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Sunday, 20 March 2016
Real time database - absolute consistency
The consistency about the state of the environment (controlled system) and its reflection in the database (controlling systems database) is absolute consistency. This ensures the freshness of the data that is read by a transaction.
A data item obeys absolute consistency when its value is updated within a predefined time interval called Absolute Validity Interval (avi). That is, the value of a data item is absolutely consistent (fresh) if and only if the difference between the time of the observation of data and the current time is not greater than the absolute validity interval value.
Absolute Validity Interval (avi) is the length of the time interval since the time of the observation of data, during which the data is considered to be fresh (not outdated) or to have absolute validity.
Absolute data consistency states that the validity interval of the most recent value of a base (or derived) item is always longer than the time interval between its absolute valid (or absolute transaction) time and “now.” This indicates that the data has absolute validity.
Data is only valid between absolute points in time. This is due to the need to keep the database consistent with the environment.
Assume that a car is manufactured with a sensor in fuel tank to monitor the availability level of the fuel at any time. Let us also assume that the reading from the sensor can be of maximum 20 msec (absolute validity interval) outdated (old) at any time. That is, if sensor gives us the current fuel level at time 100 msec as 30 litres, then the value 30 litres would be valid for next 20 seconds. The current fuel level becomes outdated (old or obsolete) after 120 msec (the data is valid for 20 msec, ie., from 100 msec to 120 msec). The consistency of the data at this absolute validity interval is called Absolute consistency.
If the sensor reads the fuel level at 100 msec, the fuel reading is 30 litres and if we assume 20 msec as the avi value, then we would represent this data as follows;
d(30 litres, 20 msec, 100msec)
The data 30 litres is valid between 100 msec and 120 msec.
If suppose the current time is 118 msec, then this is absolutely consistent. If the current time is 121 msec, then the data is not consistent and the data becomes old or archival data.
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