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Rewrite best practices section.

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Michael DeHaan 12 years ago
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docsite/rst/bestpractices.rst
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@ -7,13 +7,243 @@ Here are some tips for making the most of Ansible.
:depth: 2
:backlinks: top
Always Mention State
++++++++++++++++++++
Content Organization
++++++++++++++++++++++
The following section shows one of many possible ways to organize content. Your usage of Ansible should fit your needs,
so feel free to modify this approach and organize as you see fit.
Directory Layout
````````````````
The top level of the directory would contain files and directories like so::
production # inventory file for production servers
stage # inventory file for stage environment
group_vars/
group1 # here we assign variables to particular groups
group2 # ""
host_vars/
hostname1 # if systems need specific variables, put them here
hostname2 # ""
site.yml # master playbook
webservers.yml # playbook for webserver tier
dbservers.yml # playbook for dbserver tier
common/ # this hierarchy represents a "role"
tasks/ #
main.yml # <-- tasks file can include smaller files if warranted
handlers/ #
main.yml # <-- handlers file
templates/ # <-- files for use with the template resource
ntp.conf.j2 # <------- templates end in .j2
files/ #
bar.txt # <-- files for use with the copy resource
webtier/ # same kind of structure as "common" was above, done for the webtier role
monitoring/ # ""
fooapp/ # ""
How to Arrange Inventory, Stage vs Production
`````````````````````````````````````````````
In this example, the *production* file contains the inventory of all of your production hosts. Of course you can pull inventory from an external
data source as well, but this is just a basic example. Define groups based on purpose of the host (roles) and also geography or datacenter location::
# file: production
[atlanta-webservers]
www-atl-1.example.com
www-atl-2.example.com
[boston-webservers]
www-bos-1.example.com
www-bos-2.example.com
[atlanta-dbservers]
db-atl-1.example.com
db-atl-2.example.com
[boston-dbservers]
db-bos-1.example.com
# webservers in all geos
[webservers:children]
atlanta-webservers
boston-webservers
# dbservers in all geos
[dbservers:children]
atlanta-dbservers
boston-dbservers
# everything in the atlanta geo
[atlanta:children]
atlanta-webservers
atlanta-dbservers
# everything in the boston geo
[boston:children]
boston-webservers
boston-dbservers
Group And Host Variables
````````````````````````
Now, groups are nice for organization, but that's not all groups are good for. You can also assign variables to them! For instance, atlanta
has it's own NTP servers, so when settings up ntp.conf, we should use them. Let's set those now::
---
# file: group_vars/atlanta
ntp: ntp-atlanta.example.com
backup: backup-atlanta.example.com
Variables aren't just for geographic information either! Maybe the webservers have some configuration that doesn't make sense for the database
servers::
---
# file: group_vars/webservers
apacheMaxRequestsPerChild: 3000
apacheMaxClients: 900
If we had any default values, or values that were universally true, we would put them in a file called group_vars/all::
---
# file: group_vars/all
ntp: ntp-boston.example.com
backup: backup-boston.example.com
We can define specific hardware variance in systems in a host_vars file, but avoid doing this unless you need to::
---
# file: host_vars/db-bos-1.example.com
foo_agent_port: 86
bar_agent_port: 99
Top Level Playbooks Are Seperated By Role
`````````````````````````````````````````
In site.yml, we include a playbook that defines our entire infrastructure. Note this is SUPER short, because it's just including
some other playbooks. Remember playbooks are nothing more than lists of plays::
---
# file: site.yml
- include: webservers.yml
- include: dbservers.yml
In a file like webservers.yml (also at the top level), we simply map the configuration of the webservers group to the roles performed by the webservers group. Also notice this is incredibly short. For example::
---
# file: webservers.yml
- hosts: webservers
tasks:
- include: common/tasks/main.yml tags=common
- include: webtier/tasks/main.yml tags=webtier
handlers:
- include: common/handlers/main.yml
- include: webtier/handlers/main.yml
Task And Handler Organization For A Role
````````````````````````````````````````
This file is just mapping hosts to the roles they fulfill. Now, below is an example tasks file, that explains how a role works. Our common role here just sets up NTP, but it could do more if we wanted::
---
# file: common/tasks/main.yml
- name: be sure ntp is installed
yum: pkg=ntp state=installed
tags: ntp
- name: be sure ntp is configured
template: src=common/templates/ntp.conf.j2 dest=/etc/ntp.conf
notify:
- restart ntpd
tags: ntp
The 'state' parameter is optional to a lot of modules. Whether
'state=present' or 'state=absent', it's always best to leave that
parameter in your playbooks to make it clear, especially as some
modules support additional states.
