
__init__.py
The contents of lumberjack/__init__.py show how to import and use these
included libraries.
# Libraries included in lumberjack.zip
from lumberjack import sleep, work
def is_ok(person):
"""Checks whether a person is really a lumberjack"""
return sleep.all_night(person) and work.all_day(person)
Then, create the zip:
modules $ zip -r lumberjack lumberjack
adding: lumberjack/ (stored 0%)
adding: lumberjack/__init__.py (deflated 39%)
adding: lumberjack/sleep/ (stored 0%)
adding: lumberjack/sleep/__init__.py (deflated 7%)
adding: lumberjack/work/ (stored 0%)
adding: lumberjack/work/__init__.py (deflated 7%)
modules $ unzip -l lumberjack.zip
Archive: lumberjack.zip
Length Date Time Name
-------- ---- ---- ----
0 08-21-15 20:08 lumberjack/
348 08-21-15 20:08 lumberjack/__init__.py
0 08-21-15 19:53 lumberjack/sleep/
83 08-21-15 19:53 lumberjack/sleep/__init__.py
0 08-21-15 19:53 lumberjack/work/
81 08-21-15 19:21 lumberjack/work/__init__.py
-------- -------
512 6 files
Once placed in file_roots, Salt users can distribute and use
lumberjack.zip like any other module.
$ sudo salt minion1 saltutil.sync_modules
minion1:
- modules.lumberjack
$ sudo salt minion1 lumberjack.is_ok 'Michael Palin'
minion1:
True
Cross Calling Execution Modules¶
All of the Salt execution modules are available to each other and modules can call functions available in other execution modules.
The variable __salt__ is packed into the modules after they are loaded into
the Salt minion.
The __salt__ variable is a Python dictionary
containing all of the Salt functions. Dictionary keys are strings representing
the names of the modules and the values are the functions themselves.
Salt modules can be cross-called by accessing the value in the __salt__
dict:
def foo(bar):
return __salt__["cmd.run"](bar)
This code will call the run function in the cmd
module and pass the argument bar to it.
Calling Execution Modules on the Salt Master¶
New in version 2016.11.0.
Execution modules can now also be called via the salt-run command using the salt runner.
Preloaded Execution Module Data¶
When interacting with execution modules often it is nice to be able to read information dynamically about the minion or to load in configuration parameters for a module.
Salt allows for different types of data to be loaded into the modules by the minion.
Grains Data¶
The values detected by the Salt Grains on the minion are available in a
Python dictionary named __grains__ and can be
accessed from within callable objects in the Python modules.
To see the contents of the grains dictionary for a given system in your
deployment run the grains.items() function:
salt 'hostname' grains.items --output=pprint
Any value in a grains dictionary can be accessed as any other Python
dictionary. For example, the grain representing the minion ID is stored in the
id key and from an execution module, the value would be stored in
__grains__['id'].
Module Configuration¶
Since parameters for configuring a module may be desired, Salt allows for configuration information from the minion configuration file to be passed to execution 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
therefore strongly recommended that the values passed in the configuration
file match the module name. A value intended for the test execution module
should be named test.<value>.
The test execution 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.
__init__ Function¶
If you want your module to have different execution modes based on minion
configuration, you can use the __init__(opts) function to perform initial
module setup. The parameter opts is the complete minion configuration,
as also available in the __opts__ dict.
"""
Cheese module initialization example
"""
def __init__(opts):
"""
Allow foreign imports if configured to do so
"""
if opts.get("cheese.allow_foreign", False):
_enable_foreign_products()
Strings and Unicode¶
An execution module author should always assume that strings fed to the module
have already decoded from strings into Unicode. In Python 2, these will
be of type 'Unicode' and in Python 3 they will be of type str. Calling
from a state to other Salt sub-systems, should pass Unicode (or bytes if passing binary data). In the
rare event that a state needs to write directly to disk, Unicode should be
encoded to a string immediately before writing to disk. An author may use
__salt_system_encoding__ to learn what the encoding type of the system is.
For example, 'my_string'.encode(__salt_system_encoding__').
Outputter Configuration¶
Since execution module functions can return different data, and the way the data is printed can greatly change the presentation, Salt allows for a specific outputter to be set on a function-by-function basis.
This is done be declaring an __outputter__ dictionary in the global scope
of the module. The __outputter__ dictionary contains a mapping of function
names to Salt outputters.
