Ashes of Creation

zerodahero said:

It's not that Unreal is bad at MMOs, MMO developers almost always implement their own networking stacks. But this is a tremendous amount of work, something one would expect when developing an MMO.
[...]
Epic also has a new experimental networking stack called Iris, which fundamentally changes the networking to handle open world games and large player counts, and will completely replace the existing replication system which has its roots in the old Unreal Tournament games (push vs polling, reducing CPU load on a server).

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The biggest issue I see when you don't develop your own network stack/server and try to shoehorn an existing software to develop an MMO is the underlying persistent nature of an MMO. In normal multiplayer games--even with a large number of players or objects--once the match is over you can kill the server and start a fresh one. If it crashes just that match is affected. All players also start fresh, so it may be frustrating, but the other millions players couldn't care less.

That obviously doesn't fly in MMOs, especially when it is a competitive setting. You not only have to save every little bit of progress of any character, you also have to have a central authority. Doing server meshing is a headache in this situation, because if not watertight it can open yourself to a lot of transactional problems. The most glaring issue is duping of money/items, but also things like concurrent mining of limited resources can be an issue.

To top it off, if you do the ACID thing correctly, server meshing might not even let you scale out that much because everything is limited by the database in the first place.

zerodahero said:

Persistency can be accomplished through a variety of databases, with some sort of high performance cache service sitting between the database and game server(s). At some point in development, the game designers must choose how much of the world will live on between sessions (in an MMO, this is normally player states, quests, inventory, housing and perhaps some high end raid encounter timers).

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Caching is not the silver bullet for transactional issues. You are just moving the fundamental issue of ACID properties from the database to the "high performance cache service". We called things like this "middleware" back in the day, and I've seen and heard about horrors with in-house developed servers like those so many times (especially from Java developers that have no database or computer science background) that I stopped counting after all my fingers and toes were used up. "That transaction number, the one that shows up on customer invoices ... shouldn't that be ... I don't know ... unique?"

Remember back in the day when playing EverQuest and you got chat messages like "Saving character", and the common wisdom was when you looted a great item to drop a copper coin to force the server to update your character in the database?

Also, the issue is exacerbated when you decide to have a non-instanced + seamless world, like in the case of Ashes of Creation, because there is no easy way to partition.

zerodahero said:

At some point in development, the game designers must choose how much of the world will live on between sessions (in an MMO, this is normally player states, quests, inventory, housing and perhaps some high end raid encounter timers).

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That is a way to partition the load, but at the same time MMO systems are usually interlocking and interdependent: Quests requiring looting items (and when following the WoW design, also affecting loot tables), player states being affected by zone data (for example like M+ affixes, or jumping into water or lava), and combat being affected by items, buffs, or encounters.

zerodahero said:

The DB cache for server meshing would most certainly need to use event sourcing to keep the multiple server-nodes in synch:

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You are describing a transaction log. You know, the like the ones we are using in things like ... databases and journaling filesystems? Although filesystems usually use intent logs.