The CFG API for Ruby

The CFG reference implementation for the Ruby language assumes a Ruby version of 2.7 or later.

Installation

You can install the gem using gem install cfg-config.

There’s a minimal example of a program that uses CFG here.

Exploration

To explore CFG functionality for Ruby, we the irb program, which is a Read-Eval-Print-Loop (REPL). You can invoke a shell using

$ irb

Getting Started with CFG in Ruby

A configuration is represented by an instance of the Config class. The constructor for this class can be passed a filename or a stream which contains the text for the configuration. The text is read in, parsed and converted to an object that you can then query. A simple example:

test0.cfg

a: 'Hello, '
b: 'world!'
c: {
  d: 'e'
}
'f.g': 'h'
christmas_morning: `2019-12-25 08:39:49`
now: `date:::DateTime.now`
home: `$HOME`
foo: `$FOO|bar`

Loading a configuration

The configuration above can be loaded as shown below. In the REPL shell:

2.7.1 :001 > require 'CFG/config'
 => true
2.7.1 :002 > include CFG
 => Object
2.7.1 :003 > cfg = CFG::Config::new("test0.cfg")

Access elements with keys

Accessing elements of the configuration with a simple key is just like using a Hash:

2.7.1 :004 > cfg['a']
 => "Hello, "
2.7.1 :005 > cfg['b']
 => "world!"

Access elements with paths

As well as simple keys, elements can also be accessed using path strings:

2.7.1 :006 > cfg['c.d']
 => "e"

Here, the desired value is obtained in a single step, by (under the hood) walking the path c.d – first getting the mapping at key c, and then the value at d in the resulting mapping.

Note that you can have simple keys which look like paths:

2.7.1 :007 > cfg['f.g']
 => "h"

If a key is given that exists in the configuration, it is used as such, and if it is not present in the configuration, an attempt is made to interpret it as a path. Thus, f.g is present and accessed via key, whereas c.d is not an existing key, so is interpreted as a path.

Access to date/time objects

You can also get native Ruby date/time objects from a configuration, by using an ISO date/time pattern in a backtick-string:

2.7.1 :008 > cfg['christmas_morning']
 => #<DateTime: 2019-12-25T08:39:49+00:00 ((2458843j,31189s,0n),+0s,2299161j)>

Access to other Ruby objects

Access to other Ruby objects is also possible using the backtick-string syntax, provided that they are one of:

• Environment variables
• Public fields of public classes
• Public static methods without parameters of public classes

2.7.1 :009 > require 'date'
 => false
2.7.1 :010 > DateTime::now - cfg['now']
 => (-148657/86400000000000)

Accessing the “now” element of the above configuration invokes the DateTime::now method and returns its value. You can see from the subtraction that the times are practically equivalent (the difference is the time between the two calls to DateTime::now - once when computing the configured value, and once in the interactive session.

Access to environment variables

To access an environment variable, use a backtick-string of the form `$VARNAME`:

2.7.1 :011 > cfg['home'] == ENV['HOME']
 => true

You can specify a default value to be used if an environment variable isn’t present using the `$VARNAME|default-value` form. Whatever string follows the pipe character (including the empty string) is returned if VARNAME is not a variable in the environment.

2.7.1 :012 > cfg['foo']
 => "bar"

Access to computed values

Sometimes, it’s useful to have values computed declaratively in the configuration, rather than imperatively in the code that processes the configuration. For example, an overall time period may be specified and other configuration values are fractions thereof. It may also be desirable to perform other simple calculations declaratively, e.g. concatenation of numerous file names to a base directory to get a final pathname.

test0a.cfg

total_period : 100
header_time: 0.3 * ${total_period}
steady_time: 0.5 * ${total_period}
trailer_time: 0.2 * ${total_period}
base_prefix: '/my/app/'
log_file: ${base_prefix} + 'test.log'

When this file is read in, the computed values can be accessed directly:

2.7.1 :013 > cfg = CFG::Config::new("test0a.cfg")
2.7.1 :014 > cfg['header_time']
 => 30.0
2.7.1 :015 > cfg['steady_time']
 => 50.0
2.7.1 :016 > cfg['trailer_time']
 => 20.0
2.7.1 :017 > cfg['log_file']
 => "/my/app/test.log"

