Building A Simple Module¶

Let’s build a very basic module to get and set the system time. For starters, let’s build a module that just outputs the current time.

We are going to use Python here but any language is possible. Only File I/O and outputting to standard out are required. So, bash, C++, clojure, Python, Ruby, whatever you want is fine.

Now Python Ansible modules contain some extremely powerful shortcuts (that all the core modules use) but first we are going to build a module the very hard way. The reason we do this is because modules written in any language OTHER than Python are going to have to do exactly this. We’ll show the easy way later.

So, here’s an example. You would never really need to build a module to set the system time, the ‘command’ module could already be used to do this.

Reading the modules that come with Ansible (linked above) is a great way to learn how to write modules. Keep in mind, though, that some modules in Ansible’s source tree are internalisms, so look at service or yum, and don’t stare too close into things like async_wrapper or you’ll turn to stone. Nobody ever executes async_wrapper directly.

Ok, let’s get going with an example. We’re going to use Python. For starters, save this as a file named timetest.py

#!/usr/bin/python

import datetime
import json

date = str(datetime.datetime.now())
print(json.dumps({
    "time" : date
}))

Testing Your Module¶

There’s a useful test script in the source checkout for Ansible:

git clone git://github.com/ansible/ansible.git
source ansible/hacking/env-setup

For instructions on setting up Ansible from source, please see Installation.

Let’s run the script you just wrote with that:

ansible/hacking/test-module -m ./timetest.py

You should see output that looks something like this:

{"time": "2012-03-14 22:13:48.539183"}

If you did not, you might have a typo in your module, so recheck it and try again.

Reading Input¶

Let’s modify the module to allow setting the current time. We’ll do this by seeing if a key value pair in the form time=<string> is passed into the module.

Ansible internally saves arguments to an arguments file. So we must read the file and parse it. The arguments file is just a string, so any form of arguments are legal. Here we’ll do some basic parsing to treat the input as key=value.

The example usage we are trying to achieve to set the time is:

time time="March 14 22:10"

If no time parameter is set, we’ll just leave the time as is and return the current time.

Let’s look at the code. Read the comments as we’ll explain as we go. Note that this is highly verbose because it’s intended as an educational example. You can write modules a lot shorter than this:

#!/usr/bin/python

# import some python modules that we'll use.  These are all
# available in Python's core

import datetime
import sys
import json
import os
import shlex

# read the argument string from the arguments file
args_file = sys.argv[1]
args_data = file(args_file).read()

# For this module, we're going to do key=value style arguments.
# Modules can choose to receive json instead by adding the string:
#   WANT_JSON
# Somewhere in the file.
# Modules can also take free-form arguments instead of key-value or json
# but this is not recommended.

arguments = shlex.split(args_data)
for arg in arguments:

    # ignore any arguments without an equals in it
    if "=" in arg:

        (key, value) = arg.split("=")

        # if setting the time, the key 'time'
        # will contain the value we want to set the time to

        if key == "time":

            # now we'll affect the change.  Many modules
            # will strive to be idempotent, generally
            # by not performing any actions if the current
            # state is the same as the desired state.
            # See 'service' or 'yum' in the main git tree
            # for an illustrative example.

            rc = os.system("date -s \"%s\"" % value)

            # always handle all possible errors
            #
            # when returning a failure, include 'failed'
            # in the return data, and explain the failure
            # in 'msg'.  Both of these conventions are
            # required however additional keys and values
            # can be added.

            if rc != 0:
                print(json.dumps({
                    "failed" : True,
                    "msg"    : "failed setting the time"
                }))
                sys.exit(1)

            # when things do not fail, we do not
            # have any restrictions on what kinds of
            # data are returned, but it's always a
            # good idea to include whether or not
            # a change was made, as that will allow
            # notifiers to be used in playbooks.

            date = str(datetime.datetime.now())
            print(json.dumps({
                "time" : date,
                "changed" : True
            }))
            sys.exit(0)

