forked from Deuxfleurs/garage
Add some doc on LWW
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@ -40,7 +40,22 @@ where
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/// Last Write Win (LWW)
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///
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/// LWW is a very simple
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/// LWW is based on time, the most recent write wins.
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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 entreprise 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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@ -51,22 +66,36 @@ 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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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 value for the CRDT
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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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