Share some metrics between the Prometheus exporter and the phone home stats (#13671)

6 files changed, 146 insertions, 4 deletions

diff --git a/changelog.d/13671.misc b/changelog.d/13671.misc
new file mode 100644
index 0000000000..f1c62b5b1e
--- /dev/null
+++ b/changelog.d/13671.misc
@@ -0,0 +1 @@
+Introduce a `CommonUsageMetrics` class to share some usage metrics between the Prometheus exporter and the phone home stats.
diff --git a/synapse/app/_base.py b/synapse/app/_base.py
index 4742435d3b..9a24bed0a0 100644
--- a/synapse/app/_base.py
+++ b/synapse/app/_base.py
@@ -511,9 +511,10 @@ async def start(hs: "HomeServer") -> None:
     setup_sentry(hs)
     setup_sdnotify(hs)
 
-    # If background tasks are running on the main process, start collecting the
-    # phone home stats.
+    # If background tasks are running on the main process or this is the worker in
+    # charge of them, start collecting the phone home stats and shared usage metrics.
     if hs.config.worker.run_background_tasks:
+        await hs.get_common_usage_metrics_manager().setup()
         start_phone_stats_home(hs)
 
     # We now freeze all allocated objects in the hopes that (almost)
diff --git a/synapse/app/phone_stats_home.py b/synapse/app/phone_stats_home.py
index 40dbdace8e..51c8d15711 100644
--- a/synapse/app/phone_stats_home.py
+++ b/synapse/app/phone_stats_home.py
@@ -51,6 +51,16 @@ async def phone_stats_home(
     stats: JsonDict,
     stats_process: List[Tuple[int, "resource.struct_rusage"]] = _stats_process,
 ) -> None:
+    """Collect usage statistics and send them to the configured endpoint.
+
+    Args:
+        hs: the HomeServer object to use for gathering usage data.
+        stats: the dict in which to store the statistics sent to the configured
+            endpoint. Mostly used in tests to figure out the data that is supposed to
+            be sent.
+        stats_process: statistics about resource usage of the process.
+    """
+
     logger.info("Gathering stats for reporting")
     now = int(hs.get_clock().time())
     # Ensure the homeserver has started.
@@ -83,6 +93,7 @@ async def phone_stats_home(
     #
 
     store = hs.get_datastores().main
+    common_metrics = await hs.get_common_usage_metrics_manager().get_metrics()
 
     stats["homeserver"] = hs.config.server.server_name
     stats["server_context"] = hs.config.server.server_context
@@ -104,7 +115,7 @@ async def phone_stats_home(
     room_count = await store.get_room_count()
     stats["total_room_count"] = room_count
 