- name: be sure ntp is enabled
service: name=ntp state=running enabled=yes
tags: ntp
Here is an example handlers file. As a review, handlers are only fired when certain tasks report changes, and are run at the end
of each play::
---
# file: common/handlers/main.yml
- name: restart ntp
service: name=ntp state=restarted
What This Organization Enables (Examples)
`````````````````````````````````````````
So that's our basic organizational structure.
Now what sort of use cases does this layout enable? Lots! If I want to reconfigure my whole infrastructure, it's just::
ansible-playbook -i production site.yml
What about just reconfiguring NTP on everything? Easy.::
ansible-playbook -i production site.yml --tags ntp
What about just reconfiguring my webservers?::
ansible-playbook -i production webservers.yml
What about just my webservers in Boston?::
ansible-playbook -i production webservers.yml --limit boston
What about just the first 10, and then the next 10?::
ansible-playbook -i production webservers.yml --limit boston[0-10]
ansible-playbook -i production webservers.yml --limit boston[10-20]
And of course just basic ad-hoc stuff is also possible.::
ansible -i hosts -m ping
ansible -i hosts -m command -a '/sbin/reboot' --limit boston
And there are some useful commands (at least in 1.1 to know)::
# confirm what task names would be run if I ran this command and said "just ntp tasks"
ansible-playbook -i production webservers.yml --tags ntp --list-tasks
# confirm what hostnames might be communicated with if I said "limit to boston"
ansible-playbook -i production webservers.yml --limit boston --list-hosts
Deployment vs Configuration Organization
````````````````````````````````````````
The above setup models a typical OS configuration topology. When doing multi-tier deployments, there are going
to be some additional playbooks that hop between tiers to roll out an application. In this case, 'site.yml'
may be augmented by playbooks like 'deploy_exampledotcom.yml' but the general concepts can still apply.
Ansible allows you to deploy and configure using the same tool, so you would likely reuse groups and just
keep the OS configuration in seperate playbooks from the app deployment.
Stage vs Production
+++++++++++++++++++
As also mentioned above, a good way to keep your stage (or testing) and production environments seperate is to use a seperate inventory file for stage
and production. This way you pick with -i what you are targetting. Keeping them all in one file can lead to surprises!
Testing things in a stage environment before trying in production is always a great idea. Your environments need not be the same
size and you can use group variables to control the differences between those environments.
Rolling Updates
+++++++++++++++
Understand the 'serial' keyword. If updating a webserver farm you really want to use it to control how many machines you are
updating at once in the batch.
Always Mention The State
++++++++++++++++++++++++
The 'state' parameter is optional to a lot of modules. Whether 'state=present' or 'state=absent', it's always best to leave that
parameter in your playbooks to make it clear, especially as some modules support additional states.
Group By Roles
++++++++++++++
@ -24,66 +254,63 @@ A system can be in multiple groups. See :doc:`patterns`. Having groups named
This allows playbooks to target machines based on role, as well as to assign role specific variables
using the group variable system.
Directory Organization
++++++++++++++++++++++
Operating System and Distribution Variance
++++++++++++++++++++++++++++++++++++++++++
When dealing with a parameter that is different between two different operating systems, the best way to handle this is
by using the group_by module.
