Category Archives for "How to"

Advanced Testing: Manipulating getUserMedia and Available Devices

Philipp Hancke is not new here on our blog. He has assisted us when we wrote the series on webrtc-internals. He is also not squeamish about writing his own testing environment and sharing the love. This time, he wanted to share a piece of code that takes device availability test automation in WebRTC to a new level.

Obviously… we said yes.

We don’t have that implemented in testRTC yet, but if you are interested – just give us a shout out and we’ll prioritize it.

Both Chrome and Firefox have quite powerful mechanisms for automating getUserMedia with fake devices and skipping the permission prompt.

In Chrome this is controlled by the use-fake-device-for-media-stream and use-fake-ui-for-media-stream command line flags while Firefox offers a preferences media.navigator.streams.fake. See the webdriver.js helper in this repository for the gory details of how to use this with selenium.

However there are some scenarios which are not testable by this:

  • getUserMedia returning an error
  • restricting the list of available devices

While most of these are typically handled by unit tests sometimes it is nice to test the complete user experience for a couple of use-cases

  • test the behaviour of a client with only a microphone
  • test the behaviour of a client with only a camera
  • test the behaviour of a client with neither camera or microphone
  • combine those tests with screen sharing which in some cases replaces the video track on
  • test audio-only clients interoperating with audio-video ones. The test matrix becomes pretty big at some point.

Those tests are particularly important because as developers we tend to do some manual testing on our own machines which tend to be equipped with both devices. Automated tests running on a continuous integration server help a lot to prevent regressions.

Manipulating APIs with an extension

In order to manipulate both APIs I wrote a chrome extension (which magically works in Firefox and Edge because both support webextensions) that makes them controllable.

An extension can inject javascript into the page on page load as a content script. This has been used in the webrtc-externals extension described on webrtchacks to wrap the whole RTCPeerConnection API.

In our case, the content script replaces the getUserMedia and enumerateDevices functions with wrappers that can be modified at runtime. For example, the enumerateDevices wrapper calls the original function and then uses Javascript to modify the result before returning it to the caller:

The full extension can be found on github. The behaviour is dynamic and can be controlled via sessionStorage flags. With Selenium, one would typically navigate to a page in the same domain, execute a small script to set the session storage flags as desired and then navigate to the page that is to be tested.

We will walk through two examples now:

Use-case: Have getUserMedia return an error and change it at runtime

Let’s say we want to test the case that a user has denied permission. For this leads to a dialog that attempts to help them with the browser UX to change that.

The full test can be found here. As most selenium tests, it consists of a series of simple and straightforward steps:

  • build a selenium webdriver instance that allows permissions and loads the extension
  • go to the homepage
  • set the  List of fake devices in Chrome WebRTC testing  flag to cause a NotAllowedError (i.e. the user has denied permission) as well as an specific localStorage property that says the visitor is returning — this ensures we go into the flow we want to test and not into the “getUserMedia primer” that is shown to first-time users.
  • join an room by loading the URL directly.
  • the next step would typically be asserting the presence of certain DOM elements guiding the user to change the denied permission. This is omitted here as those elements change rather frequently and replaced with a three second sleep which allows for a visual inspection. It should look like this:
  • the  List of fake devices in Chrome WebRTC testing  flag is deleted
  • this eventually leads to the user entering the room and video showing up. We do some magic here in order to avoid having to ask the user to refresh the page.

Watch a video of this test running below:


Incidentally, that dialog had a “enter anyway” button which, due to the lack of testing, was not visible for quite some time without anyone noticing because the visual regression tests could not access this stage. Now that is possible.

Restricting the list of available devices

The fake devices in both Chrome and Firefox return a stream with exactly those properties that you ask for and they always succeed (in Chrome there is a way to make them always fail too). In the real world you need to deal with users who don’t have a microphone or a camera attached to their machine. A call to getUserMedia would fail with a NotFoundError (note the recent change in Chrome 64 or simply use adapter.js and write spec-compliant code today).

The common way to avoid this is to enumerate the list of devices to figure out what is available using enumerateDevices by pasting this into the javascript console:


When you run this together with the fake device flag you’ll notice that it provides two fake microphones and one fake camera device:

When the extension is loaded (which for manual testing can be done on chrome://extensions; see above for the selenium ways to do it) one can manipulate that list:

Paste the enumerateDevices into the console again and the audio devices no longer show up:

At we used this to replace a couple of audio-only and video-only tests that used feature flags in the application code with more realistic behaviour. The extension allows a much cleaner separation between the frontend logic and the test logic.


Using a tiny web extension we could easily extend the already powerful WebRTC testing capabilities of the browsers and cover more advanced test scenarios. Using this approach it would even be possible to simulate events like the user unplugging the microphone during the call.

