# Save from recipe data to chef server

**URL:** <https://discourse.chef.io/t/save-from-recipe-data-to-chef-server/4679>\
**Category:** Chef Infra (archive)\
**Created:** [November 15, 2013, 2:14am UTC](https://discourse.chef.io/t/save-from-recipe-data-to-chef-server/4679 "2013-11-15T02:14:16Z")\
**Posts on this page:** 6\
**Page:** 1

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**Author:** ![silviu\_dicu](https://avatars.discourse-cdn.com/v4/letter/s/c4cdca/32.png) [@silviu\_dicu](https://discourse.chef.io/u/silviu_dicu)\
**Post date:** [November 15, 2013, 2:14am UTC](https://discourse.chef.io/t/save-from-recipe-data-to-chef-server/4679/1 "2013-11-15T02:14:16Z")

</div>

Hi,

I have a set of servers that need run chef-client at 30 min. interval.  
Any of them have to execute an operation at any given time but can’t have  
more then one at the same time.

I’m looking to make a lock(mutex) that is saved onto the chef server by  
a recipe.

My question is how can I save it ?

- use a node attribute and all other will do a search for it
- make use of a databag(need to distribute keys to be a client on nodes)  
…

thanks!

-silviu

---

<div class="post-metadata">

**Author:** ![Andrew\_Gross](https://sea2.discourse-cdn.com/flex016/user_avatar/discourse.chef.io/andrew_gross/32/320_2.png) [@Andrew\_Gross](https://discourse.chef.io/u/Andrew_Gross)\
**Post date:** [November 15, 2013, 3:31pm UTC](https://discourse.chef.io/t/save-from-recipe-data-to-chef-server/4679/2 "2013-11-15T15:31:33Z")

</div>

Hey Silviu,

It is 'possible' to do this with Chef, though I can't guarantee Chef will  
do it well. This sort of coordination is not what Chef excels at, and if  
it is really important I would recommend using something better at  
coordination, like Zookeeper. That said, here are some possible Chef  
solutions:

1. Use a library file to make an API call to some endpoint that distributes  
your locks, this can be something hacked up in Chef server, or a different  
service (like Zookeeper or Chubby). Have the result from this library call  
block the recipe if you can't get the lock.

2. Node Attributes can work, but you need to be careful. You can set a  
particular attribute when you 'have the lock' and then always search for  
that attribute on any nodes before you do you dangerous operation. There is  
no built in locking mechanism, so you will need to wrap it very carefully.  
I would not recommend it.

3. Databags are another option. These have many of the same caveats as  
using Node attributes as it is hard to guarantee and atomic update. The  
most likely failure scenario is two nodes read that no one has the lock and  
both try to add it. Since databags always overwrite with the latest data,  
they will both 'write' the lock and proceed to cheerfully destroy data.

To do this sort of thing properly, you will want a system than can do an  
atomic compare-and-update, which is not what Chef was designed for in Node  
attributes and Databags. There may be some other subsystem that can  
perform this, but I cannot think of it right now.

Andrew

On Thu, Nov 14, 2013 at 9:14 PM, [silviudicu@gmail.com](mailto:silviudicu@gmail.com) wrote:

> Hi,
> 
> I have a set of servers that need run chef-client at 30 min. interval.  
> Any of them have to execute an operation at any given time but can't have  
> more then one at the same time.
> 
> I'm looking to make a lock(mutex) that is saved onto the chef server by  
> a recipe.
> 
> My question is how can I save it ?
> 
> - use a node attribute and all other will do a search for it
> - make use of a databag(need to distribute keys to be a client on nodes)  
> ...
> 
> thanks!
> 
> -silviu

---

<div class="post-metadata">

**Author:** ![Seth\_Falcon\_01](https://sea2.discourse-cdn.com/flex016/user_avatar/discourse.chef.io/seth_falcon_01/32/429_2.png) [@Seth\_Falcon\_01](https://discourse.chef.io/u/Seth_Falcon_01)\
**Post date:** [November 15, 2013, 4:48pm UTC](https://discourse.chef.io/t/save-from-recipe-data-to-chef-server/4679/3 "2013-11-15T16:48:58Z")

</div>

[andrew@yipit.com](mailto:andrew@yipit.com) writes:

> It is 'possible' to do this with Chef, though I can't guarantee Chef will  
> do it well. This sort of coordination is not what Chef excels at, and if  
> it is really important I would recommend using something better at  
> coordination, like Zookeeper. That said, here are some possible Chef  
> solutions:

+1 on the caveats. Chef Server doesn't provide orchestration primitives  
at this time.

It is something that we discuss adding and it always helps to examples  
of what would be useful.

> 1. Databags are another option. These have many of the same caveats as  
> using Node attributes as it is hard to guarantee and atomic update. The  
> most likely failure scenario is two nodes read that no one has the lock and  
> both try to add it. Since databags always overwrite with the latest data,  
> they will both 'write' the lock and proceed to cheerfully destroy  
> data.

