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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

-- Prosody IM
-- Copyright (C) 2008-2010 Matthew Wild
-- Copyright (C) 2008-2010 Waqas Hussain
--
-- This project is MIT/X11 licensed. Please see the
-- COPYING file in the source package for more information.
--

local t_insert, t_remove = table.insert, table.remove;

local _ENV = nil;
-- luacheck: std none

local new_filter_hooks = {};

local function initialize(session)
	if not session.filters then
		local filters = {};
		session.filters = filters;

		function session.filter(type, data)
			local filter_list = filters[type];
			if filter_list then
				for i = 1, #filter_list do
					data = filter_list[i](data, session);
					if data == nil then break; end
				end
			end
			return data;
		end
	end

	for i=1,#new_filter_hooks do
		new_filter_hooks[i](session);
	end

	return session.filter;
end

local function add_filter(session, type, callback, priority)
	if not session.filters then
		initialize(session);
	end

	local filter_list = session.filters[type];
	if not filter_list then
		filter_list = {};
		session.filters[type] = filter_list;
	elseif filter_list[callback] then
		return; -- Filter already added
	end

	priority = priority or 0;

	local i = 0;
	repeat
		i = i + 1;
	until not filter_list[i] or filter_list[filter_list[i]] < priority;

	t_insert(filter_list, i, callback);
	filter_list[callback] = priority;
end

local function remove_filter(session, type, callback)
	if not session.filters then return; end
	local filter_list = session.filters[type];
	if filter_list and filter_list[callback] then
		for i=1, #filter_list do
			if filter_list[i] == callback then
				t_remove(filter_list, i);
				filter_list[callback] = nil;
				return true;
			end
		end
	end
end

local function add_filter_hook(callback)
	t_insert(new_filter_hooks, callback);
end

local function remove_filter_hook(callback)
	for i=1,#new_filter_hooks do
		if new_filter_hooks[i] == callback then
			t_remove(new_filter_hooks, i);
		end
	end
end

return {
	initialize = initialize;
	add_filter = add_filter;
	remove_filter = remove_filter;
	add_filter_hook = add_filter_hook;
	remove_filter_hook = remove_filter_hook;
};