iteration-conditionals.md

Iteration and Conditionals

Iteration and Conditionals

$ cd $INTRO_ANSIBLE_DIR/working-with-playbooks
.
├── ansible.cfg
├── hosts
├── playbook-dict.yml
├── playbook-lists.yml

Iterating through lists

Iterating with_items

• Step through an array
• Each element in iteration is assigned to item variable

 vars:
  list_of_files: 
    - file1.txt
    - file2.txt
       .
       .
  tasks:
   - name: Process a list of items        
     copy:                               
       src: "/path/to/local/{{ item }}"   
       dest: "/path/to/remote/{{ item }}" 
     with_items: "{{ list_of_files }}"    

Configuring our fake application myapp

• Have a look at playbook-lists.yml
• Run the playbook and then have a look in /etc/myapp/config
• Playbook adds lines to a configuration file for a fake app
• There are a number of things we can optimise

Exercise: Remove repetitive allowed_host tasks from playbook

• Use with_items to refactor tasks
• Replace all allowed host tasks with

  
        - name: Add allowed_hosts entries
          lineinfile:
            path: /etc/myapp/config
            line: "allow_from: {{ item }}"
            insertafter: EOF
          with_items:
            - "{{ allowed_host1 }}"
            - "{{ allowed_host2 }}"
            - "{{ allowed_host3 }}"
  
  

Exercise: Remove repetitive listen tasks from playbook

• Use with_items to simply listen tasks
• Replace all the listen tasks with:

  
    - name: Add listen port entries
      lineinfile:
        path: /etc/myapp/config
        line: "listen: {{ item }}"
        insertafter: EOF
      with_items: "{{ listen_ports }}"
    

Nesting Loops with_nested

• It is possible to loop over two lists at once

  - name: Add allowed ports for each host
    lineinfile:
      path: /etc/myapp/config
      line: "host: {{ item[0] }}:{{ item[1] }}"
      insertafter: EOF
    with_nested:
      - [ "{{ allowed_host1 }}", "{{ allowed_host2 }}", "{{ allowed_host3 }}"]
      - "{{ listen_ports }}"
  
  

Iterating through dictionaries with_dict

• Takes a dictionary argument
• Each item has
  • Key
  • Value

- name: Task that iterates over a dictionary
  somemodule:
    arg: "{{ item.key }}"
    arg2: "{{ item.value }}"
  with_dict: "{{ dictionary }}"

More configuration for myapp

• `playbook-dict.yml` sets up database config for our fake application
• Adds each element of database variable to config
• Run the playbook and have a look at /etc/myapp/app_config

  ansible-playbook playbook-dict.yml
  

Exercise: Remove repetition from playbook-dict.yml

• Refactor the playbook using with_dict to remove repetitive tasks

 - name: Add database config to /etc/myapp/config
   lineinfile:
     path: /etc/myapp/app_config
     line: "database_{{ item.key }}={{ item.value }}"
     regexp: '^database_{{ item.key}}.*'
   with_dict: "{{ database[env_name] }}"

Conditionals

The when clause

• The main way to conditionally do something in Ansible is by using the when clause
• `when` has identical semantics to a Python _if_ block
  • `when: some_variable is defined`
  • `when: env_name == 'staging'`

- name: Some task
  command: do something
  when: <condition is true>

Exercise: Make our tasks conditional

• Tasks in playbook-dict.yml assume a staging environment

  vars:
    .
    env_name: staging

• Add a conditional to the lineinfile tasks
  • Only execute if env_name is production
• Re-run the playbook

  - name: Add database config to /etc/myapp/config
    lineinfile:
    .
    .
    when: env_name == 'production'

Exercise: Override default env_name to run tasks

• env_name is set to staging by default
• Override this to be production and re-run playbook
• Hint: What type of variable has precedence in this situation?

ansible-playbook playbook-dict.yml -e env_name=production