This makes a dynamic group of hosts matching certain criteria, even if that group is not defined in the inventory file::
---
# talk to all hosts just so we can learn about them
- hosts: all
tasks:
- group_by key=${ansible_distribution}
# now just on the CentOS hosts...
Playbooks should be organized like this::
# root of source control repository
├── acme/
│ ├── files/
│ │ └── some_file_path_foo.conf
│ ├── handlers/
│ │ └── main.yml
│ ├── tasks/
│ │ ├── setup.yml
│ │ └── stop.yml
│ ├── templates/
│ │ ├── etc_acme_conf_acme.conf
│ │ └── etc_other_conf_other.conf
│ ├── vars/
│ │ └── main.yml
│ ├── setup.yml
│ └── stop.yml
└── global_vars.yml
Any directories or files not needed can be omitted. Not all modules
may require 'vars' or 'files' sections, though most will require
'handlers', 'tasks', and 'templates'. To review what each of
these sections do, see :doc:`playbooks` and :doc:`playbooks2`.
The acme/setup.yml playbook would be as simple as::
- hosts: CentOS
gather_facts: False
tasks:
- # tasks that only happen on CentOS go here
If group specific settings are needed, this can also be done, for example::
---
- hosts: webservers
user: root
# file: group_vars/all
asdf: 10
vars_files:
- ../global_vars.yml
- vars/main.yml
tasks:
- include: tasks/setup.yml
handlers:
- include: handlers/main.yml
---
# file: group_vars/CentOS
asdf: 42
The tasks are individually broken out in 'acme/tasks/setup.yml', and handlers, which are common to all task files,
are contained in 'acme/handlers/main.yml'. As a reminder, handlers are mostly just used to notify services to restart
when things change, and these are described in :doc:`playbooks`.
In the above example, CentOS machines get the value of '42' for asdf, but other machines get 10.
Including more than one setup file or more than one handlers file is of course legal.
Bundling Ansible Modules With Playbooks
+++++++++++++++++++++++++++++++++++++++
.. versionadded:: 0.5
If a playbook has a "./library" directory relative to it's YAML file,
this directory can be used to add ansible modules that will
automatically be in the ansible module path. This is a great way to
keep modules that go with a playbook together.
If a playbook has a "./library" directory relative to it's YAML file, this directory can be used to add ansible modules that will
automatically be in the ansible module path. This is a great way to keep modules that go with a playbook together.
Whitespace and Comments
+++++++++++++++++++++++
Miscellaneous Tips
++++++++++++++++++
Generous use of whitespace to break things up, and use of comments (which start with '#'), is encouraged.
Always Name Tasks
+++++++++++++++++
It is possible to leave off the 'name' for a given task, though it is recommended to provide a description
about why something is being done instead. This name is shown when the playbook is run.
Keep It Simple
++++++++++++++
When you can do something simply, do something simply. Do not reach
to use every feature of Ansible together, all at once. Use what works
@ -91,40 +318,14 @@ for you. For example, you should probably not need 'vars',
'vars_files', 'vars_prompt' and '--extra-vars' all at once,
while also using an external inventory file.
Optimize for readability. Whitespace between sections of YAML
documents and in between tasks is strongly encouraged, as is usage of
YAML comments, which start with '#'. It is also useful to comment
at the top of each file the purpose of the individual file and the
author, including email address.
It is possible to leave off the 'name' for a given task, though it
is recommended to provide a descriptive comment about why something is
being done instead.
Version Control
+++++++++++++++
Use version control. Keep your playbooks and inventory file in git
(or another version control system), and commit when you make changes
to them. This way you have an audit trail describing when and why you
changed the rules automating your infrastructure.
Resist the urge to write the same playbooks and configuration files
for heterogeneous distributions. While lots of software packages
claim to make this easy on you, the configuration files are often
quite different, to the point where it would be easier to treat them
as different playbooks. This is why, for example, Ansible has a
separate 'yum' and 'apt' module. Yum and apt have different
capabilities, and we don't want to code for the least common
denominator.
Use variables for user tunable settings versus having constants in the
tasks file or templates, so that it is easy to reconfigure a playbook.
Think about this as exposing the knobs to things you would like to
tweak.
Since a system can be in more than one group, if you have multiple
datacenters or sites, consider putting systems into groups by role,
but also different groups by geography. This allows you to assign
different variables to different geographies.
.. seealso::
:doc:`YAMLSyntax`

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