System design 101

I’ve seen a certain pattern of thinking in engineering organizations that feels backwards to me but I keep running up against it. Part of the reason it feels backwards to me is probably because I tend to think axiomatically and the pattern of thinking that I’ve observed makes no sense logically. Abstractly the argument almost always involves 3 components of a system (A, B, C). If it makes things simpler you can pretend A = front-end, B = back-end, and C = database. Continue reading

Terraform as a backend (in a compiler sense)

Every time I’ve tried to use terraform proper I have failed. Fundamentally it is a non-composable tool so if you want compostionlity you have to work around its limitations. One way I’ve found to force compositionality is to not rely on any of its high level features. I only use it for managing resource graphs. That’s the only part I think it is good at and that’s the only part that consistently works. If you feed it a resource graph then it will mostly work. This means you’ll have to find some way to generate and feed it the resource graph. There are a few ways to do it and in the past I would have recommended ERB but there is actually a simpler way that just requires generating JSON. You could generate the JSON from anything you are comfortable with that provides the kind of compostionality you expect. I’m going to outline how I use Rake and Ruby to do this for a very basic VPC configuration across several regions and availability zones. Continue reading

Bootstrapping a consul cluster with cloud-init-buddy

Bootstrapping a consul cluster is a non-trivial operation mostly because it requires sequencing the startup of the cluster nodes with the starting of the consul agents. You can’t form a cluster until you know the IP addresses of the nodes to pass to the consul agent for registration and cluster formation. So you first need to start the nodes and then distribute the IP addresses to all the nodes that need to be in the cluster. This is where cloud-init-buddy comes in. Cloud-init-buddy provides the glue for the cluster nodes to discover each other and not worry about sequencing startup and setup. Everything happens in the cloud-init script with all the sequencing and discovery automatically handled by the cluster nodes themselves. Continue reading

Unsolved but tractable problems

The world is full of unsolved but tractable problems. One of my recent projects needed an AWS spot market simulator but all the obvious searches didn’t turn up anything so I went ahead and wrote one for my use cases.

Another but slightly smaller problem was about writing to and rotating logs based on number of lines instead of file size or timestamps and again the obvious searches turned up no results so again I wrote something.

Never be afraid to tackle small and tractable problems. Never let the haters get in the way of making and sharing something because there are people out there that will find your work useful and at the end of the day that’s what counts.

Project checkpoint: AWS spot market simulator

Finally settled on a structure that seems to work. My initial design was doing everything in Ruby by shuffling elements between sets and hashes and this was a little hard to follow. After stepping back from the problem I noticed that I was basically re-inventing database tables very poorly so I shifted everything to using an in-memory SQLite database.

Update: I’ve now written a simulator that can be used for testing bidding and control strategies without having to pay for actual server time.

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