Add MetricsBackend with prometheus counter support

12 files changed, 389 insertions, 21 deletions

diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index b416faaf3..710d9e13e 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -217,6 +217,26 @@ endif(ENABLE_REDIS)
 
 find_package(SQLite3 REQUIRED)
 
+OPTION(ENABLE_PROMETHEUS "Enable prometheus client support" FALSE)
+set(USE_PROMETHEUS FALSE)
+
+if(ENABLE_PROMETHEUS)
+	find_path(PROMETHEUS_CPP_INCLUDE_DIR NAMES prometheus/counter.h)
+	find_library(PROMETHEUS_PULL_LIBRARY NAMES prometheus-cpp-pull)
+	find_library(PROMETHEUS_CORE_LIBRARY NAMES prometheus-cpp-core)
+	if(PROMETHEUS_CPP_INCLUDE_DIR AND PROMETHEUS_PULL_LIBRARY AND PROMETHEUS_CORE_LIBRARY)
+		set(PROMETHEUS_LIBRARIES ${PROMETHEUS_PULL_LIBRARY} ${PROMETHEUS_CORE_LIBRARY})
+		set(USE_PROMETHEUS TRUE)
+		include_directories(${PROMETHEUS_CPP_INCLUDE_DIR})
+	endif(PROMETHEUS_CPP_INCLUDE_DIR AND PROMETHEUS_PULL_LIBRARY AND PROMETHEUS_CORE_LIBRARY)
+endif(ENABLE_PROMETHEUS)
+
+if(USE_PROMETHEUS)
+	message(STATUS "Prometheus client enabled.")
+else(USE_PROMETHEUS)
+	message(STATUS "Prometheus client disabled.")
+endif(USE_PROMETHEUS)
+
 OPTION(ENABLE_SPATIAL "Enable SpatialIndex AreaStore backend" TRUE)
 set(USE_SPATIAL FALSE)
 
@@ -597,6 +617,9 @@ if(BUILD_CLIENT)
 	if (USE_REDIS)
 		target_link_libraries(${PROJECT_NAME} ${REDIS_LIBRARY})
 	endif()
+	if (USE_PROMETHEUS)
+		target_link_libraries(${PROJECT_NAME} ${PROMETHEUS_LIBRARIES})
+	endif()
 	if (USE_SPATIAL)
 		target_link_libraries(${PROJECT_NAME} ${SPATIAL_LIBRARY})
 	endif()
@@ -632,6 +655,9 @@ if(BUILD_SERVER)
 	if (USE_REDIS)
 		target_link_libraries(${PROJECT_NAME}server ${REDIS_LIBRARY})
 	endif()
+	if (USE_PROMETHEUS)
+		target_link_libraries(${PROJECT_NAME}server ${PROMETHEUS_LIBRARIES})
+	endif()
 	if (USE_SPATIAL)
 		target_link_libraries(${PROJECT_NAME}server ${SPATIAL_LIBRARY})
 	endif()
diff --git a/src/cmake_config.h.in b/src/cmake_config.h.in
index cac6335d4..cfcee4b58 100644
--- a/src/cmake_config.h.in
+++ b/src/cmake_config.h.in
@@ -23,6 +23,7 @@
 #cmakedefine01 USE_LEVELDB
 #cmakedefine01 USE_LUAJIT
 #cmakedefine01 USE_POSTGRESQL
+#cmakedefine01 USE_PROMETHEUS
 #cmakedefine01 USE_SPATIAL
 #cmakedefine01 USE_SYSTEM_GMP
 #cmakedefine01 USE_REDIS
diff --git a/src/defaultsettings.cpp b/src/defaultsettings.cpp
index b6b1ce1f2..06daa3b94 100644
--- a/src/defaultsettings.cpp
+++ b/src/defaultsettings.cpp
@@ -334,6 +334,9 @@ void set_default_settings(Settings *settings)
 	// Server
 	settings->setDefault("disable_escape_sequences", "false");
 	settings->setDefault("strip_color_codes", "false");
+#if USE_PROMETHEUS
+	settings->setDefault("prometheus_listener_address", "127.0.0.1:30000");
+#endif
 
