— modern ops stuff —
Host Naming Schemes
10 July 2013 // Chef

You wouldn’t think that, in 2013, you’d still have to tell people the importance of a decent host-naming scheme. Surely we’ve all grown out of calling things after Star Wars characters. But, having just been in a meeting with Amazon, and hearing about its Chef-based OpsWorks project, we apparently haven’t.

I work as a contractor, and I move between a lot of sites. I appreciate documentation of all kinds, but I like nothing more than things with document themselves. The most obvious of these is the host naming scheme.

War Stories

I’ve seen more than a few ways of naming hosts on my travels. One site took “host names” literally, and used names for their hosts. People’s names. Given names and family names. Every machine had an internal and an external DNS name. So, from the inside, you’d refer to one of the webservers as “leonardo” from the inside, but from the outside it resolved to “davinci”. Obvious, right? The problem was that you needed a reasonable knowledge of literature to understand that “nadine” and “gordimer” referred to the same machine, and be a good-spelling astrophysicist to remember “subramanyan” was also known as “chandresehkar”. As a contractor, I spent over half a day on that site trying to find “george”. It turned out to be a virtual circuit on a content switch. Work that out from the name. Someone had to pay me half a day’s rate, and push everything else back half a day, because someone else thought they were being “clever”.

Another site used a lot of Sun Clusters, and the pairs were thematically named. For instance, “flyover” and “underpass”. Great, you can tell which machines form pairs with only a little lateral thinking. But what you can’t tell is what those servers might do, which business unit they belong to or, more imporantly, which server is on which site. Every cluster had a node in each of the two data centres the business owned, and there was also a SAN in each. As I was doing a SAN migration, I needed to know which SAN was local to which host. I couldn’t tell from the hostname, and no one else knew which was where either. So, lots of time finding people and asking, resolved eventually by a time-consuming trip to a data centre. We also had an alternative energy naming theme, including “solar”, “wind”, “wave”, and “tidal”. Can you spot the two nodes of a cluster in there? Me neither.

Way back in the day I took delivery of a lab full of Sun workstations. Now, here’s where the old “call them after anything” scheme isn’t too offensive. If it’s called after a colour, it’s in the second floor lab

Where, What, Who, How Many?

I built a completely new infrastructure at the site with “subramanyan chandresehkar”. We had almost all Sun gear, with a dash of Linux, and at the time things were being built, a single data centre near Coventry, with plans to move some kit to another in London. I wanted anyone to be able to look at the hostname of a machine, and tell where and what it was doing. For instance: cs-infra-01. Coventry datacentre. Solaris host. infrastructure server number 01. Simple, and the dashes, though they make it a little longer to type, make it much easier to read than csinfra01. A Linux mailhost in the London datacentre? That’ll be ll-mail-01. Obvious.

That site used a lot of Solaris zones. In fact, pretty much everything was in a non-global zone, with globals acting only as hypervisors, much in the way SmartOS does things now. Mail didn’t run under Linux in London, but under Solaris in Coventry, in a zone on each of the aforementioned infrastructure servers. cs-infra-01z-mail. It’s a mail zone, on the first infrastructure server, running Solaris, in Coventry. A bit long, I admit, but I think much more helpful than “gustav”, which it replaced.

I did run into problems when we started using zones inside Logical Domains, ending up with things like cd-01d-dbz-mysql01, which I admit needed a bit of unpicking. (Coventry Domain server 01; Domain db; zone mysql instance 01 We didn’t have many of those though, and I never said the system was perfect! Perhaps you can have too much information?

Naming Schemes and Configuration Management

Finally, I get to the point I wanted to make, which is that under configuration management, naming schemes are more important than ever.

To date, I have worked on two Chef projects of significant size and complexity. In one, the site already had a strong naming scheme of the form environment-product-service[0-9][0-9] The second had a kind of half-baked system where you could tell the physical location of the host, possibly work out its environment, and probably tell at least some of the purpose.

At the first site, a knife plugin expanded the hostname of the instance you wished to start, and could easily work out what environment the node should be in, and which roles it needed. This gave us an elegant, easy to maintain and understand system with little repetition. In the second, we had to use node files to describe everything. Time-consuming and error-prone.

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