ansible playbook run a task only once - ansible

Question

I need to run a task ,set a flag and the second time the play runs run the task only if the flag is not set
Play at a later stage
- name: Dump all databases
mysql_db:
state: dump
name: all
target: /root/mysql_all.sql
when: ansible_local.mysql.replication.setup is not defined
- name: create directory for ansible custom facts
file: state=directory recurse=yes path=/etc/ansible/facts.d
- name: install custom fact stating mysql is setup
template:
src: mysql.fact.j2
dest: /etc/ansible/facts.d/mysql.fact
The problem is that the fist time this play runs its throwing an error.
FAILED! => {"failed": true, "msg": "The conditional check 'ansible_local.mysql.replication.setup is not defined' failed. The error was: error while evaluating conditional (ansible_local.mysql.replication.setup is not defined): 'ansible_local' is undefined
What is the best way to run a task only in the first run and skip for subsequent runs.

Answer 1

you should make a task prior that registers if /root/mysql_all.sql exists then add it to your when clause.
Example:
- name: check if dump exists
stat:
path: /root/mysql_all.sql
register: mysqldump
- name: Dump all databases
mysql_db:
state: dump
name: all
target: /root/mysql_all.sql
when:
- ansible_local.mysql.replication.setup is not defined
- mysqldump.stat.exists == true
- name: create directory for ansible custom facts
file: state=directory recurse=yes path=/etc/ansible/facts.d
- name: install custom fact stating mysql is setup
template:
src: mysql.fact.j2
dest: /etc/ansible/facts.d/mysql.fact

Answer 2

One option is to utilize the fact cache. When enabled, you can set cacheable facts in your play and check for them.
Fact caching always takes place. There are various fact cache plug-ins available of which the memory plug-in is the default, and json file and redis cache are the most popular. You can only set one plug-in. Refer to
https://docs.ansible.com/ansible/latest/plugins/cache.html
When you want to explore with the json file you can set the environment variables as follows:
export ANSIBLE_CACHE_PLUGIN=jsonfile
export ANSIBLE_CACHE_PLUGIN_CONNECTION="~/ansiblefactcache"
In your play book you can check for facts and set them as cacheable, Refer to
https://docs.ansible.com/ansible/latest/collections/ansible/builtin/set_fact_module.html
A small example playbook.yaml:
- name: Example Fact Cache Playbook
hosts: all
gather_facts: false # the default is true; it gathers various host facts
tasks:
- name: Runs when examplefact equals something
debug:
msg: "Runs when examplefact equals something"
when: ansible_facts['examplefact'] is defined and ansible_facts['examplefact'] == "something"
- name: Does not run when examplefact equals something
debug:
msg: "does not run when examplefact equals something"
when: ansible_facts['examplefact'] is not defined or ansible_facts['examplefact'] != "something"
- name: Set the examplefact to something
set_fact:
examplefact: "something"
cacheable: true
Note the usage of the cacheable instruction. When true the fact goes into the cache.
After having run this small playbook you will notice the creation of a localhost file in your home's subfolder ansiblefactcache which contains your cached facts.
Also note the usage of the gather_facts instruction. The default being true will scan your machine for various details such as environment variables, network details, etc. All are cached. You can play with it and see the localhost file being populated with it.
You can also try to edit the localhost file yourself or even delete it and run the play again.
I used the following inventory file inventory.yaml:
all:
hosts:
localhost:
ansible_connection: local
And I run ansible as follows:
ansible-playbook playbook.yaml -i inventory.yaml -vvv
First run yields the following:
PLAY [Example Fact Cache Playbook] ********************************************************************************************************************************************************************************
TASK [Runs when examplefact equals something] *********************************************************************************************************************************************************************
skipping: [localhost]
TASK [Does not run when examplefact equals something] *************************************************************************************************************************************************************
ok: [localhost] => {
"msg": "does not run when examplefact equals something"
}
TASK [Set the examplefact to something] ***************************************************************************************************************************************************************************
ok: [localhost]
Second run yields the following:
PLAY [Example Fact Cache Playbook] ********************************************************************************************************************************************************************************
TASK [Runs when examplefact equals something] *********************************************************************************************************************************************************************
ok: [localhost] => {
"msg": "Runs when examplefact equals something"
}
TASK [Does not run when examplefact equals something] *************************************************************************************************************************************************************
skipping: [localhost]
TASK [Set the examplefact to something] ***************************************************************************************************************************************************************************
ok: [localhost]