Targeting

With Ansible you can “taget” (or select) hosts using a couple of patterns.

For example, here is a playbook snippet that will run on all hosts:

- name: Do something on all hosts
  hosts: all

  tasks:
    - name: Say something
      debug:
        msg: "Hello world!"

As you can see, the hosts parameter is targeting all hosts.

There are a couple of patterns that are interesting:

• all - all hosts in the inventory.
• webservers - all hosts in the webservers group. (Or any other group that you have defined in the inventory.)
• node-1 - a single host, by name. There is actually no different in targeting a group and a single host, so pick you hostnames and groupnames carefully.
• node-1:node-2 - multiple hosts, by name. You can use a colon (:) to separate multiple hosts.
• node-* - multiple hosts, by name. You can use a wildcard to select multiple hosts.
• all:!node-1 - all hosts, except node-1. You can use an exclamation mark to exclude a host.
• webservers:&production - all hosts in the webservers and production group. You can use an ampersand to make a cross-selection of groups.

Limiting

You can also restict the hosts that are targeted by using the --limit parameter. This is a reduction from the targeted pattern defined in hosts in the playbook.

Note: The --limit parameter can be shortened to -l.

For example, this inventory:

[webservers]
node-1
node-2
node-3
node-4

[databaseservers]
node-5
node-6
node-7
node-8

[development]
node-1
node-5

[test]
node-2
node-6

[accepance]
node-3
node-7

[production]
node-4
node-8

With this playbook:

- name: Do something on the webservers hosts
  hosts: webservers

  tasks:
    - name: Say something
      debug:
        msg: "Hello world!"

And this limit:

ansible-playbook playbook.yml --limit development

Would run on node-1 only.

Note: Limiting on the command line, like above, has a disadvantage of not being stored in code.

Assignment

1. Create an inventory with the following groups:

• applicationservers
• databaseservers
• production
• development

Note: You need to tell Ansible (in ansible.cfg, under the [defaults] section, to use your inventory file as the inventory. The parameter is inventory.)

1. Add the following hosts:

• node-1 (an production application server)
• node-2 (an production database server)
• node-3 (an development application server)
• node-4 (an development database server)

1. Run the playbook with a limit on development.

Note: The nodes are not reachable, so the task will fail. That’s okay.

Verify

You should see the playbook run on node-3 only.