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//! Defines the [`State`] trait and convenience type aliases.
use std::fmt::Debug;
use crate::LogEntry;
use crate::NodeInfo;
/// Type alias to extract a state's [`Context`][State::Context] type.
pub type ContextOf<S> = <S as State>::Context;
/// Type alias to extract a state's [`Frozen`][State::Frozen] type.
pub type FrozenOf<S> = <S as State>::Frozen;
/// Type alias to extract a state's [`LogEntry`][State::LogEntry] type.
pub type LogEntryOf<S> = <S as State>::LogEntry;
/// Type alias to extract [`Id`][LogEntry::Id] type of a state's
/// [`LogEntry`][State::LogEntry] type.
pub type LogEntryIdOf<S> = <LogEntryOf<S> as LogEntry>::Id;
/// Type alias to extract a state's [`Node`][State::Node] type.
pub type NodeOf<S> = <S as State>::Node;
/// Type alias to extract [`Id`][NodeInfo::Id] type of a state's
/// [`Node`][State::Node] type.
pub type NodeIdOf<S> = <NodeOf<S> as NodeInfo>::Id;
/// Type alias to extract a state's [`Outcome`][State::Outcome] type.
pub type OutcomeOf<S> = <S as State>::Outcome;
/// Type alias to extract a state's [`Effect`][State::Effect] type.
pub type EffectOf<S> = <S as State>::Effect;
/// Distributed state to which log entries are applied.
pub trait State: 'static + Debug + Send + Sized + Sync {
/// Frozen representation of this state type.
type Frozen: Frozen<Self> + Send + Sync + Debug;
/// Type of log entry that can be applied to this state.
type LogEntry: LogEntry;
/// An execution context.
///
/// The execution context commonly provides access to a working directory or
/// other state that is node and instance specific.
type Context: Debug + Send;
/// Result of applying a log entry to the state.
///
/// This result is what those actively appending to the log receive.
type Outcome: 'static + Clone + Debug + Send + Sync + Unpin;
/// Result of applying a log entry to the state.
///
/// This result is emitted as an [`Apply` event][Apply] event.
///
/// [Apply]: crate::event::Event::Apply
type Effect: 'static + Send + Debug;
/// Node descriptor type, see [`NodeInfo`].
type Node: NodeInfo;
/// Applies the given log entry to this state object.
///
/// Implementations do not have to account for out-of-order application. Log
/// entries are applied in a consistent order accross the cluster.
///
/// Note: The distributed log may contain duplicates of the same event, i.e.
/// entries `e1` and `e2` may be appended to the log for rounds `r1` and
/// `r2` such that `e1.id() == e2.id()` and `r1 != r2`. Implementations that
/// choose to ignore the application of the second entry must take care to
/// do so in a fashion that is consistent across the cluster.
fn apply(
&mut self,
log_entry: &Self::LogEntry,
context: &mut Self::Context,
) -> (Self::Outcome, Self::Effect);
/// Applies the given log entry to this state object.
///
/// This method may be implemented as an optimization. It is called instead
/// of [apply](State::apply) when there are no observers for the result
/// object.
///
/// *Careful*: Implementations must be equivalent to the default
/// implementation in their effects.
fn apply_unobserved(
&mut self,
log_entry: &Self::LogEntry,
context: &mut Self::Context,
) -> Self::Effect {
self.apply(log_entry, context).1
}
/// Returns the current level of concurrency.
///
/// Implementations must return `Some` when the argument is `Some`.
///
/// This function's contract admits two kinds of implementation. The first
/// kind returns a constant value independent of the argument. The second
/// kind returns `None` when the argument is `None` and `Some(value)`
/// otherwise. The latter kind allows for the level of concurrency to change
/// dynamically yet remain consistent across the cluster.
///
/// The default implementation always returns `Some(1)`.
#[allow(unused_variables)]
fn concurrency(this: Option<&Self>) -> Option<std::num::NonZeroUsize> {
Some(std::num::NonZeroUsize::new(1).unwrap())
}
/// Returns the set of nodes that make up the cluster at the given round
/// offset.
///
/// The round offset is guaranteed to be less than or equal to the current
/// level of concurrency. As such there must always be a known set of nodes
/// that make up the cluster for the given round.
///
/// The order of the returned set must be determisistic, meaning it must be
/// consistent across the entire cluster.
fn cluster_at(&self, round_offset: std::num::NonZeroUsize) -> Vec<Self::Node>;
/// Creates a frozen copy of this state object to create a snapshot with.
fn freeze(&self) -> Self::Frozen;
}
/// A frozen value that may be thawed.
pub trait Frozen<S> {
/// Thaws this value.
fn thaw(&self) -> S;
}
impl<S: Clone> Frozen<S> for S {
fn thaw(&self) -> S {
self.clone()
}
}
/// Determines the level of concurrency given `state`.
///
/// This is a convenience function that delegates to [`State::concurrency`] and
/// then unwraps the returned value.
pub fn concurrency_of<S: State>(state: &S) -> std::num::NonZeroUsize {
S::concurrency(Some(state)).expect("State::concurrency(Some(...)) returned None")
}