Including one configuration inside another

There are times when it’s useful to include one configuration inside another. For example, consider the following configuration files:

logging.cfg

layouts: {
  brief: {
    pattern: '%d [%t] %p %c - %m%n'
  }
},
appenders: {
  file: {
    level: 'INFO',
    layout: 'brief',
    filename: 'run/server.log',
    append: true,
    charset: 'UTF-8'
  },
  error: {
    level: 'ERROR',
    layout: 'brief',
    filename: 'run/server-errors.log',
    append: false,
    charset: 'UTF-8'
  },
  debug: {
    level: 'DEBUG',
    layout: 'brief',
    filename: 'run/server-debug.log',
    append: false,
    charset: 'UTF-8'
  }
},
loggers: {
  mylib: {
    level: 'INFO'
  }
  'mylib.detail': {
    level: 'DEBUG'
  }
},
root: {
  handlers: ['file', 'error', 'debug'],
  level: 'WARNING'
}

redirects.cfg

cookies: {
  url: 'http://www.allaboutcookies.org/',
  permanent: false
},
freeotp: {
  url: 'https://freeotp.github.io/',
  permanent: false
},
'google-auth': {
  url: 'https://play.google.com/store/apps/details?id=com.google.android.apps.authenticator2',
  permanent: false
}

main.cfg

secret: 'some random application secret',
port: 8000,
sitename: 'My Test Site',
default_access: 'public',
ignore_trailing_slashes: true,
site_options: {
  want_ipinfo: false,
  show_form: true,
  cookie_bar: true
},
debug: true,
captcha_length: 4,
captcha_timeout: 5,
session_timeout: 7 * 24 * 60 * 60,  # 7 days in seconds
redirects: @'redirects.cfg',
email: {
  sender: 'no-reply@my-domain.com',
  host: 'smtp.my-domain.com:587',
  user: 'smtp-user',
  password: 'smtp-pwd'
}
logging: @'logging.cfg'

The main.cfg contents have been kept to the highest-level values, within logging and redirection configuration relegated to other files logging.cfg and redirects.cfg which are then included in main.cfg. This allows the high-level configuration to be more readable at a glance, and even allows the separate configuration files to be e.g. maintained by different people.

The contents of the sub-configurations are easily accessible from the main configuration just as if they had been defined in the same file:

2.7.1 :018 > cfg = CFG::Config::new('main.cfg')
2.7.1 :019 > cfg['logging.layouts.brief.pattern']
 => "%d [%t] %p %c - %m%n"
2.7.1 :020 > cfg['redirects.freeotp.url']
 => "https://freeotp.github.io/"
2.7.1 :021 > cfg['redirects.freeotp.permanent']
 => false

Avoiding unnecessary repetition

Don’t Repeat Yourself (DRY) is a useful principle to follow. CFG can help with this. You may have noticed that the logging.cfg file above has some repetitive elements:

logging.cfg (partial)

appenders: {
  file: {
    level: 'INFO',
    layout: 'brief',
    filename: 'run/server.log',
    append: true,
    charset: 'UTF-8'
  },
  error: {
    level: 'ERROR',
    layout: 'brief',
    filename: 'run/server-errors.log',
    append: false,
    charset: 'UTF-8'
  },
  debug: {
    level: 'DEBUG',
    layout: 'brief',
    filename: 'run/server-debug.log',
    append: false,
    charset: 'UTF-8'
  }
},

This portion could be rewritten as:

logging.cfg (partial)

defs: {
  base_appender: {
    layout: 'brief',
    append: false,
    charset: 'UTF-8'
  }
},
appenders: {
  file: ${defs.base_appender} + {
    level: 'INFO',
    filename: 'run/server.log',
    append: true,
  },
  error: ${defs.base_appender} + {
    level: 'ERROR',
    filename: 'run/server-errors.log',
  },
  debug: ${defs.base_appender} + {
    level: 'DEBUG',
    filename: 'run/server-debug.log',
  }
},

where the common elements are separated out and just referenced where they are needed. We find it useful to put all things which will be reused like this in one place in the condiguration, so we always know where to go to make changes. The key used is conventionally defs or base, though it can be anything you like.

