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/// Observed-Remove Set With Out Tombstones (ORSWOT), ported directly from `riak_dt`.
use std::cmp::Ordering;
use std::collections::{HashMap, HashSet};
use std::fmt::{Debug, Display};
use std::hash::Hash;
use std::mem;
use serde::{Deserialize, Serialize};
use crate::ctx::{AddCtx, ReadCtx, RmCtx};
use crate::{CmRDT, CvRDT, Dot, ResetRemove, VClock};
/// `Orswot` is an add-biased or-set without tombstones ported from
/// the riak_dt CRDT library.
#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
pub struct Orswot<M: Hash + Eq, A: Ord + Hash> {
pub(crate) clock: VClock<A>,
pub(crate) entries: HashMap<M, VClock<A>>,
pub(crate) deferred: HashMap<VClock<A>, HashSet<M>>,
}
/// Op's define an edit to an Orswot, Op's must be replayed in the exact order
/// they were produced to guarantee convergence.
///
/// Op's are idempotent, that is, applying an Op twice will not have an effect
#[derive(Clone, PartialEq, Eq, Hash, Serialize, Deserialize)]
pub enum Op<M, A: Ord> {
/// Add members to the set
Add {
/// witnessing dot
dot: Dot<A>,
/// Members to add
members: Vec<M>,
},
/// Remove member from the set
Rm {
/// witnessing clock
clock: VClock<A>,
/// Members to remove
members: Vec<M>,
},
}
impl<M: Hash + Eq, A: Ord + Hash> Default for Orswot<M, A> {
fn default() -> Self {
Orswot {
clock: Default::default(),
entries: Default::default(),
deferred: Default::default(),
}
}
}
impl<M: Hash + Clone + Eq, A: Ord + Hash + Clone + Debug> CmRDT for Orswot<M, A> {
type Op = Op<M, A>;
type Validation = <VClock<A> as CmRDT>::Validation;
fn validate_op(&self, op: &Self::Op) -> Result<(), Self::Validation> {
match op {
Op::Add { dot, .. } => self.clock.validate_op(dot),
Op::Rm { .. } => Ok(()),
}
}
fn apply(&mut self, op: Self::Op) {
match op {
Op::Add { dot, members } => {
if self.clock.get(&dot.actor) >= dot.counter {
// we've already seen this op
return;
}
for member in members {
let member_vclock = self.entries.entry(member).or_default();
member_vclock.apply(dot.clone());
}
self.clock.apply(dot);
self.apply_deferred();
}
Op::Rm { clock, members } => {
self.apply_rm(members.into_iter().collect(), clock);
}
}
}
}
/// The variations that an ORSWOT may fail validation.
#[derive(Debug, PartialEq, Eq)]
pub enum Validation<M, A> {
/// We've detected that two different members were inserted with the same dot.
/// This can break associativity.
DoubleSpentDot {
/// The dot that was double spent
dot: Dot<A>,
/// Our member inserted with this dot
our_member: M,
/// Their member inserted with this dot
their_member: M,
},
}
impl<M: Debug, A: Debug> Display for Validation<M, A> {
fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
Debug::fmt(&self, f)
}
}
impl<M: Debug, A: Debug> std::error::Error for Validation<M, A> {}
impl<M: Hash + Eq + Clone + Debug, A: Ord + Hash + Clone + Debug> CvRDT for Orswot<M, A> {
type Validation = Validation<M, A>;
fn validate_merge(&self, other: &Self) -> Result<(), Self::Validation> {
for (member, clock) in self.entries.iter() {
for (other_member, other_clock) in other.entries.iter() {
for Dot { actor, counter } in clock.iter() {
if other_member != member && other_clock.get(actor) == counter {
return Err(Validation::DoubleSpentDot {
dot: Dot::new(actor.clone(), counter),
our_member: member.clone(),
their_member: other_member.clone(),
});
}
}
}
}
Ok(())
}
/// Merge combines another `Orswot` with this one.