2 Automating Your WebRTC Product Testing (Recorded session)

I took part this week in Twilio’s Signal event in London.

As with the previous Signal event I attended, this one was excellent (but that’s for some other post).

Twilio were kind enough to invite me to talk at their event, which resulted in the recorded session below:

In the first part of this session, I tried explaining the challenges that WebRTC testing and automation brings with it. I ended up talking about these 5 challenges:

  1. WebRTC being a brand new technology (=always changing)
  2. Browser based (=you don’t control your whole tech stack)
  3. Resource intensive (=need to factor that in when allocating your testing machines)
  4. Network sensitive (=need to be able to test in different network conditions)
  5. It takes two to tango (=need to synchronize across browsers during a test)

The second part was going through some of the results we’ve collected in our recent Kurento experiment, where we tried to see how much can we scale a deployed Kurento media server in different scenarios.

After the session everyone asked me how was the session. Frankly – I don’t know. I wasn’t sitting and listening there. I was talking (enjoying myself while doing so). I hope the audience in the room found the session useful. You can check it out on your own and make your own judgement.

Oh – and if you need to test your WebRTC application then you know where to find us 🙂

–> And if you don’t, then here’s our contact page.

Automated WebRTC Testing using testRTC

Yesterday, we hosted a webinar on testRTC. This time, we were really focused on showing some live demos of our service.

I wanted this one to be useful, so I sat down earlier this week, working on a general story outline with the idea of showing live how you can write a test script from scratch, building more and more capabilities and functionality into it as I went along.

It was real fun.

If you missed it, I’d like to invite you to watch the replay:

watch @ crowdcast

For the purpose of this webinar, I took Jitsi Meet ( and created the following scripts for it:

  1. Simple one-on-one test
    • Then I cleaned it up a bit from nagging warnings
    • And added a few basic expectations
  2. 4-way video test
    • For this one I’ve added some synchronization across the probes, and made sure Jitsi is the one generating the random rooms
    • I changed the script to be aware of sessions (parallel meeting rooms in the same test)
    • Then I played with the test reconfiguring it to run 40 probes, 8 in each meeting room
  3. One-on-one test with network limits
    • Switched back to a 1:1 session, this time with the flexibility we achieved in (2)
    • Increased the test length to 3 minutes
    • Injected 5% packet loss to the test in the second minute of the test

I also went over some of the results from the Kurento post we’ve published yesterday and went through the screen sharing script we’ve written recently about that uses as an example

One of the things I was asked is to share the scripts used throughout the session.

So I cleaned up the scripts a bit and placed them on our Google Drive. I am sharing them here in two forms:

  1. The GDoc file of the script – open it to read, copy+paste it to wherever
  2. The JSON file of the script – you can import this one directly into your testRTC account (you’ll need to reconfigure the probe profiles before you run it):

Here they are:

  1. Simple one-on-one test: GDocJSON
  2. 4-way video test: GDocJSON
  3. One-on-one test with network limits: GDocJSON

We’re here for any questions you may have.


How to Prepare Your WebRTC Application for a Surge in Traffic

OK, this is the moment you’ve been waiting for: there’s a huge surge in traffic on your WebRTC application. Success! You even had the prescience to place all of your web application’s assets on a CDN and whatever uptime monitoring service you use, be it New Relic, Datadog or a homegrown Nagios solution – says all is fine. But there’s just one nagging problem – users are complaining. More than they used to. Either because the service doesn’t work at all for them or the quality of the media just doesn’t cut it for them. What The–?!

We recently hosted a webinar about preparing for that big WebRTC launch. You might want to check the suggestions we made there as well.

Register now for free: WebRTC – How NOT to Fail in Your Big Launch

Let’s start by focusing on the positives here. Your service is being used be people. Then again, these people aren’t getting the real deal – the quality they are experiencing isn’t top notch. What they are experiencing is inability to join sessions, low bitrates or inexplicable packet losses. These are different than your run of the mill 500 and 502 errors, and you might not even notice something is wrong until a user complains.

So, what now?

Here’s what I’m going to cover today:

  1. Learn how to predict service hiccups
  2. Prepare your WebRTC application in advance for growth

Learn How to Predict Service Hiccups

While lots of users is probably what you are aiming for in your business, the effects they can have on your WebRTC application if unprepared for it can be devastating. Sure, sometimes they’ll force your service to go offline completely, but in many other times, the service will keep on running but it will deliver bad user experience. This can manifest itself by having users wait for long times to connect, requiring them to refresh the page to connect or just having poor audio and video quality.

Once you get to that point, it is hard to know what to do:

  • Do you throw more machines on the problem?
  • Do you need to check your network connections?
  • How do you find the affected users and tell them things have been sorted out?

This mess is going to take up a lot of your time and attention to resolve.