Access is just as before, and provides the same results:

2.7.1 :022 > cfg['logging.appenders.file.append']
 => true
2.7.1 :023 > cfg['logging.appenders.file.filename']
 => "run/server.log"
2.7.1 :024 > cfg['logging.appenders.file.level']
 => "INFO"
2.7.1 :02 > cfg['logging.appenders.debug.level']
 => "DEBUG"
2.7.1 :026 > cfg['logging.appenders.debug.filename']
 => "run/server-debug.log"
2.7.1 :027 > cfg['logging.appenders.debug.append']
 => false

The definition of logging.appenders.file as ${defs.base_appender} + { level: 'INFO', filename: 'run/server.log', append: true } has resulted in an evaluation which first fetches the defs.base_appender value, which is a mapping, and “adds” to that the literal mapping which defines the level, filename and append keys. The + operation for mappings is implemented as a copy of the left-hand side merged with the right-hand side. Note that the append value for logging.appenders.file is overridden by the right-hand side to true, whereas that for e.g. logging.appenders.error is unchanged as false.

We could do some further refinement by factoring out the common location for the log files:

logging.cfg (partial)

defs: {
  base_appender: {
    layout: 'brief',
    append: false,
    charset: 'UTF-8'
  }
  log_prefix: 'run/',
},
appenders: {
  file: ${defs.base_appender} + {
    level: 'INFO',
    filename: ${defs.log_prefix} + 'server.log',
    append: true,
  },
  error: ${defs.base_appender} + {
    level: 'ERROR',
    filename: ${defs.log_prefix} + 'server-errors.log',
  },
  debug: ${defs.base_appender} + {
    level: 'DEBUG',
    filename: ${defs.log_prefix} + 'server-debug.log',
  }
}

with the same result as before. It is slightly more verbose than before, but the location of all files can be changed in just one place now, as opposed to three, as it was before.

Classes

The Config class

This class implements a CFG configuration. You’ll generally interface to CFG files using this class. When you pass in a stream or file path to the constructor or the Config.load / Config.load_file methods, the CFG source in the stream or file is parsed and converted into an internal form that can be queried.

class Config

Variables

var no_duplicates = true

Whether duplicates keys are allowed when parsing a mapping or mapping body. If not allowed and duplicates are seen, a ConfigError is thrown.

var strict_conversions = true

If true, a ConfigError is raised if a string can’t be converted. It’s intended to help catch typos in backtick-strings.

var context

A mapping within which to look up identifiers during evaluation of AST nodes.

var cached = false

If true, an internal cache is used.

var include_path

A list of directories which is searched for included sub-configurations if the parent directory of path (or the current directory, if path isn’t set) doesn’t contain them.

var path

The path to the file from which this instance’s configuration has been loaded.

Constructors

def Config()

Constructs an instance with no actual configuration loaded. A call to load or load_file would be needed to actually make a usable instance.

def Config(stream)

Construct this instance with the configuration read from the provided stream.

def Config(path)

Construct this instance with the configuration read from a file named by the provided path.

Methods

def load(stream)

Load this instance with the configuration read from the provided stream.

Parameters:

• stream – A stream for the configuration source.

Raises:

ConfigError – If there is an I/O error when reading the source or an error in the configuration syntax.

def load_file(path)

Load this instance with the configuration read from the file at the provided path.

Parameters:

• path – A String specifying the location of the file which contains the configuration.

Raises:

ConfigError – If there is an I/O error when reading the source or an error in the configuration syntax.

def get(key)

This method implements the key access operator, e.g. mapping[key]. The value of key can be a simple key (identifier) or a valid path string.

Parameters:

• key – A string describing a simple key or a path in the configuration.

Raises:

• InvalidPathError – If key is not present in the data and it cannot be parsed as a path.
• BadIndexError – If, while traversing a path, a key or index is not of the appropriate type for its container, or if it isn’t in the required range.
• ConfigError – If a key is not found or some other semantic error occurs (for example, an operation involving incompatible types).

def get(key, default_value)

This works similarly to the method above, except that if a key isn’t found, the default_value is returned.

Parameters:

• key – A string describing a simple key or a path in the configuration.
• default_value – A value to return if the key or path is not available in a configuration.

Raises:

• InvalidPathError – If key is not present in the data and it cannot be parsed as a path.
• BadIndexError – If, while traversing a path, a key or index is not of the appropriate type for its container, or if it isn’t in the required range.
• ConfigError – If some other semantic error occurs (for example, an operation involving incompatible types).