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/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at https://mozilla.org/MPL/2.0/. */
use alloc::collections::VecDeque;
use core::{cmp::Ordering, time::Duration};
use bytes::{Bytes, BytesMut};
use crate::{Identity, Incarnation};
/// A Runtime is Foca's gateway to the real world: here is where
/// implementations decide how to interact with the network, the
/// hardware timer and the user.
///
/// Implementations may react directly to it for a fully synchronous
/// behavior or accumulate-then-drain when dispatching via fancier
/// mechanisms like async.
pub trait Runtime<T>
where
T: Identity,
{
/// Whenever something changes Foca's state significantly a
/// notification is emitted.
///
/// It's the best mechanism to watch for membership changes
/// and allows implementors to keep track of the cluster state
/// without having direct access to the running Foca instance.
///
/// Implementations may completely disregard this if desired.
fn notify(&mut self, notification: Notification<T>);
/// This is how Foca connects to an actual transport.
///
/// Implementations are responsible for the actual delivery.
fn send_to(&mut self, to: T, data: &[u8]);
/// Request to schedule the delivery of a given event after
/// a specified duration.
///
/// Implementations MUST ensure that every event is delivered.
/// Foca is very tolerant to delays, but non-delivery will
/// cause errors.
fn submit_after(&mut self, event: Timer<T>, after: Duration);
}
// A mutable reference to a Runtime is a Runtime too
impl<T, R> Runtime<T> for &mut R
where
T: Identity,
R: Runtime<T>,
{
fn notify(&mut self, notification: Notification<T>) {
R::notify(self, notification);
}
fn send_to(&mut self, to: T, data: &[u8]) {
R::send_to(self, to, data);
}
fn submit_after(&mut self, event: Timer<T>, after: Duration) {
R::submit_after(self, event, after);
}
}
/// A Notification contains information about high-level relevant
/// state changes in the cluster or Foca itself.
#[derive(Debug, Clone, PartialEq, Eq)]
#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
pub enum Notification<T> {
/// Foca discovered a new active member with identity T.
MemberUp(T),
/// A previously active member has been declared down by the cluster.
///
/// If Foca detects a down member but didn't know about its activity
/// before, this notification will not be emitted.
///
/// Can only happen if `MemberUp(T)` happened before.
MemberDown(T),
/// Foca has learned that there's a more recent identity with
/// the same address and chose to use it instead of the previous
/// one.
///
/// So `Notification::Rename(A,B)` means that we knew about a member
/// `A` but now there's a `B` with the same `Identity::Addr` and
/// foca chose to keep it. i.e. `B.win_addr_conflict(A) == true`.
///
/// This happens naturally when a member rejoins the cluster after
/// any event (maybe they were declared down and `Identity::renew`d
/// themselves, maybe it's a restart/upgrade process)
///
/// Example:
///
/// If `A` was considered Down and `B` is Alive, you'll get
/// two notifications, in order:
//
/// 1. `Notification::Rename(A,B)`
/// 2. `Notification::MemberUp(B)`
///
/// However, if there's no liveness change (both are active
/// or both are down), you'll only get the `Rename` notification
Rename(T, T),
/// Foca's current identity is known by at least one active member
/// of the cluster.
///
/// Fired when successfully joining a cluster for the first time and
/// every time after a successful identity change.
Active,
/// All known active members have either left the cluster or been
/// declared down.
Idle,
/// Foca's current identity has been declared down.
///
/// Manual intervention via `Foca::change_identity` or
/// `Foca::reuse_down_identity` is required to return to a functioning
/// state.
Defunct,
/// Foca automatically changed its identity and rejoined the cluster
/// after being declared down.
///
/// This happens instead of `Defunct` when identities opt-in on
/// `Identity::renew()` functionality.
Rejoin(T),
}
/// Timer is an event that's scheduled by a [`Runtime`]. You won't need
/// to construct or understand these, just ensure a timely delivery.
