MC-Settings — An Application Settings Manager

Table of Contents

1. Description
- 1.1. Background
- 1.2. Example
2. Usage
3. Loading Settings
- 3.1. Nested Hashes and Default Values
4. Capistrano Recommendations
5. Copyright
6. License

1. Description

This Ruby gem provides a an easy way to manage application configuration across multiple environments, such as development, QA, staging, production, etc. It should be compatible with Capistrano deployments.

1.1. Background

Applications typically rely on configuration settings, such as host names, URLs, usernames and many more. Some values change between deployment environemnts, some do not.

The library assumes that application configuration is represented by a Hash of arbitrary depth, and provides a convenient and a compact syntax to access the settings through a singleton instance inside Setting class.

Configuration is stored in one or more YAML files with the top-level Hash, having the keys as names of individual settings.

1.2. Example

For example, consider the following sample application configuration file:

 tax:
   default: 0.0
   california: 7.5
 states:
   default:
     - 'CA'
     - 'WA'
     - 'NY'
   ship_to:
     - 'CA'
     - 'NY'
 math_pi: 3.14159526

Setting Gem provides Setting.load(..) method to load configuration from files in a way that allows some configuration files to override previously loaded values, and then offers a simple method API to access the values, for example Setting.tax(:california) or Setting.tax. Supporting default values in 2nd, 3rd, .. - level hashes is one of the advantages of using this gem.

By loading configuration from YAML files, Setting gem is inherently compatible with Capistrano deployment methodology, where a certain set of files may become "activated" by simply sym-linking them into the appropriate settings folder.

Note: using example above, "1st level" hash is the one with keys tax, states and math_pi. The 2nd-level hash is, for example, the tax definition one, with keys default and california.

2. Usage

Once configuration is initialized using Setting#load or Setting#reload methods (see below), they can be used in code in the following way:

Setting.key_name is optimized to return default value if available instead of a Hash.
Setting.key_name(:sub_key_name) returns a value from the 2nd level hash.
Setting.key_name(:sub_key_name, :sub_sub_key_name) returns value from the 3rd level hash if available.

The algorithm is recursive, so only the maximum method stack depth will limit the number of nested hash values you can access this way.
Special syntax Setting[:key_name], Setting[:key_name][:sub_key_name], etc also supported. This method, however, does not support default values (see below).

Note	Method notation is recommended over square bracket accessing single values.

However, square bracket notation may be useful when you want to fetch the entire 2nd-level hash that includes the default value.

For example, given the above YAML file, you can access the settings in your code as follows:

Setting.tax => 0.0
Setting.tax(:california) => 7.5
Setting.math_pi => 3.14159526
Setting[:math_pi] => 3.14159526
Setting.states => [ 'CA', 'WA', 'NY' ]
Setting.states['ship_to'] => [ 'CA', 'NY' ]

Method-calling notation allows passing an array of keys to fetch a value from a nested hash. This method also supports returning a default value, stored against the "default" key.

  Setting.tax => 0.0

Square bracket syntax returns the actual nested hash, without any regard for the default value:

Setting[:tax] => { 'default' => 0.0, 'california' => 7.5 }

3. Loading Settings

The gem should be initialized in your environment.rb (if using Rails), or in any other application initialization block. Setting.load() method is provided for loading settings, and it can be called only once in application life cycle, or it will throw an exception.

If you need to reload settings, use the reload() method with similar arguments.

Consider an example:

Setting.load(
  path:  "#{Rails.root}/config/settings",
  files: ["default.yml", "environments/#{Rails.env}.yml"],
  local: true
)

The argument is an options hash that configures which YAML files to load, in what order, and from where.

path specifies the "root" folder where settings files will be loaded from
files is an array that lists file names relative to the path.
local can be optionally specified as a true value, and if the specified Setting gem will load all *.yml files that live under the :path/local folder.

Below is list of YAML files loaded in order specified in the above example, assuming that "development" is the Rails environment, and "local" folder exists with 3 additional YAML files in it:

config/settings/default.yml
config/settings/environments/development.yml
config/settings/local/authorize-net.yml
config/settings/local/paypal.yml
config/settings/local/other.yml

Each YML file defines a ruby Hash. During file loading, the hashes are merged, so that values loaded in early files may be overwritten by values in subsequent files. This is deliberate and by design: it allows you to create small "override" files for each environment, or even each machine you want to deploy to. Exactly how you split your application settings in files is up to you.

3.1. Nested Hashes and Default Values

MC Setting gem provides a convenient way to access nested values, including full support for the default values within nested hashes (as of 0.1.1).

Consider the following nested hash example:

default.yml

services:
  inventory:
    url: http://ims.mycompany.com:3443/inventory_manager
    name: Inventory Management
  shipping:
    url: http://ship.mycompany.com:3443/shipper
    name: Shipping

Setting.load(:files => ['default.yml'], :path => ...)

Setting.services(:inventory)
# => {
#   :url => "http://localhost:3443/inventory_manager",
#   :name => "Inventory Management"
# }

Setting.services(:inventory, :url)
# => "http://localhost:3443/inventory_manager"

staging.yml

We are changing URLs for services in staging.yml, so they work in the staging environment. Service URLs have been updated to use localhost:

services:
  inventory:
    url: http://localhost:8009/inventory_manager
  shipping:
    url: http://localhost:8008/shipper

Setting.load(:files => ['default.yml', 'staging.yml'], :path => ...)

Setting.services(:inventory)
# => {
#   :url => "http://localhost:8009/inventory_manager",
#   :name => "Inventory Management"
# }

Setting.services(:inventory, :url)
# => "http://localhost:8008/inventory_manager"

4. Capistrano Recommendations

Assume the directory structure of your Rails application is as follows:

config/settings/default.yml
config/settings/environments/development.yml
config/settings/environments/staging.yml
config/settings/environments/production.yml
config/settings/local
config/settings/systems/reporting.yml
config/settings/systems/admin.yml

Note that the "local" directory is empty, and that the "systems" directory contains several YAML files which provide alternative configuration for a reporting server and the admin server, both of which run in the "production" rails environment.

When deploying to the main production site, neither YAML files inside "systems" folder are activated or used.

But upon deployment to the admin server, Capistrano could symlink "admin.yml" from config/settings/local folder, so the Setting gem would load these values. So for each Capistrano role, you can define which files need to be symlinked into local, thus creating a flexible configuration scheme which can be easily managed by Capistrano.

5. Copyright

Authors: 2010-2020 Edwin Cruz, Colin Shield & Konstantin Gredeskoul

6. License

This software is distributed under the MIT License.

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