Ansible: Difference between revisions

Revision as of 17:09, 20 November 2020

Ansible is an open source configuration management tool written in Python and helps automate software configuration and deployment. It manages remote hosts via SSH or PowerShell and requires no agents on the client side. Changes can be executed on one or more hosts which can be on different networks, so long as it is accessible by the Ansible host.

Introduction

Ansible provides a bunch of modules that perform a certain action. For example, the package module installs a package on the host machine. Each module can take any number of parameters which are specified by a task definition. We could define a new task that passes 'vim' as a parameter to the package module so that vim gets installed. A set of roles that together implements a specific feature or sets up the 'role' of a machine can be bundled together as an Ansible role. A playbook specifies which host or host groups implement which roles.

In summary:

Inventory: file or script that tells Ansible what the hosts are. Hosts can be bundled together into groups. Groups can inherit other groups.
Playbook: A playbook tells Ansible what tasks or roles a host or host group need to implement
Role: A bundle of tasks, files, and templates that configures a specific feature or role.
Task: A list of parameters which are called against a specific Module in order to do a specific action
Module: A piece of code which does an action

Modules & Tasks

Modules can take in any number of parameters as variables. Modules can also return data back to Ansible which can be used later on by other modules. A module by itself isn't able to do anything and can only 'run' when used as a task. A Task is simply a list item that specifies a module and its parameters.

A single module can be executed via the ansible command while multiple modules can be executed through the use of a playbook which describes the list of tasks or a bundle of tasks (called roles) it needs to perform.

Ansible uses the Jinja2 template engine and all parameters that are passed to Ansible can be templated. Uses of this are found throughout the examples below and allows for the use of loops (by iterating over lists) and embedding variables within strings. Templated values must be enclosed in double quotes.

Modules which are commonly used and their commonly used parameters are listed below:

Module	parameter	Description
yum/dnf	- name: Install NTP yum: name: ntp state: present - name: Install common packages dnf: name: - git - vim - zsh	Installs a package. You may also use loops to call the module multiple times for each item. Eg: - name: Install packages dnf: name: "{{ item }}" with_items: - git - vim This is something which isn't required for the yum/dnf modules since version 2.0.
package	- name: Ensure NFS utils are installed package: name: "{{ __packages.client }}" state: "{{ 'latest' if upgrade else 'present' }}"	This module will use the proper underlying package manager suitable on the system, making cross-platform tasks easier to write. The name parameter also accepts a list, like yum/dnf.
template	- name: Configure NTP notify: restart ntp template: src: ntp.conf.j2 dest: /etc/ntp.conf	Copy a templated file. Notice that we also call 'restart ntp' when the file is changed. 'restart ntp' is a task defined under the `handlers/main.yml` file. Template may include variables passed to this playbook.
service	- name: Start the ntp service service: name=ntpd state=started enabled=yes - name: restart ntp service: name=ntpd state=restarted	Ensures something is started and enabled on startup
debug	- name: "testing something" debug: msg: "tem: {{ item }}" with_items: - 1 - 2	Debug messages
command	- name: Extract archive command: /bin/tar xvf wordpress.tar.gz args: chdir: /srv/ creates=/srv/wordpress - name: safely use templated variable to run command command: cat {{ myfile\|quote }} register: myoutput	Runs a command
shell	- name: Shell command shell: "echo $HOME \| grep something"	Similar to command, but the task is executed inside a shell. Environment variables and piping will work as a result. The `creates` argument can be used so that the command only runs when the file does not exist.
copy	- name: Copy using inline content copy: content: \| this is some file content of mine. dest: /tmp/file - name: Copy a "sudoers" file on the remote machine for editing copy: src: /etc/sudoers dest: /etc/sudoers.edit remote_src: yes validate: /usr/sbin/visudo -csf %s	Copies files or sets file contents
file	- name: Create MariaDB log file file: path=/var/log/mysqld.log state=touch owner=mysql group=mysql mode=0775 - name: Create MariaDB PID directory file: path=/var/run/mysqld state=directory owner=mysql group=mysql mode=0775	Creates a file or directory
stat	- name: Check that the somefile.conf exists stat: path: /etc/file.txt register: stat_result - name: Create the file, if it doesnt exist already file: path: /etc/file.txt state: touch when: not stat_result.stat.exists	Stats a file, which can be used to only run something based on a file's existance.
mount	- name: Mount /home mount: fstype: nfs opts: "vers=3,intr,hard" path: /home src: "netapp{{ fileserver_index }}:/ArcHome" state: present	Mounts a filesystem
firewalld	- name: insert firewalld rule firewalld: port: {{ mysql_port }}/tcp permanent: true state: enabled immediate: yes	Manages the firewall
git	- name: Copy the code from repository git: repo: { { repository }} dest: /var/www/html/	You can clone an entire git repository to some location.

