Discussion on "The Tri-Z Architecture: a Pattern for Layering ZIO Applications in Scala"

Pierre Ricadat · 2024-08-05T07:15:46.896Z

After working on several different services and spending a lot of time improving the code to make it easier to use, I discovered a pattern for layering my applications that I found very useful. First, a disclaimer: this architecture is not a one-size...

Sorry to bother you again, but I'd like to ask about cross-user processing, such as leaderboards and server-wide emails. These are very similar to singleton services. Can sharding help with this? How do you handle the pressure on these types of services?

How about discussing on Discord instead of here? Comments are not ideal for long conversations. There’s a Shardcake channel on the ZIO Discord.

Is it convenient to use your Gmail? Pierre Ricadat

ZHE ZHANG I’d rather use Discord since other people can benefit from reading it. Potentially a GitHub issue if it’s specifically about Shardcake.

Hello, I saw in your "Beautiful Domain Logic" slide that you discussed Event Sourcing, and it seems like events are written to both a database and a message queue like Kafka. My questions are: Is Event Sourcing actually used in games? Does the database encounter performance issues due to the large number of event records? How is Event Sourcing integrated with sharding (like Shardcake)?

Right, we are not using Akka persistence and instead use our own code. We're just using github.com/typelevel/doobie to communicate with the database.

We don't use STM in this part of the code actually, it's all one big state object in memory. When we load an actor, we load the latest snapshot and journals, replay those to have the latest state in memory. Then we serve reads and writes from that.

When we need a "fast access" to some data without loading the actor, we have some view tables in the database that are updated via Kafka: after the state is updated and saved in DB we publish some Kafka messages and a separate component reads those and update the views.

I'm also curious, if we use Kubernetes rolling updates, will the database read pressure be very high when the in-memory actor state is rebuilt? Pierre Ricadat

In our case it has never been a problem (since rolling updates start pods sequentially the pressure is not that high).

If ZPure is used together with Event Sourcing, because Event Sourcing has an EventHandler that folds events and applies them to the state, and ZPure also describes the change from the oldState to the newState, does this mean that the change function is provided by the State model and is called in both ZPure and the EventHandler? But is it still necessary to coordinate the changes in ZPure and the changes in Event Sourcing to be consistent?

The way we do it is that we have a function liftEvent that "logs" the event but also runs a "transition" that modifies the state. We use a typeclass for Transition[Event]. When running the ZPure we get the new state with the list of lifted events and we can write those event to our event store. On replay, we run the same transitions for each event.

Many Thanks! I feel like these tricks are worth writing a new blog post about～

I have one more question. If there are two programs, for example, Guild and Inventory, with their states being GuildState and InventoryState respectively, and if I want to combine them to write new logic, do I need some kind of "glue" program?

I usually keep the State common for all my programs. If they are really different entities, I keep the logic completely separate and have them communicate with each other (e.g. via messages).

How to mix Random/Clock(ZIO side effects) with ZPure(pure domain)? While generating some random rewards or properties for equipments in business logic.

Since they return ZIO, you can't use them directly. In our case we often include the current time in the environment R (so we get it before running the ZPure and then the value is consistent during the execution since the environment is read-only). For random, we use Scala Random directly wrapped inside ZPure.suspend.

I apologize, I'm still a bit confused about the Random part.

I understand it's like defining something like: def nextInt: Program[Int] = ZPure.suspend(ZPure.attempt(rng.nextInt()))?

Does using Scala's Random directly lead to a loss of purity? Could you please explain it a bit more concretely? Thank you.

That's right, that's how we do if we need it. It's non-deterministic but I think it's acceptable since it doesn't cause any actual side effects.

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Very Interesting, but I am still confused... If Shardcake entity handling messages one at a time, just like Akka actor does. Why do you need to handle race condition?

This allows you to handle the actor messages in parallel instead of sequentially (e.g. using mapZIOPar or just forking each message handling). Now depending on the use case that may or may not be desirable. If there would be a lot of contention (each request handling accessing the same objects), sequentially is probably better. In that case you can still use the ZIO/ZPure separation and skip the ZSTM layer. In my use cases (games), we often want to respond to client requests ASAP so parallelizing is better.

Thank you for the comprehensive explanation. I learned a lot. Much appreciated.

Thank you for this insightful review of stacked technologies. But I still wonder if guilds are contained in one server but other things are not. I guess the ZIO effect code needs to access multiple guilds from different servers. How do you solve this communication issue?

Hi, For the ZPure type W, you mentioned "I've used it for storing events in an event sourcing application, for feeding a stream of messages to players in a game, but also for storing actual logs" Isn't logging or firing events a side effect? And should no be in ZPure?

That's right, but in the ZPure layer I only accumulate them. After running the ZPure, if the program was successful only then I actually log them or do something with them.

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The Tri-Z Architecture: a Pattern for Layering ZIO Applications in Scala

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