__outputter__ = {"run": "txt"}
This will ensure that the txt outputter is used to display output from the
run function.
Virtual Modules¶
Virtual modules let you override the name of a module in order to use the same name to refer to one of several similar modules. The specific module that is loaded for a virtual name is selected based on the current platform or environment.
For example, packages are managed across platforms using the pkg module.
pkg is a virtual module name that is an alias for the specific package
manager module that is loaded on a specific system (for example, yumpkg on RHEL/CentOS systems , and aptpkg on Ubuntu).
Virtual module names are set using the __virtual__ function and the
virtual name.
__virtual__ Function¶
The __virtual__ function returns either a string,
True, False, or False with an error
string. If a string is returned then the module is loaded
using the name of the string as the virtual name. If True is returned the
module is loaded using the current module name. If False is returned the
module is not loaded. False lets the module perform system checks and
prevent loading if dependencies are not met.
Since __virtual__ is called before the module is loaded, __salt__ will
be unreliable as not all modules will be available at this point in time. The
__pillar__ and __grains__ "dunder" dictionaries
are available however.
Note
Modules which return a string from __virtual__ that is already used by
a module that ships with Salt will _override_ the stock module.
Returning Error Information from __virtual__¶
Optionally, Salt plugin modules, such as execution, state, returner, beacon,
etc. modules may additionally return a string containing the reason that a
module could not be loaded. For example, an execution module called cheese
and a corresponding state module also called cheese, both depending on a
utility called enzymes should have __virtual__ functions that handle
the case when the dependency is unavailable.
"""
Cheese execution (or returner/beacon/etc.) module
"""
try:
import enzymes
HAS_ENZYMES = True
except ImportError:
HAS_ENZYMES = False
def __virtual__():
"""
only load cheese if enzymes are available
"""
if HAS_ENZYMES:
return "cheese"
else:
return (
False,
"The cheese execution module cannot be loaded: enzymes unavailable.",
)
def slice():
pass
"""
Cheese state module. Note that this works in state modules because it is
guaranteed that execution modules are loaded first
"""
def __virtual__():
"""
only load cheese if enzymes are available
"""
# predicate loading of the cheese state on the corresponding execution module
if "cheese.slice" in __salt__:
return "cheese"
else:
return False, "The cheese state module cannot be loaded: enzymes unavailable."
Examples¶
The package manager modules are among the best examples of using the
__virtual__ function. A table of all the virtual pkg modules can be
found here.
Overriding Virtual Module Providers¶
Salt often uses OS grains (os, osrelease, os_family, etc.) to
determine which module should be loaded as the virtual module for pkg,
service, etc. Sometimes this OS detection is incomplete, with new distros
popping up, existing distros changing init systems, etc. The virtual modules
likely to be affected by this are in the list below (click each item for more
information):
If Salt is using the wrong module for one of these, first of all, please
report it on the issue tracker, so that this issue can be resolved for a
future release. To make it easier to troubleshoot, please also provide the
grains.items output, taking care to
redact any sensitive information.
Then, while waiting for the SaltStack development team to fix the issue, Salt
can be made to use the correct module using the providers option
in the minion config file:
providers:
service: systemd
pkg: aptpkg
The above example will force the minion to use the systemd module to provide service management, and the
aptpkg module to provide package management.
For per-state provider overrides, see documentation on state providers.
Logging Restrictions¶
As a rule, logging should not be done anywhere in a Salt module before it is
loaded. This rule apples to all code that would run before the __virtual__()
function, as well as the code within the __virtual__() function itself.
If logging statements are made before the virtual function determines if the module should be loaded, then those logging statements will be called repeatedly. This clutters up log files unnecessarily.
Exceptions may be considered for logging statements made at the trace level.
However, it is better to provide the necessary information by another means.
One method is to return error information in the
__virtual__() function.
__virtualname__¶
__virtualname__ is a variable that is used by the documentation build
system to know the virtual name of a module without calling the __virtual__
function. Modules that return a string from the __virtual__ function
must also set the __virtualname__ variable.
To avoid setting the virtual name string twice, you can implement
__virtual__ to return the value set for __virtualname__ using a pattern
similar to the following:
# Define the module's virtual name
__virtualname__ = "pkg"
def __virtual__():
"""
Confine this module to Mac OS with Homebrew.
"""
if salt.utils.path.which("brew") and __grains__["os"] == "MacOS":
return __virtualname__
return False
The __virtual__() function can return a True or False boolean, a tuple,
or a string. If it returns a True value, this __virtualname__ module-level
attribute can be set as seen in the above example. This is the string that the module
should be referred to as.
