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<title>Not Your Dad's Scala: Flying Objects and Proactive Streams</title>
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<div class="reveal">
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<section>
<h2>Not Your Dad's Scala: Flying Objects and Proactive Streams</h2>
<h5>Jakob Odersky</h5>
<h5>
<a href="https://github.com/jodersky">@jodersky</a>
</h5>
<p>
<h4>April 18, 2016</h4>
</p>
</section>
<section id="or-navigating-a-drone-with-scala" class="level2">
<h2>Or, <br/> Navigating a Drone with Scala</h2>
</section>
<section id="initial-idea" class="level2">
<h2>Initial Idea</h2>
<ul>
<li>Build a quadrotor</li>
<li>From scratch
<ul>
<li>hardware</li>
<li>software</li>
</ul></li>
<li>Control with a joystick, no remote</li>
<li>⇒ Project Condor</li>
</ul>
</section>
<section id="two-parts" class="level2">
<h2>Two Parts</h2>
<ol type="1">
<li>The Quadrotor</li>
<li>The Base Station</li>
</ol>
</section>
<section id="system-overview" class="level2">
<h2>System Overview</h2>
<figure>
<img src="images/functional.svg" alt="overview" /><figcaption>overview</figcaption>
</figure>
</section>
<section id="the-quadrotor" class="level2">
<h2>1. The Quadrotor</h2>
<section id="the-flying-object-no-scala-here" class="level3">
<h3><em>(the flying object)</em> <br/> No Scala Here</h3>
<p>Move on to the Scala parts →</p>
<p>See backstory ↓</p>
</section>
<section id="frame" class="level3">
<h3>Frame</h3>
<figure>
<img src="images/structure.png" alt="Structure" /><figcaption>Structure</figcaption>
</figure>
</section>
<section id="frame-1" class="level3">
<h3>Frame</h3>
<figure>
<img src="images/prototype.jpg" alt="Unit" /><figcaption>Unit</figcaption>
</figure>
</section>
<section id="electronics" class="level3">
<h3>Electronics</h3>
<figure>
<img src="images/trifle.jpg" alt="Trifle Board" /><figcaption>Trifle Board</figcaption>
</figure>
</section>
<section id="flight-stack" class="level3">
<h3>Flight Stack</h3>
<ul>
<li>And then I found PX4, exactly what I was trying to build.</li>
</ul>
</section>
<section id="alternative-px4" class="level3">
<h3>Alternative: PX4</h3>
<ul>
<li>Open Source Software</li>
<li>Developed by Computer Vision and Geometry Group, ETHZ</li>
<li>Uses open, wide-spread protocol MAVLink</li>
<li>Runs on free RTOS <a href="http://nuttx.org/">NuttX</a>, available for AVR, ARM and more.</li>
</ul>
</section>
<section id="pixhawk-hardware" class="level3">
<h3>Pixhawk Hardware</h3>
<ul>
<li>Developed by the same group</li>
<li>Expensive</li>
<li>I already have a microcontroller and all sensors</li>
</ul>
<figure>
<img src="images/pixhawk.jpg" alt="Pixhawk" /><figcaption>Pixhawk</figcaption>
</figure>
</section>
<section id="nuttx-beginning-2015" class="level3">
<h3>NuttX (beginning 2015)</h3>
<ul>
<li>Our board uses the Teensy 3.1:
<ul>
<li>ARM Cortex M4</li>
<li>Freescale Kinetis K20 family</li>
</ul></li>
<li>NuttX (beginning 2015) has support for K40 family</li>
</ul>
</section>
<section id="lets-port-it" class="level3">
<h3>Let's port it!</h3>
<ul>
<li>Subtle differences in processor families, K20 != K40</li>
<li>I was never much involved with kernel development beforehand</li>
<li>Debugging microcontrollers is no fun
<ul>
<li>(without JTAG or other fancy equipment)</li>
</ul></li>
</ul>
</section>
<section id="fast-forward-2-months" class="level3">
<h3>Fast-forward 2 months</h3>
</section>
<section id="get-the-pixhawk-and-move-on" class="level3">
<h3>Get the Pixhawk and move on</h3>
</section>
</section>
<section id="quadrotor-take-away" class="level2">
<h2>Quadrotor Take-Away:</h2>
<ul>
