Code:play 

package main

import (
	"context"
	"fmt"
	"log"

	"gocloud.dev/blob/memblob"
	"gocloud.dev/runtimevar"
	"gocloud.dev/runtimevar/blobvar"
)

// MyConfig is a sample configuration struct.
type MyConfig struct {
	Server string
	Port   int
}

func main() {
	// Create a *blob.Bucket.
	// Here, we use an in-memory implementation and write a sample
	// configuration value.
	bucket := memblob.OpenBucket(nil)
	ctx := context.Background()
	err := bucket.WriteAll(ctx, "cfg-variable-name", []byte(`{"Server": "foo.com", "Port": 80}`), nil)
	if err != nil {
		log.Fatal(err)
	}

	// Create a decoder for decoding JSON strings into MyConfig.
	decoder := runtimevar.NewDecoder(MyConfig{}, runtimevar.JSONDecode)

	// Construct a *runtimevar.Variable that watches the blob.
	v, err := blobvar.NewVariable(bucket, "cfg-variable-name", decoder, nil)
	if err != nil {
		log.Fatal(err)
	}
	defer v.Close()

	// We can now read the current value of the variable from v.
	snapshot, err := v.Watch(ctx)
	if err != nil {
		log.Fatal(err)
	}
	// runtimevar.Snapshot.Value is decoded to type MyConfig.
	cfg := snapshot.Value.(MyConfig)
	fmt.Printf("%s running on port %d", cfg.Server, cfg.Port)

}

Output:

foo.com running on port 80