 	// Network
 	settings->setDefault("enable_ipv6", "true");
diff --git a/src/map.cpp b/src/map.cpp
index 12e122124..5f1b984a4 100644
--- a/src/map.cpp
+++ b/src/map.cpp
@@ -144,7 +144,7 @@ bool Map::isNodeUnderground(v3s16 p)
 {
 	v3s16 blockpos = getNodeBlockPos(p);
 	MapBlock *block = getBlockNoCreateNoEx(blockpos);
-	return block && block->getIsUnderground(); 
+	return block && block->getIsUnderground();
 }
 
 bool Map::isValidPosition(v3s16 p)
@@ -1187,7 +1187,7 @@ bool Map::isBlockOccluded(MapBlock *block, v3s16 cam_pos_nodes)
 	ServerMap
 */
 ServerMap::ServerMap(const std::string &savedir, IGameDef *gamedef,
-		EmergeManager *emerge):
+		EmergeManager *emerge, MetricsBackend *mb):
 	Map(dout_server, gamedef),
 	settings_mgr(g_settings, savedir + DIR_DELIM + "map_meta.txt"),
 	m_emerge(emerge)
@@ -1221,6 +1221,8 @@ ServerMap::ServerMap(const std::string &savedir, IGameDef *gamedef,
 	m_savedir = savedir;
 	m_map_saving_enabled = false;
 
+	m_save_time_counter = mb->addCounter("minetest_core_map_save_time", "Map save time (in nanoseconds)");
+
 	try {
 		// If directory exists, check contents and load if possible
 		if (fs::PathExists(m_savedir)) {
@@ -1777,6 +1779,8 @@ void ServerMap::save(ModifiedState save_level)
 		return;
 	}
 
+	u64 start_time = porting::getTimeNs();
+
 	if(save_level == MOD_STATE_CLEAN)
 		infostream<<"ServerMap: Saving whole map, this can take time."
 				<<std::endl;
@@ -1835,6 +1839,9 @@ void ServerMap::save(ModifiedState save_level)
 		infostream<<"Blocks modified by: "<<std::endl;
 		modprofiler.print(infostream);
 	}
+
+	auto end_time = porting::getTimeNs();
+	m_save_time_counter->increment(end_time - start_time);
 }
 
 void ServerMap::listAllLoadableBlocks(std::vector<v3s16> &dst)
diff --git a/src/map.h b/src/map.h
index ff6b20c4f..77ee4da9e 100644
--- a/src/map.h
+++ b/src/map.h
@@ -31,6 +31,7 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #include "voxel.h"
 #include "modifiedstate.h"
 #include "util/container.h"
+#include "util/metricsbackend.h"
 #include "nodetimer.h"
 #include "map_settings_manager.h"
 #include "debug.h"
@@ -45,6 +46,7 @@ class NodeMetadata;
 class IGameDef;
 class IRollbackManager;
 class EmergeManager;
+class MetricsBackend;
 class ServerEnvironment;
 struct BlockMakeData;
 
@@ -324,7 +326,7 @@ public:
 	/*
 		savedir: directory to which map data should be saved
 	*/
-	ServerMap(const std::string &savedir, IGameDef *gamedef, EmergeManager *emerge);
+	ServerMap(const std::string &savedir, IGameDef *gamedef, EmergeManager *emerge, MetricsBackend *mb);
 	~ServerMap();
 
 	s32 mapType() const
@@ -449,6 +451,8 @@ private:
 	bool m_map_metadata_changed = true;
 	MapDatabase *dbase = nullptr;
 	MapDatabase *dbase_ro = nullptr;
+
+	MetricCounterPtr m_save_time_counter;
 };
 
 
diff --git a/src/server.cpp b/src/server.cpp
index af6d3e40d..a7eb52837 100644
--- a/src/server.cpp
+++ b/src/server.cpp
@@ -229,18 +229,46 @@ Server::Server(
 	m_nodedef(createNodeDefManager()),
 	m_craftdef(createCraftDefManager()),
 	m_thread(new ServerThread(this)),
-	m_uptime(0),
 	m_clients(m_con),
 	m_admin_chat(iface),
 	m_modchannel_mgr(new ModChannelMgr())
 {
-	m_lag = g_settings->getFloat("dedicated_server_step");
-
 	if (m_path_world.empty())
 		throw ServerError("Supplied empty world path");
 
