Creating a Go based plugin for snap through a stripped down example

1. Choose a repository and plugin name

• It is suggested that your name include the following convention snap-plugin-[plugin-type]-[plugin-name]
• For example: snap-plugin-collector-foo
• Plugin types: collector, processor, publisher

2. Implement the CollectorPlugin interface (cpuid.go)

// Collector plugin
type CollectorPlugin interface {
   Plugin
   CollectMetrics([]PluginMetricType) ([]PluginMetricType, error)
   GetMetricTypes(PluginConfigType) ([]PluginMetricType, error)
}```
```go
type Plugin interface {
   GetConfigPolicy() (*cpolicy.ConfigPolicy, error)
}

• GetMetricTypes([]PluginMetricType) ([]PluginMetricType, error) communicates what metrics the plugin collects through returning an array of plugin.PluginMetricType
  • Through the plugin.PluginMetricType we are providing the namespace and version of the metric(s) our plugin will provide

// GetMetricTypes returns the metrics this plugin provides through an array of plugin.PluginMetricType
func (c *CPUID) GetMetricTypes(_ plugin.PluginConfigType) ([]plugin.PluginMetricType, error) {
   return []plugin.PluginMetricType{
   	plugin.PluginMetricType{
   		Namespace_: []string{"jcooklin", "cpuid", "VendorIdentificationString"},
   		Version_: 1,
   	},
   }, nil
}

• CollectMetrics(mts []plugin.PluginMetricType) ([]plugin.PluginMetricType, error) returns the requested metrics

  • The argument to CollectMetrics is an array of plugin.PluginMetricType where the Namespace field communicates what metric is being requested
  • The result of CollectMetrics is also an array of plugin.PluginMetricType where the fields Data, TimeStamp and Source communicate the value, time of collection and source of the metric

  // CollectMetrics returns []plugin.PluginMetrictType

func (c *CPUID) CollectMetrics(mts []plugin.PluginMetricType) ([]plugin.PluginMetricType, error) { hostname, _ := os.Hostname() for idx, m := range mts { switch m.Namespace_[len(m.Namespace_)-1] { case "VendorIdentificatorString": mts[idx].Data_ = cpuid.VendorIdentificatorString mts[idx].Timestamp_ = time.Now() mts[idx].Source_ = hostname } } return mts, nil } ```

• GetConfigPolicy() (*cpolicy.ConfigPolicy, error) returns the required config policy, if any, for the metrics provided by the plugin
  • This plugin does not require a config policy so an empty cpolicy is returned

  // GetConfigPolicy returns the policy for this plugin

func (c *CPUID) GetConfigPolicy() (*cpolicy.ConfigPolicy, error) { return cpolicy.New(), nil } ``` * Note: a config policy provides the plugin author a mechanism which forces the users of the plugin to provide configuration detail such as username, password, address, port, etc when their plugin is used

• Start the plugin (main.go)
  • plugin.Start(m *PluginMeta, c Plugin, requestString string) starts the plugin

    func main() {
    c := &cpuid.CPUID{}
    plugin.Start(
    	cpuid.Meta(),
    	c,
    	os.Args[1],
    )

}

    * `cpuid.Meta()` returns `*PluginMeta`    	   
  	```go
	func Meta() *plugin.PluginMeta {
		return plugin.NewPluginMeta(
			name,
			version,
			pluginType,
			[]string{plugin.SnapGOBContentType},
			[]string{plugin.SnapGOBContentType})
	}

Download, build and use the plugin

• Prerequisite
  • Go is properly installed and configured
  • Snap binaries snapd, snapctl and the plugin file publisher plugin are available
    • Download the latest snap release or build [snap]((https://github.com/intelsdi-x/snap) yourself

1. Use 'go get' to get the plugin

go get github.com/jcooklin/snap-plugin-collector-cpuid

Note The rest of the commands below are run from the root of the snap-plugin-collector-cpuid which should now be located at $GOPATH/src/github.com/jcooklin/

• Build the plugin

make

• Start snapd

snapd -t 0 -l 1

• -t 0 disables plugin trust

• -l 1 starts snapd with debug logging

• Load plugins

snapctl plugin load build/rootfs/snap-plugin-collector-cpuid
snapctl plugin load ../../intelsdi-x/snap/build/plugin/snap-publisher-file

Note: You may need to reference a different path for the snap-publisher-file. For instance, if you downloaded a release.

• Start task

snapctl task create -t example/task.json

• Watch task

snapctl task watch <TASK_ID>