<li>A lot of work has gone into existing hardware platforms</li>
<li>⇒ Use existing Pixhawk controller</li>
</ul>
<figure>
<img src="images/pixhawk.jpg" alt="Pixhawk" /><figcaption>Pixhawk</figcaption>
</figure>
</section>
<section id="the-base-station" class="level2">
<h2>2. The Base Station</h2>
<ul>
<li>Control drone with joystick/gamepad</li>
<li>Display live flight information
<ul>
<li>"virtual cockpit"</li>
</ul></li>
<li>Underlying process: <em>(proactive)</em> message streaming</li>
</ul>
</section>
<section id="mavlink-protocol" class="level2">
<h2>MAVLink Protocol</h2>
<ul>
<li>Wide-spread use, including the PX4</li>
<li>Published 2009 by Lorenz Meier, ETHZ</li>
<li>Very simple and extensible</li>
</ul>
</section>
<section id="mavlink-frame" class="level2">
<h2>MAVLink Frame</h2>
<ul>
<li>Header-only structure</li>
<li>Every entity on network has 2-byte address</li>
<li>Low overhead: 6 bytes</li>
</ul>
<pre class="scala"><code>+-----+-----+-----+-------+--------+-------+---------+-----+
| Stx | Len | Seq | SysID | CompID | MsgID | Payload | CRC |
+-----+-----+-----+-------+--------+-------+---------+-----+
| 1 | 1 | 1 | 1 | 1 | 1 | n | 2 |
+-----+-----+-----+-------+--------+-------+---------+-----+</code></pre>
</section>
<section id="mavlink-messages" class="level2">
<h2>MAVLink Messages</h2>
<ul>
<li>Message consists of:
<ul>
<li>ID/name</li>
<li>collection of fields (name, value) pairs</li>
</ul></li>
<li><p>Common message definitions in XML</p></li>
<li><p><a href="https://github.com/mavlink/mavlink">Official repository</a> contains bindings for various languages</p></li>
</ul>
<pre class="xml"><code><message id="0" name="HEARTBEAT">
<description>Description...</description>
<field type="uint8_t" name="type">...</field>
<field type="uint8_t" name="autopilot">...</field>
<field type="uint8_t" name="base_mode">...</field>
<field type="uint32_t" name="custom_mode">...</field>
<field type="uint8_t" name="system_status">...</field>
</message></code></pre>
</section>
<section id="mavlink-via-the-base-station" class="level2">
<h2>MAVLink via the Base Station</h2>
<ul>
<li>Send movement messages to drone:
<ul>
<li>standalone program</li>
<li>⇒ <a href="https://github.com/project-condor/maverick">Maverick</a></li>
</ul></li>
<li>View and send non-movement messages:
<ul>
<li>⇒ <a href="https://github.com/project-condor/mavigator">Mavigator</a> (rest of this presentation)</li>
</ul></li>
</ul>
</section>
<section id="mavigator" class="level2">
<h2>Mavigator</h2>
<table>
<thead>
<tr class="header">
<th style="text-align: left;">Requirement</th>
<th style="text-align: left;">add Scala</th>
<th style="text-align: left;">Area</th>
</tr>
</thead>
<tbody>
<tr class="odd">
<td style="text-align: left;">Modern, portable UI</td>
<td style="text-align: left;">Scala.js</td>
<td style="text-align: left;">frontend</td>
</tr>
<tr class="even">
<td style="text-align: left;">Web server</td>
<td style="text-align: left;">Akka Http</td>
<td style="text-align: left;">backend</td>
</tr>
<tr class="odd">
<td style="text-align: left;">Message streaming</td>
<td style="text-align: left;">Akka Streams</td>
<td style="text-align: left;">backend</td>
</tr>
</tbody>
</table>
</section>
<section id="live-demo" class="level2">
<h2>Live Demo</h2>
<section id="alternatives" class="level4">
<h4>Alternatives</h4>
<ul>
<li>online: <a href="http://mavigator.jodersky.ch" class="uri">http://mavigator.jodersky.ch</a></li>
<li>run locally: <code>sbt mavigator-server/run</code></li>
</ul>
</section>
</section>
<section id="frontend-scala.js" class="level2">
<h2>Frontend: Scala.js</h2>
<ul>
<li>A JavaScript backend to the Scala compiler.
