apecs

Asyncronous Parallel Entity Component System

apecs is an entity-component system written in Rust that supports traditional syncronous systems as well as asyncronous systems that can evolve over time. This makes it great for general applications, quick game prototypes, DIY engines and any simulation that has discrete steps.

Why

Most ECS libraries (and game main-loops in general) are polling based. This is great for certain tasks, but things get complicated when programming in the time domain. Async / await is great for programming in the time domain without explicitly spawning new threads or blocking, but it isn't supported by ECS libraries. apecs was designed to allow async / await programming within ECS systems.

What and How

At its core apecs is a library for sharing resources across disparate polling and async loops. It uses derivable traits and channels to orchestrate systems' access to resources and uses rayon (where available) for concurrency.

Asyncronous systems

Async systems are system functions that are async. Specifically async systems have this type signature:

use apecs::{Facade, anyhow};

async fn my_system(mut facade: Facade) -> anyhow::Result<()> {
    //...
    Ok(())
}

The Facade type is like a window into the world. It can visit bundles of resources in the world asyncronously. This allows your async system to affect different parts of the world at different times.

Syncronous systems are great for tight loops that are always iterating over the same data. In other words sync systems are highly optimized algorithms that run in the hot path. But they don't quite fit in situations where the system's focus changes over time, or when the system needs to wait for some condition before doing something different. Async systems are a good fit for these situations. Async systems are therefore a good fit for high-level orchestration.

For example you might use an async system to setup your title screen, wait for user input and then start the main game simulation by injecting your game entities.

Another example might be an app that has a dependency graph of work to complete. An Async system can hold the dependencies as a series of async operations that it is awaiting, while syncronous systems do the hot-path work that completes those async operations as fast as possible.

Goals

• productivity
• flexibility
• observability
• very well rounded performance, competitive with inspirational ECS libraries
  • like specs, bevy_ecs, hecs, legion, shipyard, planck_ecs

Features

• syncronous systems with early exit and failure

  use apecs::*;
  
  #[derive(Clone, Copy, Debug, Default, PartialEq)]
  struct Number(u32);
  
  let mut world = World::default();
  world
      .with_system("demo", |mut u32_number: Write<Number>| -> anyhow::Result<ShouldContinue> {
          u32_number.0 += 1;
          if u32_number.0 == 3 {
              end()
          } else {
              ok()
          }
      })
      .unwrap();
  world.run();
  assert_eq!(Number(3), *world.resource::<Number>().unwrap());
  

• async systems, ie systems that end and/or change over time (for scenes, stories, etc)
  • fetch and visit resources from the world asyncronously. If they have not been added and can be created by default, they will be. Write and Read will create default resources during fetching if possible.
  • resources are acquired without lifetimes
  • when fetched resources are dropped they are sent back into the world

  use apecs::*;
  
  #[derive(Clone, Copy, Debug, Default, PartialEq)]
  struct Number(u32);
  
  async fn demo(mut facade: Facade) -> anyhow::Result<()> {
      loop {
          let i = facade.visit(|mut u32_number: Write<Number>| {
              u32_number.0 += 1;
              Ok(u32_number.0)
          }).await?;
          if i > 5 {
              break;
          }
      }
      Ok(())
  }
  
  let mut world = World::default();
  world
      .with_async("demo", demo)
      .unwrap();
  world.run();
  assert_eq!(Number(6), *world.resource::<Number>().unwrap());
  

• support for async futures

  use apecs::*;
  let mut world = World::default();
  world
      .spawn(async {
          log::trace!("hello");
      });
  world.run();
  

• fetch data (system data) derive macros

  use apecs::*;
  
  #[derive(CanFetch)]
  struct MyData {
      entities: Read<Entities>,
      u32_number: Write<u32>,
  }
  
  let mut world = World::default();
  let mut my_data: MyData = world.fetch().unwrap();
  *my_data.u32_number = 1;
  

• system scheduling
  • compatible systems are placed in parallel batches (a batch is a group of systems that can run in parallel, ie they don't have conflicting borrows)
  • systems may depend on other systems running before or after
  • barriers

  use apecs::*;
  
  fn one(mut u32_number: Write<u32>) -> anyhow::Result<ShouldContinue> {
      *u32_number += 1;
      end()
  }
  
  fn two(mut u32_number: Write<u32>) -> anyhow::Result<ShouldContinue> {
      *u32_number += 1;
      end()
  }
  
  fn exit_on_three(mut f32_number: Write<f32>) -> anyhow::Result<ShouldContinue> {
      *f32_number += 1.0;
      if *f32_number == 3.0 {
          end()
      } else {
          ok()
      }
  }
  
  fn lastly(
      (u32_number, f32_number): (Read<u32>, Read<f32>),
  ) -> anyhow::Result<ShouldContinue> {
      if *u32_number == 2 && *f32_number == 3.0 {
          end()
      } else {
          ok()
      }
  }
  
  let mut world = World::default();
  world
      // one should run before two
      .with_system_with_dependencies("one", one, &[], &["two"])
      .unwrap()
      // two should run after one - this is redundant but good for illustration
      .with_system_with_dependencies("two", two, &["one"], &[])
      .unwrap()
      // exit_on_three has no dependencies
      .with_system("exit_on_three", exit_on_three)
      .unwrap()
      // all systems after a barrier run after the systems before a barrier
      .with_system_barrier()
      .with_system("lastly", lastly)
      .unwrap();
  
  assert_eq!(
      vec![
          vec!["one", "exit_on_three"],
          vec!["two"],
          vec!["lastly"],
      ],
      world.get_sync_schedule_names()
  );
  
  world.tick().unwrap();
  
  assert_eq!(
      vec![vec!["exit_on_three"], vec!["lastly"],],
      world.get_sync_schedule_names()
  );
  
  world.tick().unwrap();
  world.tick().unwrap();
  assert!(world.get_sync_schedule_names().is_empty());
  

