Intro to Playbooks¶
About Playbooks¶
Playbooks are a completely different way to use ansible than in ad-hoc task execution mode, and are particularly powerful.
Simply put, playbooks are the basis for a really simple configuration management and multi-machine deployment system, unlike any that already exist, and one that is very well suited to deploying complex applications.
Playbooks can declare configurations, but they can also orchestrate steps of any manual ordered process, even as different steps must bounce back and forth between sets of machines in particular orders. They can launch tasks synchronously or asynchronously.
While you might run the main /usr/bin/ansible
program for ad-hoc
tasks, playbooks are more likely to be kept in source control and used
to push out your configuration or assure the configurations of your
remote systems are in spec.
There are also some full sets of playbooks illustrating a lot of these techniques in the ansible-examples repository. We’d recommend looking at these in another tab as you go along.
There are also many jumping off points after you learn playbooks, so hop back to the documentation index after you’re done with this section.
Playbook Language Example¶
Playbooks are expressed in YAML format (see YAML Syntax) and have a minimum of syntax, which intentionally tries to not be a programming language or script, but rather a model of a configuration or a process.
Note
Some editors have add-ons that can help you write clean YAML syntax in your playbooks. See Other Tools And Programs for details.
Each playbook is composed of one or more ‘plays’ in a list.
The goal of a play is to map a group of hosts to some well defined roles, represented by things ansible calls tasks. At a basic level, a task is nothing more than a call to an ansible module.
By composing a playbook of multiple ‘plays’, it is possible to orchestrate multi-machine deployments, running certain steps on all machines in the webservers group, then certain steps on the database server group, then more commands back on the webservers group, etc.
“plays” are more or less a sports analogy. You can have quite a lot of plays that affect your systems to do different things. It’s not as if you were just defining one particular state or model, and you can run different plays at different times.
For starters, here’s a playbook, verify-apache.yml
that contains just one play:
---
- hosts: webservers
vars:
http_port: 80
max_clients: 200
remote_user: root
tasks:
- name: ensure apache is at the latest version
yum:
name: httpd
state: latest
- name: write the apache config file
template:
src: /srv/httpd.j2
dest: /etc/httpd.conf
notify:
- restart apache
- name: ensure apache is running
service:
name: httpd
state: started
handlers:
- name: restart apache
service:
name: httpd
state: restarted
Playbooks can contain multiple plays. You may have a playbook that targets first the web servers, and then the database servers. For example:
---
- hosts: webservers
remote_user: root
tasks:
- name: ensure apache is at the latest version
yum:
name: httpd
state: latest
- name: write the apache config file
template:
src: /srv/httpd.j2
dest: /etc/httpd.conf
- hosts: databases
remote_user: root
tasks:
- name: ensure postgresql is at the latest version
yum:
name: postgresql
state: latest
- name: ensure that postgresql is started
service:
name: postgresql
state: started
You can use this method to switch between the host group you’re targeting, the username logging into the remote servers, whether to sudo or not, and so forth. Plays, like tasks, run in the order specified in the playbook: top to bottom.
Below, we’ll break down what the various features of the playbook language are.
Basics¶
Hosts and Users¶
For each play in a playbook, you get to choose which machines in your infrastructure to target and what remote user to complete the steps (called tasks) as.
The hosts
line is a list of one or more groups or host patterns,
separated by colons, as described in the Patterns: targeting hosts and groups
documentation. The remote_user
is just the name of the user account:
---
- hosts: webservers
remote_user: root
Note
The remote_user
parameter was formerly called just user
. It was renamed in Ansible 1.4 to make it more distinguishable from the user module (used to create users on remote systems).
Remote users can also be defined per task:
---
- hosts: webservers
remote_user: root
tasks:
- name: test connection
ping:
remote_user: yourname
Support for running things as another user is also available (see Understanding privilege escalation: become):
---
- hosts: webservers
remote_user: yourname
become: yes
You can also use keyword become
on a particular task instead of the whole play:
---
- hosts: webservers
remote_user: yourname
tasks:
- service:
name: nginx
state: started
become: yes
become_method: sudo
You can also login as you, and then become a user different than root:
---
- hosts: webservers
remote_user: yourname
become: yes
become_user: postgres
You can also use other privilege escalation methods, like su:
---
- hosts: webservers
remote_user: yourname
become: yes
become_method: su
If you need to specify a password for sudo, run ansible-playbook
with --ask-become-pass
or -K
.
If you run a playbook utilizing become
and the playbook seems to hang, it’s probably stuck at the privilege
escalation prompt and can be stopped using Control-C, allowing you to re-execute the playbook adding the
appropriate password.
Important
When using become_user
to a user other than root, the module
arguments are briefly written into a random tempfile in /tmp
.