fn merge(&mut self, other: Self) {
self.entries = mem::take(&mut self.entries)
.into_iter()
.filter_map(|(entry, mut clock)| {
if !other.entries.contains_key(&entry) {
// other doesn't contain this entry because it:
// 1. has seen it and dropped it
// 2. hasn't seen it
if other.clock >= clock {
// other has seen this entry and dropped it
None
} else {
// the other map has not seen this version of this
// entry, so add it. But first, we have to remove any
// information that may have been known at some point
// by the other map about this key and was removed.
clock.reset_remove(&other.clock);
Some((entry, clock))
}
} else {
Some((entry, clock))
}
})
.collect();
for (entry, mut clock) in other.entries {
if let Some(our_clock) = self.entries.get_mut(&entry) {
// SUBTLE: this entry is present in both orswots, BUT that doesn't mean we
// shouldn't drop it!
// Perfectly possible that an item in both sets should be dropped
let mut common = VClock::intersection(&clock, our_clock);
common.merge(clock.clone_without(&self.clock));
common.merge(our_clock.clone_without(&other.clock));
if common.is_empty() {
// both maps had seen each others entry and removed them
self.entries.remove(&entry).unwrap();
} else {
// we should not drop, as there is information still tracked in
// the common clock.
*our_clock = common;
}
} else {
// we don't have this entry, is it because we:
// 1. have seen it and dropped it
// 2. have not seen it
if self.clock >= clock {
// We've seen this entry and dropped it, we won't add it back
} else {
// We have not seen this version of this entry, so we add it.
// but first, we have to remove the information on this entry
// that we have seen and deleted
clock.reset_remove(&self.clock);
self.entries.insert(entry, clock);
}
}
}
// merge deferred removals
for (rm_clock, members) in other.deferred {
self.apply_rm(members, rm_clock);
}
self.clock.merge(other.clock);
self.apply_deferred();
}
}
impl<M: Hash + Clone + Eq, A: Ord + Hash> ResetRemove<A> for Orswot<M, A> {
fn reset_remove(&mut self, clock: &VClock<A>) {
self.clock.reset_remove(clock);
self.entries = mem::take(&mut self.entries)
.into_iter()
.filter_map(|(val, mut val_clock)| {
val_clock.reset_remove(clock);
if val_clock.is_empty() {
None
} else {
Some((val, val_clock))
}
})
.collect();
self.deferred = mem::take(&mut self.deferred)
.into_iter()
.filter_map(|(mut vclock, deferred)| {
vclock.reset_remove(clock);
if vclock.is_empty() {
None
} else {
Some((vclock, deferred))
}
})
.collect();
}
}
impl<M: Hash + Clone + Eq, A: Ord + Hash + Clone> Orswot<M, A> {
/// Returns a new `Orswot` instance.
pub fn new() -> Self {
Default::default()
}
/// Return a snapshot of the ORSWOT clock
pub fn clock(&self) -> VClock<A> {
self.clock.clone()
}
/// Add a single element.
pub fn add(&self, member: M, ctx: AddCtx<A>) -> Op<M, A> {
Op::Add {
dot: ctx.dot,
members: std::iter::once(member).collect(),
}
}
/// Add multiple elements.
pub fn add_all<I: IntoIterator<Item = M>>(&self, members: I, ctx: AddCtx<A>) -> Op<M, A> {
Op::Add {
dot: ctx.dot,
members: members.into_iter().collect(),
}
}
/// Remove a member with a witnessing ctx.
pub fn rm(&self, member: M, ctx: RmCtx<A>) -> Op<M, A> {
Op::Rm {
clock: ctx.clock,
members: std::iter::once(member).collect(),
}
}
/// Remove members with a witnessing ctx.
pub fn rm_all<I: IntoIterator<Item = M>>(&self, members: I, ctx: RmCtx<A>) -> Op<M, A> {
Op::Rm {
clock: ctx.clock,
members: members.into_iter().collect(),
}
}
/// Remove members using a witnessing clock.