 	if (!gamespec.isValid())
 		throw ServerError("Supplied invalid gamespec");
+
+#if USE_PROMETHEUS
+	m_metrics_backend = std::unique_ptr<MetricsBackend>(createPrometheusMetricsBackend());
+#else
+	m_metrics_backend = std::unique_ptr<MetricsBackend>(new MetricsBackend());
+#endif
+
+	m_uptime_counter = m_metrics_backend->addCounter("minetest_core_server_uptime", "Server uptime (in seconds)");
+	m_player_gauge = m_metrics_backend->addGauge("minetest_core_player_number", "Number of connected players");
+
+	m_timeofday_gauge = m_metrics_backend->addGauge(
+			"minetest_core_timeofday",
+			"Time of day value");
+
+	m_lag_gauge = m_metrics_backend->addGauge(
+			"minetest_core_latency",
+			"Latency value (in seconds)");
+
+	m_aom_buffer_counter = m_metrics_backend->addCounter(
+			"minetest_core_aom_generated_count",
+			"Number of active object messages generated");
+
+	m_packet_recv_counter = m_metrics_backend->addCounter(
+			"minetest_core_server_packet_recv",
+			"Processable packets received");
+
+	m_packet_recv_processed_counter = m_metrics_backend->addCounter(
+			"minetest_core_server_packet_recv_processed",
+			"Valid received packets processed");
+
+	m_lag_gauge->set(g_settings->getFloat("dedicated_server_step"));
 }
 
 Server::~Server()
@@ -353,7 +381,7 @@ void Server::init()
 	MutexAutoLock envlock(m_env_mutex);
 
 	// Create the Map (loads map_meta.txt, overriding configured mapgen params)
-	ServerMap *servermap = new ServerMap(m_path_world, this, m_emerge);
+	ServerMap *servermap = new ServerMap(m_path_world, this, m_emerge, m_metrics_backend.get());
 
 	// Initialize scripting
 	infostream << "Server: Initializing Lua" << std::endl;
@@ -511,9 +539,7 @@ void Server::AsyncRunStep(bool initial_step)
 	/*
 		Update uptime
 	*/
-	{
-		m_uptime.set(m_uptime.get() + dtime);
-	}
+	m_uptime_counter->increment(dtime);
 
 	handlePeerChanges();
 
@@ -527,11 +553,13 @@ void Server::AsyncRunStep(bool initial_step)
 	*/
 
 	m_time_of_day_send_timer -= dtime;
-	if(m_time_of_day_send_timer < 0.0) {
+	if (m_time_of_day_send_timer < 0.0) {
 		m_time_of_day_send_timer = g_settings->getFloat("time_send_interval");
 		u16 time = m_env->getTimeOfDay();
 		float time_speed = g_settings->getFloat("time_speed");
 		SendTimeOfDay(PEER_ID_INEXISTENT, time, time_speed);
+
+		m_timeofday_gauge->set(time);
 	}
 
 	{
@@ -603,7 +631,7 @@ void Server::AsyncRunStep(bool initial_step)
 	}
 	m_clients.step(dtime);
 
-	m_lag += (m_lag > dtime ? -1 : 1) * dtime/100;
+	m_lag_gauge->increment((m_lag_gauge->get() > dtime ? -1 : 1) * dtime/100);
 #if USE_CURL
 	// send masterserver announce
 	{
@@ -614,9 +642,9 @@ void Server::AsyncRunStep(bool initial_step)
 						ServerList::AA_START,
 					m_bind_addr.getPort(),
 					m_clients.getPlayerNames(),
-					m_uptime.get(),
+					m_uptime_counter->get(),
 					m_env->getGameTime(),
-					m_lag,
+					m_lag_gauge->get(),
 					m_gamespec.id,
 					Mapgen::getMapgenName(m_emerge->mgparams->mgtype),
 					m_modmgr->getMods(),
@@ -638,6 +666,7 @@ void Server::AsyncRunStep(bool initial_step)
 		const RemoteClientMap &clients = m_clients.getClientList();
 		ScopeProfiler sp(g_profiler, "Server: update objects within range");
 
+		m_player_gauge->set(clients.size());
 		for (const auto &client_it : clients) {
 			RemoteClient *client = client_it.second;
 
@@ -703,6 +732,8 @@ void Server::AsyncRunStep(bool initial_step)
 			message_list->push_back(aom);
 		}
 