<ul>
<li>All the goodies from Scala: type safety, collections, etc</li>
<li>Outputs plain JavaScript</li>
<li>Can interface with plain JavaScript</li>
</ul></li>
<li>Developed at EPFL by Sebastien Doeraene</li>
<li>Still in 0.x but totally usable!</li>
</ul>
</section>
<section id="frontend-design" class="level2">
<h2>Frontend: Design</h2>
<ul>
<li>One-page applications</li>
<li>Static scaffolding with <a href="https://github.com/playframework/twirl">Twirl</a> (Play! templates)</li>
<li>Common launcher for all applications</li>
<li>Extensible "cake" pattern:
<ul>
<li>Providing web page environment</li>
<li>Reading messages</li>
<li>Displaying instruments</li>
<li>Laying out elements</li>
</ul></li>
</ul>
</section>
<section id="backend-akka-streams" class="level2">
<h2>Backend: Akka Streams</h2>
<ul>
<li>Compatible with Reactive Streams
<ul>
<li>standard for aynchronous, non-blocking, back-pressures streaming</li>
<li><a href="http://www.reactive-streams.org/" class="uri">http://www.reactive-streams.org/</a></li>
</ul></li>
<li>"Natural" Streaming API</li>
</ul>
</section>
<section id="streams-as-graphs" class="level2">
<h2>Streams as Graphs</h2>
<figure>
<img src="images/stream-graphs.svg" alt="overview" /><figcaption>overview</figcaption>
</figure>
</section>
<section id="streams-in-code" class="level2">
<h2>Streams in Code</h2>
<figure>
<img src="images/counter.svg" />
</figure>
<pre class="scala"><code>//Simple source
val src: Source[Int, _] = Source.tick[Int](100millis, 100millis, 1)
//Nested source
val inc: Source[Int, _] = src.scan(0)(_+_)
//A discrete flow
val flow = Flow[Int].map(_*2)
//Simple sink
val sink = Sink.foreach(println)
//Print all even numbers, ad infinitum
(inc via flow to sink).run()
</code></pre>
</section>
<section id="more-streams" class="level2">
<h2>More Streams</h2>
<ul>
<li>Arbitrary graphs</li>
<li>Feedback loops</li>
<li>Materialization</li>
<li>Much more...</li>
</ul>
<p><a href="http://akka.io" class="uri">http://akka.io</a></p>
</section>
<section id="backend-conceptual-model" class="level2">
<h2>Backend: Conceptual Model</h2>
<figure>
<img src="images/model.svg" alt="Conceptual stream model" /><figcaption>Conceptual stream model</figcaption>
</figure>
</section>
<section id="backend-actual-model" class="level2">
<h2>Backend: Actual Model</h2>
<figure>
<img src="images/core.svg" alt="Actual stream model" /><figcaption>Actual stream model</figcaption>
</figure>
</section>
<section id="serial-backend" class="level2">
<h2>Serial Backend</h2>
<pre class="scala"><code>Serial().watch(Set("/dev/ttyUSB0")).runForeach{ port =>
//replace backend on connection
val multiplexer: Flow[Bytes, Bytes, NotUsed] = setBackend()
//open actual connection to drone
val uav: Flow[Bytes, Bytes, NotUsed] =
Serial().open(port, serialSettings)
(uav join multiplexer).run()
}</code></pre>
<ul>
<li><code>Serial()</code> is a custom Akka extension</li>
<li>See <a href="https://www.jodersky.ch/flow" class="uri">https://www.jodersky.ch/flow</a> for implementation</li>
</ul>
</section>
<section id="web-sockets" class="level2">
<h2>Web Sockets</h2>
<pre class="scala"><code>path("/mavlink") {
get {
val fromWebSocket = Flow[Message].collect{
case BinaryMessage.Strict(data) => data
}
val toWebSocket = Flow[Bytes].map{bytes =>
BinaryMessage(bytes)
}
//request connection to multiplexer
val backend: Flow[Bytes, Bytes, NotUsed] = Uav().connect()
handleWebSocketMessages(fromWebSocket via backend via toWebSocket)
}
}</code></pre>
<ul>
<li>Akka Route DSL</li>
<li>Graph is run by Akka Http</li>
</ul>
</section>
<section id="mavlink-codegen" class="level2">
<h2>MAVLink CodeGen</h2>
<ul>
<li><a href="https://github.com/project-condor/sbt-mavlink">sbt-mavlink</a>:
<ul>
<li>generates data parser and assembler</li>
<li>message maps to case class</li>
</ul></li>
</ul>
<pre class="scala"><code>case class Attitude(
timeBootMs: Int,
roll: Float, pitch: Float, yaw: Float,
rollspeed: Float, pitchspeed: Float, yawspeed: Float)
extends Message</code></pre>
</section>
<section id="demo-do-a-barrel-roll" class="level2">
<h2>Demo: Do a Barrel Roll</h2>
<ul>
<li>Define "unstable" message</li>
<li>Add extra UI element to notify user</li>
<li>Inject attitude messages</li>
</ul>
</section>
<section id="message" class="level2">
<h2>Message</h2>
<p>Add to dialect definition:</p>
<pre class="xml"><code><message id="150" name="STABILITY">
<description>Stability issues</description>
<field type="uint8_t" name="stable">Is it stable?</field>
</message></code></pre>
</section>
<section id="new-ui-element" class="level2">
<h2>New UI element</h2>
<ul>
<li>Danger indicator</li>
<li>Illuminates on stability issues</li>
</ul>
</section>
<section id="barrel-roll-injection" class="level2">
<h2>Barrel Roll: Injection</h2>
<figure>
<img src="images/core-new.svg" alt="overview" /><figcaption>overview</figcaption>
</figure>
</section>
<section id="live-demo-1" class="level2">
<h2>Live Demo</h2>
<section id="alternatives-1" class="level4">
<h4>Alternatives</h4>
<ul>
<li><code>git checkout demo1-before</code></li>
<li><code>git checkout demo1-after</code></li>
</ul>
</section>
</section>
<section id="references" class="level2">
<h2>References</h2>
<ul>
<li><p>Mavigator repository:</p>
<p><a href="https://github.com/project-condor/mavigator" class="uri">https://github.com/project-condor/mavigator</a></p></li>
<li><p>Project website:</p>
<p><a href="https://project-condor.github.io" class="uri">https://project-condor.github.io</a></p></li>
</ul>
</section>
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