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12.3. Windows and Statistics

As mentioned above, the hierarchical database measures the values for each metric every 5 seconds. But it would take vast amounts of space to keep all these measurements around for long periods of time. Instead, the hierarchical database calculates the statistics for various intervals, and then rolls up the statistics into different time windows .

The statistics are straightforward:

Expand

Statistic	Data type	Description
Count	int	The number of samples.
Maximum	long	The maximum value from the samples.
Minimum	long	The minimum value from the samples.
Mean	double	The mean (or average) value from the samples.
Variance	double	The average of the squared differences from the mean .
Standard Deviation	double	A measure of how spread out the samples are and is the square root of the variance

and are represented in the API by the org.modeshape.jcr.api.monitor.Statistics interface (with getter methods for each statistic). The statistics were chosen because multiple Statistics objects can easily be rolled-up into a single Statistic object.

The rollup process is pretty simple. For each metric:

The value is captured every 5 seconds, and recorded a Statistics instance with a single sample. This is repeated 9 more times.
After 60 seconds, the 10 Statistics objects recorded in the previous step are rolled-up into a single Statistics object for this minute. This is repeated 59 more times.
After 60 minutes, the 60 Statistics objects recorded in the previous step are rolled-up into a single Statistics object for this hour. This is repeated 23 more times.
After 24 hours, the 24 Statistics objects recorded in the previous step are rolled-up into a single Statistics object for the day. This is repeated 7 more times.
After 7 days, the 7 Statistics objects recorded in the previous step are rolled-up into a single Statistics object for the week. This is repeated 52 more times.

Each of these periods represents a window in time during with the Statistics are captured:

Expand

Window timeframe	Description
60 seconds	A `Statistics` for each of the ten 5-second intervals during the last minute.
60 minutes	A `Statistics` for each minute during the last hour.
24 hours	A `Statistics` for each hour during the last day.
7 days	A `Statistics` for each day during the last week.
52 weeks	A `Statistics` for each week during the last year.

The org.modeshape.jcr.api.monitor.Window enumeration is used to represent each of these windows in time.

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12.3. Windows and Statistics

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