final class LWWRegister[A] extends ReplicatedData with ReplicatedDataSerialization

Implements a 'Last Writer Wins Register' CRDT, also called a 'LWW-Register'.

It is described in the paper A comprehensive study of Convergent and Commutative Replicated Data Types.

Merge takes the register with highest timestamp. Note that this relies on synchronized clocks. LWWRegister should only be used when the choice of value is not important for concurrent updates occurring within the clock skew.

Merge takes the register updated by the node with lowest address (UniqueAddress is ordered) if the timestamps are exactly the same.

Instead of using timestamps based on System.currentTimeMillis() time it is possible to use a timestamp value based on something else, for example an increasing version number from a database record that is used for optimistic concurrency control.

The defaultClock is using max value of System.currentTimeMillis() and currentTimestamp + 1. This means that the timestamp is increased for changes on the same node that occurs within the same millisecond. It also means that it is safe to use the LWWRegister without synchronized clocks when there is only one active writer, e.g. a Cluster Singleton. Such a single writer should then first read current value with ReadMajority (or more) before changing and writing the value with WriteMajority (or more).

For first-write-wins semantics you can use the LWWRegister#reverseClock instead of the LWWRegister#defaultClock

This class is immutable, i.e. "modifying" methods return a new instance.

Annotations
@SerialVersionUID()
Source
LWWRegister.scala
Linear Supertypes
ReplicatedDataSerialization, Serializable, Serializable, ReplicatedData, AnyRef, Any
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  1. LWWRegister
  2. ReplicatedDataSerialization
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  4. Serializable
  5. ReplicatedData
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Type Members

  1. type T = LWWRegister[A]

    The type of the concrete implementation, e.g.

    The type of the concrete implementation, e.g. GSet[A]. To be specified by subclass.

    Definition Classes
    LWWRegister → ReplicatedData

Value Members

  1. def equals(o: Any): Boolean
    Definition Classes
    LWWRegister → AnyRef → Any
  2. def getValue(): A

    Java API

  3. def hashCode(): Int
    Definition Classes
    LWWRegister → AnyRef → Any
  4. def merge(that: LWWRegister[A]): LWWRegister[A]

    Monotonic merge function.

    Monotonic merge function.

    Definition Classes
    LWWRegister → ReplicatedData
  5. val timestamp: Long
  6. def toString(): String
    Definition Classes
    LWWRegister → AnyRef → Any
  7. def updatedBy: UniqueAddress

    The current value was set by this node.

  8. val value: A
  9. def withValue(node: Cluster, value: A, clock: Clock[A]): LWWRegister[A]

    Change the value of the register.

    Change the value of the register.

    You can provide your clock implementation instead of using timestamps based on System.currentTimeMillis() time. The timestamp can for example be an increasing version number from a database record that is used for optimistic concurrency control.

  10. def withValue(node: Cluster, value: A): LWWRegister[A]

    Change the value of the register.

  11. def withValue(value: A)(implicit node: Cluster, clock: Clock[A] = defaultClock[A]): LWWRegister[A]

    Change the value of the register.

    Change the value of the register.

    You can provide your clock implementation instead of using timestamps based on System.currentTimeMillis() time. The timestamp can for example be an increasing version number from a database record that is used for optimistic concurrency control.