Run once with when statement causes only skipped action to run

[ddrak...@gmail.com]

Hey,

I'm seeing some weird behavior that does not seem intuitive when trying to use run_once with when. Intuitively I think this should run an action one time when the conditional is met, however what I'm seeing is that a skipped action is ran (because the conditional is false) and nothing actually changes. Is there anyway to work around this? Should I file a bug?

Thanks,

Dave

[Matt Martz]

By default `run_once` only executes the task against the 1st host in the list of hosts in the play batch.  As such, the `when` statement only uses the variables from that 1 host.

Effectively it is doing:

run_once: true

delegate_to: "{{ ansible_play_batch[0] }}"

So you will either need to ensure that your `when` statement applies for that first host, or potentially use `delegate_to` to specify another host.

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[Josh Smift]

There's a bunch of tickets about run_once and skipped hosts;
https://github.com/ansible/ansible/issues/19966 is my personal favorite.

-Josh (j...@care.com)

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[ddrak...@gmail.com]

Thanks for the replies. That's an interesting idea about using delegate_to, perhaps I can use a jinja2 filter to only delegate_to a host matching the criteria in the when. 

I was considering creating an in-memory group of only the hosts that matched the when but that sounds a bit verbose and a pain every time I want to run something once on a specific set of hosts.

Dave

[ddrak...@gmail.com]

I can't think of a clever way to use delegate_to, I really need the run_once functionality since these are all running in parallel.

Weirdly, when I try to create an in-memory host group using when and add_host the task only gets executed one time (on a skipped host). I don't have run_once on this task so I'm not sure why all my nodes aren't executing it. Any ideas?

Anyone have a good workaround for this issue?

Thanks

[Josh Smift]

> Weirdly, when I try to create an in-memory host group using when and
> add_host the task only gets executed one time (on a skipped host). I
> don't have run_once on this task so I'm not sure why all my nodes aren't
> executing it. Any ideas?

add_host is implicitly run_once; there's a note about it at the bottom of
http://docs.ansible.com/ansible/latest/add_host_module.html saying "This
module bypasses the play host loop and only runs once for all the hosts in
the play, if you need it to iterate use a with_ directive." Something like

- add_host:
groups: cool_new_group
name: "{{ item }}"
changed_when: False
with_items: "{{ play_hosts }}"

might do what you need, depending what you need. :^)

[ddrak...@gmail.com]

Thanks for the help, Josh. 

I actually am trying to run a command once on a certain set of EC2 instances I'm managing that match a specific tag. This seems like a very reasonable thing to do and yet the run_once skipping functionality makes this quite difficult. I managed to find two similar but different ways to do this (though not exactly elegant), which I'll post for future reference. I was inspired by https://groups.google.com/forum/#!topic/ansible-project/cpnrBRxLy0E and https://imil.net/blog/2016/08/05/Ansible_and_AWS_ASG

Method 1 (serial 1 is important due to the add_host behavior Josh pointed out). Run the playbook with the AWS dynamic inventory script (-i ec2.py)

- hosts: ec2

user: ubuntu

serial: 1

gather_facts: false

tasks:

- name: add only nodes that don't match your_tag_name to in-memory host group

add_host:

name: "{{ inventory_hostname }}"

groups: your_hosts

when: ec2_tag_Name != your_tag_name

and then run the rest of your playbook not serially and perform run_once actions like this:

- hosts: ec2

user: ubuntu

gather_facts: true

tasks:

- name: Run once on your_tag_name

shell: echo yes >> yes.txt

run_once: true

delegate_to: "{{ groups['your_hosts'][0] }}"

Method 2: use the ec2_instance_facts module to create the in-memory group. This requires a tag name variable or constant you can reference to match instances with

- hosts: localhost

user: ubuntu

connection: local

gather_facts: true

tasks:

- name: get ec2 remote facts

ec2_instance_facts:

region: "{{ aws_region }}"

register: ec2

- name: create an in memory group of only nodes with your_tag_name

add_host:

name: "{{ item }}"

groups: your_hosts

with_items: "{{ ec2.instances | selectattr('state.name', 'equalto', 'running') | selectattr('tags.Name', 'equalto', your_tag_name ) | map(attribute='public_ip_address')|list }}"

Then performing the run_once is the same. 

