# Chose random host **URL:** \ **Category:** Ansible Project\ **Created:** [May 18, 2013, 1:05am UTC](https://forum.ansible.com/t/chose-random-host/12826 "2013-05-18T01:05:56Z")\ **Posts on this page:** 11\ **Page:** 1
**Author:** ![Curtis](https://avatars.discourse-cdn.com/v4/letter/c/58956e/32.png) [@Curtis](https://forum.ansible.com/u/Curtis)\ **Post date:** [May 18, 2013, 1:05am UTC](https://forum.ansible.com/t/chose-random-host/12826/1 "2013-05-18T01:05:56Z")
Hi All, What I need to do is pick one of the ansible\_hosts group members randomly to run some cluster bootstrap code on it. I don’t care which one, but it can be only one. Any suggestions as to how to do that? Thanks, Curtis. ---
**Author:** ![Michael\_DeHaan2](https://avatars.discourse-cdn.com/v4/letter/m/da6949/32.png) [@Michael\_DeHaan2](https://forum.ansible.com/u/Michael_DeHaan2)\ **Post date:** [May 18, 2013, 5:27pm UTC](https://forum.ansible.com/t/chose-random-host/12826/2 "2013-05-18T17:27:32Z")
There’s presently no mechanism to do this. Is there anything wrong with always picking the first one, perhaps? ---
**Author:** ![Curtis](https://avatars.discourse-cdn.com/v4/letter/c/58956e/32.png) [@Curtis](https://forum.ansible.com/u/Curtis)\ **Post date:** [May 18, 2013, 9:12pm UTC](https://forum.ansible.com/t/chose-random-host/12826/3 "2013-05-18T21:12:50Z")
> There's presently no mechanism to do this. > > Is there anything wrong with always picking the first one, perhaps? That would work. Do you happen to know if there are any examples as to how I might go about doing that? 🙂 Thanks, Curtis. ---
**Author:** ![Michael\_DeHaan](https://avatars.discourse-cdn.com/v4/letter/m/c4cdca/32.png) [@Michael\_DeHaan](https://forum.ansible.com/u/Michael_DeHaan)\ **Post date:** [May 18, 2013, 9:46pm UTC](https://forum.ansible.com/t/chose-random-host/12826/4 "2013-05-18T21:46:00Z")
Sure thing, this is pretty straightforward. Group names in host groups can take offsets, like so: hosts: groupname[0] And even: # first five hosts hosts: groupname[0-4] So the first example is an easy way to target just the first host in a group. ---
**Author:** ![Serge\_van\_Ginderacht](https://avatars.discourse-cdn.com/v4/letter/s/b782af/32.png) [@Serge\_van\_Ginderacht](https://forum.ansible.com/u/Serge_van_Ginderacht)\ **Post date:** [May 19, 2013, 12:11am UTC](https://forum.ansible.com/t/chose-random-host/12826/5 "2013-05-19T00:11:55Z")
​nice trick, didn't know pattern took this Downside of this method, however, is that ​it forces you to repeat that for every group of clusternodes. At work we have many groups containing clusternodes. Almost all of those need some single action on the clusterlevel, and it would be neat to be able to say e.g. "create proxy configurations for all tomcat applications clusters in all environments" we have many groups like "app" containg "app-dev" and "app-test" and "app-prod", and I would want to run a playbook for the proxy configuration on the master group containing \*all\* "app" groups. I've played a little adapting group\_by to a group\_by\_pool.py plugin, where I loop through all the hosts groups, and pick those with the highest depth (1) to create a "pool" group. which now gives me this: $ ansible -m group\_by\_pool mkros --list-hosts     mkros-oe-1-mgt     mkros-oe-2-mgt     mkros-on-1-mgt     mkros-on-2-mgt     mkros-pr-1-mgt     mkros-pr-2-mgt $ ansible -m group\_by\_pool mkros -a key=POOL mkros-oe-1-mgt | success \>\> {     "changed": true,     "groups": {         "POOL\_mkros-oe": [             "mkros-oe-1-mgt"         ],         "POOL\_mkros-on": [             "mkros-on-1-mgt"         ],         "POOL\_mkros-pr": [             "mkros-pr-1-mgt"         ]     } } ​Would this be interesting?    Serge​ ​(1) I'm sometimes seeing the same effect with depth I mentioned on another thread ---
**Author:** ![Michael\_DeHaan2](https://avatars.discourse-cdn.com/v4/letter/m/da6949/32.png) [@Michael\_DeHaan2](https://forum.ansible.com/u/Michael_DeHaan2)\ **Post date:** [May 19, 2013, 12:21am UTC](https://forum.ansible.com/t/chose-random-host/12826/6 "2013-05-19T00:21:16Z")
“Downside of this method, however, is that it forces you to repeat that for every group of clusternodes.” Depends on what you are doing. If you have a 50 node Riak cluster or a 100 node Hadoop or something, and you want to run something on only one node, that’s nothing to repeat. Not sure if I understand what the group by pool does. ---
