diff options
| -rw-r--r-- | src/table/crdt.rs | 31 |
1 files changed, 30 insertions, 1 deletions
diff --git a/src/table/crdt.rs b/src/table/crdt.rs index 3e5e8317..b0323a66 100644 --- a/src/table/crdt.rs +++ b/src/table/crdt.rs @@ -40,7 +40,22 @@ where /// Last Write Win (LWW) /// -/// LWW is a very simple +/// LWW is based on time, the most recent write wins. +/// As multiple computers clocks are always desynchronized, +/// when operations are close enough, it is equivalent to +/// take one copy and drop the other one. +/// +/// Given that clocks are not too desynchronized, this assumption +/// is enough for most cases, as there is few chance that two humans +/// coordonate themself faster than the time difference between two NTP servers. +/// +/// As a more concret example, let's suppose you want to upload a file +/// with the same key (path) in the same bucket at the very same time. +/// For each request, the file will be timestamped by the receiving server +/// and may differ from what you observed with your atomic clock! +/// +/// This scheme is used by AWS S3 or Soundcloud and often without knowing +/// in entreprise when reconciliating databases with ad-hoc scripts. #[derive(Clone, Debug, Serialize, Deserialize, PartialEq)] pub struct LWW<T> { ts: u64, @@ -51,22 +66,36 @@ impl<T> LWW<T> where T: CRDT, { + /// Creates a new CRDT + /// + /// CRDT's internal timestamp is set with current node's clock. pub fn new(value: T) -> Self { Self { ts: now_msec(), v: value, } } + + /// Build a new CRDT from a previous non-compatible one + /// + /// Compared to new, the CRDT's timestamp is not set to now + /// but must be set to the previous, non-compatible, CRDT's timestamp. pub fn migrate_from_raw(ts: u64, value: T) -> Self { Self { ts, v: value } } + + /// Update the LWW CRDT while keeping some causal ordering. pub fn update(&mut self, new_value: T) { self.ts = std::cmp::max(self.ts + 1, now_msec()); self.v = new_value; } + + /// Get the CRDT value pub fn get(&self) -> &T { &self.v } + + /// Get a mutable value for the CRDT pub fn get_mut(&mut self) -> &mut T { &mut self.v } |