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hosts Step 03 and 04 Jan 29, 2016

README.md

Ansible tutorial

Ansible playbooks

Playbook concept is very simple: it's just a series of ansible commands (tasks), like the ones we used with the ansible CLI tool. These tasks are targeted at a specific set of hosts/groups.

The necessary files for this step should have appeared magically and you don't even have to type them.

Apache example (a.k.a. Ansible's "Hello World!")

We assume we have the following inventory file (let's name it hosts):

[web]
host1.example.org

and all hosts are debian-like.

Note: remember you can (and in our exercise we do) use ansible_host to set the real IP of the host. You can also change the inventory and use a real hostname. In any case, use a non-critical machine to play with! In the real hosts file, we also have ansible_user=root to cope with potential different ansible default configurations.

Lets build a playbook that will install apache on machines in the web group.

- hosts: web
  tasks:
    - name: Installs apache web server
      apt:
        pkg: apache2
        state: present
        update_cache: true

We just need to say what we want to do using the right ansible modules. Here, we're using the apt module that can install debian packages. We also ask this module to update the package cache.

We also added a name for this task. While this is not necessary, it's very informative when the playbook runs, so it's highly recommended.

All in all, this was quite easy!

You can run the playbook (lets call it apache.yml):

ansible-playbook -i step-04/hosts -l host1.example.org step-04/apache.yml

Here, step-04/hosts is the inventory file, -l limits the run only to host1.example.org and apache.yml is our playbook.

When you run the above command, you should see something like:

PLAY [web] ********************* 

GATHERING FACTS ********************* 
ok: [host1.example.org]

TASK: [Installs apache web server] ********************* 
changed: [host1.example.org]

PLAY RECAP ********************* 
host1.example.org              : ok=2    changed=1    unreachable=0    failed=0    

Note: You might see a cow passing by if you have cowsay installed. You can get rid of it with export ANSIBLE_NOCOWS="1" if you don't like it.

Let's analyse the output one line at a time.

PLAY [web] ********************* 

Ansible tells us it's running the play on hosts web. A play is a suite of ansible instructions related to a host. If we'd have another -host: blah line in our playbook, it would show up too (but after the first play has completed).

GATHERING FACTS ********************* 
ok: [host1.example.org]

Remember when we used the setup module? Before each play, ansible runs it on necessary hosts to gather facts. If this is not required because you don't need any info from the host, you can just add gather_facts: no below the host entry (same level as tasks:).

TASK: [Installs apache web server] ********************* 
changed: [host1.example.org]

Next, the real stuff: our (first and only) task is run, and because it says changed, we know that it changed something on host1.example.org.

PLAY RECAP ********************* 
host1.example.org              : ok=2    changed=1    unreachable=0    failed=0 

Finally, ansible outputs a recap of what happened: two tasks have been run and one of them changed something on the host (our apache task, setup module doesn't change anything).

Now let's try to run it again and see what happens:

$ ansible-playbook -i step-04/hosts -l host1.example.org step-04/apache.yml

PLAY [web] ********************* 

GATHERING FACTS ********************* 
ok: [host1.example.org]

TASK: [Installs apache web server] ********************* 
ok: [host1.example.org]

PLAY RECAP ********************* 
host1.example.org              : ok=2    changed=0    unreachable=0    failed=0    

Now changed is '0'. This is absolutely normal and is one of the core feature of ansible: the playbook will act only if there is something to do. It's called idempotency, and means that you can run your playbook as many times as you want, you will always end up in the same state (well, unless you do crazy things with the shell module of course, but this is beyond ansible's control).

Refining things

Sure our playbook can install apache server, but it could be a bit more complete. It could add a virtualhost, ensure apache is restarted. It could even deploy our web site from a git repository. Lets "make it so"

Head to next step in step-05.