///
/// **Warning:** This type implements [`Ord`] to facilitate correcting
/// for out-of-order delivery due to the runtime lagging for whatever
/// reason. It assumes the events being sorted come from the same foca
/// instance and are not being persisted after being handled
/// via [`crate::Foca::handle_timer`]. Any use outside of this scenario
/// will likely lead to unintended consequences.
#[derive(Debug, Clone, PartialEq, Eq)]
#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
pub enum Timer<T> {
/// Pick a random active member and initiate the probe cycle.
ProbeRandomMember(TimerToken),
/// Send indirect probes if the direct one hasn't completed yet.
SendIndirectProbe {
/// The current member being probed
probed_id: T,
/// See `TimerToken`
token: TimerToken,
},
/// Transitions member T from Suspect to Down if the incarnation is
/// still the same.
ChangeSuspectToDown {
/// Target member identity
member_id: T,
/// Its Incarnation the moment the suspicion was raised. If the
/// member refutes the suspicion (by increasing its Incarnation),
/// this won't match and it won't be declared Down.
incarnation: Incarnation,
/// See `TimerToken`
token: TimerToken,
},
/// Sends a [`crate::Message::Announce`] to randomly chosen members as
/// specified by [`crate::Config::periodic_announce`]
PeriodicAnnounce(TimerToken),
/// Sends a [`crate::Message::Announce`] to randomly chosen members
/// that are condidered [`crate::State::Down`] as specified by
/// [`crate::Config::periodic_announce_to_down_members`]
PeriodicAnnounceDown(TimerToken),
/// Sends a [`crate::Message::Gossip`] to randomly chosen members as
/// specified by [`crate::Config::periodic_gossip`]
PeriodicGossip(TimerToken),
/// Forgets about dead member `T`, allowing them to join the
/// cluster again with the same identity.
RemoveDown(T),
}
impl<T> Timer<T> {
fn seq(&self) -> u8 {
match self {
Timer::SendIndirectProbe {
probed_id: _,
token: _,
} => 0,
Timer::ProbeRandomMember(_) => 1,
Timer::ChangeSuspectToDown {
member_id: _,
incarnation: _,
token: _,
} => 2,
Timer::PeriodicAnnounce(_) => 3,
Timer::PeriodicGossip(_) => 4,
Timer::RemoveDown(_) => 5,
Timer::PeriodicAnnounceDown(_) => 6,
}
}
}
impl<T: PartialEq> core::cmp::PartialOrd for Timer<T> {
fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
self.seq().partial_cmp(&other.seq())
}
}
impl<T: Eq> core::cmp::Ord for Timer<T> {
fn cmp(&self, other: &Self) -> Ordering {
self.partial_cmp(other).expect("total ordering")
}
}
/// `TimerToken` is simply a bookkeeping mechanism to try and prevent
/// reacting to events dispatched that aren't relevant anymore.
///
/// Certain interactions may cause Foca to decide to disregard every
/// event it scheduled previously- so it changes the token in order
/// to drop everything that doesn't match.
///
/// Similar in spirit to [`crate::ProbeNumber`].
pub type TimerToken = u8;
/// A `Runtime` implementation that's good enough for simple use-cases.
///
/// It accumulates all events that happen during an interaction with
/// `crate::Foca` and users must drain those and react accordingly.
///
/// Better runtimes would react directly to the events, intead of
/// needlessly storing the events in a queue.
///
/// Users must drain the runtime immediately after interacting with
/// foca. Example:
///
/// See it in use at `examples/foca_insecure_udp_agent.rs`
pub struct AccumulatingRuntime<T> {
to_send: VecDeque<(T, Bytes)>,
to_schedule: VecDeque<(Duration, Timer<T>)>,
notifications: VecDeque<Notification<T>>,
buf: BytesMut,
}
impl<T> Default for AccumulatingRuntime<T> {
fn default() -> Self {
Self {
to_send: Default::default(),
to_schedule: Default::default(),
notifications: Default::default(),
buf: Default::default(),
}
}
}
impl<T: Identity> Runtime<T> for AccumulatingRuntime<T> {
fn notify(&mut self, notification: Notification<T>) {
self.notifications.push_back(notification);
}
fn send_to(&mut self, to: T, data: &[u8]) {
self.buf.extend_from_slice(data);
let packet = self.buf.split().freeze();
self.to_send.push_back((to, packet));
}
fn submit_after(&mut self, event: Timer<T>, after: Duration) {
// We could spawn+sleep here
self.to_schedule.push_back((after, event));
}
}
impl<T> AccumulatingRuntime<T> {
/// Create a new `AccumulatingRuntime`
pub fn new() -> Self {
Self::default()
}
/// Yields data to be sent to a cluster member `T` in the
/// order they've happened.