• component storage
  • optimized for space and iteration time as archetypes
  • queries with "maybe" and "without" semantics
  • queries can find a single entity without iteration or filtering
  • add and modified time tracking
  • parallel queries (inner parallelism)

  use apecs::*;
  
  // Make a type for tracking changes
  #[derive(Default)]
  struct MyTracker(u64);
  
  // Entities and Components (which stores components) are default
  // resources
  let mut world = World::default();
  world
      .with_system("create", |mut entities: Write<Entities>| {
          for mut entity in (0..100).map(|_| entities.create()) {
              entity.insert_bundle((0.0f32, 0u32, format!("{}:0", entity.id())));
          }
          end()
      }).unwrap()
      .with_system_with_dependencies("progress", |q_f32s: Query<&mut f32>| {
          for f32 in q_f32s.query().iter_mut() {
              **f32 += 1.0;
          }
          ok()
      }, &["create"], &[]).unwrap()
      .with_system_with_dependencies(
          "sync",
          |(q_others, mut tracker): (Query<(&f32, &mut String, &mut u32)>, Write<MyTracker>)| {
              for (f32, string, u32) in q_others.query().iter_mut() {
                  if f32.was_modified_since(tracker.0) {
                      **u32 = **f32 as u32;
                      **string = format!("{}:{}", f32.id(), **u32);
                  }
              }
              tracker.0 = apecs::current_iteration();
              ok()
          },
          &["progress"],
          &[]
      ).unwrap();
  
  assert_eq!(
      vec![
          vec!["create"],
          vec!["progress"],
          vec!["sync"],
      ],
      world.get_sync_schedule_names()
  );
  
  world.tick(); // entities are created, components applied lazily
  world.tick(); // f32s are modified, u32s and strings are synced
  world.tick(); // f32s are modified, u32s and strings are synced
  
  let q_bundle: Query<(&f32, &u32, &String)> = world.fetch().unwrap();
  assert_eq!(
      (2.0f32, 2u32, "13:2".to_string()),
      q_bundle.query().find_one(13).map(|(f, u, s)| (**f, **u, s.to_string())).unwrap()
  );
  

• outer parallelism (running systems in parallel)
  • parallel system scheduling
  • parallel execution of async futures
  • parallelism is configurable (can be automatic or a requested number of threads, including 1)

  use apecs::*;
  
  #[derive(Default)]
  struct F32(f32);
  
  let mut world = World::default();
  world
      .with_system("one", |mut f32_number: Write<F32>| {
          f32_number.0 += 1.0;
          ok()
      }).unwrap()
      .with_system("two", |f32_number: Read<F32>| {
          println!("system two reads {}", f32_number.0);
          ok()
      }).unwrap()
      .with_system("three", |f32_number: Read<F32>| {
          println!("system three reads {}", f32_number.0);
          ok()
      }).unwrap()
      .with_parallelism(Parallelism::Automatic);
  world.tick();
  

• plugins (groups of systems, resources and sub-plugins)

  use apecs::*;
  
  #[derive(Default)]
  struct MyTracker(u64);
  
  #[derive(CanFetch)]
  struct SyncData {
      q_dirty_f32s: Query<(&'static f32, &'static mut String)>,
      tracker: Write<MyTracker>,
  }
  
  fn create(
      (mut entities, mut components): (Write<Entities>, Write<Components>),
  ) -> anyhow::Result<ShouldContinue> {
      // create a bunch of entities in bulk, which is quite fast
      let ids = entities.create_many(1000);
      components.extend::<(f32, String)>((
          Box::new(ids.clone().into_iter().map(|id| Entry::new(id, id as f32))),
          Box::new(
              ids.into_iter()
                  .map(|id| Entry::new(id, format!("{}:{}", id, id))),
          ),
      ));
      end()
  }
  
  fn progress(q: Query<&mut f32>) -> anyhow::Result<ShouldContinue> {
      for f32 in q.query().iter_mut() {
          **f32 += 10.0;
      }
      ok()
  }
  
  fn sync(mut data: SyncData) -> anyhow::Result<ShouldContinue> {
      for (f32, string) in data.q_dirty_f32s.query().iter_mut() {
          if f32.was_modified_since(data.tracker.0) {
              **string = format!("{}:{}", f32.id(), **f32);
          }
      }
      data.tracker.0 = apecs::current_iteration();
      ok()
  }
  
  // now we can package it all up into a plugin
  let my_plugin = Plugin::default()
      .with_system("create", create, &[], &[])
      .with_system("progress", progress, &["create"], &[])
      .with_system("sync", sync, &[], &[]);
  
  let mut world = World::default();
  world.with_plugin(my_plugin).unwrap();
  world.tick();
  

• fully compatible with WASM and runs in the browser

Roadmap

• your ideas go here

Tests

cargo test
wasm-pack test --firefox crates/apecs

I like firefox, but you can use different browsers for the wasm tests. The tests make sure apecs works on wasm.

Benchmarks

The apecs benchmarks measure itself against my favorite ECS libs: specs, bevy, hecs, legion, shipyard and planck_ecs.

cargo bench -p benchmarks

Minimum supported Rust version 1.65

apecs uses generic associated types for its component iteration traits.