## Real time database - Relative consistency, What is relative consistency in real time database, Explain relative consistency with an example

## Relative consistency

A set of data items are relatively
consistent when their values are updated within a certain time interval, called
the relative validity interval (rvi).

A set of data items are relatively
consistent if they are temporally correlated to each other. So, a derived data
will be correct, only if the data items on which it is derived are relatively
consistent with each other, or in other words, a set of data items would be
called as relatively consistent, if they are contemporary data item.

Different data items that are used to
derive new data must be temporally consistent with each other. This requires
that a set of data items used to derive a new data item form a relative
consistency set R.

**Data item d is temporally consistent if and only if d is absolutely consistent and relatively consistent.**

**Example:**
Assume that sensors are embedded in a
car to monitor its fuel efficiency. They read the current fuel level and the
distance travelled to derive the fuel efficiency. The car may go faster or
slower. Depends on the speed there will be slight variation in the fuel
consumption. And this variation may not go abnormal in any cases.

Let us assume that the current fuel
level is 30 litres at time 100msec and the distance travelled is 10 kilo meters.
Also assume that the avi is 20 msec. These real-time data can be specified as
follows;

**d**

_{fuel}(30 litres, 20 msec, 100 msec)**d**

_{distance}(10 kms, 20 msec, 110 msec)
We read these two values from two
different sensors. So, there will be a difference or delay in data arrival at
the controlling system. We have to accept the fact that both data are valid
during this time delay. That acceptable time delay is called relative validity
interval (rvi). Hence, if rvi is 15 msec for example, then the given data are
relatively consistent. This time rvi is the difference between the time at
which we read 30 litres and the time at which we read 10 kilo meters.

*If the time difference between these two data is not more than the rvi value, then the data are consisdered to be relatively consistent.*

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