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+---
+title: "Not Your Dad's Scala: Flying Objects and Proactive Streams"
+author: Jakob Odersky
+github: jodersky
+date: April 18, 2016
+---
+
+## Or, <br/> Navigating a Drone with Scala
+
+## Initial Idea
+
+- Build a quadrotor
+- From scratch
+	- hardware
+	- software
+- Control with a joystick, no remote
+- ⇒ Project Condor
+
+## Two Parts
+
+1. The Quadrotor
+2. The Base Station
+
+## System Overview
+
+![overview](images/functional.svg)
+
+## 1. The Quadrotor
+
+### *(the flying object)* <br/> No Scala Here
+
+Move on to the Scala parts →
+
+See backstory
+↓
+
+### Frame
+![Structure](images/structure.png)
+
+### Frame
+![Unit](images/prototype.jpg)
+
+### Electronics
+![Trifle Board](images/trifle.jpg)
+
+### Flight Stack
+- And then I found PX4, exactly what I was trying to build.
+
+### Alternative: PX4
+- Open Source Software
+- Developed by Computer Vision and Geometry Group, ETHZ
+- Uses open, wide-spread protocol MAVLink
+- Runs on free RTOS [NuttX](http://nuttx.org/), available for AVR, ARM and more.
+
+### Pixhawk Hardware
+- Developed by the same group
+- Expensive
+- I already have a microcontroller and all sensors
+
+![Pixhawk](images/pixhawk.jpg)
+
+### NuttX (beginning 2015)
+- Our board uses the Teensy 3.1:
+	- ARM Cortex M4
+	- Freescale Kinetis K20 family
+
+- NuttX (beginning 2015) has support for K40 family
+
+### Let's port it!
+
+- Subtle differences in processor families, K20 != K40
+- I was never much involved with kernel development beforehand
+- Debugging microcontrollers is no fun
+	- (without JTAG or other fancy equipment)
+
+### Fast-forward 2 months
+
+### Get the Pixhawk and move on
+
+## Quadrotor Take-Away:
+
+- A lot of work has gone into existing hardware platforms
+- ⇒ Use existing Pixhawk controller
+
+![Pixhawk](images/pixhawk.jpg)
+
+## 2. The Base Station
+
+- Control drone with joystick/gamepad
+- Display live flight information
+	- "virtual cockpit"
+
+- Underlying process: *(proactive)* message streaming
+
+
+## MAVLink Protocol
+
+- Wide-spread use, including the PX4
+- Published 2009 by Lorenz Meier, ETHZ
+- Very simple and extensible
+
+## MAVLink Frame
+
+- Header-only structure
+- Every entity on network has 2-byte address
+- Low overhead: 6 bytes
+
+~~~{.scala}
++-----+-----+-----+-------+--------+-------+---------+-----+
+| Stx | Len | Seq | SysID | CompID | MsgID | Payload | CRC |
++-----+-----+-----+-------+--------+-------+---------+-----+
+| 1   | 1   | 1   | 1     | 1      | 1     | n       | 2   |
++-----+-----+-----+-------+--------+-------+---------+-----+
+~~~
+
+## MAVLink Messages
+
+- Message consists of:
+	- ID/name
+	- collection of fields (name, value) pairs
+
+- Common message definitions in XML
+
+- [Official repository](https://github.com/mavlink/mavlink) contains bindings for various languages
+
+~~~{.xml}
+<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>
+~~~
+
+## MAVLink via the Base Station
+
+- Send movement messages to drone:
+    - standalone program
+    - ⇒ [Maverick](https://github.com/project-condor/maverick)
+
+- View and send non-movement messages:
+	- ⇒ [Mavigator](https://github.com/project-condor/mavigator) (rest of this presentation)
+
+## Mavigator
+
+| Requirement | add Scala | Area
+|-------------|-----------|-----|
+| Modern, portable UI | Scala.js | frontend |
+| Web server | Akka Http | backend |
+| Message streaming | Akka Streams | backend |
+
+
+## Live Demo
+
+#### Alternatives
+- online: <http://mavigator.jodersky.ch>
+- run locally: `sbt mavigator-server/run`
+
+## Frontend: Scala.js
+- A JavaScript backend to the Scala compiler.
+	- All the goodies from Scala: type safety, collections, etc
+	- Outputs plain JavaScript
+	- Can interface with plain JavaScript
+
+- Developed at EPFL by Sebastien Doeraene
+- Still in 0.x but totally usable!
+
+## Frontend: Design
+- One-page applications
+- Static scaffolding with [Twirl](https://github.com/playframework/twirl) (Play! templates)
+- Common launcher for all applications
+- Extensible "cake" pattern:
+	- Providing web page environment
+	- Reading messages
+	- Displaying instruments
+	- Laying out elements
+
+## Backend: Akka Streams
+- Compatible with Reactive Streams
+	- standard for aynchronous, non-blocking, back-pressures streaming
+	- <http://www.reactive-streams.org/>
+- "Natural" Streaming API
+
+## Streams as Graphs
+
+![overview](images/stream-graphs.svg)
+
+## Streams in Code
+
+![](images/counter.svg)
+
+~~~{.scala}
+//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()
+
+~~~
+
+## More Streams
+
+- Arbitrary graphs
+- Feedback loops
+- Materialization
+- Much more...
+
+<http://akka.io>
+
+## Backend: Conceptual Model
+
+![Conceptual stream model](images/model.svg)
+
+## Backend: Actual Model
+
+![Actual stream model](images/core.svg)
+
+## Serial Backend
+
+~~~~ {.scala}
+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()
+}
+~~~~
+
+- `Serial()` is a custom Akka extension
+- See <https://www.jodersky.ch/flow> for implementation
+
+## Web Sockets
+
+~~~~ {.scala}
+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)
+  }
+}
+~~~~
+- Akka Route DSL
+- Graph is run by Akka Http
+
+## MAVLink CodeGen
+- [sbt-mavlink](https://github.com/project-condor/sbt-mavlink):
+    - generates data parser and assembler
+	- message maps to case class
+
+~~~{.scala}
+case class Attitude(
+  timeBootMs: Int,
+  roll: Float, pitch: Float, yaw: Float,
+  rollspeed: Float, pitchspeed: Float, yawspeed: Float)
+    extends Message
+~~~
+
+
+## Demo: Do a Barrel Roll
+- Define "unstable" message
+- Add extra UI element to notify user
+- Inject attitude messages
+
+## Message
+
+Add to dialect definition:
+
+~~~{.xml}
+<message id="150" name="STABILITY">
+	<description>Stability issues</description>
+	<field type="uint8_t" name="stable">Is it stable?</field>
+</message>
+~~~
+
+## New UI element
+- Danger indicator
+- Illuminates on stability issues
+
+## Barrel Roll: Injection
+![overview](images/core-new.svg)
+
+## Live Demo
+
+#### Alternatives
+- `git checkout demo1-before`
+- `git checkout demo1-after`
+
+## References
+
+- Mavigator repository:
+
+	<https://github.com/project-condor/mavigator>
+
+- Project website:
+
+	<https://project-condor.github.io>