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A Quick Thought About Mesos-DNS

A colleague recently pointed me to the recent Mesosphere announcement of Mesos-DNS, a DNS-based service discovery mechanism for Apache Mesos clusters. A comment made in the announcement got me thinking, and I wanted to briefly share my thoughts.

The comment that got me thinking was this:

Mesos-DNS is simple and stateless. Unlike Consul and SkyDNS, it does not require consensus mechanisms, persistent storage, or a replicated log.

If you’ve been following along here on my site, you know that I’ve written about Consul before (see here), and I have more Consul content planned. I’m reasonably familiar with Consul’s architecture and requirements. Likewise, although I haven’t specifically written about SkyDNS, it’s based on etcd, which I have talked about (see here). The Mesos-DNS article seems to imply that Mesos-DNS is somehow less complex than either of these two solutions because it doesn’t require consensus mechanisms, persistent storage, or a replicated log.

However, in my mind that’s a misleading statement. Yes, Consul does require a consensus mechanism (it uses Raft, like etcd). SkyDNS (as I understand it, at least) simply leverages etcd, so technically SkyDNS itself doesn’t require a consensus mechanism. And the assertion that SkyDNS itself doesn’t require a consensus mechanism is no different than the assertion that Mesos-DNS itself doesn’t require one either—both of them rely on something else that provides this functionality. In the case of SkyDNS, it’s etcd; in the case of Mesos-DNS, it’s Mesos, which does require a consensus mechanism. Therefore, to say that Mesos-DNS doesn’t require a consensus mechanism (when in fact it relies on one in Mesos) while SkyDNS does (when in fact that functionality is provided by etcd) is misleading. You can either say that neither Mesos-DNS nor SkyDNS require a consensus mechanism (because it is provided by a different component), or you can say that both Mesos-DNS and SkyDNS require a consensus mechanism because both of them rely on another component that incorporates consensus functionality.

Now, this statement is fair when discussing Consul, which does incorporate its own consensus and clustering technologies. Of course, the key difference between Mesos-DNS and Consul is that Mesos-DNS is purpose-built for Mesos environments, whereas Consul could work with any kind of environment. Therefore, Consul had to build in its own consensus/clustering functionality, while Mesos-DNS knew it could rely on the underlying functionality found in Mesos.

I know this doesn’t seem like a very big deal, but I felt like it needed to be mentioned. There is a saying that goes something like this:

“We judge others by their behavior. We judge ourselves by our intentions.”

(The earliest known attribution for this quote is 1917, by Montague Jocelyn King-Harmon in “British Boys: Their Training and Prospects.”)

I think a variation of this saying is applicable here: you can’t chide other products for their external dependencies while overlooking your own.

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