Tuesday, January 24, 2017

Interview Question: How would you architect Uber?

There was recently a thread on hacker news about what questions you would ask a software architect. One of the questions was how you would architect Uber:

I would probably divide up the architecture into the following pieces:

  • Driver App
  • Passenger App
  • Backend Server

Let’s architect out the driver experience first. Drivers need to be onboarded so figure a whole system just for onboarding a driver. Once drivers are in the system we need to be able to track their location whenever they are ready to accept rides. So the app at a bare minimum needs to be able to constantly relay the driver’s current location to the backend server.

This needs to be updated in real time. I may look toward a messaging bus/queue for these types of updates so I can throw location updates out if get too behind. I would look at how multiplayer games like Quake implemented reporting real time location to the server as fast as possible and with as little delay as possible.

Once the backend server has the location we can look to the passenger app to do almost the same thing. The passenger app at a minimum needs to report trips that people want to take. Basically source and destination location.

Once we have both in the system we can now creating a matching algorithm or module. I would base this on how stock markets match and route orders. There are a number of different techniques for this. I would model the drivers as ask orders and the passenger requests as buy orders. Once a driver has been matched to a passenger we can send them both notifications and provide updates along the way.

Keep in mind that this is really only for traditional Ubers with one passenger. With pools the matching algorithm becomes more complex as well as the infrastructure to support it.

Monday, January 23, 2017

Developing for the NRF51822 on Mac OS X

Over the weekend I worked on trying to get a development environment up and running for the Nordic nRF51822 system on a chip.

This outlines how I did it:

Need to Download Segger JLink. This is used to flash the board.

J-Link Software and Documentation pack for MacOSX

For Mac I installed: JLink_MacOSX_V612f.pkg

After it was installed I was able to run JLinkExe (it put it in my path)

$ JLinkExe 
SEGGER J-Link Commander V6.12f (Compiled Jan 13 2017 16:41:09)
DLL version V6.12f, compiled Jan 13 2017 16:40:56

Connecting to J-Link via USB...Updating firmware:  J-Link OB-SAM3U128-V2-NordicSemi compiled Nov 14 2016 16:58:29
Replacing firmware: J-Link OB-SAM3U128-V2-NordicSemi compiled May  4 2015 13:48:48
Waiting for new firmware to boot
New firmware booted successfully
Firmware: J-Link OB-SAM3U128-V2-NordicSemi compiled Nov 14 2016 16:58:29
Hardware version: V1.00
S/N: 681696138
VTref = 3.300V

Type "connect" to establish a target connection, '?' for help


I had to use SWD:

JLinkExe -device nrf51822 -if swd -speed 2000

J-Link>loadfile nrf51422_xxac.hex
Downloading file [nrf51422_xxac.hex]...
Comparing flash   [100%] Done.
Erasing flash     [100%] Done.
Programming flash [100%] Done.
Verifying flash   [100%] Done.
J-Link: Flash download: Flash programming performed for 1 range (4096 bytes)
J-Link: Flash download: Total time needed: 0.132s (Prepare: 0.043s, Compare: 0.004s, Erase: 0.000s, Program: 0.075s, Verify: 0.001s, Restore: 0.008s)