Sharing ansible variable between plays

Question

This is a follow up to this question.

I have a play that provisions a bunch of EC2 instances. By necessity, the hosts value is localhost (because when it's run there are no hosts), and the play assembles a new host list called ec2hosts, and generates a host ip to hostname map because this is the first and only time that information is made available, and saves it using set_fact. This hosts list is then the subject of a follow-up play. The problem is that I need to use the hostname_map dict created during the provisioning play in the second play, and I can't see how to do this.

Here's the first play:

- hosts: localhost
  connection: local
  gather_facts: False

  tasks:
    - name: Provision a set of instances
      ec2:
        key_name: marcus
        instance_type: t2.micro
        image: "{{ ami_id }}"
        wait: true
        exact_count: "{{ server_count }}"
        count_tag:
          Tutorial: "{{ tutorial_name }}"
        instance_tags:
          Tutorial: "{{ tutorial_name }}"
        groups: ['SSH', 'Web']
      register: ec2

    - name: Add all instance public IPs to host group
      add_host:
        hostname: "{{ item.public_ip }}"
        groups: ec2hosts
      loop: "{{ ec2.instances }}"

    - name: Build an IP to hostname map
      set_fact:
        hostname_map: "{{ hostname_map | combine({item.0.public_ip: (item.1 + '.' + tutorial_domain)}) }}"
      loop: "{{ ec2.instances|zip(hostnames)|list }}"

    - name: Debug hostname_map
      debug:
        msg: "{{ hostname_map }}"

At the end of this, hostname_map contains a map like:

{
    "18.184.109.70": "host1.example.com", 
    "18.196.135.59": "host2.example.com"
}

From reading the ansible docs on variable scope, it says that variables defined in a play are not available outside that play unless it's being applied to the same set of hosts. That's not possible in this case, so I need to use a var with global scope, and from what I've read, set_fact is the appropriate way to do that. So I created an empty variable in /group_vars/all, so that the variable is accessible to all plays:

hostname_map: {}

The next play connects to each newly-created instance (using the host list we created dynamically) and sets its hostname from the inside:

- hosts: ec2hosts
  gather_facts: yes
  tasks:
    - name: Debug hostname_map
      debug:
        msg: "{{ hostname_map }}"
    - name: Set hostnames
      hostname:
        name: "{{ hostname_map[ansible_host] }}"

However, this fails because hostname_map is empty

TASK [Debug hostname_map]
ok: [18.184.109.70] => {
    "msg": {}
}
ok: [18.196.135.59] => {
    "msg": {}
}

so I get this error:

fatal: [18.184.109.70]: FAILED! => {"msg": "The task includes an option with an undefined variable. The error was: 'dict object' has no attribute u'18.184.109.70'

Curiously I get the same debug output and error even when I don't define the variable globally.

Other articles I've read all suggest that the values should be enumerated explicitly in vars, but I can't do that because the data is dynamic and unknown until runtime. Similarly, I can't provide it using extra_vars on a command line for the same reason.

How can I make this variable available within the second play? I'd like to avoid clunky solutions like writing a local file out and then reading it back in!

I'm also open to suggestions for doing this a completely different way, where "this" is: create an arbitrary number of EC2 instances and assign them hostnames taken from a static list.

Answer 1

A value you set with set_fact will be available between different plays. Keep in mind that set_fact are set for a specific host. Your first play is run against localhost so the fact is part of the localhosts variables. So it the following play you should be able to access it with a task like this.

- debug:
    var: hostvars['localhost']['hostmap']

Answer 2

You could use dummy host which will store all the variables you want to share with a help of add_host. Then just access your variables with hostvars

- hosts: "18.184.109.70"
  user: ubuntu
  tasks:
    - name: set a variable
      set_fact:
        shared_variable: "Some value"
    - name: add variables to dummy host
      add_host:
        name: "variable_holder"
        shared_variable:  "{{ shared_variable }}"

- hosts: "18.196.135.59"
  user: ubuntu
  vars:
    shared_variable: "{{ hostvars['variable_holder']['shared_variable'] }}"
  tasks:
    - debug:
        msg: "{{ shared_variable }}"

Answer 3

Depends on how you define play. If you have multiple plays with a single ansible run it works. However does not work with multiple sequential plays. What I mean it doesn't work when you run ansible-playbook playbook1.yml, and afterwards ansible-playbook playbook2.yml