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https://github.com/facebook/chef-cookbooks

Open source chef cookbooks.
https://github.com/facebook/chef-cookbooks

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README 

        
# Facebook Cookbooks Suite



[![Continuous Integration](https://github.com/facebook/chef-cookbooks/workflows/Continuous%20Integration/badge.svg?event=push)](https://github.com/facebook/chef-cookbooks/actions?query=workflow%3A%22Continuous+Integration%22)



This repo contains attribute-driven-API cookbooks maintained by Facebook. It's

a large chunk of what we refer to as our "core cookbooks."



It's worth reading our

[Philosophy.md](https://github.com/facebook/chef-utils/blob/main/Philosophy.md)

doc before reading this. It covers how we use Chef and is important context

before reading this.



It is important to note that these cookbooks are built using a very specific

model that's very different from most other cookbooks in the community.

Specifically:



* It assumes an environment in which you want to delegate. The cookbooks are

  designed to be organized in a "least specific to most specific" order in the

  run-list. The run-list starts with the "core cookbooks" that setup APIs and

  enforce a base standard, which can be adjusted by the service owners using

  cookbooks later in the run-list.

* It assumes a "run from main" type environment. At Facebook we use [Grocery

  Delivery](http://www.github.com/facebook/grocery-delivery) to sync the main

  branch of our source control repo with all of our Chef servers. Grocery

  Delivery is not necessary to use these cookbooks, but since they were built

  with this model in mind, the versions never change (relatedly: we do not use

  environments).

* It assumes you have a testing toolset that allows anyone modifying later

  cookbooks to ensure that their use of the API worked as expected on a live

  node before committing. For this, we use [Taste

  Tester](http://www.github.com/facebook/taste-tester).



Cookbooks in this repo all being with `fb_` to denote that not only do they

use the Facebook Cookbook Model, but that they are maintained in this repo.



Local cookbooks or cookbooks in other repositories that implement this model

should not use this prefix, but should reference this document in their docs.



## APIs



Unlike other cookbook models, we do not use resources as APIs, we use the node

object. Configuration is modeled in arrays and hashes as closely and thinly as

possible to the service we are configuring. Ideally, you should only have to

read the docs to the service to configure it, not the docs to the cookbook.



For example, if the service we are configuring has a key-value pair

configuration file, we will provide a simple hash where keys and values will be

directly put into the necessary configuration file.



There are two reasons we use attribute-driven APIs:



1. Cascading configuration

   Since our cookbooks are ordered least specific (core team that owns Chef) to

   most specific (the team that owns this machine or service) it means that the

   team who cares about this specific instance can always override anything.

   This enables stacking that is not possible in many other models. For

   example, you can have a run-list that looks like:



   * Core cookbooks (the ones in this repo)

   * Site/Company cookbooks (site-specific settings)

   * Region cookbooks (overrides for a given region/cluster)

   * Application Category cookbooks (webserver, mail server, etc.)

   * Specific Application cookbook ("internal app1 server")



   So let's say that you want a specific amount of shared memory by default,

   but in some region you know you have different size machines, so you shrink

   it, but web servers need a further different setting, and then finally some

   specific internal webserver needs an even more specific setting... this all

   just works.



   Further, a cookbook can see the value that was set before it modifies

   things, so the 'webserver' cookbook could look to see what the value was

   (small or large) before modifying it and adjust it accordingly (so it could

   be relative to the size of memory that the 'region' cookbook set).



   Using resources for this does not allow this "cascading", it instead creates

   "fighting". If you use the cron resource to setup an hourly job, and then

   someone else creates a cron for that same job but only twice a day, then

   during each Chef run the cron job gets modified to hourly, then re-modified

   to twice a day.



2. Allows for what we refer to as "idempotent systems" instead of "idempotent

   settings." In other words, if you only manage a specific item in a larger

   config, and then you stop managing it, it should either revert to a

   less-specific setting (see #1) or be removed, as necessary.



   For example let's say you want to set a cron job. If you use the internal

   cron resource, and then delete the recipe code that adds that cronjob, that

   cron isn't removed from your production environment - it's on all existing

   nodes, but not on any new nodes.



   For this reason we use templates to take over a whole configuration wherever

   possible. All cron jobs in our `fb_cron` API are written to

   `/etc/cron.d/fb_crontab`. If you delete the lines adding a cronjob, since

   they are just entries in a hash, when the template is generated on the next

   Chef run, those crons go away.



   Alternatively, consider a sysctl set by the "site" cookbook, then

   overwritten by a later cookbook. When that later code is removed, the entry

   in the hash falls back to being set again by the next-most-specific value

   (i.e. the "site" cookbook in this case).



## Run-lists



How you formulate your run-lists is up to your site, as long as you follow the

basic rule that core cookbooks come first and you order least-specific to

most-specific. At Facebook, all of our run-lists are:



    recipe[fb_init], recipe[$SERVICE]



Where `fb_init` is similar to the sample provided in this repo, but with extra

"core cookbooks."