fn apply_rm(&mut self, members: HashSet<M>, clock: VClock<A>) {
for member in members.iter() {
if let Some(member_clock) = self.entries.get_mut(member) {
member_clock.reset_remove(&clock);
if member_clock.is_empty() {
self.entries.remove(member);
}
}
}
match clock.partial_cmp(&self.clock) {
None | Some(Ordering::Greater) => {
if let Some(existing_deferred) = self.deferred.get_mut(&clock) {
existing_deferred.extend(members);
} else {
self.deferred.insert(clock, members);
}
}
_ => { /* we've already seen this remove */ }
}
}
/// Check if the set contains a member
pub fn contains(&self, member: &M) -> ReadCtx<bool, A> {
let member_clock_opt = self.entries.get(member);
let exists = member_clock_opt.is_some();
ReadCtx {
add_clock: self.clock.clone(),
rm_clock: member_clock_opt.cloned().unwrap_or_default(),
val: exists,
}
}
/// Gets an iterator over the entries of the `Map`.
///
/// # Examples
///
/// ```rust
/// use crdts::{Orswot, CmRDT};
///
/// let actor = "actor";
///
/// let mut set: Orswot<u8, &'static str> = Default::default();
///
/// let add_ctx = set.read_ctx().derive_add_ctx(actor);
/// set.apply(set.add(100, add_ctx));
///
/// let add_ctx = set.read_ctx().derive_add_ctx(actor);
/// set.apply(set.add(50, add_ctx));
///
/// let mut items: Vec<_> = set
/// .iter()
/// .map(|item_ctx| *item_ctx.val)
/// .collect();
///
/// items.sort();
///
/// assert_eq!(items, &[50, 100]);
/// ```
pub fn iter(&self) -> impl Iterator<Item = ReadCtx<&M, A>> {
self.entries.iter().map(move |(m, clock)| ReadCtx {
add_clock: self.clock.clone(),
rm_clock: clock.clone(),
val: m,
})
}
/// Retrieve the current members.
pub fn read(&self) -> ReadCtx<HashSet<M>, A> {
ReadCtx {
add_clock: self.clock.clone(),
rm_clock: self.clock.clone(),
val: self.entries.keys().cloned().collect(),
}
}
/// Retrieve the current read context
pub fn read_ctx(&self) -> ReadCtx<(), A> {
ReadCtx {
add_clock: self.clock.clone(),
rm_clock: self.clock.clone(),
val: (),
}
}
fn apply_deferred(&mut self) {
let deferred = mem::take(&mut self.deferred);
for (clock, entries) in deferred.into_iter() {
self.apply_rm(entries, clock)
}
}
}
#[cfg(feature = "quickcheck")]
use quickcheck::{Arbitrary, Gen};
#[cfg(feature = "quickcheck")]
impl<A: Ord + Hash + Arbitrary + Debug, M: Hash + Eq + Arbitrary> Arbitrary for Op<M, A> {
fn arbitrary(g: &mut Gen) -> Self {
let dot = Dot::arbitrary(g);
let clock = VClock::arbitrary(g);
let mut members_set = HashSet::new();
for _ in 0..u8::arbitrary(g) % 10 {
members_set.insert(M::arbitrary(g));
}
let members: Vec<_> = members_set.into_iter().collect();
match u8::arbitrary(g) % 2 {
0 => Op::Add { members, dot },
1 => Op::Rm { members, clock },
_ => panic!("tried to generate invalid op"),
}
}
fn shrink(&self) -> Box<dyn Iterator<Item = Self>> {
let mut shrunk_ops = Vec::new();
match self {
Op::Add { members, dot } => {
for (i, _m) in members.iter().enumerate() {
let mut shrunk_members = members.clone();
shrunk_members.remove(i);
shrunk_ops.push(Op::Add {
members: shrunk_members,
dot: dot.clone(),
});
}
dot.shrink().for_each(|shrunk_dot| {
shrunk_ops.push(Op::Add {
members: members.clone(),
dot: shrunk_dot,
})
});
}
Op::Rm { members, clock } => {
for (i, _m) in members.iter().enumerate() {
let mut shrunk_members = members.clone();
shrunk_members.remove(i);
shrunk_ops.push(Op::Rm {
members: shrunk_members,
clock: clock.clone(),
});
}
clock.shrink().for_each(|shrunk_clock| {
shrunk_ops.push(Op::Rm {
members: members.clone(),
clock: shrunk_clock,
})
});
}
}
Box::new(shrunk_ops.into_iter())
}
}
impl<M: Debug, A: Ord + Hash + Debug> Debug for Op<M, A> {
fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
match self {
Op::Add { dot, members } => write!(f, "Add({:?}, {:?})", dot, members),
Op::Rm { clock, members } => write!(f, "Rm({:?}, {:?})", clock, members),
}
}
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
// a bug found with rust quickcheck where deferred operations
// are not carried over after a merge.
