Some Random Idiot

embroidery, networks, programming, feelings

A Quick Guide to Quick Changes in MirageOS

MirageOS is a collection of libraries and a system for assembling them into unikernels. What happens if you want to make changes to those libraries and test them with a new unikernel?

Say, for example, I have a static website (like this blog) that I build in MirageOS. I want to make some changes to the TCP implementation against which the blog is built. In order to do that, I need to do all the following:

  • figure out which module to change
  • figure out which package provides that module
  • get the source for that package and instruct the package manager to use it instead of the release
  • make changes
  • reinstall the package with your changes
  • rebuild the unikernel completely
  • see whether changes had the desired effect

Here’s a quick primer on how.

OCaml Workshop and Strange Loop Talks

As a result of great encouragement from colleagues and friends, I gave a few talks in September. Persistent Networking with Irmin and MirageOS, which I gave at the OCaml Workshop, is a talk on sticking a persistent database into various levels of the network stack. (It includes demonstrations from What a Distributed, Version-Controlled ARP Cache Gets You, as well as an Irmin-ified NAT device that I haven’t yet written up here.

Fun with Opam: Advice to my Past Self

Most instructions on how to get started with OCaml packages now advise the user to get started with opam, which is excellent advice. Getting up and running with opam is pretty easy, but I wasn’t sure where to go from there when I wanted to modify other people’s packages and use the modifications in my environment. I wish I’d realized that the documentation for making packages has a lot of applicable advice for that use case, as well as the apparent target (making your own packges from scratch).

Retrospective

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:

  • How was the Recurse Center?
  • Was attending RC worth your time?
  • What did you learn at the Recurse Center?

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.

What a Distributed, Version-Controlled ARP Cache Gets You

git (and its distributed version control system friends hg and darcs) have some great properties. Not only do you get a full history of changes on objects stored in them, you can get comments on changes, as well as branching and merging, which lets you do intermediate changes without messing up state for other entities which want to work with the repository.

That’s all pretty cool. I actually want that for some of my data structures, come to think of it. Say, for example, a boring ol’ key-value store which can be updated from a few different threads – in this case, a cache that stores values it gets from the network and the querying/timeout code around it. It would be nice if each thread could make a new branch, make its changes, then merge them into the primary branch once it’s done.

It turns out you can totally do that with Irmin, “the database that never forgets”! I did (and am still doing) a bit of work on sticking a modified version of the MirageOS address resolution protocol code’s data structures into Irmin:

$ git log --all --decorate --oneline --graph
* 68216f3 (HEAD, primary, expire_1429688434.645130) Arp.tick: updating to age out old entries
* ec10c9a entry added: 192.168.3.1 -> 02:50:2a:16:6d:01
* 6446cef entry added: 10.20.254.2 -> 02:50:2a:16:6d:01
* 81cfa43 entry added: 10.50.20.22 -> 02:50:2a:16:6d:01
*   4e1e1c7 Arp.tick: merge expiry branch
|\  
| * cd787a0 (expire_1429688374.601896) Arp.tick: updating to age out old entries
* | 8df2ef7 entry added: 10.23.10.1 -> 02:50:2a:16:6d:01
|/  
* 8d11bba Arp.create: Initial empty cache