Plugin Dependencies

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Link to code: Adding dependencies to your plugin

Your control plane agent will typically consist of one or more plugins that contain the application logic. In addition there could be a suite of Ligato plugins and components providing various services to your application plugins. Examples include a KV data store, message bus adapters, loggers and health monitors

This tutorial shows how to add dependencies to your plugins.

Requirements:

• Complete the ‘Hello World Agent’ tutorial

The Ligato infrastructure uses the dependency injection design pattern to manage dependencies. In other words, dependencies on other plugins are injected into your plugin when it is initialized. You should use dependency injection to manage all dependencies in your plugin.

One of the most commonly used dependencies in your plugins will likely be PluginDeps defined in the github.com/ligato/cn-infra/infra package. It is a struct that aggregates three plugin essentials:

• plugin name
• logging
• plugin configuration.

It is defined as:

type PluginDeps struct {
    PluginName
    Log logging.PluginLogger
    Cfg config.PluginConfig
}

You embed it into your plugin as follows:

type HelloWorld struct {
    infra.PluginDeps
}

PluginName, which is embedded in the PluginDeps struct, provides the String() method for obtaining the name of the plugin. The plugin name is set using the SetName(name string) method defined for PluginName:

p.SetName("helloworld")

The two other components in PluginDeps are Log and Cfg. Log is the plugin’s logger used to log message at different log levels. Cfg is used to load configuration data from a configuration file in YAML format.

PluginDeps has the Setup() method, which initializes Log and Cfg with the name from PluginName. It is typically called in the plugin’s constructor:

func NewHelloWorld() *HelloWorld {
    p := new(HelloWorld)
    p.SetName("helloworld")
    p.Setup()
    return p
}

After initializing Log and Cfg, they are ready for action.

Let’s log a few messages:

func (p *HelloWorld) Init() error {
    p.Log.Info("System ready.")
    p.Log.Warn("Problems found!")
    p.Log.Error("Errors encountered!")
}

For more details on the Log API see infra/logging/log_api.go.

Now, let’s load configuration from a file. By default, the name of the config file will be derived from the plugin name with extension .conf. In our example, the configuration file name will be helloworld.conf.

type Config struct {
    MyValue int `json:"my-value"`
}

func (p *HelloWorld) Init() error {
    cfg := new(Config)
    found, err := p.Cfg.LoadValue(cfg)
    // ...
}

If the config file is not found, the LoadValue will return false. If the configuration cannot be parsed, the function will return an error.

The complete working example can be found at examples/tutorials/02_plugin-deps.