While it really is not a good idea, if you are going go down this route,  
I'd recommend using the presence/absence of a data bag (not data bag  
item).

```
api.post("data", {:name => "lock1"})

```

This will only succeed if there is not already such a data bag (returns  
409 if a bag named lock1 already exists). Top-level data bags are about  
as light weight as you can get and get you closest to the atomic CAS  
thing.

- seth

--  
Seth Falcon | Development Lead | Opscode | @sfalcon

---

<div class="post-metadata">

**Author:** ![silviu\_dicu](https://avatars.discourse-cdn.com/v4/letter/s/c4cdca/32.png) [@silviu\_dicu](https://discourse.chef.io/u/silviu_dicu)\
**Post date:** [November 16, 2013, 6:11pm UTC](https://discourse.chef.io/t/save-from-recipe-data-to-chef-server/4679/4 "2013-11-16T18:11:12Z")

</div>

Hi,

Can you elaborate on 'this can be something hacked up in Chef server'  
From a node/client that is not admin on the open source chef you are limited to node attributes - unless there is a different way to do it.

The strategy check if exists is fine (I imagined the same).

thanks!

> On Nov 15, 2013, at 11:48 AM, Seth Falcon [seth@opscode.com](mailto:seth@opscode.com) wrote:
> 
> [andrew@yipit.com](mailto:andrew@yipit.com) writes:
> 
> > It is 'possible' to do this with Chef, though I can't guarantee Chef will  
> > do it well. This sort of coordination is not what Chef excels at, and if  
> > it is really important I would recommend using something better at  
> > coordination, like Zookeeper. That said, here are some possible Chef  
> > solutions:
> 
> +1 on the caveats. Chef Server doesn't provide orchestration primitives  
> at this time.
> 
> It is something that we discuss adding and it always helps to examples  
> of what would be useful.
> 
> > 1. Databags are another option. These have many of the same caveats as  
> > using Node attributes as it is hard to guarantee and atomic update. The  
> > most likely failure scenario is two nodes read that no one has the lock and  
> > both try to add it. Since databags always overwrite with the latest data,  
> > they will both 'write' the lock and proceed to cheerfully destroy  
> > data.
> 
> While it really is not a good idea, if you are going go down this route,  
> I'd recommend using the presence/absence of a data bag (not data bag  
> item).
> 
> api.post("data", {:name =\> "lock1"})
> 
> This will only succeed if there is not already such a data bag (returns  
> 409 if a bag named lock1 already exists). Top-level data bags are about  
> as light weight as you can get and get you closest to the atomic CAS  
> thing.
> 
> - seth
> 
> --  
> Seth Falcon | Development Lead | Opscode | @sfalcon

---

<div class="post-metadata">

**Author:** ![Seth\_Falcon\_01](https://sea2.discourse-cdn.com/flex016/user_avatar/discourse.chef.io/seth_falcon_01/32/429_2.png) [@Seth\_Falcon\_01](https://discourse.chef.io/u/Seth_Falcon_01)\
**Post date:** [November 17, 2013, 4:59pm UTC](https://discourse.chef.io/t/save-from-recipe-data-to-chef-server/4679/5 "2013-11-17T16:59:28Z")

</div>

Hi,

[silviudicu@gmail.com](mailto:silviudicu@gmail.com) writes:

> Can you elaborate on 'this can be something hacked up in Chef server'  
> From a node/client that is not admin on the open source chef you are  
> limited to node attributes - unless there is a different way to do it.

I'm assuming you're asking about my reply, but not clear on what you'd  
like more detail on.

- seth

---

<div class="post-metadata">

**Author:** ![silviu\_dicu](https://avatars.discourse-cdn.com/v4/letter/s/c4cdca/32.png) [@silviu\_dicu](https://discourse.chef.io/u/silviu_dicu)\
**Post date:** [November 18, 2013, 10:51pm UTC](https://discourse.chef.io/t/save-from-recipe-data-to-chef-server/4679/6 "2013-11-18T22:51:43Z")

</div>

Hi,

Sorry - I didn't include the original text.

"

1. Use a library file to make an API call to some endpoint that distributes your locks, this can be something hacked up in Chef server, or a different service (like Zookeeper or Chubby). Have the result from this library call block the recipe if you can't get the lock.  
"

So what I inquired is what else could be hacked up into the chef server except what was covered, node attributes and data bags.

thanks!

- silviu

> On Nov 17, 2013, at 11:59 AM, Seth Falcon [seth@opscode.com](mailto:seth@opscode.com) wrote:
> 
> Hi,
> 
> [silviudicu@gmail.com](mailto:silviudicu@gmail.com) writes:
> 
> > Can you elaborate on 'this can be something hacked up in Chef server'  
> > From a node/client that is not admin on the open source chef you are  
> > limited to node attributes - unless there is a different way to do it.
> 
> I'm assuming you're asking about my reply, but not clear on what you'd  
> like more detail on.
> 
> - seth