Beyond ThunderDock

I Plugged Some Stuff Into A Thunderbolt Dock. You Won’t Believe what Happens Next

This weekend I found myself pleased to receive a Kensington SD5000T Thunderbolt 3 Docking Station.

Some of its functionality was a bit of a weird surprise.

The Setup

Due to my ... accretive history with computer purchases, I have 3 things on my desk at home: a USB-C macbook pro, a 27" Thunderbolt iMac, and an older 27" Dell display, which is old enough at this point that I can’t link it to you. Please do not take this to be some kind of totally sweet setup. It would just be somewhat pointlessly expensive to replace this jumble with something nicer. I purchased the dock because I want to have one cable to connect me to power & both displays.

For those not familiar, iMacs of a certain vintage1 can be jury-rigged to behave as Thunderbolt displays with limited functionality (no access from the guest system to the iMac’s ethernet port, for example), using Target Display Mode, which extends their useful lifespan somewhat. (This machine is still, relatively speaking, a powerhouse, so it’s not quite dead yet; but it’s nice to be able to swap in my laptop and use the big screen.)

The Link-up

On the back of the Thunderbolt dock, there are 2 Thunderbolt 3 ports. I plugged the first one into a Thunderbolt 3 to Thunderbolt 2 adapter which connects to the back of the iMac, and the second one into the Macbook directly. The Dell display plugs into the DisplayPort; I connected my network to the Ethernet port of the dock. My mouse, keyboard, and iPhone were plugged into the USB ports on the dock.

The Problem

I set it up and at first it seemed to be delivering on the “one cable” promise of thunderbolt 3. But then I switched WiFi off to test the speed of the wired network and was surprised to see that it didn’t see the dock’s ethernet port at all. Flipping wifi back on, I looked over at my router’s control panel and noticed that a new device (with the expected manufacturer) was on my network. nmap seemed to indicate that it was... running exactly the network services I expected to see on my iMac. VNCing into the iMac to see what was going on, I popped open the Network system preference pane, and right there alongside all the other devices, was the thunderbolt dock’s ethernet device.

The Punch Line

Despite the miasma of confusion surrounding USB-C and Thunderbolt 32, the surprise here is that apparently Thunderbolt is Thunderbolt, and (for this device at least) Thunderbolt devices connected across the same bus can happily drive whatever they’re plugged in to. The Thunderbolt 2 to 3 adapter isn’t just a fancy way of plugging in hard drives and displays with the older connector; as far as I can tell all the functionality of the Thunderbolt interface remains intact as both “host” and “guest”. It’s like having an ethernet switch for your PCI bus.

What this meant is that when I unplugged everything and then carefully plugged in the iMac before the Macbook, it happily lit up the Dell display, and connected to all the USB devices plugged into the USB hub. When I plugged the laptop in, it happily started charging, but since it didn’t “own” the other devices, nothing else connected to it.


This dock works a little bit too well; when I “dock” now I have to carefully plug in the laptop first, give it a moment to grab all the devices so that it “owns” them, then plug in the iMac, then use this handy app to tell the iMac to enter Target Display mode.

On the other hand, this does also mean that I can quickly toggle between “everything is plugged in to the iMac” and “everything is plugged in to the MacBook” just by disconnecting and reconnecting a single cable, which is pretty neat.

  1. Sadly, not the most recent fancy 5K ones. 

  2. which are, simultaneously, both the same thing and not the same thing. 

As some of you may have guessed from the unintentional recent flurry of activity on my Twitter account, twitter feed, the service I used to use to post blog links automatically, is getting end-of-lifed. I've switched to for the time being, unless they send another unsolicited tweetstorm out on my behalf...

Sorry about the noise! In the interests of putting some actual content here, maybe you would be interested to know that I was recently interviewed for PyDev of the Week?

Probably best to get this out of the way before this weekend:

If I meet you at a technical conference, you’ll probably see me extend my elbow in your direction, rather than my hand. This is because I won’t shake your hand at a conference.

People sometimes joke about “con crud”, but the amount of lost productivity and human misery generated by conference-transmitted sickness is not funny. Personally, by the time the year is out, I will most likely have attended 5 conferences. This means that if I get sick at each one, I will spend more than a month out of the year out of commission being sick.

When I tell people this, they think I’m a germophobe. But, in all likelihood, I won’t be the one getting sick. I already have 10 years of building up herd immunity to the set of minor ailments that afflict the international Python-conference-attending community. It’s true that I don’t particularly want to get sick myself, but I happily shake people’s hands in more moderately-sized social gatherings. I’ve had a cold before and I’ve had one again; I have no illusion that ritually dousing myself in Purell every day will make me immune to all disease.

I’m not shaking your hand because I don’t want you to get sick. Please don’t be weird about it!

Remember that thing I said in my pycon talk about native packaging being the main thing to worry about, and single-file binaries being at best a stepping stone to that and at worst a bit of a red herring? You don’t have to take it from me. From the authors of a widely-distributed command-line application that was rewritten from Python into Go specifically for easier distribution, and then rewritten in Python:

... [the] majority of people prefer native packages so distributing precompiled binaries wasn’t a big win for this type of project1 ...

I don’t want to say “I told you so”, but... no. Wait a second. That is exactly what I want to do. That is what I am doing.

I told you so.

Hello lazyweb,

I want to run some “legacy” software (Trac, specifically) on a Swarm cluster. The files that it needs to store are mostly effectively write-once (it’s the attachments database) but may need to be deleted (spammers and purveyors of malware occasionally try to upload things for spamming or C&C) so while mutability is necessary, there’s a very low risk of any write contention.

I can’t use a networked filesystem, or any volume drivers, so no easy-mode solutions. Basically I want to be able to deploy this on any swarm cluster, so no cheating and fiddling with the host.

Is there any software that I can easily run as a daemon that runs in the background, synchronizing the directory of a data volume between N containers where N is the number of hosts in my cluster?

I found this but it strikes me as ... somehow wrong ... to use that as a critical part of my data-center infrastructure. Maybe it would actually be a good start? But in addition to not being really designed for this task, it’s also not open source, which makes me a little nervous. This list, or even this smaller one is huge and bewildering. So I was hoping you could drop me a line if you’ve got an idea what I could use for this.