Roles

Roles define a set of tasks that should be done. I like to think of Roles as a bundle or grouping of tasks. Ansible Galaxy contains a bunch of other roles that can be used. GitHub also has lots of other roles users have made public which may do what you want.

Roles help organize tasks into a single unit. For example, you may want to create a webserver role that contains instructions on setting up Apache and another database role with instructions on setting up MySQL. Creating a new role involves creating the following directory structure:

roles/
   installapache/
     tasks/
     handlers/
     templates/
     vars/
     defaults/
   installtomcat/
     tasks/
     meta/

Each role should be its own subdirectory in the roles directory. Each role should also have folders for each of: tasks for all ansible tasks the role should perform, handlers containing all handlers in this role, files for files needed by the role, such as configuration files, templates containing templating data, vars containing values that override defaults set in the defaults directory, meta for any metadata needed for this role, such as dependency data. In each of these directories, the main.yml file will be read and used if it exists.

In the example above, we have a installapache role and installtomcat role.

Inside installapache/tasks/main.yml, we can load the appropriate set of tasks depending on the distribution:

- name: import a tasks based on OS platform 
  import_tasks: centos.yml 
  when: ansible_distribution == 'CentOS' 
- import_tasks: ubuntu.yml 
  when: ansible_distribution == 'Ubuntu'

We would then create a centos.yml and ubuntu.yml within this same tasks directory with all the tasks that should be done in this role. Eg.

---
- name: Install Apache using yum
  yum:
    name: "httpd"
    state: latest
- name: Start the Apache server
  service:
    name: httpd
    state: started

As Dependencies

Roles can be included as dependencies. Under meta, define something like:

dependencies:
  - role: common
    vars:
      some_parameter: 3
  - role: apache
    vars:
      apache_port: 80
  - role: postgres
    vars:
      dbname: blarg
      other_parameter: 12

Inventory

Inventory contains a list of hosts managed by Ansible. By default, it uses the hosts file at /etc/ansible/hosts. However, this can be changed to something else such as a static .ini file, a .yaml file using the -i parameter or to some dynamic method of obtaining hosts. Hosts can be targeted specifically or by groups defined in an inventory file.

An example .ini inventory file is given below:

[webservers]
web1.example.com ansible_user=bob
web2.example.com  ansible_host=192.168.81.142 ansible_port=3333

[apservers]
ap1.example.com
ap2.example.com

[dbservers]
db[01:10].example.com

Can be grouped (webserers, apservers), or left ungrouped at the very top (for .ini files only). The 'all' group captures everything too. Groups can be nested. Use the ':' in group names for .ini files to represent hierarchy. Ranges can also be specified with brackets.

ansible -i /path/to/inventory all -m shell -a 'echo hello world' -f 5

Additional variables can be set as well. Variables can either be set to each host, or to a group. Since groups can be nested, the most immediate group variable takes precident.

Playbook

Playbooks define what tasks (or roles which is just a bundle of tasks) a host or host group Ansible needs to perform. When running a playbook with ansible-playbook, tasks are executed in the order they appear and in the order of the host pattern that is given, ensuring that runs are repeatable and reproduceable.

Playbooks can be applied with the ansible-playbook command. After a playbook run, a recap will output the following:

ok - task ran successfully with no changes being made
changed - task ran successfully with changes being made
failed - task failed to run
unreachable - host was unreachable
skipped - task was skipped
ignored - task was ignored
rescued - ?