+		m_aom_buffer_counter->increment(buffered_messages.size());
+
 		m_clients.lock();
 		const RemoteClientMap &clients = m_clients.getClientList();
 		// Route data to every client
@@ -943,7 +974,9 @@ void Server::Receive()
 			}
 
 			peer_id = pkt.getPeerId();
+			m_packet_recv_counter->increment();
 			ProcessData(&pkt);
+			m_packet_recv_processed_counter->increment();
 		} catch (const con::InvalidIncomingDataException &e) {
 			infostream << "Server::Receive(): InvalidIncomingDataException: what()="
 					<< e.what() << std::endl;
@@ -3127,7 +3160,7 @@ std::wstring Server::getStatusString()
 	// Version
 	os << L"version=" << narrow_to_wide(g_version_string);
 	// Uptime
-	os << L", uptime=" << m_uptime.get();
+	os << L", uptime=" << m_uptime_counter->get();
 	// Max lag estimate
 	os << L", max_lag=" << (m_env ? m_env->getMaxLagEstimate() : 0);
 
diff --git a/src/server.h b/src/server.h
index b995aba28..71059dd30 100644
--- a/src/server.h
+++ b/src/server.h
@@ -33,6 +33,7 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #include "util/numeric.h"
 #include "util/thread.h"
 #include "util/basic_macros.h"
+#include "util/metricsbackend.h"
 #include "serverenvironment.h"
 #include "clientiface.h"
 #include "chatmessage.h"
@@ -203,7 +204,7 @@ public:
 
 	// Connection must be locked when called
 	std::wstring getStatusString();
-	inline double getUptime() const { return m_uptime.m_value; }
+	inline double getUptime() const { return m_uptime_counter->get(); }
 
 	// read shutdown state
 	inline bool isShutdownRequested() const { return m_shutdown_state.is_requested; }
@@ -591,9 +592,6 @@ private:
 	float m_step_dtime = 0.0f;
 	std::mutex m_step_dtime_mutex;
 
-	// current server step lag counter
-	float m_lag;
-
 	// The server mainly operates in this thread
 	ServerThread *m_thread = nullptr;
 
@@ -602,8 +600,6 @@ private:
 	*/
 	// Timer for sending time of day over network
 	float m_time_of_day_send_timer = 0.0f;
-	// Uptime of server in seconds
-	MutexedVariable<double> m_uptime;
 
 	/*
 	 	Client interface
@@ -677,6 +673,19 @@ private:
 
 	// ModChannel manager
 	std::unique_ptr<ModChannelMgr> m_modchannel_mgr;
+
+	// Global server metrics backend
+	std::unique_ptr<MetricsBackend> m_metrics_backend;
+
+	// Server metrics
+	MetricCounterPtr m_uptime_counter;
+	MetricGaugePtr m_player_gauge;
+	MetricGaugePtr m_timeofday_gauge;
+	// current server step lag
+	MetricGaugePtr m_lag_gauge;
+	MetricCounterPtr m_aom_buffer_counter;
+	MetricCounterPtr m_packet_recv_counter;
+	MetricCounterPtr m_packet_recv_processed_counter;
 };
 
 /*
diff --git a/src/server/mods.cpp b/src/server/mods.cpp
index c8d8a28e2..6ac530739 100644
--- a/src/server/mods.cpp
+++ b/src/server/mods.cpp
@@ -23,6 +23,7 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #include "scripting_server.h"
 #include "content/subgames.h"
 #include "porting.h"
+#include "util/metricsbackend.h"
 
 /**
  * Manage server mods
diff --git a/src/server/mods.h b/src/server/mods.h
index 2bc1aa22f..54774bd86 100644
--- a/src/server/mods.h
+++ b/src/server/mods.h
@@ -20,7 +20,10 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #pragma once
 
 #include "content/mods.h"
+#include <memory>
 
+class MetricsBackend;
+class MetricCounter;
 class ServerScripting;
 