**Author:** ![Serge\_van\_Ginderacht](https://avatars.discourse-cdn.com/v4/letter/s/b782af/32.png) [@Serge\_van\_Ginderacht](https://forum.ansible.com/u/Serge_van_Ginderacht)\ **Post date:** [May 19, 2013, 12:40am UTC](https://forum.ansible.com/t/chose-random-host/12826/7 "2013-05-19T00:40:25Z")
> "Downside of this method, however, is that it forces you to repeat that > for every group of clusternodes." > > Depends on what you are doing. > > If you have a 50 node Riak cluster or a 100 node Hadoop or something, and > you want to run something on only one node, that's nothing to repeat. But then you'd ​​probably just have a situation where you only manage those nodes in 1 big group, as they are all equivalent. Say you have three environments, a 50 node development hadoop cluster, a 50 node testing hadoop cluster and a 50 node production hadoop cluster (I don't know what hadoop is, so forgive me if that doesn't make sense. s/hadoop/apache/ You could then do: - hosts: hadoop-dev[0]   tasks:   - action: do something on the cluster - hosts: hadoop-test[0]   tasks:   - action: do something on the cluster - hosts: hadoop-prod[0]   tasks:   - action: do something on the cluster Or: - hosts: hadoop   tasks:   - action: group\_by\_pool # this would group by hadoop-dev/test/prod and target just one node per new group - hosts: POOL-hadoop-\*   tasks:   - action: do something on the cluster In my environment I have over 3 environments with each 100 clusters, so effectively over 300 clusters, for which I would need to do the same action, so that last solution with two plays would scale a lot better for me. Not sure if I understand what the group by pool does. > ​It basically takes eacht child group, where 1 group is 1 cluster, and which has all the nodes for that cluster, and creates a new group containing just the first node of that original group. That was just a quick idea and proof of concept, that seems to do what I might need. Not saying right now all the details are optimal though. I possibly should get a life on Saturday nights.   Serge ---
**Author:** ![Michael\_DeHaan2](https://avatars.discourse-cdn.com/v4/letter/m/da6949/32.png) [@Michael\_DeHaan2](https://forum.ansible.com/u/Michael_DeHaan2)\ **Post date:** [May 19, 2013, 1:40pm UTC](https://forum.ansible.com/t/chose-random-host/12826/8 "2013-05-19T13:40:16Z")
I think you are conflating the problem. He requested how to address one node out of a group, not how to manage lots of first nodes in lost of groups. Then you’re going on into a long discussion about something he wasn’t talking about. Let’s keep things simple, please 🙂 ---
**Author:** ![Serge\_van\_Ginderacht](https://avatars.discourse-cdn.com/v4/letter/s/b782af/32.png) [@Serge\_van\_Ginderacht](https://forum.ansible.com/u/Serge_van_Ginderacht)\ **Post date:** [May 19, 2013, 5:33pm UTC](https://forum.ansible.com/t/chose-random-host/12826/9 "2013-05-19T17:33:50Z")
​The solution you gave for the initial question is a perfect fit. I'm just telling I have the same problem, just on a bigger scale, where that solution doesn't fit in, and was thinking about how I could solve this.​ I'd thought this would be an interesting use case and this was merely FYI, asking if it would be of any interest to anyone. ​ ​ Serge​ ---
**Author:** ![Michael\_DeHaan](https://avatars.discourse-cdn.com/v4/letter/m/c4cdca/32.png) [@Michael\_DeHaan](https://forum.ansible.com/u/Michael_DeHaan)\ **Post date:** [May 19, 2013, 6:09pm UTC](https://forum.ansible.com/t/chose-random-host/12826/10 "2013-05-19T18:09:03Z")
In your case, I would just make a group of all of the head nodes. ---
**Author:** ![Rene\_Moser](https://avatars.discourse-cdn.com/v4/letter/r/7bcc69/32.png) [@Rene\_Moser](https://forum.ansible.com/u/Rene_Moser)\ **Post date:** [December 13, 2016, 4:43pm UTC](https://forum.ansible.com/t/chose-random-host/12826/11 "2016-12-13T16:43:29Z")
Better late than never - hosts: localhost tasks: - add\_host: name: “{{ item }}” groups: elected\_leader with\_random\_choice: “{{ groups[‘web’] }}” - hosts: elected\_leader gather\_facts: no tasks: - debug: var=inventory\_hostname