org.multiverse.utils.clock
Interface PrimitiveClock

All Known Implementing Classes:

RelaxedPrimitiveClock, SingleThreadedPrimitiveClock, StrictPrimitiveClock

public interface PrimitiveClock

A PrimitiveClock: a logical timer based on a primitive long. It is to make a sense of time possible so that concurrent transactions that commits are ordered. Unlike normal clocks that change based on linear time, a logical clock is non linear; sometimes there are bursts and some there is not much to do. Time is increased by calling tick() or strictTick(). In the future also different forms of clocks like the vector clock will be added to Multiverse.

Concurrent transactions are only interesting when there is shared state. Transactions that don't share state are not ordered. So the relation partially orders transactions in time. Only transactions that share state are fully ordered. It they weren't, the system would start to suffer from isolation problems:

1. transactions seeing state of other transactions that completed before it did.
2. a transaction not seeing state of other transactions that started after the transaction but completed before (in databases this is called the lost update problem).

Small Warning

The PrimitiveClock is based on a long, so there is a limit on the total time a clock is able to live. If a clock lives longer, the time could go 'back' when the long flows over to the negative side. In most cases this won't cause problems; if the dawn time is zero and there are 1.000.0000.0000 transactions per second, only after Long.MAX_VALUE/1.000.0000.000 transactions per second, the clock is able to run for 9223372036 second; which is roughly 292 years.

All clocks always start from 0. An stm could give a clock extra ticks when it starts, so that some versions for the time can be (miss)used to describe some state.

Author:

Peter Veentjer.

Method Summary

long	getVersion() Returns the current version of this PrimitiveClock.
long	strictTick() Executes a strict clock tick by increasing the version.
long	tick() Executes a clock tick by increasing the version.

Method Detail

tick

long tick()

Executes a clock tick by increasing the version. The returned value will always be bigger than the current version. Once the tick method completes, the getVersion() method will always return a time equal or larger than.the last tick.

The returned value could be stale as soon as it is received.

Returns:

the time after the tick.

See Also:

strictTick()

strictTick

long strictTick()

Executes a strict clock tick by increasing the version. The big difference between the strictTick() and the tick() is that the former will always increase the version for every calling thread and the is allowed to return the same version for concurrent ticks.

The returned value could be stale as soon as it is received.

Returns:

the time after the tick.

See Also:

tick()

getVersion

long getVersion()

Returns the current version of this PrimitiveClock.

The returned value could be stale as soon as it is received.

Returns:

the current version.