 /**
diff --git a/src/util/CMakeLists.txt b/src/util/CMakeLists.txt
index 199d3aeaa..cd2e468d1 100644
--- a/src/util/CMakeLists.txt
+++ b/src/util/CMakeLists.txt
@@ -5,6 +5,7 @@ set(UTIL_SRCS
 	${CMAKE_CURRENT_SOURCE_DIR}/directiontables.cpp
 	${CMAKE_CURRENT_SOURCE_DIR}/enriched_string.cpp
 	${CMAKE_CURRENT_SOURCE_DIR}/ieee_float.cpp
+	${CMAKE_CURRENT_SOURCE_DIR}/metricsbackend.cpp
 	${CMAKE_CURRENT_SOURCE_DIR}/numeric.cpp
 	${CMAKE_CURRENT_SOURCE_DIR}/pointedthing.cpp
 	${CMAKE_CURRENT_SOURCE_DIR}/quicktune.cpp
diff --git a/src/util/metricsbackend.cpp b/src/util/metricsbackend.cpp
new file mode 100644
index 000000000..4454557a3
--- /dev/null
+++ b/src/util/metricsbackend.cpp
@@ -0,0 +1,140 @@
+/*
+Minetest
+Copyright (C) 2013-2020 Minetest core developers team
+
+This program is free software; you can redistribute it and/or modify
+it under the terms of the GNU Lesser General Public License as published by
+the Free Software Foundation; either version 2.1 of the License, or
+(at your option) any later version.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU Lesser General Public License for more details.
+
+You should have received a copy of the GNU Lesser General Public License along
+with this program; if not, write to the Free Software Foundation, Inc.,
+51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+*/
+
+#include "metricsbackend.h"
+#if USE_PROMETHEUS
+#include <prometheus/exposer.h>
+#include <prometheus/registry.h>
+#include <prometheus/counter.h>
+#include <prometheus/gauge.h>
+#include "log.h"
+#include "settings.h"
+#endif
+
+MetricCounterPtr MetricsBackend::addCounter(
+		const std::string &name, const std::string &help_str)
+{
+	return std::make_shared<SimpleMetricCounter>(name, help_str);
+}
+
+MetricGaugePtr MetricsBackend::addGauge(
+		const std::string &name, const std::string &help_str)
+{
+	return std::make_shared<SimpleMetricGauge>(name, help_str);
+}
+
+#if USE_PROMETHEUS
+
+class PrometheusMetricCounter : public MetricCounter
+{
+public:
+	PrometheusMetricCounter() = delete;
+
+	PrometheusMetricCounter(const std::string &name, const std::string &help_str,
+			std::shared_ptr<prometheus::Registry> registry) :
+			MetricCounter(),
+			m_family(prometheus::BuildCounter()
+							.Name(name)
+							.Help(help_str)
+							.Register(*registry)),
+			m_counter(m_family.Add({}))
+	{
+	}
+
+	virtual ~PrometheusMetricCounter() {}
+
+	virtual void increment(double number) { m_counter.Increment(number); }
+	virtual double get() const { return m_counter.Value(); }
+
+private:
+	prometheus::Family<prometheus::Counter> &m_family;
+	prometheus::Counter &m_counter;
+};
+
+class PrometheusMetricGauge : public MetricGauge
+{
+public:
+	PrometheusMetricGauge() = delete;
+
+	PrometheusMetricGauge(const std::string &name, const std::string &help_str,
+			std::shared_ptr<prometheus::Registry> registry) :
+			MetricGauge(),
+			m_family(prometheus::BuildGauge()
+							.Name(name)
+							.Help(help_str)
+							.Register(*registry)),
+			m_gauge(m_family.Add({}))
+	{
+	}
+
+	virtual ~PrometheusMetricGauge() {}
+
+	virtual void increment(double number) { m_gauge.Increment(number); }
+	virtual void decrement(double number) { m_gauge.Decrement(number); }
+	virtual void set(double number) { m_gauge.Set(number); }
+	virtual double get() const { return m_gauge.Value(); }
+
+private:
+	prometheus::Family<prometheus::Gauge> &m_family;
+	prometheus::Gauge &m_gauge;
+};
+
+class PrometheusMetricsBackend : public MetricsBackend
+{
+public:
+	PrometheusMetricsBackend(const std::string &addr) :
+			MetricsBackend(), m_exposer(std::unique_ptr<prometheus::Exposer>(
+							  new prometheus::Exposer(addr))),
+			m_registry(std::make_shared<prometheus::Registry>())
+	{
+		m_exposer->RegisterCollectable(m_registry);
+	}
+
+	virtual ~PrometheusMetricsBackend() {}
+
+	virtual MetricCounterPtr addCounter(
+			const std::string &name, const std::string &help_str);
+	virtual MetricGaugePtr addGauge(
