Ha! I've been agonizing on this one. Half of my brain was telling me: "You are being silly. It took you a bit to figure out what "updater" really meant but eventually it makes sense". The other half of my brain however just didn't want to believe that a better term could be found even thought I didn't feel comfortable with my final choice.

I think there is an ambiguity stemming from the term "updatER" perhaps implying too much a "subject updating an object" meaning, when in reality here the update() method hints to the updater updating itself rather than a separate object. 

Now, I can see how a component system can be an updater of components. But this concept is a bit removed from this particular piece of code. "updatABLE" or "updateABLE" would make more sense to me given the UpdateSubscriberSystem interface. Would "updatable" be acceptable?