///
/// Users are expected to drain it until it yields `None`
/// after every interaction with `crate::Foca`
pub fn to_send(&mut self) -> Option<(T, Bytes)> {
self.to_send.pop_front()
}
/// Yields timer events and how far in the future they
/// must be given back to the foca instance that produced it
///
/// Users are expected to drain it until it yields `None`
/// after every interaction with `crate::Foca`
pub fn to_schedule(&mut self) -> Option<(Duration, Timer<T>)> {
self.to_schedule.pop_front()
}
/// Yields event notifications in the order they've happened
///
/// Users are expected to drain it until it yields `None`
/// after every interaction with `crate::Foca`
pub fn to_notify(&mut self) -> Option<Notification<T>> {
self.notifications.pop_front()
}
/// Returns how many unhandled events are left in this runtime
///
/// Should be brought down to zero after every interaction with
/// `crate::Foca`
pub fn backlog(&self) -> usize {
self.to_send.len() + self.to_schedule.len() + self.notifications.len()
}
}
#[cfg(test)]
impl<T: PartialEq> AccumulatingRuntime<T> {
pub(crate) fn clear(&mut self) {
self.notifications.clear();
self.to_send.clear();
self.to_schedule.clear();
}
pub(crate) fn is_empty(&self) -> bool {
self.notifications.is_empty() && self.to_send.is_empty() && self.to_schedule.is_empty()
}
pub(crate) fn take_all_data(&mut self) -> VecDeque<(T, Bytes)> {
core::mem::take(&mut self.to_send)
}
pub(crate) fn take_data(&mut self, dst: T) -> Option<Bytes> {
let position = self.to_send.iter().position(|(to, _data)| to == &dst)?;
self.to_send.remove(position).map(|(_, data)| data)
}
pub(crate) fn take_notification(&mut self, wanted: Notification<T>) -> Option<Notification<T>> {
let position = self
.notifications
.iter()
.position(|notification| notification == &wanted)?;
self.notifications.remove(position)
}
pub(crate) fn take_scheduling(&mut self, timer: Timer<T>) -> Option<Duration> {
let position = self
.to_schedule
.iter()
.position(|(_when, event)| event == &timer)?;
self.to_schedule.remove(position).map(|(when, _)| when)
}
pub(crate) fn find_scheduling<F>(&self, predicate: F) -> Option<&Timer<T>>
where
F: Fn(&Timer<T>) -> bool,
{
self.to_schedule
.iter()
.find(|(_, timer)| predicate(timer))
.map(|(_, timer)| timer)
}
}
#[cfg(test)]
mod tests {
use super::Timer;
#[test]
fn timers_sort() {
// What we really care about is SendIndirectProbe
// appearing before ProbeRandomMember,
// Foca tolerates other events in arbitrary order
// without emitting scary errors / traces.
let mut out_of_order = alloc::vec![
Timer::<u8>::RemoveDown(0),
Timer::<u8>::ChangeSuspectToDown {
member_id: 0,
incarnation: 0,
token: 0,
},
Timer::<u8>::ProbeRandomMember(0),
Timer::<u8>::SendIndirectProbe {
probed_id: 0,
token: 0
},
];
out_of_order.sort_unstable();
assert_eq!(
alloc::vec![
Timer::<u8>::SendIndirectProbe {
probed_id: 0,
token: 0
},
Timer::<u8>::ProbeRandomMember(0),
Timer::<u8>::ChangeSuspectToDown {
member_id: 0,
incarnation: 0,
token: 0,
},
Timer::<u8>::RemoveDown(0),
],
out_of_order
);
}
}