zubrin

Zubrin

Zubrin is a configuration loader for Golang.

Overview

Zubrin populates application configuration from a configurable source and injects the values into an (initially zero-valued) struct requestedd by the application.

Usage

We use the following configuration struct as an example.

type Config struct {
    A string        `env:"X"`
    B bool          `env:"Y" default:"true"`
    C string        `env:"Z" required:"true"`
    D []string      `env:"W" default:"[\"foo\", \"bar\", \"baz\"]"`
}

When pulling values from a variable source, a missing value (or empty string) will use the default value, if provided. If no value is set for a required configuration value, a fatal error will occur. String values will retrieve the variable value unaltered. All other field types will attempt to unmarshal the variable value as JSON.

Sources

When creating a Config object, you can supply a Sourcer interface which pulls values from a specific place (the environment, a file, the network, etc).

config := NewConfig(NewMultiSourcer(
    NewYAMLFileSourcer("config.yaml"), // lower priority
    NewEnvSourcer("APP"),              // higher priority
))

The following struct loads a variable X from the environment or loads the path a.b.c from a configuration file (this assumes the configuration file contains a nested dictionary structure with the path a.b.c).

type Config struct {
    X string `env:"x" file:"a.b.c"`
}

PostLoading Configuration Structs

After hydration, the PostLoad method will be invoked on all registered configuration structs (where such a method exists). This allows additional non-type validation to occur, and to create any types which are not directly/easily encodable as JSON.

func (c *Config) PostLoad() error {
    if c.Field != "foo" && c.Field != "bar" {
        return fmt.Errorf("field value must be foo or bar")
    }

    return nil
}

Embedded Configs

It is possible to embed anonymous configuration structs in order to get configuration reusability. Embedded config structs have the same set of struct tags.

type (
    BaseConfig struct {
        X string `env:"X"`
        Y string `env:"Y"`
        Z string `env:"Z"`
    }

    ProducerConfig struct {
        BaseConfig
        W string `env:"W"`
    }

    ConsumerConfig struct {
        BaseConfig
        Q string `env:"Q"`

    }
)

Config Tags

In some circumstances, it may be necessary to dynamically alter the tags on a configuration struct. This has become an issue in two circumstances so far. First, two instances of the same configuration struct may need to be registered but must be configured separately (i.e. they need to look at distinct environment variables). This is a particular problem when running two HTTP servers with the same base, for example. Second, the default value of a field may need to be altered. This issue can also arise when two instances of the same configuration struct are registered but shouldn't get clashing defaults (e.g. a default listening port).

Zubrin provides two tag modifiers which can be applied at configuration registration time. In the following, the configuration struct is loaded such that the environment variables used to hydrate the object are Q_X, Q_Y, Q_Z, Q_W, instead of X, Y, Z, and W the default value of the struct field B (loaded through the environment variable Q_Y) is false instead of true.

target := &Config{}

if err := config.Load(
    target,
    zubrin.NewEnvTagPrefixer("Q")
    zubrin.NewDefaultTagSetter("B", "false"),
); err != nil {
    // ...
}

// target is hydrated
// ...

If other dynamic modifications of a configuration struct is necessary, simply implement the TagModifier interface and use it in the same way.

License

Copyright (c) 2018 Eric Fritz

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.