We generally think of this way: `fb_init` should make you a "Facebook server"

and the rest should make you a whatever-kind-of-server-you-are.



## Getting started



Grab a copy of the repo, rename `fb_init_sample` to `fb_init`, and follow the

instructions in its [README.md](https://github.com/facebook/chef-cookbooks/blob/main/cookbooks/fb_init_sample/README.md)

(coordinating guidance is in comments in the default recipe).



## Other Guidelines



### Modules and classes in cookbooks



It is often useful to factor out logic into a library - especially logic that

doesn't create resources. Doing so makes this logic easier to unit test and

makes the recipe or resource cleaner.



Our standard is that all cookbooks use the top-level container of `module FB`,

and then create a class for their cookbook under that. For example, `fb_fstab`

creates a `class Fstab` inside of the `module FB`. We will refer to this as

the cookbook class from here.



We require all cookbooks use this model for consistency.



Since we don't put anything other than other classes inside the top-level

object, it's clear that a `module` is the right choice.



While there is no reason that a cookbook class can't be one designed to be

instantiated, more often than not it is simply a collection of class methods

and constants (i.e. static data and methods that can then be called both from

this cookbook and others).



Below the cookbook class, the author is free to make whatever class or methods

they desire.



When building a complicated Custom Resource, the recommended pattern is to

factor out the majority of the logic into a module, inside of the cookbook

class, that can be `include`d in the `action_class`. This allows the logic to

be easily unit tested using standard rspec. It is preferred for this module to

be in its own library file, and for its name to end in `Provider`, ala

`FB::Thing::ThingProvider`.



When more than 1 or 2 methods from this module are called from the custom

resource itself, it is highly recommended you include it in a Helper class for

clarity, ala:



```

action_class do

  class ThingHelper

    include FB::Thing::ThingProvider

  end

end

```



In this way, it is clear where methods come from.



### Extending the node vs self-contained classes



You may have noticed that some of our cookbooks will extend the `node` object,

while others have self-contained classes that sometimes require the `node` be

passed as a parameter to some methods.



In general, the **only** time when extending the `node` is acceptable is when

you are simply making a convenience function around using the node object. So,

for example, instead of making people do `node['platform_family'] = 'debian'`,

there's a `node.debian?`. This is simply syntactic sugar on top of data

entirely in the node.



In all other cases, one should simply have the `node` be an argument passed on,

so as to not pollute the node namespace. For example, a method that looks at

the node attributes, but also does a variety of other logic, should be in a

cookbook class and take the node as an argument (per standard programming

paradigms about clear dependencies).



### Methods in recipes



Sometimes it is convenient to put a method directly in a recipe. It is strongly

preferred to put these methods in the cookbook class, however there are some

cases where methods directly in recipes make sense. The primary example is a

small method which creates a resource based on some input to make a set of

loops more readable.



### Methods in templates



Methods should not be put into templates. In general, as little logic as

possible should be in templates. In general the easiest way to do this is to

put the complex logic into methods in your cookbook class and call them from

the templates.



### Err on the side of fail



Chef is an ordered system and thus is designed to fail a run if a resource

cannot be converged. The reason for this is that if one step in an ordered

list cannot be completed, it's likely not safe to do at least some of the

following steps. For example, if you were not able to write the correct

configuration for a service, then starting it may open up a security

vulnerability.



Likewise, the Facebook cookbooks will err on the side of failing if something

seems wrong. This is both in line with the Chef philosophy we just outlined,

but also because this model assumes that code is being tested on real systems

before being released using something like [taste-tester](https://github.com/facebook/taste-tester/) and that monitoring is

in place to know if your machines are successfully running Chef.



Here are some examples of this philosophy in practice:



* If a cookbook is included on a platform it does not support, we `fail`. It

  might seem like `return`ing in this case is reasonable but there is a good

  indication the run-list isn't as-expected, so it's a great idea to bail out

  before this machine is mis-configured.

* If a configuration was passed in that we don't support, rather than ignore it

  we `fail`.



### Validation of inputs and `whyrun_safe_ruby_blocks`



Many cookbooks rely on the service underneath and the testing of the user to be

the primary validator of inputs. Is the software we just configured, behaving

as expected?



However, sometimes it's useful to do our own validation because there are

certain configurations we don't want to support, because the software may

accepted dangerous configurations we want to catch, or because the user could

pass us a combination of configurations that is conflicting or impossible to

implement.



In this model, however, this must be done at runtime. If your implementation is

done primarily inside of an internally-called resource, then this validation

can also be done there. However, if your implementation is primarily a recipe

and templates, doing the validation in templates is obviously not desirable.

This is where `whyrun_safe_ruby_blocks` come in.