An example playbook looks like this:

---
- name: Handler demo 1
  hosts: frt01.example.com
  gather_facts: no
  become: yes

  tasks:
    - name: Update Apache configuration
      template:
        src: template.j2
        dest: /etc/httpd/httpd.conf
      notify: Restart Apache

    - name: restart all services
      command: echo "this task will restart all services"
      notify: "restart all services"

  handlers:
    - name: Restart Apache
      service:
        name: httpd
        state: restarted
      listen: "restart all services"

Handlers can be called with notify. Handlers can also listen for a specific event with the listen directive.

Facts

You can get all facts by running:

# ansible all -m setup -i inventory.ini

You can determine if the environment is a VM or container by reading the value set under ansible_virtualization_type.

When defining tasks, you can conditionally run something with the when parameter followed by a truth statement. For example, to only run on Red Hat based systems, add when: "ansible_facts['os_family'] == 'RedHat'"

Loops

More about loops https://docs.ansible.com/ansible/latest/user_guide/playbooks_loops.html#playbooks-loops

Loops can be done by specifying a list of items and passing it to loop, or with_items.

---
- name: Install Apache
  hosts: frt01.example.com
  gather_facts: no
  become: yes

  tasks:
    - name: Install Apache package
      yum:
        name: httpd
        state: latest
    - name: Open firewall for Apache
      firewalld:
        service: "[[:Template:Item]]"
        permanent: yes
        state: enabled
        immediate: yes
      loop:
        - "http"
        - "https"
    - name: Restart and enable the service
      service:
        name: httpd
        state: restarted
        enabled: yes

Quick Demo

The quickest way to get your toes wet with Ansible is to try it out with a small set of Docker containers. Using the two docker images (TODO: link to docker files) and docker-compose, we can bring up a small lab to test Ansible with (TODO: link to docker-compose.yml).

Each container has an ansible account with ansible as the password. Using ansible, you can reach all of them using password authentication with the --ask-pass flag. To make the containers use SSH keys going forward, generate a new SSH key on the head node, then run this (TODO: link to playbook) playbook.

You may want to turn off SSH host key checking by creating an ansible.cfg file:

[defaults]
host_key_checking = false

You can then ping all the nodes by running:

# ansible -i inventory.ini all -m shell -a 'hostname'

Tips & Tricks

Ansible

An entire task can be written in one line as a key=value space delimited string. Ansible will parse the string out into a map before passing it to its module.
When defining tasks, you can also call the import_tasks or import_role modules to load a task file or role directory.
Ansible will load all group_vars/group_name/*.yml files as variables. The group_vars directory is relative to the inventory file.

Jinja

Comma Delimited List

Use the loop.last variable to determine whether this is the last item in the list. You may use either the if-expression, or an actual if block.

{% for item in my_list %}
{{ item }}
{{ "," if not loop.last }}
{% if not loop.last %},{% endif %}
{% endfor %}

Idempotent Random Number

Random numbers are useful when configuring many hosts since it lets you spread out tasks over time (in the case of cronjobs) or over different resources (in the case of mounting network filesystems from different endpoints). However, we want the same random number for each host so that each run through of the playbook remains the same. To accomplish this, we can make use of Jinja's random function seeded with the host's inventory name.

vars:
  fileserver_index: {{ 10 | random(seed=inventory_hostname) }}

Doing this, we can use fileserver_index, which contains a value between 0 through 9.

Troubleshooting

Import tasks not honoring when condition

Given the following task, the when condition is not being honored and is still evaluated:

- name: load tasks if condition is true
  import_tasks: tasks.yml
  when: some_condition_var is defined and some_condition_var is sameas true

The problem here is that import_tasks will load all tasks in tasks.yml at parse time rather than at runtime. This means that all tasks within tasks.yml will be loaded with the when condition applied to each task. With this example, if the condition is false, all tasks that were imported will be skipped.

To not load any of the tasks in tasks.yml when the condition is false, use include_tasks instead.

@@ Line 111: / Line 111: @@
 - name: safely use templated variable to run command
-   command: cat {{ myfile|quote }}
+   command: cat <nowiki>{{ myfile|quote }}</nowiki>
    register: myoutput
 | lang = yaml
@@ Line 201: / Line 201: @@
 | code = - name: insert firewalld rule
    firewalld:
-<nowiki>    port: { { mysql_port }}/tcp</nowiki>
+    port: <nowiki>{{ mysql_port }}/tcp</nowiki>
      permanent: true
      state: enabled