Config sourcer implementations
The github.com/go-nacelle/config package provides the following sourcer implementations.
Environment sourcer
An environment sourcer reads the env tag and looks up the corresponding value in the process’s environment. An expected prefix may be supplied in order to namespace application configuration from the rest of the system. A sourcer instantiated with NewEnvSourcer("APP") will load the env tag fetch_limit from the environment variable APP_FETCH_LIMIT and falling back to the environment variable FETCH_LIMIT.
Test environment sourcer
A test environment sourcer reads the env tag but looks up the corresponding value from a literal map. This sourcer can be used in unit tests where the full construction of a nacelle process is too burdensome.
Flag sourcer
A flag sourcer reads the flag tag and looks up the corresponding value attached to the process’s command line arguments.
File sourcer
A file sourcer reads the file tag and returns the value at the given path. A filename and a file parser musts be supplied on instantiation. Both ParseYAML and ParseTOML are supplied file parsers – note that as JSON is a subset of YAML, ParseYAML will also correctly parse JSON files. If a nil file parser is supplied, one is chosen by the filename extension. A file sourcer will load the file tag api.timeout from the given file by parsing it into a map of values and recursively walking the (keys separated by dots). This can return a primitive type or a structured map, as long as the target field has a compatible type. The constructor NewOptionalFileSourcer will return a no-op sourcer if the filename does not exist.
Multi sourcer
A multi-sourcer is a sourcer wrapping one or more other sourcers. For each configuration struct field, each sourcer is queried in reverse order of registration and the first value to exist is returned. This is useful to allow a chain of configuration files in which some files or directories take precedence over others, or to allow environment variables to take precedence over files.
Directory sourcer
A directory sourcer creates a multi-sourcer by reading each file in a directory in alphabetical order. The constructor NewOptionalDirectorySourcer will return a no-op sourcer if the directory does not exist.
Glob sourcer
A glob sourcer creates a multi-sourcer by reading each file that matches a given glob pattern. Each matching file creates a distinct file sourcer and does so in alphabetical order.