Trigger ansible handler when a variable is changed

You should be able to watch the file for changes using incrond. For example (from the linked documenation)

_You need to run program 'abc' with the full file path as an argument every time a file is changed in /var/mail. One of the solutions follows:

/var/mail IN_CLOSE_WRITE abc $@/$#  

There incrontab(5) man page is also useful and contains further examples.

This will only be able to tell you that the file has changed (close_write) it won't be able to tell you what was changed. To find out what was changed I think you're going to need to write some scripts.

For a pure Ansible solution you could set the innodb-log-file-size with lineinfile module

Like this:

- name: Set innodb-log-file-size for MySQL.
  lineinfile:
    dest: /etc/mysql/my.cnf
    line: 'innodb-log-file-size = {{ innodb_log_file_size }}'
  notify: restart mysql

You would then need to create a handler for each action you listed. The task above would only return CHANGED and trigger the configured handlers when the value of innodb_log_file_size changed.

I assuming here that you are using template module to create the mysql configuration. template module returns CHANGED when any parameter set via Ansible in the config file changed. lineinfile module enables you to trigger handlers for a specific change.

This strategy has however the bad side effect that you can't mix template and lineinfile module because in subsequent Ansible runs both task would always return CHANGED and therefor break idempotence of the play.

edit

After thinking a bit about the problem I would recommend the following strategy: check via command module, create via template, check again via command module and notify if value changed.

- name: Register innodb-log-file-size in my.cnf
  command: grep -Fxq "innodb-log-file-size" /etc/mysql/my.cnf
  register: innodb_log_file_size_pre
  always_run: True
  ignore_errors: True
  changed_when: False

- name: Create my.cnf via template.
  template:
    src: my.cnf.j2
    dest: /etc/mysql/my.cnf

- name: Restart mysql service when innodb_log_file_size changed
  debug: msg="Restart mysql to activate innodb_log_file_size change"
  when: innodb_log_file_size_pre !== innodb_log_file_size
  notify: restart mysql

Based on the help received from the other answers I decided to move the option in a separate file and trigger the restart+log_expansion when this file is changed. Bellow are the details.

In /etc/my.cnf file, I added:

!includedir /etc/my.cnf.d

In roles/mysql/tasks/main.yml I added:

- name: Create /etc/my.cnf.d MySQL config dir
  file: state=directory name=/etc/my.cnf.d owner=mysql group=mysql mode="u=rwx,go=rx" setype=mysqld_etc_t

- name: Configure innodb_log_file
  template: dest=/etc/my.cnf.d/set_innodb_log_file.cnf src=set_innodb_log_file.cnf.j2 backup=yes mode="u=rw,go=r"
  when: innodb_log_file_size_mb is defined
  register: innodb_log_file

- name: Expand innodb_log_file
  include: expand-innodb-log-file.yml
  static: no
  when: innodb_log_file.changed

roles/mysql/tasks/expand-innodb-log-file.yml:

- name: Stop MySQL
  service: name=mysql state=stopped

- name: Rename old log files
  shell: |
    cd /var/lib/mysql/
    for file in ib_logfile*;do
      mv $file remove-me-$file
    done
  when: mysql_minor_version == 5

- name: Start MySQL
  service: name=mysql state=started

- name: Remove old log files
  shell: rm -f /var/lib/mysql/remove-me-ib_logfile*
  args:
    warn: no
  when: mysql_minor_version == 5

roles/mysql/templates/set_innodb_log_file.cnf.j2:

[mysqld]
innodb_log_file_size = {{ innodb_log_file_size_mb }}M
innodb_log_files_in_group = 2