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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::vec::Vec;
use core::{cmp::Ordering, fmt};
use bytes::BufMut;
/// A type capable of decoding a (associated) broadcast from a buffer
/// and deciding whether to keep disseminating it for other members
/// of the cluster (when it's new information) or to discard it (when
/// its outdated/stale).
pub trait BroadcastHandler<T> {
/// A unique identifier for the broadcasts this handler manages
///
/// It should be able to compare itself against an arbitrary number
/// of other [`Self::Key`] instances and decide wether it
/// replaces it or not so conflicting/stale information isn't
/// disseminated.
type Key: Invalidates;
/// The error type that `receive_item` may emit. Will be wrapped
/// by [`crate::Error::CustomBroadcast`].
type Error: fmt::Debug + fmt::Display + Send + Sync + 'static;
/// Decodes a [`Self::Key`] from a buffer and either discards
/// it or tells Foca to persist and disseminate it.
///
/// `Sender` is `None` when you're adding broadcast data directly,
/// via [`crate::Foca::add_broadcast`], otherwise it will be the
/// address of the member that sent the data. Notice that Foca
/// doesn't track the origin of packets- if you need it you
/// have to add it to the data you're broadcasting.
///
/// When you receive a broadcast you have to decide whether it's
/// new information that needs to be disseminated (`Ok(Some(...))`)
/// or not (`Ok(None)`).
///
/// Always yielding `Some(...)` is wrong because Foca will never
/// know when to stop sending this information to other members.
///
/// Example: Assume your custom broadcast is a simple Set-Key-Value
/// operation. When you receive it you should check if your map
/// contains the Key-Value pair; If it didn't, you yield
/// `Some`, otherwise the operation is stale, so you yield `None`.
///
/// The `data` parameter is the exact data provided to
/// `crate::Foca::add_broadcast`. When Foca receives N custom
/// broadcasts at once, this gets called N times.
fn receive_item(
&mut self,
data: &[u8],
sender: Option<&T>,
) -> Result<Option<Self::Key>, Self::Error>;
/// Decides whether Foca should add broadcast data to the message
/// it's about to send to active member `T`.
///
/// Normally when Foca sends a message it always tries to include
/// custom broadcasts alongside the information it actually
/// cares about; This allows implementations to override this
/// logic with something else.
///
/// Example: You are running a heterogeneous cluster and some nodes
/// are always very busy and you'd rather they never have to deal
/// with the extra cpu/bandwidth cost of receiving/sending
/// your custom broadcasts.
///
/// Returning `true` tells Foca to proceed as it would normally,
/// including broadcasts in the messages it sends when it can.
///
/// Returning `false` tells Foca to not include such broadcasts
/// in the message. It does *not* prevent the message from being
/// sent, just keeps Foca from attaching extra data to them.
fn should_add_broadcast_data(&self, _member: &T) -> bool {
true
}
}
/// A type that's able to look at another and decide wether it's
/// newer/fresher (i.e. invalidates) than it.
///
/// As you send/receive broadcasts, Foca will hold them for a while
/// as it disseminates the data to other cluster members. This trait
/// helps with replacing data that hasn't been fully disseminated
/// yet but you already know it's stale.
///
/// Example: Assume a versioned broadcast like `{key,version,...}`;
/// After you receive `{K, 0, ...}` and keep it, Foca will be
/// disseminating it. Soon after you receive `{K, 1, ...}` which
/// is a newer version for this broadcast. This trait enables
/// Foca to immediately stop disseminating the previous version,
/// even if it hasn't sent it to everyone it can yet.
pub trait Invalidates {
/// When `item.invalidates(&other)` it means that Foca will
/// keep `item` and discard `other` from its dissemination
/// backlog.
