forked from Deuxfleurs/garage
Alex Auvolat
1bcd6fabbd
- Fix bucket delete
- fix merge of bucket creation date
- Replace deletable with option in aliases
Rationale: if two aliases point to conflicting bucket, resolving
by making an arbitrary choice risks making data accessible when it
shouldn't be. We'd rather resolve to deleting the alias until
someone puts it back.
87 lines
3.2 KiB
Rust
87 lines
3.2 KiB
Rust
/// Definition of a CRDT - all CRDT Rust types implement this.
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///
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/// A CRDT is defined as a merge operator that respects a certain set of axioms.
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///
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/// In particular, the merge operator must be commutative, associative,
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/// idempotent, and monotonic.
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/// In other words, if `a`, `b` and `c` are CRDTs, and `⊔` denotes the merge operator,
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/// the following axioms must apply:
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///
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/// ```text
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/// a ⊔ b = b ⊔ a (commutativity)
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/// (a ⊔ b) ⊔ c = a ⊔ (b ⊔ c) (associativity)
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/// (a ⊔ b) ⊔ b = a ⊔ b (idempotence)
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/// ```
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///
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/// Moreover, the relationship `≥` defined by `a ≥ b ⇔ ∃c. a = b ⊔ c` must be a partial order.
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/// This implies a few properties such as: if `a ⊔ b ≠ a`, then there is no `c` such that `(a ⊔ b) ⊔ c = a`,
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/// as this would imply a cycle in the partial order.
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pub trait Crdt {
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/// Merge the two datastructures according to the CRDT rules.
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/// `self` is modified to contain the merged CRDT value. `other` is not modified.
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///
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/// # Arguments
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///
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/// * `other` - the other CRDT we wish to merge with
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fn merge(&mut self, other: &Self);
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}
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/// Option<T> implements Crdt for any type T, even if T doesn't implement CRDT itself: when
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/// different values are detected, they are always merged to None. This can be used for value
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/// types which shoulnd't be merged, instead of trying to merge things when we know we don't want
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/// to merge them (which is what the AutoCrdt trait is used for most of the time). This cases
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/// arises very often, for example with a Lww or a LwwMap: the value type has to be a CRDT so that
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/// we have a rule for what to do when timestamps aren't enough to disambiguate (in a distributed
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/// system, anything can happen!), and with AutoCrdt the rule is to make an arbitrary (but
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/// determinstic) choice between the two. When using an Option<T> instead with this impl, ambiguity
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/// cases are explicitely stored as None, which allows us to detect the ambiguity and handle it in
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/// the way we want. (this can only work if we are happy with losing the value when an ambiguity
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/// arises)
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impl<T> Crdt for Option<T>
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where
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T: Eq,
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{
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fn merge(&mut self, other: &Self) {
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if self != other {
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*self = None;
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}
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}
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}
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/// All types that implement `Ord` (a total order) can also implement a trivial CRDT
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/// defined by the merge rule: `a ⊔ b = max(a, b)`. Implement this trait for your type
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/// to enable this behavior.
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pub trait AutoCrdt: Ord + Clone + std::fmt::Debug {
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/// WARN_IF_DIFFERENT: emit a warning when values differ. Set this to true if
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/// different values in your application should never happen. Set this to false
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/// if you are actually relying on the semantics of `a ⊔ b = max(a, b)`.
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const WARN_IF_DIFFERENT: bool;
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}
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impl<T> Crdt for T
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where
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T: AutoCrdt,
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{
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fn merge(&mut self, other: &Self) {
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if Self::WARN_IF_DIFFERENT && self != other {
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warn!(
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"Different CRDT values should be the same (logic error!): {:?} vs {:?}",
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self, other
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);
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if other > self {
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*self = other.clone();
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}
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warn!("Making an arbitrary choice: {:?}", self);
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} else if other > self {
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*self = other.clone();
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}
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}
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}
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impl AutoCrdt for String {
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const WARN_IF_DIFFERENT: bool = true;
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}
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impl AutoCrdt for bool {
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const WARN_IF_DIFFERENT: bool = true;
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}
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