# if no parameters are sent, the module may or
# may not error out, this one will just
# return the time

date = str(datetime.datetime.now())
print(json.dumps({
    "time" : date
}))

Let’s test that module:

ansible/hacking/test-module -m ./timetest.py -a "time=\"March 14 12:23\""

This should return something like:

{"changed": true, "time": "2012-03-14 12:23:00.000307"}

{
    "data": "pong",
    "_ansible_verbosity": 4,
    "_ansible_diff": false,
    "_ansible_debug": false,
    "_ansible_check_mode": false,
    "_ansible_no_log": false
}

Module Provided ‘Facts’¶

The setup module that ships with Ansible provides many variables about a system that can be used in playbooks and templates. However, it’s possible to also add your own facts without modifying the system module. To do this, just have the module return a ansible_facts key, like so, along with other return data:

{
    "changed" : true,
    "rc" : 5,
    "ansible_facts" : {
        "leptons" : 5000,
        "colors" : {
            "red"   : "FF0000",
            "white" : "FFFFFF"
        }
    }
}

These ‘facts’ will be available to all statements called after that module (but not before) in the playbook. A good idea might be to make a module called ‘site_facts’ and always call it at the top of each playbook, though we’re always open to improving the selection of core facts in Ansible as well.

Returning a new fact from a python module could be done like:

module.exit_json(msg=message, ansible_facts=dict(leptons=5000, colors=my_colors))

Common Module Boilerplate¶

As mentioned, if you are writing a module in Python, there are some very powerful shortcuts you can use. Modules are still transferred as one file, but an arguments file is no longer needed, so these are not only shorter in terms of code, they are actually FASTER in terms of execution time.

Rather than mention these here, the best way to learn is to read some of the source of the modules that come with Ansible.

The ‘group’ and ‘user’ modules are reasonably non-trivial and showcase what this looks like.

Key parts include always importing the boilerplate code from ansible.module_utils.basic like this:

from ansible.module_utils.basic import AnsibleModule
if __name__ == '__main__':
    main()

from ansible.module_utils.basic import *

And instantiating the module class like:

def main():
    module = AnsibleModule(
        argument_spec = dict(
            state     = dict(default='present', choices=['present', 'absent']),
            name      = dict(required=True),
            enabled   = dict(required=True, type='bool'),
            something = dict(aliases=['whatever'])
        )
    )

The AnsibleModule provides lots of common code for handling returns, parses your arguments for you, and allows you to check inputs.

Successful returns are made like this:

module.exit_json(changed=True, something_else=12345)

And failures are just as simple (where msg is a required parameter to explain the error):

module.fail_json(msg="Something fatal happened")

There are also other useful functions in the module class, such as module.sha1(path)(). See lib/ansible/module_utils/basic.py in the source checkout for implementation details.

Again, modules developed this way are best tested with the hacking/test-module script in the git source checkout. Because of the magic involved, this is really the only way the scripts can function outside of Ansible.

If submitting a module to Ansible’s core code, which we encourage, use of AnsibleModule is required.

Supporting Check Mode¶

Modules may optionally support check mode. If the user runs Ansible in check mode, a module should try to predict and report whether changes will occur but not actually make any changes (modules that do not support check mode will also take no action, but just will not report what changes they might have made).

For your module to support check mode, you must pass supports_check_mode=True when instantiating the AnsibleModule object. The AnsibleModule.check_mode attribute will evaluate to True when check mode is enabled. For example:

module = AnsibleModule(
    argument_spec = dict(...),
    supports_check_mode=True
)

if module.check_mode:
    # Check if any changes would be made but don't actually make those changes
    module.exit_json(changed=check_if_system_state_would_be_changed())

Remember that, as module developer, you are responsible for ensuring that no system state is altered when the user enables check mode.

If your module does not support check mode, when the user runs Ansible in check mode, your module will simply be skipped.