Hello World#

---

Tutorial code: Hello World

In this tutorial, you will create a Ligato agent that contains a plugin called HelloWorld. This plugin prints “Hello World” to the log.

---

Let’s start with the plugin. Every plugin must implement the Plugin interface defined in the cn-infra/infra package:

type Plugin interface {
    // Init is called in the agent`s startup phase.
    Init() error
    // Close is called in the agent`s cleanup phase.
    Close() error
    // String returns unique name of the plugin.
    String() string
}

Implement the Plugin interface methods for the HelloWorld plugin:

type HelloWorld struct{}

func (p *HelloWorld) String() string {
    return "HelloWorld"
}

func (p *HelloWorld) Init() error {
    log.Println("Hello World!")
    return nil
}

func (p *HelloWorld) Close() error {
    log.Println("Goodbye World!")
    return nil
}

Note that the HelloWorld struct {} is empty. Your plugin does not have any data, so all you need is an empty struct that supports the Plugin interface.

---

Some plugins require additional initialization after the base system is up. If required, you can optionally define the AfterInit method for your plugin. This method will be executed after the Init method has been called for all plugins.

The AfterInit() method originates from the PostInit interface defined in the cn-infra/infra package:

type PostInit interface {
    // AfterInit is called once Init() of all plugins have returned without error.
    AfterInit() error
}

---

Next, create an instance of the HelloWorld plugin. Then, create a new agent and tell it about the HelloWorld plugin:

func main() {
        p := new(HelloWorld)    
        a := agent.NewAgent(agent.Plugins(p))
        // ...
}

You can use agent options to add the list of plugins to the agent at the agent’s creation time. The code block above demonstrates how the agent.Plugins() option adds an instance of the HelloWorld plugin to the agent.

agent.AllPlugins is another option to add the HelloWorld plugin instance to the agent. In addition, the agent.AllPlugins() option can add any dependencies that the HelloWorld plugin might have. Since your plugin has no dependencies, the simpler agent.Plugins() option will suffice.

---

Finally, start the agent using the agent’s Run() method. This method will initialize the agent’s plugins by calling their Init() and AfterInit() methods, and then wait for an interrupt from the user.

if err := a.Run(); err != nil {
    log.Fatalln(err)
}

When an interrupt, such as ctrl-c, arrives from the user, the Close() method will be called on the agent’s plugins, and the agent will exit.

---

Run the Hello World tutorial code

1. Open a terminal session.
   
   
2. Change to the 01_hello-world folder:

cn-infra git:(master) cd examples/tutorials/01_hello-world

3. Run code:

go run main.go

Example output:

INFO[0000] Starting agent version: v0.0.0-dev            BuildDate= CommitHash= loc="agent/agent.go(134)" logger=agent
2020/01/17 10:25:02 Hello World!
2020/01/17 10:25:02 All systems go!
INFO[0000] Agent started with 1 plugins (took 0s)        loc="agent/agent.go(179)" logger=agent

Example output after interrupt:

^CINFO[0030] Signal interrupt received, stopping.          loc="agent/agent.go(196)" logger=agent
INFO[0030] Stopping agent                                loc="agent/agent.go(269)" logger=agent
2020/01/17 10:25:32 Goodbye World!
INFO[0030] Agent stopped                                 loc="agent/agent.go(291)" logger=agent