# Run a command only on hosts that have low CPU usage **URL:** \ **Category:** Ansible Project\ **Created:** [January 30, 2020, 12:48am UTC](https://forum.ansible.com/t/run-a-command-only-on-hosts-that-have-low-cpu-usage/32094 "2020-01-30T00:48:58Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![Eric\_S](https://avatars.discourse-cdn.com/v4/letter/e/aca169/32.png) [@Eric\_S](https://forum.ansible.com/u/Eric_S)\ **Post date:** [January 30, 2020, 12:48am UTC](https://forum.ansible.com/t/run-a-command-only-on-hosts-that-have-low-cpu-usage/32094/1 "2020-01-30T00:48:58Z")
Hi Folks, I am looking for a simple way to have Ansible perform the following automation for me: 1. From a set of N hosts, find out which host has the lowest CPU usage at the moment 2. Run a command on **only that host**. This pattern could be extended to a general case of: > Run a command on a subset of hosts matching a condition which is not static. This could apply for the following types of use cases: - Find out which hosts are running low on disk space, and only expand disk space on the hosts which are running low - Find out which hosts are lacking a particular upgrade, and only upgrade those hosts - Only run a command on machines which are not currently under heavy load This seems like a common and desirable enough pattern, but as far as I can tell there is not a well defined way to execute this with Ansible. Is there a simple way to do this with a playbook, or do I need to write a strategy plugin? Thanks, Eric ---
**Author:** ![racke](https://sea1.discourse-cdn.com/ansible/user_avatar/forum.ansible.com/racke/32/4767_2.png) [@racke](https://forum.ansible.com/u/racke)\ **Post date:** [January 30, 2020, 6:27am UTC](https://forum.ansible.com/t/run-a-command-only-on-hosts-that-have-low-cpu-usage/32094/2 "2020-01-30T06:27:33Z")
> Hi Folks, > I am looking for a simple way to have Ansible perform the following automation for me: > > 1. From a set of N hosts, find out which host has the lowest CPU usage at the moment > 2. Run a command on \*only that host\*. > > This pattern could be extended to a general case of: > >     Run a command on a subset of hosts matching a condition which is not static. > > This could apply for the following types of use cases: > >   \* Find out which hosts are running low on disk space, and only expand disk space on the hosts which are running low >   \* Find out which hosts are lacking a particular upgrade, and only upgrade those hosts >   \* Only run a command on machines which are not currently under heavy load > > This seems like a common and desirable enough pattern, but as far as I can tell there is not a well defined way to > execute this with Ansible. Is there a simple way to do this with a playbook, or do I need to write a strategy plugin? > > Thanks, > Eric Hello Eric, you could try to add the hosts to a new group with the "add\_host" module and execute the command with delegate\_to in a loop over the hosts in that group. Regards          Racke ---
**Author:** ![Eric\_S](https://avatars.discourse-cdn.com/v4/letter/e/aca169/32.png) [@Eric\_S](https://forum.ansible.com/u/Eric_S)\ **Post date:** [January 30, 2020, 4:24pm UTC](https://forum.ansible.com/t/run-a-command-only-on-hosts-that-have-low-cpu-usage/32094/3 "2020-01-30T16:24:14Z")
Racke, Thanks for your response. In practice, what would this look like? So far, I have a playbook that I can use to get a list of CPU values. However, I am having trouble correlating the value of CPU to its corresponding hostname. ---
**Author:** ![racke](https://sea1.discourse-cdn.com/ansible/user_avatar/forum.ansible.com/racke/32/4767_2.png) [@racke](https://forum.ansible.com/u/racke)\ **Post date:** [January 30, 2020, 4:42pm UTC](https://forum.ansible.com/t/run-a-command-only-on-hosts-that-have-low-cpu-usage/32094/4 "2020-01-30T16:42:26Z")
> Racke, > Thanks for your response. > > In practice, what would this look like? > > So far, I have a playbook that I can use to get a list of CPU values. However, I am having trouble correlating the value > of CPU to its corresponding hostname. > > --- > - hosts: host\_list > gather\_facts: false > > tasks: > - name: Get CPU usage > shell: "top -b -n 1 | head -n3 | tail -n1 | awk '{print $2}'" > register: top > > - name: Set CPU fact > set\_fact: > cpu\_list: "{{ ansible\_play\_hosts | map('extract', hostvars, 'top') | map(attribute='stdout') | list }}" > > Is this on the right track, or do I want a different approach entirely? > > Thanks, > Eric Hello Eric, this can be done, but I doubt that it makes sense to do it with Ansible as above (especially with a lot of hosts). Collect system metrics on each server with something like Telegraf and base your decisions on that. Regards        Racke ---
**Author:** ![vbotka](https://sea1.discourse-cdn.com/ansible/user_avatar/forum.ansible.com/vbotka/32/359_2.png) [@vbotka](https://forum.ansible.com/u/vbotka)\ **Post date:** [January 30, 2020, 6:07pm UTC](https://forum.ansible.com/t/run-a-command-only-on-hosts-that-have-low-cpu-usage/32094/5 "2020-01-30T18:07:59Z")
Create list of dictionaries   - name: Set CPU fact     set\_fact:       cpu\_list: "{{ ansible\_play\_hosts|                      map('extract', hostvars)|                      list\>                      json\_query('.{host: inventory\_hostname,                                      cpu: top.stdout}') }}"     run\_once: true Find the minimal load   - set\_fact:       cpu\_min: "{{ cpu\_list|json\_query('.cpu')|min }}"     run\_once: true and find the host with this minimal load   - set\_fact:       host\_min: "{{ cpu\_list|json\_query(query)|first }}"     vars:       query: "[?cpu == '{{ cpu\_min }}'].host"     run\_once: true HTH,   -vlado