In 2014, I spent 12 weeks at the Recurse Center, formerly (and at the time) known as Hacker School. After finishing up my time there in May of that year, a lot of people asked me reasonable questions like:
My response to these questions was “I don’t know yet! It’s too early to say.” Now that more than a year has passed, I think I might have some idea of where to start.
On the last day of our Hacker School batch, we got some cool commemorative T-shirts with the stylish Hacker School logo on them. Now, this is an excellent T-shirt, and the only way I know of to get an official one is to attend the last day of a batch of Hacker School, so it’s already a pretty rad shirt. I wanted to make it radder, and I did, and now I want to tell you how.
This blog has been running on a Mirage OS unikernel hosted on Amazon EC2 since April 3rd:
$ ec2-get-console-output --region the-best-region i-0123abcd
2014-04-03T16:42:58+0000
Xen Minimal OS!
In that time, I’ve done some stuff:
I figured it was time to tell you about some of it, but first I did some other stuff:
You’d rather hear about all of that, right?
“This is a pretty strange piece of code, and it may take a few moments of thought to figure out what’s going on.”
– Real World OCaml
A few weeks ago, fellow Hacker Schooler Chen Lin and I were trying to solve a simple graph problem in Haskell. I was all ready to charge forward with something quite like the Java implementation I learned back in undergrad, but my fellow Hacker Schooler had some hesitation around whether this kind of structure would work in Haskell.
After a little bit of Googling, I found out that the canonical solution in Haskell involves something intriguingly dubbed tying the knot. I stared blankly at this HaskellWiki page with my fellow Hacker Schooler, trying to understand it quickly enough to have a useful conversation about it, and failed. We threw a couple of other ideas around and then decided to both pursue other projects. I moved on, Chen moved on, and I’m not sure either of us thought much about it…
…until yesterday, when I ran into tying the knot again. This time, it was hiding deep within (of all things!) the chapter on imperative programming in Real World OCaml, and I was unhurried and determined. “Abstract concept, I am going to understand you so hard,” I thought, jaw set.
When last we left our hero, I was strugging valiantly to get a Mirage unikernel version of this blog running on Amazon EC2. All unikernels built and shipped off to EC2 would begin booting, but never become pingable or reachable on TCP port 80. ec2-get-console-output on any instance running a Mirage unikernel would show the beginning stages of a DHCP transaction, then the disappointing RX exn Invalid_argument("String.sub"), then… silence.
When all you had for many years was a hammer, stuff is still going to look an awful lot like nails to you, even if it’s pretty distinctly screw-shaped. I wanted to take a packet trace of this transaction pretty badly. I could do three things that were almost like this:
Trying to draw some conclusions from the first option above led me down the wrong path for about a day or so. I did manage to cause the DHCP client to fail on my local Xen server by sending a DHCP reply packet with no server-identifier set, using scapy and some hackery to cause the xid to always match:
I left off last time telling you about getting Mirage to not work. I’m still working hard to get this blog – yes, this one you’re reading now – up and running as a unikernel on EC2.
It became clear to me last week that I needed to fork my own instance of the mirage-tcpip repository and compile my kernels with it, if I were to make any progress in debugging the DHCP problems I was having. A few naive attempts to monkey with version of mirage-tcpip downloaded by opam weren’t successful, so I set about to figure out how actual OCaml developers develop in OCaml with opam.
First stop: the opam documentation on doing tricky things. This is a little short of a step-by-step “do this, dorp” guide, unfortunately; here’s what I end up doing, and it sorta seems to work.
A week or so ago, I heard about the Mirage project, a library OS project that makes tiny virtual machines running on top of Xen to run a given application, and do nothing else. I was intrigued, and started working through the excellent intro documentation, and got to the point where I wanted to replace my ho-hum statically-compiled blog hosted from Ubuntu LTS with a unikernel that would serve my static site and do nothing else.
There are excellent instructions on doing this with a Jekyll site on Amir Chaudhry’s blog. Octopress, which I use to generate this site, is built on top of Jekyll, and I only had a few extra goodies to throw in before I was able to make a unikernel that would run my blog with a few rake invocations. After getting the first unikernel up and running via Xen on my laptop, I entertained myself by throwing a few nmap commands at it; I was particularly curious to see whether my unikernel knew what to do with UDP packets:
sudo nmap -sO 192.168.2.13
Starting Nmap 6.40 ( http://nmap.org ) at 2014-03-14 23:26 EDT
Nmap scan report for 192.168.2.13
Host is up (0.00037s latency).
Not shown: 254 open|filtered protocols
PROTOCOL STATE SERVICE
1 open icmp
6 open tcp
MAC Address: 00:16:3E:53:E0:1B (Xensource)
Nmap done: 1 IP address (1 host up) scanned in 17.72 seconds
Hee hee hee.
Before I get into the gory details of learning Elm via robots, I should tell you that my implementation is available for free play (edit: sorry, this has bitrotted too much to be included anymore), and you can also go look at the source code.
I got a really wonderful introduction to Elm when Evan Czaplicki came to Hacker School in our second week. We got a slightly adapted version of this talk from StrangeLoop 2013, which moved me to make a browser game (something I’ve never wanted to do at any previous point in life). The language seemed elegant and expressive, for lack of less cliched words, and I thought it might be relatively simple to make a succinct Robot Finds Kitten clone.
I promised a lot of people that I would let them know how Hacker School is. It’s difficult for me to answer this question (although Julia Evans, a previous batch process, has done a fantastic job), both because it feels so early in the batch and because Hacker School is a lot of things. I also promised a lot of people that I would let them know how New York is, and that’s a little easier, so I’ll start there and then move on.
I got my acceptance notification for the winter 2014 batch of Hacker School on January 3rd, six weeks ago. Right after being accepted, I wrote a bit in the same directory where I’d saved my application answers: