Communication between instances

For some test plans, it is useful to pass information from one instance to another. In addition to direct network connectivity, test plans can pass information between instances using the Testground sync service.

In this tutorial, we will explore typed message passing through the Testground sync service.

Lets create a plan in which one of the plans produces a struct which is re-constructed on the distant end. First, I'll show short snippets with the relevant information, and the whole test plan will be shown at the end.

Setting up thetopic

Transferrable is the value type we will be transferring.

type Transferrable struct {
Name string
FavoriteSport int
CareWhoKnows bool
}

The value will be transferred over a topic. Think of the topic as a named and typed channel for transferring values between plan instances. This topic is named transfer-key and the value type I expect to get out of it is pointer to Transferrable.

st := sync.NewTopic("transfer-key", &Transferrable{})

Publishing to a topic

To write to a topic, create a bounded client and use it to publish to the topic we have just defined.

ctx := context.Background()
client := sync.MustBoundClient(ctx, runenv)
defer client.Close()
client.Publish(ctx, st, &Transferrable{"Guy#1", 1, false})

Reading from a topic

Subscribe to the topic we created earlier and set up a channel to receive the values.

tch := make(chan *Transferrable)
_, err = client.Subscribe(ctx, st, tch)
if err != nil {
panic(err)
}

Who subscribes and who publishes?

This question is left up to the plan writer, and certainly different situations will call for different implementations. In this example, all the plans will publish and all will subscribe, but there are scenarios where this is inappropriate.

Full Example

package main
import (
"context"
"fmt"
"math/rand"
"time"
"github.com/testground/sdk-go/runtime"
"github.com/testground/sdk-go/sync"
)
type Sport int
const (
football Sport = iota
tennis
hockey
golf
)
func (s Sport) String() string {
return [...]string{"football", "tennis", "hockey", "golf"}[s]
}
type Transferrable struct {
Name string
FavoriteSport Sport
CareWhoKnows bool
}
func (t *Transferrable) String() string {
msg := fmt.Sprintf("%s: I like %s", t.Name, t.FavoriteSport)
if t.CareWhoKnows {
return msg + " and I really care!"
}
return msg + " and I don't care who knows!"
}
func main() {
runtime.Invoke(run)
}
func run(runenv *runtime.RunEnv) error {
rand.Seed(time.Now().UnixNano())
ctx := context.Background()
client := sync.MustBoundClient(ctx, runenv)
defer client.Close()
st := sync.NewTopic("transfer-key", &Transferrable{})
// Configure the test
myName := fmt.Sprintf("Guy#%d", rand.Int()%100)
mySport := Sport(rand.Int() % 4)
howMany := runenv.TestInstanceCount
// Publish my entry
client.Publish(ctx, st, &Transferrable{myName, mySport, false})
// Wait until all instances have published entries
readyState := sync.State("ready")
client.MustSignalEntry(ctx, readyState)
<-client.MustBarrier(ctx, readyState, howMany).C
// Subscribe to the `transfer-key` topic
tch := make(chan *Transferrable)
client.Subscribe(ctx, st, tch)
for i := 0; i < howMany; i++ {
t := <-tch
runenv.RecordMessage("%s", t)
}
return nil
}

Run with multiple instances:

$ testground run single -p quickstart -t quickstart -b exec:go -r local:exec -i 4

Notice that instances is set to 4. Four instances will run at the same time.