# Histograms

Histograms are used to record the distribution of a piece of data over time.

## Creating

(use '[metrics.histograms :only (histogram)])

(def search-results-returned
(histogram "search-results-returned"))


You can create an unbiased histogram by passing an extra boolean argument (though you probably don't want to):

(def search-results-returned-biased
(histogram "search-results-returned-unbiased" false))


You can also use the defhistogram macro to create a histogram and bind it to a var in one concise, easy step:

(use '[metrics.histograms :only (defhistogram)])

(defhistogram search-results-returned)


All the def[metric] macros do some :ref:magic <desugaring> to the metric title to make it easier to define.

## Writing

Update the histogram when you have a new value to record:

(use '[metrics.histograms :only (update!)])

(update! search-results-returned 10)


The data of a histogram metrics can be retrived in a bunch of different ways.

### percentiles

The function you'll usually want to use to pull data from a histogram is percentiles:

(use '[metrics.histograms :only (percentiles)])

(percentiles search-results-returned)
=> { 0.75   180
0.95   299
0.99   300
0.999  340
1.0   1345 }


This returns a map of the percentiles you probably care about. The keys are the percentiles (doubles between 0 and 1 inclusive) and the values are the maximum value for that percentile. In this example:

• 75% of searches returned 180 or fewer results.
• 95% of searches returned 299 or fewer results.
• ... etc.

If you want a different set of percentiles just pass them as a sequence:

(use '[metrics.histograms :only (percentiles)])

(percentiles search-results-returned [0.50 0.75])
=> { 0.50 100
0.75 180 }


### number-recorded

To get the number of data points recorded over the entire lifetime of this histogram:

(use '[metrics.histograms :only (number-recorded)])

(number-recorded search-results-returned)
=> 12882


### smallest

To get the smallest data point recorded over the entire lifetime of this histogram:

(use '[metrics.histograms :only (smallest)])

(smallest search-results-returned)
=> 4


### largest

To get the largest data point recorded over the entire lifetime of this histogram:

(use '[metrics.histograms :only (largest)])

(largest search-results-returned)
=> 1345


### mean

To get the mean of the data points recorded over the entire lifetime of this histogram:

(use '[metrics.histograms :only (mean)])

(mean search-results-returned)
=> 233.12


### std-dev

To get the standard deviation of the data points recorded over the entire lifetime of this histogram:

(use '[metrics.histograms :only (std-dev)])

(std-dev search-results-returned)
=> 80.2


### sample

You can get the current sample points the histogram is using with sample, but you almost certainly don't care about this. If you use it make sure you know what you're doing.

(use '[metrics.histograms :only (sample)])

(sample search-results-returned)
=> [12 2232 234 122]