2 unstable releases
0.5.0 | Sep 1, 2024 |
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0.3.0 | May 5, 2024 |
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Aper
Aper is a Rust library for data synchronization using state machines. Aper provides mechanisms to represent common data structures in terms of state machines, as well as a transport-agnostic protocol for keeping multiple instances of a state machine synchronized across a network.
Use-cases include real-time multiplayer applications that operate on shared state, client-server applications that want to share state updates incrementally and bidirectionally, and multiplayer turn-based games.
What is a state machine?
For the purposes of Aper, a state machine is simply a struct
or enum
that
implements StateMachine
and has the following properties:
- It defines a
StateMachine::Transition
type, through which every possible change to the state can be described. It is usually useful, though not required, that this be anenum
type. - It defines a
StateMachine::Conflict
type, which describes a conflict which may occur when a transition is applied that is not valid at the time it is applied. For simple types where a conflict is impossible, you can useNeverConflict
for this. - All state updates are deterministic: if you clone a
StateMachine
and aTransition
, the result of applying the cloned transition to the cloned state must be identical to applying the original transition to the original state.
Here's an example StateMachine
implementing a counter:
use aper::{Aper, AperSync};
use serde::{Serialize, Deserialize};
#[derive(Serialize, Deserialize, Clone, Debug, Default, AperSync)]
struct Counter { value: i64 }
#[derive(Serialize, Deserialize, Clone, Debug, PartialEq)]
enum CounterTransition {
Reset,
Increment(i64),
Decrement(i64),
}
impl Aper for Counter {
type Transition = CounterTransition;
type Conflict = NeverConflict;
fn apply(&self, event: &CounterTransition) -> Result<Counter, NeverConflict> {
match event {
CounterTransition::Reset => Ok(Counter {value: 0}),
CounterTransition::Increment(amount) => Ok(Counter {value: self.value + amount}),
CounterTransition::Decrement(amount) => Ok(Counter {value: self.value - amount}),
}
}
}
Why not CRDT?
Conflict-free replicated data types are a really neat way of representing data that's shared between peers. In order to avoid the need for a central “source of truth”, CRDTs require that update operations (i.e. state transitions) be commutative. This allows them to represent a bunch of common data structures, but doesn't allow you to represent arbitrarily complex update logic. By relying on a central authority, a state-machine approach allows you to implement data structures with arbitrary update logic, such as atomic moves of a value between two data structures, or the rules of a board game.
Aper is rapidly evolving. Consider this a technology preview. See the list of issues outstanding for version 1.0
Dependencies
~11–18MB
~254K SLoC