Category Archives: Design

Keyspace on the back of an envelope

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One question that always plagues me: should I use 32-bit or 64-bit keys. I can never make a firm decision on this point. But I’ve just done some back of the envelope calculations in thinking about it.

If you issued one identity per second, every second of every day, it would take 136 years to exhaust the keyspace.

If you issued one thousand identities per second, it would take 50 days to exhaust the keyspace.

Computer coffee table

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Here’s something fun. I had a computer sitting on the floor in my lab for a few months while I waited for some parts to arrive so I could service it. In the intervening time I discovered that it was rather handy, and functioned as a small “coffee table” in the center of my room which was good for storing books and magazines on. So I taped some cardboard to the base (for when I stub my toe on it!) and have kept it as a permanent fixture now.
John's new coffee table made from an old computer case

Target then source

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I’m surprised it took me this long to figure this out. But there’s a very big problem with a command like this:

rsync var/data/ target:/var/data/

The problem is that in order to type that command, you first have to type the first part of the command, which includes this command:

rsync var/data/ target:/

That first part of the command is in fact a valid command, and if you pressed ENTER accidentally before you had finished typing the whole command, then rsync would begin to replace your entire root file system with the contents of var. That’s the sort of thing that will ruin your day.

So from now on, when I write command-line tools, I will nominate the target *before* the source, and if both the target and source are not specified then I will return an error.

p.s. Yes, the reverse problem exists, that when you nominate the target first you can still fuck up with the source, but given that you are operating on the target from the source, if you get the source wrong, you can just fix your command and run it again and it will be fixed up.