Here is something you can do to try and predict when these hiccups are about to hit you:

Establish a Baseline

I’ve said it before and I’ll say it again. You need to understand the performance metrics of your WebRTC service. In order to do that, the best thing is to run it a bit with the acceptable load that you have and writing down for yourself the important metrics.

A few that come to mind:

  • Bitrate of the channels
  • Average packet loss
  • Jitter

Now that you have your baseline, take the time to gauge what exactly your WebRTC application is capable of doing in terms of traffic. How much load can it carry as you stack up more users?

One neat trick you can do is place a testRTC monitor and use rtcSetTestExpectation() to indicate the thresholds you’ve selected for your baseline. Things like “I don’t expect more than 0.5% packet loss on average” or “average bitrate must be above 500kbps”. The moment these thresholds are breached – you’ll get notified and able to see if this is caused by growth in your traffic, changes in usage behavior, etc.

Prepare Your WebRTC Application in Advance for Growth

There aren’t always warning signs that let you know when a rampaging horde of users may come at your door. And even when there is, you better have some kind of a solution in place and be prepared to react. This preparation can be just knowing your numbers and have a resolution plan in place that you can roll out or it can be an automated solution that doesn’t require any additional effort on your end.

To get there, here are some suggestions I have for you.

Find Your System’s Limits

In general, there are 3 main limits to look at:

  1. How big can a single session grow?
  2. How many users can I cram into a single server?
  3. How many users can my service serve concurrently?

You can read more on strategies and planning for stress testing and sizing WebRTC services. I want to briefly touch these limits though.

1. How big can a single session grow?

Being able to handle 500 1:1 sessions doesn’t always scale to 100 groups of 10 users sessions. The growth isn’t linear in nature. On top of it, the end result might choke your server or just provide bitrates that are just too low above a certain number of users.

Make sure you know what’s the biggest session size you are willing to run.

Besides doing automated testing and checking the metrics against the baseline you want, you can always run an automated test using testRTC and at the same time join from your own browser to get a real feeling of what’s going on. Doing that will add the human factor into the mix.

2. How many users can I cram into a single server?

Most sizing testing are about understanding how many sessions/users/whatever can you fit in a single server. Once you hit that number, you should probably launch another server and use a load balancer to scale out.

Getting that number figured out based on your specific scenario and infrastructure is important.

3. How many users can my service serve concurrently?

Now that you know how to scale out from a single server, growing can be thought of as linearly (up to a point). So it is probably time to put in place automatic scale out and scale down and test that this thing works nicely.

Doing so will greatly reduce the potential and destruction that a service hiccup can cause.

CDN and Caching

Make sure all of the HTML assets of your WebRTC application that static are served through a CDN.

In some cases, when we stress test services, just putting 200 browsers in front of an HTML page that serves a WebRTC application can cause intermittent failures in loading the pages. That’s because the web serving part of the application is often neglected by WebRTC developers who are focusing their time and energy on the bigger resource hogs.

We’ve had numerous cases where the first roadblock we’ve hit with a customer was him forgetting to place a minor javascript file in the CDN.

Don’t be that person.

Geographically Distributed Deployment

The web and WebRTC are global, but traffic is local.

You don’t want to send users to the other side of the globe unnecessarily in your service. You want your media and NAT traversal servers to be as close to the users as possible. This gives you the flexibility of optimizing the backend network when needed.

Make sure your deployment is distributed along multiple datacenters, and that the users are routed to the correct one.

Philipp Hancke wrote how they do it at for their TURN servers.

Monitor Everything

CPU. Memory. Storage. Network. The works.

Add application metrics you collect from your servers on top of it.

And then add a testRTC monitor to check media quality end-to-end to make sure everything run consistently.

Check across large time spans if there’s an improvement or degradation of your service quality.

Stress Testing

Check your system for the load you expect to be able to handle.

Do it whenever you upgrade pieces of your backend, as minor changes there may cause large changes in performance.

Don’t Let Things Out of Your Control

WebRTC has a lot of moving parts in it so deploying it isn’t as easy as putting up a WordPress site. You should be prepared for that surge in traffic, and that means:

  1. Understanding the baseline quality of your service
  2. Knowing where you stand with your sizing and scale out strategy
  3. Monitoring your service quality so you can react before customers start complaining

Do it on your own. Use testRTC. Use whatever other tool there is at your disposal.

Just make sure you take this seriously.

We recently hosted a webinar about preparing for that big WebRTC launch. You might want to check the suggestions we made there as well.

Register now for free: WebRTC – How NOT to Fail in Your Big Launch


Just Landed: Automated WebRTC Screen Sharing Testing in testRTC

Well… this week we had a bit of a rough start, but we’re here. We just updated our production version of testRTC with some really cool capabilities. The time was selected to fit with the vacation schedule of everyone in this hectic summer and also because of some nagging Node.js security patch.