Using an ordinary `ruby_block` would suffice to have ruby code run at runtime

to validate the attributes, however that means that the error would not be

caught in whyrun mode. Since this validation does not change the system, it is

safe to execute in whyrun mode, and that's why we use `whyrun_safe_ruby_block`s:

they are run in whyrun mode.



It is worth noting that this is also where you can take input that perhaps was

in a structure convenient for users and build out a different data structure

that's more convenient to use in your template.



### Implementing runtime-safe APIs



This model intentionally draws the complexity of Chef into the "core cookbooks"

(those implementing APIs) so that the user experience of maintaining systems is

simple and (usually) requires little more than writing to the node object.

However, the trade-off for that simplicity is that implementing the API

properly can be quite tricky.



How to do this is a large enough topic that it gets [its own document](https://github.com/facebook/chef-utils/blob/main/Compile-Time-Run-Time.md).

However, some style guidance is also useful. This section assumes you have read

the aforementioned document.



The three main ways that runtime-safety is achieved are `lazy`, `templates`, and

`custom resources`. When should you use which?



The template case is fairly straight forward - if you have a template, read the

node object from the within the template source instead of using `variables`

on the template resource, and all data read is inherently runtime safe since

templates run at runtime.



But what about `lazy` vs `custom resources`? For example, in a recipe you might

do:



```ruby

package 'thingy packages' do

  package_name lazy {

    pkgs = 'thingy'

    if node['fb_thingy']['want_devel']

      pkgs << 'thingy-devel'

    end

    pkgs

  }

  action :upgrade

end

```



Where as inside of a custom resource you could instead do:



```ruby

pkgs = 'thingy'

if node['fb_thingy']['want_devel']

  pkgs << 'thingy-devel'

end



package pkgs do

  action :upgrade

end

```



Which one is better? There's not an exact answer, both work, so it's a style

consideration. In general, there are two times when we suggest a custom

resource:



The first is when you need to loop over the node in order to even know what

resources to create. Since this isn't possible to (well, technically it's

possible with some ugliness, but by and large not using the standard DSL), this

must go into a custom resource. Example might be:



```ruby

# This MUST be inside of a custom resource!

node['fb_thingy']['instances'].each do |name, config|

  template "/etc/thingy/#{instance}.conf" do

    owner 'root'

    group 'root'

    mode '644'

    variables({:config => config})

  end

end

```



The second is when you're using `lazy` on the majority of the resources in

your recipe. If your recipe has 15 resources and you've had to pepper all of

them with `lazy`, it's a bit cleaner to make a custom resource that you call

in your recipe.



It's important here to reiterate: we're **not** referring to using a Custom

Resource as an API, but simply making an internal custom resource, called

only by your own recipe, as a way to simplify runtime safety.



Outside of these two cases, you should default to implementations inside of

recipes. This is for a few reasons.



The first reason is that dropping entire implementations in custom resources

leads to confusion and sets a bad precedent for how runtime-safety works. For

example, consider the custom resource code we saw earlier where you assemble

the package list in "naked" ruby:



```ruby

pkgs = 'thingy'

if node['fb_thingy']['want_devel']

  pkgs << 'thingy-devel'

end

```



This code works fine in a resource, but serves as a bad reference for others -

since this absolutely won't work in a recipe (even though it'll run).



The second reason is that quite often implementations need both compile-time

and runtime code, and by blindly dropping the implementation into a custom

resource, you can often miss this and create bugs like this:



```ruby

# only safe because we're in a custom resource

packages = FB::Thingy.determine_packages(node)



package packages do

  action :upgrade

end



if node['fb_thingy']['want_cron']

  node.default['fb_cron']['jobs']['thingy_runner'] = {

    'time' => '* * * * *',

    'command' => '/usr/bin/thingy --quiet',

  }

end



service 'thingy' do

  action [:enable, :start]

end

```



Note here that while this code all seems reasonable in a custom resource (`if`

statements are runtime safe when inside of a custom resource), that cronjob

will never get picked up, because you're using an API at runtime, but APIs must

be called at compile time and consumed at runtime. In reality, this needs to be

in the recipe in order to work, and should look like this, in a *recipe*:



```ruby

package 'thingy packages' do

  package_name lazy { FB::Thingy.determine_packages(node) }

  action :upgrade

end



node.default['fb_cron']['jobs']['thingy_runner'] = {

  'only_if' => proc { node['fb_thingy']['want_cron'] },

  'time' => '* * * * *',

  'command' => '/usr/bin/thingy --quiet',

}



service 'thingy' do

  action [:enable, :start]

end

```



In general, always start your implementation as a recipe and then escalate to

Custom Resources where necessary.



## Debugging kitchen runs



You can set up kitchen using the same commands as in `.github/workflows/ci.yml`,

but once Chef runs you won't have access to connect, so modify

`fb_sudo/attributes/default.rb` and uncomment the kitchen block.



Then you can do `bundle exec kitchen login ` after a failed

run, and sudo will be passwordless so you can debug.



## License



See the LICENSE file in this directory