Thoughtless Banter Musings from a software engineer.

Hydration of a Chef Server

At Bloomberg my team is tasked with supporting infrastructure for all of our consumer and subscription web properties. We have been using Chef to solve some of our problems around configuration management. Most of our recipes were built using Chef Solo and eventually deployed on our cloud infrastructure without a Chef Server. As we began to plan to deploy software services to the teams that we support we quickly began to realize that it would be nearly impossible to manage multiple node deployments without Chef Server.

Our immediate need was to provision a Chef Server for our build infrastructure that we would be distributing to about twenty teams. But in addition to this we have brought in Chef to train several classes of our developers. After the classes concluded there were many requests for individual Chef Server tenancies for testing.

Using Chef Server within our infrastructure was an afterthought because most of our virtual machines had a very short lifespan. Since we could have several dozen development teams wanting their own Chef Server deployments we needed a solution to not only easily deploy a Chef Server, but to hydrate it with the data necessary for each OpenStack tenancy. This included both user and client authentication information.

After a bit of back and forth I was able to isolate the requirements for our Chef Server infrastructure:

  1. It must use the Chef Server community cookbook and the Omnibus packages provided by Chef;

  2. It should be quick and easy to deploy a new instance because we may be doing so for several dozen tenancies in OpenStack;

  3. All user and client keys should be generated locally and the server should be hydrated during the initial convergence run;

I was first introduced to Chef Metal at this past year’s ChefConf where John Keiser gave an excellent talk. My team has a lot of experience using Vagrant, but it made much more sense to me to include a metal recipe with our Chef infrastructure cookbook. It could serve as both a teaching aide, and would also make our internal cookbooks much more versatile. Throughout the discovery process I found the community Chef Server Populator cookbook, ended up completely rewriting it to use Chef resources to modify the Chef Server, and ultimately used Chef Zero to bootstrap everything.

Playing with Metal

Using Chef Metal I was able to easily distribute a recipe with our Chef infrastructure cookbook that would provision a Chef Server very easily using the machine resource. The recipe that I began with looked very similar to what is below. It first laid down changes to the base operating system using our base cookbook, and simply used the community cookbook to install the Chef Server from the Omnibus packages. It was very straight forward.

Unfortunately I still needed to hydrate the local data into the newly minted Chef Server. Luckily for me the cats over at Heavy Water Operations have solved this problem and provided the Chef Server Populator cookbook. There were only a few minor modifications that I needed to make to support client validation keys. But other than that everything seemed to be smooth sailing.

Chef Zero

At first, with only a few minor changes, the Chef Server Populator cookbook seemed to be the answer to my problems. I had a few cases that were not covered, but these were all pretty simple extensions. It was natural and made sense to convert our Chef infrastructure cookbook into a Chef repository and use Chef Zero as an in-memory Chef Server during the initial provisioning.

That worked out quite well at first. I was able to cobble together a simple Rake task to generate and write out the private and public keys to JSON data bag items. My metal recipe started to look a lot more complex than I had initially intended, and it didn’t seem as clean as I thought it could be. As I was reading through the code for the Chef Metal Fog driver a light bulb went off in my head: I could use the Cheffish gem Chef resources and build the data on the fly!

Chef within Chef

I ended up completely rewriting the Chef Server Populator cookbook. My fork now used the Chef resources provided by the Cheffish gem to read from data bag items and create the user and client records against the Chef Server node. The beauty of this approach is that I no longer had to execute clunky knife commands since in Chef 11 erchef exposed an API to pass both user and client public keys. The cookbook is now much more clean and simple to grok.

The code for creating the user and client keys was also able to use the same Chef resources from Cheffish. Instead of operating against our newly provisioned Chef Server it executed against the in-memory server provided by Chef Zero.

Using keys which are generated inside of the metal recipe is not idempotent. There were two ways that I went about solving this problem. This first was merely reading in the private keys off disk and delegating the generation to some external process. The second would be to use the Chef resource to generate private keys and allow it to manage if they needed to be updated or not. Ultimately it worked out that if the keys were generated outside of the metal recipe it was much more straightforward. Here is a full example where keys for clients were generated outside, but user keys are generated on the fly:

Tying the Bow

The final result was a complete cookbook refactor which uses Chef resources to hydrate a newly provisioned Chef Server. The default recipe uses node attributes and creates both client and user resources on the new server. A data bag recipe uses a Chef data bag search to find items and applies them to the node attributes prior to including the default recipe.

Since Chef Zero provides a fully functioning in-memory Chef Server we are able to use Chef resources to add data bag items with locally generated public keys. When using the Chef Metal recipe this data is now made accessible to the data bag search, and thus it gives us an elegant way to hydrate a Chef Server. I have provided an example of this inside of my fork of the cookbook which illustrates all of the steps above.

As I mentioned at the top of this post I originally started work on this solution to allow us to quickly deploy our build infrastructure to multiple tenancies within our private cloud. In a future post I will explain how my team was able to create an easily deployable continuous integration framework using the community Jenkins recipe.

If you are reading this post and are interested in joining our growing group of Chef technologists at Bloomberg feel free to reach out to me. We are hiring!