As always, our new release comes with too many features to enumerate, but I do want to highlight something we’ve added recently because of a couple of customers that really really really wanted it.

Screen sharing.

Yap. You can now use testRTC to validate the screen sharing feature of your WebRTC application. And like everything else with testRTC, you can do it at scale.

This time, we’ve decided to take for a spin (without even hinting anything to Philipp Hancke, so we’ll see how this thing goes).

First, a demo. Here’s a screencast of how this works, if you’re into such a thing:

Testing WebRTC Screen Sharing

There are two things to do when you want to test WebRTC screen sharing using testRTC:

  1. “Install” your WebRTC Chrome extension
  2. Show something interesting

#1 – “Install” your WebRTC Chrome extension

There are a couple of things you’ll need to do in the run options of the test script if you want to use screen sharing.

This is all quite arcane, so just follow the instructions and you’ll be good to go in no time.

Here’s what we’ve placed in the run options for

The #chrome-cli thingy stands for parameters that get passed to Chrome during execution. We need these to get screen sharing to work and to make sure Chrome doesn’t pop up any nagging selection windows when the user wants to screen share (these kills any possibility of automation here). Which is why we set the following parameters:

  • auto-select-desktop-capture-source=Entire screen – just to make sure the entire screen is automatically selected
  • use-fake-ui-for-media-stream – just add it if you want this thing to work
  • enable-usermedia-screen-capturing – just add it if you want this thing to work

The #extension bit is a new thing we just added in this release. It will tell testRTC to pre-install any Chrome extensions you wish on the browser prior to running your test script. And since screen sharing in Chrome requires an extension – this will allow you to do just that.

What we pass to #extension is the location of a .tar.gz file that holds the extension’s code.

Need to know how to obtain a .tar.gz file of your Chrome extension? Check out our Chrome extension extraction guide.

Now that we’ve got everything enabled, we can focus on the part of running a test that uses screen sharing.

#2 – Show something interesting

Screen sharing requires something interesting on the screen, preferably not an infinite video recursion of the screen being shared in one of the rectangles. Here’s what you want to avoid:

And this is what we really want to see instead:

The above is a screenshot that got captured by testRTC in a test scenario.

You can see here 4 participants where the top right one is screen sharing coming from one of the other participants.

How did we achieve this in the code?

Here are the code snippets we used in the script to get there:

We start by selecting the URL that will show some movement on the screen. In our case, an arbitrary YouTube video link.

Once we activate screen sharing in, we call rtcEvent which we’ve seen last time (and is also a new trick in this new release). This will add a vertical line on the resulting graphs so we know when we activated screen sharing (more on this one later).

We call execute to open up a new tab with our YouTube link. I decided to use the URL to get the video to work close to full screen.

Then we switch to the YouTube in the first windowHandles call.

We pause for a minute, and then go back to the tab in the browser.

Let’s analyze the results – shall we?

Reading WebRTC screen sharing stats

Screen sharing is similar to a regular video channel. But it may vary in resolution, frame rate or bitrate.

Here’s how the graphs look like on one of the receiving browsers in this test run. Let’s start with the frame rate this time:

Two things you want to watch for here:

  1. The vertical green line – that’s where we’ve added the rtcEvent call. While it was added to the browser who is sending screen sharing, we can see it on one of the receiving browsers as well. It gets us focused on the things of interest in this test
  2. The incoming blue line. It starts off nicely, oscillating at 25-30 frames per second, but once screen sharing kicks in – it drops to 2-4 frames per second – which is to be expected in most scenarios

The interesting part? made a decision to use the same video channel to send screen sharing. They don’t open an additional video channel or an additional peer connection to send screen sharing, preferring to repurpose an existing one (not all services behave like that).

Now let’s look at the video bitrate and number of packets graphs:

The video bitrate still runs at around 280 kbps, but it oscillates a lot more. BTW – I am using the mesh version of here with 4 participants, so it is going low on bitrate to accommodate for it.

The number of video packets per second on that incoming blue line goes down from around 40 to around 25. Probably due to the lower number of frames per second.

What else is new in testRTC?

Here’s a partial list of some new things you can do with testRTC

  • Manual testing service
  • Custom network profiles (more about it here)
  • Machine performance collection and visualization
  • Min/max bands on high level graphs
  • Ignore browser warnings and errors
  • Self service API key regeneration
  • Show elapsed time on running tests
  • More information in test runs on the actual script and run options used
  • More information across different tables and data views

Want to check screen sharing at scale?

You can now use testRTC to automate your screen sharing tests. And the best part? If you’re doing broadcast or multiparty, you can now test these scales easily for screen sharing related issues as well.

If you need a hand in setting up screen sharing in our account, then give us a shout and we’ll be there for you.