fn invalidates(&self, other: &Self) -> bool;
}
impl<'a> Invalidates for &'a [u8] {
fn invalidates(&self, other: &Self) -> bool {
self.eq(other)
}
}
#[allow(dead_code)]
pub(crate) struct Broadcasts<V> {
flip: alloc::collections::BinaryHeap<Entry<V>>,
flop: alloc::collections::BinaryHeap<Entry<V>>,
}
impl<T> Broadcasts<T>
where
T: Invalidates,
{
pub(crate) fn new() -> Self {
Self {
flip: Default::default(),
flop: Default::default(),
}
}
pub(crate) fn len(&self) -> usize {
self.flip.len()
}
pub(crate) fn is_empty(&self) -> bool {
self.flip.is_empty()
}
pub(crate) fn add_or_replace(&mut self, item: T, data: Vec<u8>, max_tx: usize) {
debug_assert!(max_tx > 0);
self.flip.retain(|node| !item.invalidates(&node.item));
self.flip.push(Entry {
remaining_tx: max_tx,
item,
data,
});
}
pub(crate) fn fill(&mut self, mut buffer: impl BufMut, max_items: usize) -> usize {
if self.flip.is_empty() {
return 0;
}
debug_assert!(self.flop.is_empty());
let mut num_taken = 0;
let mut remaining = max_items;
while buffer.has_remaining_mut() && remaining > 0 {
let Some(mut node) = self.flip.pop() else {
break;
};
debug_assert!(node.remaining_tx > 0);
if buffer.remaining_mut() >= node.data.len() {
num_taken += 1;
remaining -= 1;
buffer.put_slice(&node.data);
node.remaining_tx -= 1;
}
if node.remaining_tx > 0 {
self.flop.push(node);
}
}
self.flip.append(&mut self.flop);
num_taken
}
pub(crate) fn fill_with_len_prefix(
&mut self,
mut buffer: impl BufMut,
max_items: usize,
) -> usize {
if self.flip.is_empty() {
return 0;
}
debug_assert!(self.flop.is_empty());
let mut num_taken = 0;
let mut remaining = max_items;
while buffer.has_remaining_mut() && remaining > 0 {
let Some(mut node) = self.flip.pop() else {
break;
};
debug_assert!(node.remaining_tx > 0);
if buffer.remaining_mut() >= node.data.len() + 2 {
num_taken += 1;
remaining -= 1;
debug_assert!(node.data.len() <= core::u16::MAX as usize);
buffer.put_u16(node.data.len() as u16);
buffer.put_slice(&node.data);
node.remaining_tx -= 1;
}
if node.remaining_tx > 0 {
self.flop.push(node);
}
}
self.flip.append(&mut self.flop);
num_taken
}
}
#[derive(Debug, Clone)]
struct Entry<T> {
remaining_tx: usize,
// XXX could be Bytes, or keep a pool in parent
data: Vec<u8>,
// ignored for eq/ord. sorting is unstable
item: T,
}
impl<T> PartialEq for Entry<T> {
fn eq(&self, other: &Self) -> bool {
self.cmp(other).is_eq()
}
}
impl<T> Eq for Entry<T> {}
impl<T> PartialOrd for Entry<T> {
fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
Some(self.cmp(other))
}
}
impl<T> Ord for Entry<T> {
fn cmp(&self, other: &Self) -> Ordering {
self.remaining_tx
.cmp(&other.remaining_tx)
.then_with(|| self.data.len().cmp(&other.data.len()))
}
}
#[cfg(test)]
mod tests {
use super::*;
struct Key(&'static str);
impl Invalidates for Key {
fn invalidates(&self, other: &Self) -> bool {
self.0 == other.0
}
}
#[test]
fn piggyback_behaviour() {
let max_tx = 5;
let mut piggyback = Broadcasts::new();
assert!(piggyback.is_empty(), "Piggyback starts empty");
piggyback.add_or_replace(Key("AA"), b"AAabc".to_vec(), max_tx);
