Configuration files specification

All Services applications need to use the same configuration file format. This document specifies it.

Syntax

The configuration file is a ini-based file. (See http://en.wikipedia.org/wiki/INI_file for more details.) Variable names can be assigned values, and grouped into sections.

A line that starts with “#” is commented out. Empty lines are also removed.

Example:

[section1]
# comment
name = value
name2 = "other value"

[section2]
foo = bar

Ini readers in Python, PHP and other languages understand this syntax. Although, there are subtle differences in the way they interpret values and in particular if/how they convert them.

Values conversion

Here are a set of rules for converting values:

  • If value is quoted with ” chars, it’s a string. This notation is useful to include “=” characters in the value. In case the value contains a ” character, it must be escaped with a “” character.
  • When the value is composed of digits and optionally prefixed by “-”, it’s tentatively converted to an integer or a long depending on the language. If the number exceeds the range available in the language, it’s left as a string.
  • If the value is “true” or “false”, it’s converted to a boolean, or 0 and 1 when the language does not have a boolean type.
  • A value can be an environment variable : “${VAR}” is replaced by the value of VAR if found in the environment. If the variable is not found, an error must be raised.
  • A value can contain multiple lines. When read, lines are converted into a sequence of values. Each new line for a multiple lines value must start with at least one space or tab character.

Examples:

[section1]
# comment
a_flag = True
a_number = 1
a_string = "other=value"
another_string = other value
a_list = one
        two
        three

user = ${USERNAME}

Extending a file

An INI file can extend another file. For this, a “DEFAULT” section must contain an “extends” variable that can point to one or several INI files which will be merged into the current file by adding new sections and values.

If the file pointed to in “extends” contains section/variable names that already exist in the original file, they will not override existing ones.

file_one.ini:

[section1]
name2 = "other value"

[section2]
foo = baz
bas = bar

file_two.ini:

[DEFAULT]
extends = file_one.ini

[section2]
foo = bar

Result:

[section1]
name2 = "other value"

[section2]
foo = bar
bas = bar

To point to several files, the multi-line notation can be used:

[DEFAULT]
extends = file_one.ini
          file_two.ini

When several files are provided, they are processed sequentially. So if the first one has a value that is also present in the second, the second one will be ignored. This means that the configuration goes from the most specialized to the most common.

Implementations

There’s one implementation in the core package of the Python server, but it could be moved to a standalone distribution if another project wants to use it.

http://bitbucket.org/tarek/sync-core/src/tip/services/config.py