forked from Deuxfleurs/garage
114 lines
4.2 KiB
Rust
114 lines
4.2 KiB
Rust
use serde::{Deserialize, Serialize};
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use garage_util::time::now_msec;
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use crate::crdt::crdt::*;
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/// Last Write Win (LWW)
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///
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/// An LWW CRDT associates a timestamp with a value, in order to implement a
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/// time-based reconciliation rule: the most recent write wins.
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/// For completeness, the LWW reconciliation rule must also be defined for two LWW CRDTs
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/// with the same timestamp but different values.
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///
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/// In our case, we add the constraint that the value that is wrapped inside the LWW CRDT must
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/// itself be a CRDT: in the case when the timestamp does not allow us to decide on which value to
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/// keep, the merge rule of the inner CRDT is applied on the wrapped values. (Note that all types
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/// that implement the `Ord` trait get a default CRDT implemetnation that keeps the maximum value.
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/// This enables us to use LWW directly with primitive data types such as numbers or strings. It is
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/// generally desirable in this case to never explicitly produce LWW values with the same timestamp
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/// but different inner values, as the rule to keep the maximum value isn't generally the desired
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/// semantics.)
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///
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/// As multiple computers clocks are always desynchronized,
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/// when operations are close enough, it is equivalent to
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/// take one copy and drop the other one.
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///
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/// Given that clocks are not too desynchronized, this assumption
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/// is enough for most cases, as there is few chance that two humans
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/// coordonate themself faster than the time difference between two NTP servers.
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///
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/// As a more concret example, let's suppose you want to upload a file
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/// with the same key (path) in the same bucket at the very same time.
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/// For each request, the file will be timestamped by the receiving server
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/// and may differ from what you observed with your atomic clock!
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///
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/// This scheme is used by AWS S3 or Soundcloud and often without knowing
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/// in enterprise when reconciliating databases with ad-hoc scripts.
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#[derive(Clone, Debug, Serialize, Deserialize, PartialEq)]
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pub struct LWW<T> {
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ts: u64,
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v: T,
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}
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impl<T> LWW<T>
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where
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T: CRDT,
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{
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/// Creates a new CRDT
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///
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/// CRDT's internal timestamp is set with current node's clock.
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pub fn new(value: T) -> Self {
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Self {
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ts: now_msec(),
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v: value,
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}
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}
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/// Build a new CRDT from a previous non-compatible one
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///
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/// Compared to new, the CRDT's timestamp is not set to now
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/// but must be set to the previous, non-compatible, CRDT's timestamp.
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pub fn migrate_from_raw(ts: u64, value: T) -> Self {
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Self { ts, v: value }
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}
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/// Update the LWW CRDT while keeping some causal ordering.
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///
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/// The timestamp of the LWW CRDT is updated to be the current node's clock
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/// at time of update, or the previous timestamp + 1 if that's bigger,
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/// so that the new timestamp is always strictly larger than the previous one.
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/// This ensures that merging the update with the old value will result in keeping
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/// the updated value.
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pub fn update(&mut self, new_value: T) {
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self.ts = std::cmp::max(self.ts + 1, now_msec());
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self.v = new_value;
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}
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/// Get the CRDT value
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pub fn get(&self) -> &T {
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&self.v
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}
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/// Get a mutable reference to the CRDT's value
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///
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/// This is usefull to mutate the inside value without changing the LWW timestamp.
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/// When such mutation is done, the merge between two LWW values is done using the inner
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/// CRDT's merge operation. This is usefull in the case where the inner CRDT is a large
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/// data type, such as a map, and we only want to change a single item in the map.
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/// To do this, we can produce a "CRDT delta", i.e. a LWW that contains only the modification.
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/// This delta consists in a LWW with the same timestamp, and the map
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/// inside only contains the updated value.
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/// The advantage of such a delta is that it is much smaller than the whole map.
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///
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/// Avoid using this if the inner data type is a primitive type such as a number or a string,
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/// as you will then rely on the merge function defined on `Ord` types by keeping the maximum
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/// of both values.
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pub fn get_mut(&mut self) -> &mut T {
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&mut self.v
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}
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}
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impl<T> CRDT for LWW<T>
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where
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T: Clone + CRDT,
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{
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fn merge(&mut self, other: &Self) {
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if other.ts > self.ts {
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self.ts = other.ts;
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self.v = other.v.clone();
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} else if other.ts == self.ts {
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self.v.merge(&other.v);
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}
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}
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}
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