Either way, if you want to use the information from ec2.py then you'll end up having two hosts sections and some verbosity. I suppose with the ec2_instance_facts one could potentially create groups of EC2 hosts and run the playbook all with hosts: localhost and delegate tasks to certain groups? I haven't tried this as I've spend enough time getting run_once to work for now.

Dave

On Friday, December 8, 2017 at 3:20:20 PM UTC-7, ddrak...@gmail.com wrote:

Thanks for the help, Josh. 

I actually am trying to run a command once on a certain set of EC2 instances I'm managing that match a specific tag. This seems like a very reasonable thing to do and yet the run_once skipping functionality makes this quite difficult. I managed to find two similar but different ways to do this (though not exactly elegant), which I'll post for future reference. I was inspired by https://groups.google.com/forum/#!topic/ansible-project/cpnrBRxLy0E and https://imil.net/blog/2016/08/05/Ansible_and_AWS_ASG

Method 1 (serial 1 is important due to the add_host behavior Josh pointed out). Run the playbook with the AWS dynamic inventory script (-i ec2.py)

- hosts: ec2

user: ubuntu

serial: 1

gather_facts: false

tasks:

- name: add only nodes that don't match your_tag_name to in-memory host group

add_host:

name: "{{ inventory_hostname }}"

groups: your_hosts

when: ec2_tag_Name != your_tag_name

and then run the rest of your playbook not serially and perform run_once actions like this:

- hosts: ec2

user: ubuntu

gather_facts: true

tasks:

- name: Run once on your_tag_name

shell: echo yes >> yes.txt

run_once: true

delegate_to: "{{ groups['your_hosts'][0] }}"

Method 2: use the ec2_instance_facts module to create the in-memory group. This requires a tag name variable or constant you can reference to match instances with

- hosts: localhost

user: ubuntu

connection: local

gather_facts: true

tasks:

- name: create an in memory group of only nodes with your_tag_name

add_host:

name: "{{ item }}"

groups: your_hosts

with_items: "{{ ec2.instances | selectattr('state.name', 'equalto', 'running') | selectattr('tags.Name', 'equalto', your_tag_name ) | map(attribute='public_ip_address')|list }}"

Either way, if you want to use the information from ec2.py then you'll end up having two hosts sections and some verbosity. I suppose with the ec2_instance_facts one could potentially create groups of EC2 hosts and run the playbook all with hosts: localhost and delegate tasks to certain groups? I haven't tried this as I've spend enough time getting run_once to work for now.

Dave

[Pshem Kowalczyk]

If you're using ec2.py to get your dynamic inventory you might use the dynamic groups created from tags (tag_NAME_VALUE). If you dynamically create the host before having to access it you can use:

- meta: refresh_inventory

to re-run the inventory script (which should discover newly created EC2 instances).

in that case you simply run the new play in the playbook against the hosts matching the tag:

hosts: tag_NAME_VALUE

kind regards

Pshem

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[ddrak...@gmail.com]

Thanks for the suggestion. This would be nice except I am using variable tag names defined in a config file, furthermore the tag names can't have underscores (a requirement of the AWS ELB module) only dashes so I don't even think I could map things even if it was possible to do something like the below although I'm not even sure you can use variables in hosts definitions like this?  

- hosts: "'tag_Name_' + {{ my_tag_variable }}"

Even if this worked I'd be writing another whole hosts section every time I wanted to run once on a given tag. The 'run once on a tag' commands are interspersed throughout the playbook so this would get annoying. At least with the way above I have at most one extra hosts section.

Thanks,
Dave