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Statistics: Rewrite statistics backends to use OpenMetrics The metric subsystem of Prosody has had some shortcomings from the perspective of the current state-of-the-art in metric observability. The OpenMetrics standard [0] is a formalization of the data model (and serialization format) of the well-known and widely-used Prometheus [1] software stack. The previous stats subsystem of Prosody did not map well to that format (see e.g. [2] and [3]); the key reason is that it was trying to do too much math on its own ([2]) while lacking first-class support for "families" of metrics ([3]) and structured metric metadata (despite the `extra` argument to metrics, there was no standard way of representing common things like "tags" or "labels"). Even though OpenMetrics has grown from the Prometheus world of monitoring, it maps well to other popular monitoring stacks such as: - InfluxDB (labels can be mapped to tags and fields as necessary) - Carbon/Graphite (labels can be attached to the metric name with dot-separation) - StatsD (see graphite when assuming that graphite is used as backend, which is the default) The util.statsd module has been ported to use the OpenMetrics model as a proof of concept. An implementation which exposes the util.statistics backend data as Prometheus metrics is ready for publishing in prosody-modules (most likely as mod_openmetrics_prometheus to avoid breaking existing 0.11 deployments). At the same time, the previous measure()-based API had one major advantage: It is really simple and easy to use without requiring lots of knowledge about OpenMetrics or similar concepts. For that reason as well as compatibility with existing code, it is preserved and may even be extended in the future. However, code relying on the `stats-updated` event as well as `get_stats` from `statsmanager` will break because the data model has changed completely; in case of `stats-updated`, the code will simply not run (as the event was renamed in order to avoid conflicts); the `get_stats` function has been removed completely (so it will cause a traceback when it is attempted to be used). Note that the measure_*_event methods have been removed from the module API. I was unable to find any uses or documentation and thus deemed they should not be ported. Re-implementation is possible when necessary. [0]: https://openmetrics.io/ [1]: https://prometheus.io/ [2]: #959 [3]: #960

local math_abs = math.abs;
local math_ceil = math.ceil;
local math_floor = math.floor;
local math_log = math.log;
local math_max = math.max;
local math_min = math.min;
local unpack = table.unpack or unpack; --luacheck: ignore 113

if math_log(10, 10) ~= 1 then
	-- Lua 5.1 COMPAT
	local log10 = math.log10;
	function math_log(n, base)
		return log10(n) / log10(base);
	end
end

local large = {
	"k", 1000,
	"M", 1000000,
	"G", 1000000000,
	"T", 1000000000000,
	"P", 1000000000000000,
	"E", 1000000000000000000,
	"Z", 1000000000000000000000,
	"Y", 1000000000000000000000000,
}
local small = {
	"m", 0.001,
	"μ", 0.000001,
	"n", 0.000000001,
	"p", 0.000000000001,
	"f", 0.000000000000001,
	"a", 0.000000000000000001,
	"z", 0.000000000000000000001,
	"y", 0.000000000000000000000001,
}

local binary = {
	"Ki", 2^10,
	"Mi", 2^20,
	"Gi", 2^30,
	"Ti", 2^40,
	"Pi", 2^50,
	"Ei", 2^60,
	"Zi", 2^70,
	"Yi", 2^80,
}

local function adjusted_unit(n, b)
	local round = math_floor;
	local prefixes = large;
	local logbase = 1000;
	if b == 'b' then
		prefixes = binary;
		logbase = 1024;
	elseif n < 1 then
		prefixes = small;
		round = math_ceil;
	end
	local m = math_max(0, math_min(8, round(math_abs(math_log(math_abs(n), logbase)))));
	local prefix, multiplier = unpack(prefixes, m * 2-1, m*2);
	return multiplier or 1, prefix;
end

-- n: number, the number to format
-- unit: string, the base unit
-- b: optional enum 'b', thousands base
local function format(n, unit, b) --> string
	local fmt = "%.3g %s%s";
	if n == 0 then
		return fmt:format(n, "", unit);
	end
	local multiplier, prefix = adjusted_unit(n, b);
	return fmt:format(n / multiplier, prefix or "", unit);
end

return {
	adjust = adjusted_unit;
	format = format;
};