When __virtual__() returns a tuple, the first item should be a boolean and the
second should be a string. This is typically done when the module should not load. The
first value of the tuple is False and the second is the error message to display
for why the module did not load.
For example:
def __virtual__():
"""
Only load if git exists on the system
"""
if salt.utils.path.which("git") is None:
return (False, "The git execution module cannot be loaded: git unavailable.")
else:
return True
Documentation¶
Salt execution modules are documented. The sys.doc() function will return
the documentation for all available modules:
salt '*' sys.doc
The sys.doc function simply prints out the docstrings found in the modules;
when writing Salt execution modules, please follow the formatting conventions
for docstrings as they appear in the other modules.
Adding Documentation to Salt Modules¶
It is strongly suggested that all Salt modules have documentation added.
To add documentation add a Python docstring to the function.
def spam(eggs):
"""
A function to make some spam with eggs!
CLI Example::
salt '*' test.spam eggs
"""
return eggs
Now when the sys.doc call is executed the docstring will be cleanly returned to the calling terminal.
Documentation added to execution modules in docstrings will automatically be added to the online web-based documentation.
Add Execution Module Metadata¶
When writing a Python docstring for an execution module, add information about the module using the following field lists:
:maintainer: Thomas Hatch <thatch@saltstack.com, Seth House <shouse@saltstack.com>
:maturity: new
:depends: python-mysqldb
:platform: all
The maintainer field is a comma-delimited list of developers who help maintain this module.
The maturity field indicates the level of quality and testing for this module. Standard labels will be determined.
The depends field is a comma-delimited list of modules that this module depends on.
The platform field is a comma-delimited list of platforms that this module is known to run on.
Log Output¶
You can call the logger from custom modules to write messages to the minion logs. The following code snippet demonstrates writing log messages:
import logging
log = logging.getLogger(__name__)
log.info("Here is Some Information")
log.warning("You Should Not Do That")
log.error("It Is Busted")
Aliasing Functions¶
Sometimes one wishes to use a function name that would shadow a python built-in.
A common example would be set(). To support this, append an underscore to
the function definition, def set_():, and use the __func_alias__ feature
to provide an alias to the function.
__func_alias__ is a dictionary where each key is the name of a function in
the module, and each value is a string representing the alias for that function.
When calling an aliased function from a different execution module, state
module, or from the cli, the alias name should be used.
__func_alias__ = {
"set_": "set",
"list_": "list",
}
Private Functions¶
In Salt, Python callable objects contained within an execution 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¶
def foo(bar):
return bar
Objects NOT Loaded into the Salt Minion¶
def _foobar(baz): # Preceded with an _
return baz
cheese = {} # Not a callable Python object
Useful Decorators for Modules¶
Depends Decorator¶
When writing execution modules there are many times where some of the module will work on all hosts but some functions have an external dependency, such as a service that needs to be installed or a binary that needs to be present on the system.
Instead of trying to wrap much of the code in large try/except blocks, a decorator can be used.
If the dependencies passed to the decorator don't exist, then the salt minion will remove those functions from the module on that host.
If a fallback_function is defined, it will replace the function instead of
removing it
import logging
from salt.utils.decorators import depends
log = logging.getLogger(__name__)
try:
import dependency_that_sometimes_exists
except ImportError as e:
log.trace("Failed to import dependency_that_sometimes_exists: {0}".format(e))
@depends("dependency_that_sometimes_exists")
def foo():
"""
Function with a dependency on the "dependency_that_sometimes_exists" module,
if the "dependency_that_sometimes_exists" is missing this function will not exist
"""
return True
def _fallback():
"""
Fallback function for the depends decorator to replace a function with
"""
return '"dependency_that_sometimes_exists" needs to be installed for this function to exist'
@depends("dependency_that_sometimes_exists", fallback_function=_fallback)
def foo():
"""
Function with a dependency on the "dependency_that_sometimes_exists" module.
If the "dependency_that_sometimes_exists" is missing this function will be
replaced with "_fallback"
"""
return True
In addition to global dependencies the depends decorator also supports raw booleans.
from salt.utils.decorators import depends
HAS_DEP = False
try:
import dependency_that_sometimes_exists
HAS_DEP = True
except ImportError:
pass
@depends(HAS_DEP)
def foo():
return True
