Levels

Levels is a tool for merging configuration data. A level is a set of key/value pairs that represent your data. Multiple levels, written in a variety of formats can be merged in a predictable, useful way to form a final configuration.

KRAMER: I'm completely changing the configuration of the apartment. You're not gonna believe it when you see it. A whole new lifestyle.

JERRY: What are you doing?

KRAMER: Levels.

Creating a level

A level is made up of one or more groups. A group is a set of key/value pairs. To describe a very simple web application made up of a server and a task queue, you could write this (in JSON).

{
  "server": {
    "hostname": "example.com"
  },
  "task_queue": {
    "workers": 5,
    "queues": ["high", "low"]
  }
}

Now consider having a common "base" configuration, with slight differences in development and production. Our base configuration defines the possible keys, with default values.

A "production" level can override the relevant values like this.

{
  "server": {
    "hostname": "example.com"
  },
  "task_queue": {
    "workers": 5
  }
}

The system's environment may be used as a level. To alter any value at runtime, follow a convention to set the appropriate environment variable.

TASK_QUEUE_WORKERS="10"

Writing a level

A level may be written in one of many formats.

• RUBY is the most common and powerful for hand written configs.
• JSON is convenient for machine generated configs.
• YAML is good for both hand written and machine generated configs.
• Environment Variables are useful for local or runtime configuration. This syntax may not be used for the "base" level.

Data Types

Levels has a limited understanding of data types by design. The guiding principles are:

• It must be possible to represent any value in an environment variable.
• Use only types that are native in JSON.

Therefore, Levels only supports the following types:

• string (Ruby String)
• integer (Ruby Fixnum)
• float (Ruby Float)
• boolean (Ruby TrueClass or FalseClass)
• array (Ruby Array) of values, which are also typed.
• null (Ruby NilClass)

Notice that JSON's Object is not supported. This is because groups are objects, so key/values pairs are already available. It's difficult to represent key/value pairs in an environment variable, so it fails that test as well.

Fortunately, these simple types are perfectly adequate for the purposes of system configuration.

Ruby Syntax

The Ruby DSL is a clean, simple format. It aims to be readable, writable and editable. It looks like this:

group :server
  set hostname: "example.com"

group :task_queue
  set workers: 5
  set queues: ["high", "low"]

The Ruby syntax supports computed values.

group :task_queue
  set queues: -> { [server.hostname, "high", "low"] }

Extending the Ruby Runtime

To extend the runtime environment, add methods to Levels::Runtime. Those methods can return a value directly, or return a Proc for lazy evaluation.

module Levels::Runtime
  # This helper decrypts a value using the merged value of
  # `secret_keys.sha_key`.
  def encrypted(encrypted_value)
    -> { SHA.decrypt(encrypted_value, secret_keys.sha_key) }
  end
end

With this runtime helper, you can now write:

group :aws
  set secret_key: encrypted("your encrypted aws secret key")

Builtin runtime extensions

These functions are provided by the default Levels Runtime.

• file(path) reads the value from a file. The file path is interpreted as relative to the Ruby file unless it begins with '/'. File storage can be useful when configuring large strings such as SSL keys.

JSON Syntax

JSON syntax is straightforward. Because the datatypes supported by Levels are the same as supported by JSON, there's nothing else you need to know.

{
  "server": {
    "hostname": "example.com"
  },
  "task_queue": {
    "workers": 5,
    "queues": ["high", "low"]
  }
}

YAML Syntax

YAML syntax is also exactly as you would expect.

---
server:
  hostname: example.com
task_queue:
  workers: 5
  queues:
  - high
  - low

Environment Variables syntax

The environment variables syntax has rules for defining keys and values.

The format of each key is [PREFIX]<GROUP>_<KEY>.

• PREFIX is an optional prefix for all keys.
• GROUP is the name of the group in all caps.
• KEY is the name of the key in all caps.
• GROUP and KEY are separated by an underscore (_).

The example looks like this (without a prefix).

SERVER_HOSTNAME="example.com"
TASK_QUEUE_WORKERS="5"
TASK_QUEUE_QUEUES="high:low"

Typecasting

You'll notice that TASK_QUEUE_WORKERS should be an integer, and TASK_QUEUE_QUEUES should be an array. Levels will typecast each value based on the key's type in the "base" level. Or, you may define each value's type explicitly.

To set the type of a value, set <GROUP>_<KEY>_TYPE to one of the following:

• string - The value is taken as is.
• integer - The value is converted to an integer via Ruby's to_i.
• float - The value is converted to a float via Ruby's to_f.
• boolean - The value is true if it's "true" or "1", else false.
• array - The value is split using colon (:) or <GROUP>_<KEY>_DELIMITER. The values of the resulting array may be typecast using <GROUP>_<KEY>_TYPE_TYPE.

Any value may be set to Ruby's nil (NULL) by setting it to an empty string.

Some examples:

SAMPLE_MY_NULL=""

SAMPLE_MY_INT="123"
SAMPLE_MY_INT_TYPE="integer"

SAMPLE_MY_BOOL="true"
SAMPLE_MY_BOOL_TYPE="boolean"

SAMPLE_MY_STRING_ARRAY="a:b:c"
SAMPLE_MY_STRING_ARRAY_TYPE="array"

SAMPLE_MY_INT_ARRAY="1:2:3"
SAMPLE_MY_INT_ARRAY_TYPE="array"
SAMPLE_MY_INT_ARRAY_TYPE_TYPE="integer"

SAMPLE_MY_CSV_ARRAY="one,two,three"
SAMPLE_MY_CSV_ARRAY_TYPE="array"
SAMPLE_MY_CSV_ARRAY_DELIMITER=","

Using a Configuration

Once a level has been written, you can read and merge it. Once merged into a Configuration, you can use it at runtime in a Ruby process, or output it as JSON, YAML or environment variables.

Any number of levels, including the system environment, may be merged. The system environment is typically merged last, but it's not required.

From the command line, Levels can generate JSON, YAML or environment variables. The generated configuration is written to STDOUT. Both JSON and Environment Variables look exactly like the input formats above.

levels \
  --output json \
  --level "Base" \
  --level "Prod" \
  --system \
  base.rb \
  prod.json

Within a Ruby program, a Levels::Configuration is an object. You can build one with Levels.merge.

# Merge multiple input levels from various sources - file, API and
# environment variables.
config = Levels.merge do |levels|
  levels.add "Base", HTTP.get("https://server/config.json")
  levels.add "Prod", "prod.json"
  levels.add_system
end

The resulting config object works like this.

# Dot syntax.
config.server.hostname        # => "example.com"
config.task_queue.workers     # => 5
config.task_queue.queues      # => ["high", "low"]

# Hash syntax.
config[:server][:hostname]    # => "example.com"
config[:task_queue][:workers] # => 5
config[:task_queue][:queues]  # => ["high", "low"]

An attempt to read an unknown group or key will throw an exception.

config.some_group        # raises Levels::UnknownGroup
config.server.some_value # raises Levels::UnknownKey

You can find out if a group or key exists.

config.defined?(:other)           # => false
config.defined?(:server)          # => true
config.server.defined?(:other)    # => false
config.server.defined?(:hostname) # => true

Author

Ryan Carver / @rcarver

Copyright (c) Ryan Carver 2012. Made available under the MIT license.