// symptoms:
// if nothing is added, it works
// if removed elem is added first, it only misses one
// if non-related elem is added, it misses both
fn ensure_deferred_merges() {
let mut a = Orswot::new();
let mut b = Orswot::new();
b.apply(b.add("element 1", b.read().derive_add_ctx("A")));
// remove with a future context
b.apply(b.rm(
"element 1",
RmCtx {
clock: Dot::new("A", 4).into(),
},
));
a.apply(a.add("element 4", a.read().derive_add_ctx("B")));
// remove with a future context
b.apply(b.rm(
"element 9",
RmCtx {
clock: Dot::new("C", 4).into(),
},
));
let mut merged = Orswot::new();
merged.merge(a);
merged.merge(b);
merged.merge(Orswot::new());
assert_eq!(merged.deferred.len(), 2);
}
// a bug found with rust quickcheck where deferred removals
// were not properly preserved across merges.
#[test]
fn preserve_deferred_across_merges() {
let mut a = Orswot::new();
let mut b = a.clone();
let mut c = a.clone();
// add element 5 from witness 1
a.apply(a.add(5, a.read().derive_add_ctx("A")));
// on another clock, remove 5 with an advanced clock for witnesses A and B
let mut vc = VClock::new();
vc.apply(Dot::new("A", 3));
vc.apply(Dot::new("B", 8));
// remove from b (has not yet seen add for 5) with advanced ctx
b.apply(b.rm(5, RmCtx { clock: vc }));
assert_eq!(b.deferred.len(), 1);
// ensure that the deferred elements survive across a merge
c.merge(b);
assert_eq!(c.deferred.len(), 1);
// after merging the set with deferred elements with the set that contains
// an inferior member, ensure that the member is no longer visible and
// the deferred set still contains this info
a.merge(c);
assert!(a.read().val.is_empty());
}
// port from riak_dt
// Bug found by EQC, not dropping dots in merge when an element is
// present in both Sets leads to removed items remaining after merge.
#[test]
fn test_present_but_removed() {
let mut a = Orswot::new();
let mut b = Orswot::new();
a.apply(a.add(0, a.read().derive_add_ctx("A")));
// Replicate it to C so A has 0->{a, 1}
let c = a.clone();
a.apply(a.rm(0, a.contains(&0).derive_rm_ctx()));
assert_eq!(a.deferred.len(), 0);
b.apply(b.add(0, b.read().derive_add_ctx("B")));
// Replicate B to A, so now A has a 0
// the one with a Dot of {b,1} and clock
// of [{a, 1}, {b, 1}]
a.merge(b.clone());
b.apply(b.rm(0, b.contains(&0).derive_rm_ctx()));
// Both C and A have a '0', but when they merge, there should be
// no '0' as C's has been removed by A and A's has been removed by
// C.
a.merge(b);
a.merge(c);
assert!(a.read().val.is_empty());
}
}