These are deleted immediately after the command is executed. This
only occurs when changing privileges from a user like ‘bob’ to ‘timmy’,
not when going from ‘bob’ to ‘root’, or logging in directly as ‘bob’ or
‘root’. If it concerns you that this data is briefly readable
(not writable), avoid transferring unencrypted passwords with
become_user set. In other cases, /tmp
is not used and this does
not come into play. Ansible also takes care to not log password
parameters.
New in version 2.4.
You can also control the order in which hosts are run. The default is to follow the order supplied by the inventory:
- hosts: all
order: sorted
gather_facts: False
tasks:
- debug:
var: inventory_hostname
Possible values for order are:
- inventory:
The default. The order is ‘as provided’ by the inventory
- reverse_inventory:
As the name implies, this reverses the order ‘as provided’ by the inventory
- sorted:
Hosts are alphabetically sorted by name
- reverse_sorted:
Hosts are sorted by name in reverse alphabetical order
- shuffle:
Hosts are randomly ordered each run
Tasks list¶
Each play contains a list of tasks. Tasks are executed in order, one at a time, against all machines matched by the host pattern, before moving on to the next task. It is important to understand that, within a play, all hosts are going to get the same task directives. It is the purpose of a play to map a selection of hosts to tasks.
When running the playbook, which runs top to bottom, hosts with failed tasks are taken out of the rotation for the entire playbook. If things fail, simply correct the playbook file and rerun.
The goal of each task is to execute a module, with very specific arguments. Variables can be used in arguments to modules.
Modules should be idempotent, that is, running a module multiple times in a sequence should have the same effect as running it just once. One way to achieve idempotency is to have a module check whether its desired final state has already been achieved, and if that state has been achieved, to exit without performing any actions. If all the modules a playbook uses are idempotent, then the playbook itself is likely to be idempotent, so re-running the playbook should be safe.
The command and shell modules will typically rerun the same command again,
which is totally ok if the command is something like
chmod
or setsebool
, etc. Though there is a creates
flag available which can
be used to make these modules also idempotent.
Every task should have a name
, which is included in the output from
running the playbook. This is human readable output, and so it is
useful to provide good descriptions of each task step. If the name
is not provided though, the string fed to ‘action’ will be used for
output.
Tasks can be declared using the legacy action: module options
format, but
it is recommended that you use the more conventional module: options
format.
This recommended format is used throughout the documentation, but you may
encounter the older format in some playbooks.
Here is what a basic task looks like. As with most modules,
the service module takes key=value
arguments:
tasks:
- name: make sure apache is running
service:
name: httpd
state: started
The command and shell modules are the only modules that just take a list
of arguments and don’t use the key=value
form. This makes
them work as simply as you would expect:
tasks:
- name: enable selinux
command: /sbin/setenforce 1
The command and shell module care about return codes, so if you have a command whose successful exit code is not zero, you may wish to do this:
tasks:
- name: run this command and ignore the result
shell: /usr/bin/somecommand || /bin/true
Or this:
tasks:
- name: run this command and ignore the result
shell: /usr/bin/somecommand
ignore_errors: True
If the action line is getting too long for comfort you can break it on a space and indent any continuation lines:
tasks:
- name: Copy ansible inventory file to client
copy: src=/etc/ansible/hosts dest=/etc/ansible/hosts
owner=root group=root mode=0644
Variables can be used in action lines. Suppose you defined
a variable called vhost
in the vars
section, you could do this:
tasks:
- name: create a virtual host file for {{ vhost }}
template:
src: somefile.j2
dest: /etc/httpd/conf.d/{{ vhost }}
Those same variables are usable in templates, which we’ll get to later.
Now in a very basic playbook all the tasks will be listed directly in that play, though it will usually make more sense to break up tasks as described in Creating Reusable Playbooks.
Action Shorthand¶
New in version 0.8.
Ansible prefers listing modules like this:
template:
src: templates/foo.j2
dest: /etc/foo.conf
Early versions of Ansible used the following format, which still works:
action: template src=templates/foo.j2 dest=/etc/foo.conf
Handlers: Running Operations On Change¶
As we’ve mentioned, modules should be idempotent and can relay when they have made a change on the remote system. Playbooks recognize this and have a basic event system that can be used to respond to change.
These ‘notify’ actions are triggered at the end of each block of tasks in a play, and will only be triggered once even if notified by multiple different tasks.
For instance, multiple resources may indicate that apache needs to be restarted because they have changed a config file, but apache will only be bounced once to avoid unnecessary restarts.
Here’s an example of restarting two services when the contents of a file change, but only if the file changes:
- name: template configuration file
template:
src: template.j2
dest: /etc/foo.conf
notify:
- restart memcached
- restart apache
The things listed in the notify
section of a task are called
handlers.
Handlers are lists of tasks, not really any different from regular tasks, that are referenced by a globally unique name, and are notified by notifiers. If nothing notifies a handler, it will not run. Regardless of how many tasks notify a handler, it will run only once, after all of the tasks complete in a particular play.
