chapter03

README

Chapter III: starting to simulate a real torsf experiment

In this chapter we will improve upon what we did in the previous chapter by creating runner code for the torsf experiment. We will not, yet, run the real experiment, but we will instead write simple code that pretends to run a tor bootstrap using snowflake.

(This file is auto-generated from the corresponding source file, so make sure you don't edit it manually.)

The TestKeys structure

Let us start by defining the TestKeys structure that contains the experiment specific results. As we have already seen in Chapter I, this structure must contain two fields. The bootstrap time for the experiment and the failure.

type TestKeys struct {
	BootstrapTime float64 `json:"bootstrap_time"`
	Failure       *string `json:"failure"`
}

Rewriting the Run method

Next we will rewrite the Run method. We will arrange for this method to fill the measurement, to setup the timeout, and to print periodic updates via the callbacks. We will defer the real work to a private function called run.

func (m *Measurer) Run(
	ctx context.Context, sess model.ExperimentSession,
	measurement *model.Measurement, callbacks model.ExperimentCallbacks,
) error {

Let's create an instance of TestKeys and let's modify the measurement to refer to such an instance.

	testkeys := &TestKeys{}
	measurement.TestKeys = testkeys

Next, we record the current time and we modify the context to have a timeout after 300 seconds. Because Snowflake may take a long time to bootstrap, we need to specify a generous timeout here.

	start := time.Now()
	const maxRuntime = 300 * time.Second
	ctx, cancel := context.WithTimeout(ctx, maxRuntime)
	defer cancel()

Okay, now we are ready to defer the real work to the internal run function. We first create a channel to receive the result of run. Then, we create a ticker to emit periodic updates. We emit an update every 250 milliseconds, which is a reasonably smooth way of increasing a progress bar (progress is indeed used to move progress bars both in OONI Probe Desktop and mobile.)

	errch := make(chan error)
	ticker := time.NewTicker(250 * time.Millisecond)
	defer ticker.Stop()

Now we defer the real work to run, which will run in a background goroutine.

	go m.run(ctx, sess, testkeys, errch)

While run is running, we loop and check which channel has become ready.

If the errch channel is ready, it means that run is terminated, so we return to the caller.

Instead, if ticker.C is ready, we emit a progress update using the callbacks.

	for {
		select {
		case err := <-errch:
			callbacks.OnProgress(1.0, "torsf experiment is finished")
			return err
		case <-ticker.C:
			progress := time.Since(start).Seconds() / maxRuntime.Seconds()
			callbacks.OnProgress(progress, "torsf experiment is running")
		}
	}
}

The run function

We will now implement the run function. For now, this function will not do any real work, but it will just pretend to do work.

Note how we sleep for some time, set the BootstrapTime field of the TestKeys, and then return using errch.

func (m *Measurer) run(ctx context.Context,
	sess model.ExperimentSession, testkeys *TestKeys, errch chan<- error) {
	fakeBootstrapTime := 10 * time.Second
	time.Sleep(fakeBootstrapTime)
	testkeys.BootstrapTime = fakeBootstrapTime.Seconds()
	errch <- nil
}

Running the code

It's now time to run the new code we've written:

$ go run ./experiment/torsf/chapter03 | jq
2021/06/21 21:21:18  info [  0.1%] torsf experiment is running
2021/06/21 21:21:19  info [  0.2%] torsf experiment is running
[...]
2021/06/21 21:21:28  info [  3.3%] torsf experiment is running
2021/06/21 21:21:28  info [100.0%] torsf experiment is finished
{
  "data_format_version": "",
  "input": null,
  "measurement_start_time": "",
  "probe_asn": "",
  "probe_cc": "",
  "probe_network_name": "",
  "report_id": "",
  "resolver_asn": "",
  "resolver_ip": "",
  "resolver_network_name": "",
  "software_name": "",
  "software_version": "",
  "test_keys": {
  "bootstrap_time": 10,
  "failure": null
  },
  "test_name": "",
  "test_runtime": 0,
  "test_start_time": "",
  "test_version": ""
}

You see that now we're filling the bootstrap time and we're also printing progress using callbacks.

In the next chapter, we'll replace the stub run implementation with a real implementation using Snowflake.