Fork of Paper which adds regionised multithreading to the dedicated server.

Overview

Folia groups nearby loaded chunks to form an "independent region." See REGION_LOGIC.md for exact details on how Folia will group nearby chunks. Each independent region has its own tick loop, which is ticked at the regular Minecraft tickrate (20TPS). The tick loops are executed on a thread pool in parallel. There is no main thread anymore, as each region effectively has its own "main thread" that executes the entire tick loop.

For a server with many spread out players, Folia will create many spread out regions and tick them all in parallel on a configurable sized threadpool. Thus, Folia should scale well for servers like this.

Folia is also its own project, this will not be merged into Paper for the foreseeable future.

Plugin compatibility

There is no more main thread. I expect every single plugin that exists to require some level of modification to function in Folia. Additionally, multithreading of any kind introduces possible race conditions in plugin held data - so, there are bound to be changes that need to be made.

So, have your expectations for compatibility at 0.

API plans

Currently, there is a lot of API that relies on the main thread. I expect basically zero plugins that are compatible with Paper to be compatible with Folia. However, there are plans to add API that would allow Folia plugins to be compatible with Paper.

For example, the Bukkit Scheduler. The Bukkit Scheduler inherently relies on a single main thread. Folia's RegionisedScheduler and Folia's EntityScheduler allow scheduling of tasks to the "next tick" of whatever region "owns" either a location or an entity. These could be implemented on regular Paper, except they schedule to the main thread - in both cases, the execution of the task will occur on the thread that "owns" the location or entity. This concept applies in general, as the current Paper (single threaded) can be viewed as one giant "region" that encompasses all chunks in all worlds.

It is not yet decided whether to add this API to Paper itself directly or to Paperlib.

The new rules

The other important rule is that the regions tick in parallel, and not concurrently. They do not share data, they do not expect to share data, and sharing of data will cause data corruption. Code that is running in one region under no circumstance can be accessing or modifying data that is in another region. Just because multithreading is in the name, it doesn't mean that everything is now thread-safe. In fact, there are only a few things that were made thread-safe to make this happen. As time goes on, the number of thread context checks will only grow, even if it comes at a performance penalty - nobody is going to use or develop for a server platform that is buggy as hell, and the only way to prevent and find these bugs is to make bad accesses fail hard at the source of the bad access.

This means that Folia compatible plugins need to take advantage of API like the RegionisedScheduler and the EntityScheduler to ensure their code is running on the correct thread context.

In general, it is safe to assume that a region owns chunk data in an approximate 8 chunks from the source of an event (i.e player breaks block, can probably access 8 chunks around that block). But, this is not guaranteed - plugins should take advantage of upcoming thread-check API to ensure correct behavior.

The only guarantee of thread-safety comes from the fact that a single region owns data in certain chunks - and if that region is ticking, then it has full access to that data. This data is specifically entity/chunk/poi data, and is entirely unrelated to ANY plugin data.

Normal multithreading rules apply to data that plugins store/access their own data or another plugin's - events/commands/etc are called in parallel because regions are ticking in parallel (we CANNOT call them in a synchronous fashion, as this opens up deadlock issues and would handicap performance). There are no easy ways out of this, it depends solely on what data is being accessed. Sometimes a concurrent collection (like ConcurrentHashMap) is enough, and often a concurrent collection used carelessly will only hide threading issues, which then become near impossible to debug.

Current API additions

• RegionisedScheduler and EntityScheduler acting as a replacement for the BukkitScheduler, however they are not yet fully featured.

Current broken API

• Most API that interacts with portals / respawning players / some player login API is broken.
• ALL scoreboard API is considered broken (this is global state that I've not figured out how to properly implement yet)
• World loading/unloading
• Entity#teleport. This will NEVER UNDER ANY CIRCUMSTANCE come back, use teleportAsync
• Could be more

Planned API additions

• Proper asynchronous events. This would allow the result of an event to be completed later, on a different thread context. This is required to implement some things like spawn position select, as asynchronous chunk loads are required when accessing chunk data out-of-region.
• World loading/unloading
• TickThread#isTickThread overloads to API
• More to come here

Planned API changes

• Super aggressive thread checks across the board. This is absolutely required to prevent plugin devs from shipping code that may randomly break random parts of the server in entirely undiagnosable manners.
• More to come here