assert_eq!(1, piggyback.len());
piggyback.add_or_replace(Key("AA"), b"AAcba".to_vec(), max_tx);
assert_eq!(
1,
piggyback.len(),
"add_or_replace with same key should replace"
);
let mut buf = Vec::new();
for _i in 0..max_tx {
buf.clear();
let num_items = piggyback.fill(&mut buf, usize::MAX);
assert_eq!(1, num_items);
assert_eq!(
b"AAcba",
&buf[..],
"Should transmit an item at most max_tx times"
);
}
assert!(
piggyback.is_empty(),
"Should remove item after being used max_tx times"
);
}
#[test]
fn fill_does_nothing_if_buffer_full() {
let mut piggyback = Broadcasts::new();
piggyback.add_or_replace(Key("a "), b"a super long value".to_vec(), 1);
let buf = bytes::BytesMut::new();
let mut limited = buf.limit(5);
let num_items = piggyback.fill(&mut limited, usize::MAX);
assert_eq!(0, num_items);
assert_eq!(5, limited.remaining_mut());
assert_eq!(1, piggyback.len());
}
#[test]
fn piggyback_consumes_largest_first() {
let max_tx = 10;
let mut piggyback = Broadcasts::new();
piggyback.add_or_replace(Key("00"), b"00hi".to_vec(), max_tx);
piggyback.add_or_replace(Key("01"), b"01hello".to_vec(), max_tx);
piggyback.add_or_replace(Key("02"), b"02hey".to_vec(), max_tx);
let mut buf = Vec::new();
let num_items = piggyback.fill(&mut buf, usize::MAX);
assert_eq!(3, num_items);
assert_eq!(b"01hello02hey00hi", &buf[..]);
}
#[test]
fn highest_max_tx_is_consumed_first() {
let mut piggyback = Broadcasts::new();
// 3 items, same byte size, distinct max_tx
piggyback.add_or_replace(Key("10"), b"100".to_vec(), 1);
piggyback.add_or_replace(Key("20"), b"200".to_vec(), 2);
piggyback.add_or_replace(Key("30"), b"300".to_vec(), 3);
let mut buf = Vec::new();
piggyback.fill(&mut buf, usize::MAX);
assert_eq!(b"300200100", &buf[..]);
buf.clear();
piggyback.fill(&mut buf, usize::MAX);
assert_eq!(b"300200", &buf[..]);
buf.clear();
piggyback.fill(&mut buf, usize::MAX);
assert_eq!(b"300", &buf[..]);
assert_eq!(0, piggyback.len());
}
#[test]
fn piggyback_respects_limit() {
let max_tx = 10;
let mut piggyback = Broadcasts::new();
piggyback.add_or_replace(Key("fo"), b"foo".to_vec(), max_tx);
piggyback.add_or_replace(Key("ba"), b"bar".to_vec(), max_tx);
piggyback.add_or_replace(Key("ba"), b"baz".to_vec(), max_tx);
let mut buf = Vec::new();
let num_items = piggyback.fill(&mut buf, 0);
assert_eq!(0, num_items);
assert!(buf.is_empty());
let num_items = piggyback.fill(&mut buf, 2);
assert_eq!(2, num_items);
}
#[test]
fn fill_with_len_prefix() {
let mut bcs = Broadcasts::new();
bcs.add_or_replace(Key("fo"), b"foo".to_vec(), 10);
bcs.add_or_replace(Key("ba"), b"barr".to_vec(), 10);
bcs.add_or_replace(Key("ba"), b"bazz".to_vec(), 10);
let mut buf = Vec::new();
let num_items = bcs.fill_with_len_prefix(&mut buf, 0);
assert_eq!(0, num_items);
assert!(buf.is_empty());
let num_items = bcs.fill_with_len_prefix(&mut buf, 2);
assert_eq!(2, num_items);
use bytes::Buf;
let mut buf = &buf[..];
assert_eq!(4, buf.get_u16());
assert_eq!(&b"bazz"[..], &buf[..4]);
buf.advance(4);
assert_eq!(3, buf.get_u16());
assert_eq!(&b"foo"[..], &buf[..3]);
buf.advance(3);
assert!(buf.is_empty());
}
}