+			const std::string &name, const std::string &help_str);
+
+private:
+	std::unique_ptr<prometheus::Exposer> m_exposer;
+	std::shared_ptr<prometheus::Registry> m_registry;
+};
+
+MetricCounterPtr PrometheusMetricsBackend::addCounter(
+		const std::string &name, const std::string &help_str)
+{
+	return std::make_shared<PrometheusMetricCounter>(name, help_str, m_registry);
+}
+
+MetricGaugePtr PrometheusMetricsBackend::addGauge(
+		const std::string &name, const std::string &help_str)
+{
+	return std::make_shared<PrometheusMetricGauge>(name, help_str, m_registry);
+}
+
+MetricsBackend *createPrometheusMetricsBackend()
+{
+	std::string addr;
+	g_settings->getNoEx("prometheus_listener_address", addr);
+	return new PrometheusMetricsBackend(addr);
+}
+
+#endif
diff --git a/src/util/metricsbackend.h b/src/util/metricsbackend.h
new file mode 100644
index 000000000..c37306392
--- /dev/null
+++ b/src/util/metricsbackend.h
@@ -0,0 +1,140 @@
+/*
+Minetest
+Copyright (C) 2013-2020 Minetest core developers team
+
+This program is free software; you can redistribute it and/or modify
+it under the terms of the GNU Lesser General Public License as published by
+the Free Software Foundation; either version 2.1 of the License, or
+(at your option) any later version.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU Lesser General Public License for more details.
+
+You should have received a copy of the GNU Lesser General Public License along
+with this program; if not, write to the Free Software Foundation, Inc.,
+51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+*/
+
+#pragma once
+#include <memory>
+#include "config.h"
+#include "util/thread.h"
+
+class MetricCounter
+{
+public:
+	MetricCounter() = default;
+
+	virtual ~MetricCounter() {}
+
+	virtual void increment(double number = 1.0) = 0;
+	virtual double get() const = 0;
+};
+
+typedef std::shared_ptr<MetricCounter> MetricCounterPtr;
+
+class SimpleMetricCounter : public MetricCounter
+{
+public:
+	SimpleMetricCounter() = delete;
+
+	virtual ~SimpleMetricCounter() {}
+
+	SimpleMetricCounter(const std::string &name, const std::string &help_str) :
+			MetricCounter(), m_name(name), m_help_str(help_str),
+			m_counter(0.0)
+	{
+	}
+
+	virtual void increment(double number)
+	{
+		MutexAutoLock lock(m_mutex);
+		m_counter += number;
+	}
+	virtual double get() const
+	{
+		MutexAutoLock lock(m_mutex);
+		return m_counter;
+	}
+
+private:
+	std::string m_name;
+	std::string m_help_str;
+
+	mutable std::mutex m_mutex;
+	double m_counter;
+};
+
+class MetricGauge
+{
+public:
+	MetricGauge() = default;
+	virtual ~MetricGauge() {}
+
+	virtual void increment(double number = 1.0) = 0;
+	virtual void decrement(double number = 1.0) = 0;
+	virtual void set(double number) = 0;
+	virtual double get() const = 0;
+};
+
+typedef std::shared_ptr<MetricGauge> MetricGaugePtr;
+
+class SimpleMetricGauge : public MetricGauge
+{
+public:
+	SimpleMetricGauge() = delete;
+
+	SimpleMetricGauge(const std::string &name, const std::string &help_str) :
+			MetricGauge(), m_name(name), m_help_str(help_str), m_gauge(0.0)
+	{
+	}
+
+	virtual ~SimpleMetricGauge() {}
+
+	virtual void increment(double number)
+	{
+		MutexAutoLock lock(m_mutex);
+		m_gauge += number;
+	}
+	virtual void decrement(double number)
+	{
+		MutexAutoLock lock(m_mutex);
+		m_gauge -= number;
+	}
+	virtual void set(double number)
+	{
+		MutexAutoLock lock(m_mutex);
+		m_gauge = number;
+	}
+	virtual double get() const
+	{
+		MutexAutoLock lock(m_mutex);
+		return m_gauge;
+	}
+
+private:
+	std::string m_name;
+	std::string m_help_str;
+
+	mutable std::mutex m_mutex;
+	double m_gauge;
+};
+
+class MetricsBackend
+{
+public:
+	MetricsBackend() = default;
+
+	virtual ~MetricsBackend() {}
+
+	virtual MetricCounterPtr addCounter(
+			const std::string &name, const std::string &help_str);
+	virtual MetricGaugePtr addGauge(
+			const std::string &name, const std::string &help_str);
+};
+
+#if USE_PROMETHEUS
+MetricsBackend *createPrometheusMetricsBackend();
+#endif