CCP Games have a very interesting set of articles on their developer blog about how they tackle lag in EVE Online. EVE Online is interesting in that they claim that the world isn’t sharded, but its not the case that any player can interact instantaneously with any other player with the full game mechanics (i.e. ship to ship space combat in this case). In the book we called this primary awareness. The world is effectively partitioned in to individual solar systems, and there is primary awareness within a solar system, but only secondary awareness amongst the rest of the cohort. Its obviously not “sharded” in the same way as other games, but calling it “shardless” is a little bit of a stretch in our opinion.
Anyway, the articles are an interesting overview of where the lag comes from, the server/cluster architecture, how they test failure conditions under congestion events and how the architecture interacts with game play and bug fixes.
The EVE Online website is full of interesting articles, it is well worth poking around. For example, you can read about their cluster network and their three-layer server architecture.