November 2009

FAM09: Metadata Aggregation

Metadata aggregation as a route to cross-federation inter-operation continues to be my main focus for the year, and yesterday I delivered a presentation on the subject at JISC's Federating the next generation event.

I think the talk went reasonably well; a couple of people remarked that they liked having the key concepts separated out and clarified. People even chuckled in the right places a couple of times.

Checking Twitter for the #FAM09 tag I find that the main thing a couple of people took away from the talk was a snarky remark I made about XSLT. Curiously, I find that I'm fine with that.

As usual, here's a PDF version of my slides from the presentation:

20091123-Metadata-Aggregation.pdf

There are a fair number of animated diagrams in this talk, and not as many words as usual. That might mean that some parts are hard to follow without hearing me talk. I'm going to try and get hold of the audio recording made at the time and will upload a slide-synchronised version of the talk later if possible.

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No Hats

Please do not take photos with hats on

Seen on a recent trip to San Antonio, Texas, which is probably the last place you'd expect to see any attempt to constrain the use of any kind of headwear.

Obviously they don't mean you can't wear your own hat while taking photos; what they want to prevent is people wearing the display hats for the purposes of having their photographs taken. Or at least I think so; there were no hats in the vicinity of this particular notice.

Imperfect

Many of us, particularly if we have been programmers, have got into the habit of regarding computers as flawless execution engines. People with more of an electronics background tend to be a bit more sceptical, I think.

I've been trying to figure out why I couldn't burn a Fedora 11 DVD to upgrade one of my oldest machines for several months now. I had checked the SHA-256 hash of the download then copied the file from the server where I run BitTorrent across to a desktop machine's external hard drive. The burned disk verified against the image on the machine that created it but the installation self-test always failed, claiming the disk was corrupt. I tried burning from the same image on another machine; I tried burning at different speeds; I tried different blank DVDs. No change.

Finally, today, I thought to try verifying the hash on the copied image rather than the original one. It was different. Comparing the original download with the copy, I discovered two locations in the copy where byte 0x12 of a block had dropped the 0x08 bit.

It's probably not a coincidence that the machine on which I made the corrupted copy has recently come back from a couple of extended "warranty repair" holidays during which first the main system logic board and then (at my strong and repeated insistence) the actual DRAM were replaced. The machine had been having some intermittent problems involving applications shutting down unexpectedly; these looked like memory issues to me but the manufacturer's diagnostics had always given it a clean bill of health. As an old-school computer guy, of course, I know that the manufacturer's diagnostics never detect real memory issues.

The moral of the story? I'm not sure there is one: "faulty hardware sometimes gives the wrong answer" seems rather an obvious thing to say. On the other hand, if you are aware of the concept of metastability in electronics, you know that there's no such thing as perfect hardware as long as the logic needs to talk to the outside world. So we can reduce the frequency of odd weirdness to the point where we never expect to encounter it, but we can never make it go away altogether.

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