Here’s an example handlers section:
handlers:
- name: restart memcached
service:
name: memcached
state: restarted
- name: restart apache
service:
name: apache
state: restarted
You may want your Ansible handlers to use variables. For example, if the name of a service varies slightly by distribution, you want your output to show the exact name of the restarted service for each target machine. Avoid placing variables in the name of the handler. Since handler names are templated early on, Ansible may not have a value available for a handler name like this:
handlers:
# this handler name may cause your play to fail!
- name: restart "{{ web_service_name }}"
If the variable used in the handler name is not available, the entire play fails. Changing that variable mid-play will not result in newly created handler.
Instead, place variables in the task parameters of your handler. You can load the values using include_vars
like this:
tasks: - name: Set host variables based on distribution include_vars: "{{ ansible_facts.distribution }}.yml" handlers: - name: restart web service service: name: "{{ web_service_name | default('httpd') }}" state: restarted
As of Ansible 2.2, handlers can also “listen” to generic topics, and tasks can notify those topics as follows:
handlers:
- name: restart memcached
service:
name: memcached
state: restarted
listen: "restart web services"
- name: restart apache
service:
name: apache
state: restarted
listen: "restart web services"
tasks:
- name: restart everything
command: echo "this task will restart the web services"
notify: "restart web services"
This use makes it much easier to trigger multiple handlers. It also decouples handlers from their names, making it easier to share handlers among playbooks and roles (especially when using 3rd party roles from a shared source like Galaxy).
Note
Notify handlers are always run in the same order they are defined, not in the order listed in the notify-statement. This is also the case for handlers using listen.
Handler names and listen topics live in a global namespace.
Handler names are templatable and listen topics are not.
Use unique handler names. If you trigger more than one handler with the same name, the first one(s) get overwritten. Only the last one defined will run.
You cannot notify a handler that is defined inside of an include. As of Ansible 2.1, this does work, however the include must be static.
Roles are described later on, but it’s worthwhile to point out that:
handlers notified within
pre_tasks
,tasks
, andpost_tasks
sections are automatically flushed in the end of section where they were notified,handlers notified within
roles
section are automatically flushed in the end oftasks
section, but before anytasks
handlers,handlers are play scoped and as such can be used outside of the role they are defined in.
If you ever want to flush all the handler commands immediately you can do this:
tasks:
- shell: some tasks go here
- meta: flush_handlers
- shell: some other tasks
In the above example any queued up handlers would be processed early when the meta
statement was reached. This is a bit of a niche case but can come in handy from
time to time.
Executing A Playbook¶
Now that you’ve learned playbook syntax, how do you run a playbook? It’s simple. Let’s run a playbook using a parallelism level of 10:
ansible-playbook playbook.yml -f 10
Ansible-Pull¶
Should you want to invert the architecture of Ansible, so that nodes check in to a central location, instead of pushing configuration out to them, you can.
The ansible-pull
is a small script that will checkout a repo of configuration instructions from git, and then
run ansible-playbook
against that content.
Assuming you load balance your checkout location, ansible-pull
scales essentially infinitely.
Run ansible-pull --help
for details.
There’s also a clever playbook available to configure ansible-pull
via a crontab from push mode.
Linting playbooks¶
You can use ansible-lint to run a detail check of your playbooks before you execute them.
For example, if you run ansible-lint
on the verify-apache.yml playbook introduced earlier in this section, you’ll get the following results:
$ ansible-lint verify-apache.yml
[403] Package installs should not use latest
verify-apache.yml:8
Task/Handler: ensure apache is at the latest version
The ansible-lint default rules page describes each error. For [403]
, the recommended fix is to change state: latest
to state: present
in the playbook.
Other playbook verification options¶
See Tools for Validating Playbooks for a detailed list of tools you can use to verify your playbooks. Here are some others that you should consider:
To check the syntax of a playbook, use
ansible-playbook
with the--syntax-check
flag. This will run the playbook file through the parser to ensure its included files, roles, etc. have no syntax problems.Look at the bottom of the playbook execution for a summary of the nodes that were targeted and how they performed. General failures and fatal “unreachable” communication attempts are kept separate in the counts.
If you ever want to see detailed output from successful modules as well as unsuccessful ones, use the
--verbose
flag. This is available in Ansible 0.5 and later.To see what hosts would be affected by a playbook before you run it, you can do this:
ansible-playbook playbook.yml --list-hosts
See also
- ansible-lint
Learn how to test Ansible Playbooks syntax
- YAML Syntax
Learn about YAML syntax
- Best Practices
Various tips about managing playbooks in the real world
- All modules
Learn about available modules
- Should you develop a module?
Learn how to extend Ansible by writing your own modules
- Patterns: targeting hosts and groups
Learn about how to select hosts
- GitHub examples directory
Complete end-to-end playbook examples
- Mailing List
Questions? Help? Ideas? Stop by the list on Google Groups