[ansible] How to run only one task in ansible playbook?

Is there a way to only run one task in ansible playbook?

For example, in roles/hadoop_primary/tasks/hadoop_master.yml. I have "start hadoop job tracker services" task. Can I just run that one task?

hadoop_master.yml file:

# Playbook for  Hadoop master servers

- name: Install the namenode and jobtracker packages
  apt: name={{item}} force=yes state=latest
  with_items: 
   - hadoop-0.20-mapreduce-jobtracker
   - hadoop-hdfs-namenode
   - hadoop-doc
   - hue-plugins

- name: start hadoop jobtracker services
  service: name=hadoop-0.20-mapreduce-jobtracker state=started
  tags:
   debug

There is a way, although not very elegant:

1. ansible-playbook roles/hadoop_primary/tasks/hadoop_master.yml --step --start-at-task='start hadoop jobtracker services'
2. You will get a prompt: Perform task: start hadoop jobtracker services (y/n/c)
3. Answer y
4. You will get a next prompt, press Ctrl-C

I would love the ability to use a role as a collection of tasks such that, in my playbook, I can choose which subset of tasks to run. Unfortunately, the playbook can only load them all in and then you have to use the --tags option on the cmdline to choose which tasks to run. The problem with this is that all of the tasks will run unless you remember to set --tags or --skip-tags.

I have set up some tasks, however, with a when: clause that will only fire if a var is set.

e.g.

# role/stuff/tasks/main.yml
- name: do stuff
  when: stuff|default(false)

Now, this task will not fire by default, but only if I set the stuff=true

$ ansible-playbook -e '{"stuff":true}'

or in a playbook:

roles:
- {"role":"stuff", "stuff":true}

This can be easily done using the tags

The example of tags is defined below:

---
hosts: localhost
tasks:
 - name: Creating s3Bucket
   s3_bucket:
        name: ansiblebucket1234567890
   tags: 
       - createbucket

 - name: Simple PUT operation
   aws_s3:
       bucket: ansiblebucket1234567890
       object: /my/desired/key.txt
       src: /etc/ansible/myfile.txt
       mode: put
   tags:
      - putfile

 - name: Create an empty bucket
   aws_s3:
       bucket: ansiblebucket12345678901234
       mode: create
       permission: private
   tags:
       - emptybucket

to execute the tags we use the command

ansible-playbook creates3bucket.yml --tags "createbucket,putfile"

FWIW with Ansible 2.2 one can use include_role:

playbook test.yml:

- name: test
  hosts:
    - 127.0.0.1
  connection: local
  tasks:
    - include_role:
        name: test
        tasks_from: other

then in roles/test/tasks/other.yml:

- name: say something else
  shell: echo "I'm the other guy"

And invoke the playbook with: ansible-playbook test.yml to get:

TASK [test : say something else] *************
changed: [127.0.0.1]

are you familiar with handlers? I think it's what you are looking for. Move the restart from hadoop_master.yml to roles/hadoop_primary/handlers/main.yml:

- name: start hadoop jobtracker services
  service: name=hadoop-0.20-mapreduce-jobtracker state=started

and now call use notify in hadoop_master.yml:

- name: Install the namenode and jobtracker packages
  apt: name={{item}} force=yes state=latest
  with_items: 
   - hadoop-0.20-mapreduce-jobtracker
   - hadoop-hdfs-namenode
   - hadoop-doc
   - hue-plugins
  notify: start hadoop jobtracker services

See my answer here: Run only one task and handler from ansible playbook

It is possible to run separate role (from roles/ dir):

ansible -i stage.yml -m include_role -a name=create-os-user localhost

and separate task file:

ansible -i stage.yml -m include_tasks -a file=tasks/create-os-user.yml localhost

If you externalize tasks from role to root tasks/ directory (reuse is achieved by import_tasks: ../../../tasks/create-os-user.yml) you can run it independently from playbook/role.