akka.cluster.ddata

Class LWWRegister<A>

java.lang.Object

akka.cluster.ddata.LWWRegister<A>

All Implemented Interfaces:

ReplicatedData, ReplicatedDataSerialization, java.io.Serializable

public final class LWWRegister<A>
extends java.lang.Object
implements ReplicatedData, 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 reverseClock() instead of the defaultClock()

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

See Also:

Serialized Form

Nested Class Summary

Nested Classes

Modifier and Type	Class and Description
static interface	LWWRegister.Clock<A>

Constructor Summary

Constructors

Constructor and Description
LWWRegister(UniqueAddress node, A value, long timestamp)

Method Summary

Modifier and Type	Method and Description
static <A> LWWRegister<A>	apply(A initialValue, Cluster node, LWWRegister.Clock<A> clock)
static <A> LWWRegister<A>	apply(UniqueAddress node, A initialValue, LWWRegister.Clock<A> clock) INTERNAL API
static <A> LWWRegister<A>	create(Cluster node, A initialValue) Java API
static <A> LWWRegister<A>	create(Cluster node, A initialValue, LWWRegister.Clock<A> clock) Java API
static <A> LWWRegister.Clock<A>	defaultClock() The default LWWRegister.Clock is using max value of System.currentTimeMillis() and currentTimestamp + 1.
boolean	equals(java.lang.Object o)
A	getValue() Java API
int	hashCode()
LWWRegister<A>	merge(LWWRegister<A> that) INTERNAL API
UniqueAddress	node()
static <A> LWWRegister.Clock<A>	reverseClock() This LWWRegister.Clock can be used for first-write-wins semantics.
long	timestamp()
java.lang.String	toString()
static <A> scala.Option<A>	unapply(LWWRegister<A> c) Extract the value().
UniqueAddress	updatedBy() The current value was set by this node.
A	value()
LWWRegister<A>	withValue(A value, Cluster node, LWWRegister.Clock<A> clock) Change the value of the register.
LWWRegister<A>	withValue(Cluster node, A value) Change the value of the register.
LWWRegister<A>	withValue(Cluster node, A value, LWWRegister.Clock<A> clock) Change the value of the register.
LWWRegister<A>	withValue(UniqueAddress node, A value, LWWRegister.Clock<A> clock) INTERNAL API

Methods inherited from class java.lang.Object

clone, finalize, getClass, notify, notifyAll, wait, wait, wait

Methods inherited from interface akka.cluster.ddata.ReplicatedData

merge

Constructor Detail

LWWRegister

public LWWRegister(UniqueAddress node,
                   A value,
                   long timestamp)

Method Detail

defaultClock

public static <A> LWWRegister.Clock<A> defaultClock()

The default LWWRegister.Clock is using max value of System.currentTimeMillis() and currentTimestamp + 1.

Returns:

(undocumented)

reverseClock

public static <A> LWWRegister.Clock<A> reverseClock()

This LWWRegister.Clock can be used for first-write-wins semantics. It is using min value of -System.currentTimeMillis() and currentTimestamp + 1, i.e. it is counting backwards.

Returns:

(undocumented)

apply

public static <A> LWWRegister<A> apply(UniqueAddress node,
                                       A initialValue,
                                       LWWRegister.Clock<A> clock)

INTERNAL API

Parameters:

node - (undocumented)

initialValue - (undocumented)

clock - (undocumented)

Returns:

(undocumented)

apply

public static <A> LWWRegister<A> apply(A initialValue,
                                       Cluster node,
                                       LWWRegister.Clock<A> clock)

create

public static <A> LWWRegister<A> create(Cluster node,
                                        A initialValue)

Java API

Parameters:

node - (undocumented)

initialValue - (undocumented)

Returns:

(undocumented)

create

public static <A> LWWRegister<A> create(Cluster node,
                                        A initialValue,
                                        LWWRegister.Clock<A> clock)

Java API

Parameters:

node - (undocumented)

initialValue - (undocumented)

clock - (undocumented)

Returns:

(undocumented)

unapply

public static <A> scala.Option<A> unapply(LWWRegister<A> c)

Extract the value().

Parameters:

c - (undocumented)

Returns:

(undocumented)

node

public UniqueAddress node()

value

public A value()

timestamp

public long timestamp()

getValue

public A getValue()

Java API

Returns:

(undocumented)

withValue

public LWWRegister<A> withValue(A value,
                                Cluster node,
                                LWWRegister.Clock<A> clock)

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.

Parameters:

value - (undocumented)

node - (undocumented)

clock - (undocumented)

Returns:

(undocumented)

withValue

public LWWRegister<A> withValue(Cluster node,
                                A value)

Change the value of the register.

Parameters:

node - (undocumented)

value - (undocumented)

Returns:

(undocumented)

withValue

public LWWRegister<A> withValue(Cluster node,
                                A value,
                                LWWRegister.Clock<A> clock)

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.

Parameters:

node - (undocumented)

value - (undocumented)

clock - (undocumented)

Returns:

(undocumented)

updatedBy

public UniqueAddress updatedBy()

The current value was set by this node.

Returns:

(undocumented)

withValue

public LWWRegister<A> withValue(UniqueAddress node,
                                A value,
                                LWWRegister.Clock<A> clock)

INTERNAL API

Parameters:

node - (undocumented)

value - (undocumented)

clock - (undocumented)

Returns:

(undocumented)

merge

public LWWRegister<A> merge(LWWRegister<A> that)

INTERNAL API

Parameters:

that - (undocumented)

Returns:

(undocumented)

toString

public java.lang.String toString()

Overrides:

toString in class java.lang.Object

equals

public boolean equals(java.lang.Object o)

Overrides:

equals in class java.lang.Object

hashCode

public int hashCode()

Overrides:

hashCode in class java.lang.Object