-    stats["daily_active_users"] = await store.count_daily_users()
+    stats["daily_active_users"] = common_metrics.daily_active_users
     stats["monthly_active_users"] = await store.count_monthly_users()
     daily_active_e2ee_rooms = await store.count_daily_active_e2ee_rooms()
     stats["daily_active_e2ee_rooms"] = daily_active_e2ee_rooms
diff --git a/synapse/metrics/common_usage_metrics.py b/synapse/metrics/common_usage_metrics.py
new file mode 100644
index 0000000000..0a22ea3d92
--- /dev/null
+++ b/synapse/metrics/common_usage_metrics.py
@@ -0,0 +1,79 @@
+# Copyright 2022 The Matrix.org Foundation C.I.C
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+from typing import TYPE_CHECKING
+
+import attr
+
+from synapse.metrics.background_process_metrics import run_as_background_process
+
+if TYPE_CHECKING:
+    from synapse.server import HomeServer
+
+from prometheus_client import Gauge
+
+# Gauge to expose daily active users metrics
+current_dau_gauge = Gauge(
+    "synapse_admin_daily_active_users",
+    "Current daily active users count",
+)
+
+
+@attr.s(auto_attribs=True)
+class CommonUsageMetrics:
+    """Usage metrics shared between the phone home stats and the prometheus exporter."""
+
+    daily_active_users: int
+
+
+class CommonUsageMetricsManager:
+    """Collects common usage metrics."""
+
+    def __init__(self, hs: "HomeServer") -> None:
+        self._store = hs.get_datastores().main
+        self._clock = hs.get_clock()
+
+    async def get_metrics(self) -> CommonUsageMetrics:
+        """Get the CommonUsageMetrics object. If no collection has happened yet, do it
+        before returning the metrics.
+
+        Returns:
+            The CommonUsageMetrics object to read common metrics from.
+        """
+        return await self._collect()
+
+    async def setup(self) -> None:
+        """Keep the gauges for common usage metrics up to date."""
+        await self._update_gauges()
+        self._clock.looping_call(
+            run_as_background_process,
+            5 * 60 * 1000,
+            desc="common_usage_metrics_update_gauges",
+            func=self._update_gauges,
+        )
+
+    async def _collect(self) -> CommonUsageMetrics:
+        """Collect the common metrics and either create the CommonUsageMetrics object to
+        use if it doesn't exist yet, or update it.
+        """
+        dau_count = await self._store.count_daily_users()
+
+        return CommonUsageMetrics(
+            daily_active_users=dau_count,
+        )
+
+    async def _update_gauges(self) -> None:
+        """Update the Prometheus gauges."""
+        metrics = await self._collect()
+
+        current_dau_gauge.set(float(metrics.daily_active_users))
diff --git a/synapse/server.py b/synapse/server.py
index c2e55bf0b1..5a99c0b344 100644
--- a/synapse/server.py
+++ b/synapse/server.py
@@ -105,6 +105,7 @@ from synapse.handlers.typing import FollowerTypingHandler, TypingWriterHandler
 from synapse.handlers.user_directory import UserDirectoryHandler
 from synapse.http.client import InsecureInterceptableContextFactory, SimpleHttpClient
 from synapse.http.matrixfederationclient import MatrixFederationHttpClient
+from synapse.metrics.common_usage_metrics import CommonUsageMetricsManager
 from synapse.module_api import ModuleApi
 from synapse.notifier import Notifier
 from synapse.push.bulk_push_rule_evaluator import BulkPushRuleEvaluator
@@ -829,3 +830,8 @@ class HomeServer(metaclass=abc.ABCMeta):
             self.config.ratelimiting.rc_message,
             self.config.ratelimiting.rc_admin_redaction,
         )
+
+    @cache_in_self
+    def get_common_usage_metrics_manager(self) -> CommonUsageMetricsManager:
+        """Usage metrics shared between phone home stats and the prometheus exporter."""
+        return CommonUsageMetricsManager(self)
diff --git a/tests/test_phone_home.py b/tests/test_phone_home.py
index b01cae6e5d..cc1a98f1c4 100644
--- a/tests/test_phone_home.py
+++ b/tests/test_phone_home.py
@@ -15,8 +15,14 @@
 import resource
 from unittest import mock
 
+from twisted.test.proto_helpers import MemoryReactor
+
 from synapse.app.phone_stats_home import phone_stats_home
+from synapse.rest import admin
+from synapse.rest.client import login, sync
+from synapse.server import HomeServer
 from synapse.types import JsonDict
+from synapse.util import Clock
 
 from tests.unittest import HomeserverTestCase
 
@@ -47,5 +53,43 @@ class PhoneHomeStatsTestCase(HomeserverTestCase):
         stats: JsonDict = {}
         self.reactor.advance(1)
         # `old_resource` has type `Mock` instead of `struct_rusage`
-        self.get_success(phone_stats_home(self.hs, stats, past_stats))  # type: ignore[arg-type]
+        self.get_success(
+            phone_stats_home(self.hs, stats, past_stats)  # type: ignore[arg-type]
+        )
         self.assertApproximates(stats["cpu_average"], 100, tolerance=2.5)
+
+
+class CommonMetricsTestCase(HomeserverTestCase):
+    servlets = [
+        admin.register_servlets,
+        login.register_servlets,
+        sync.register_servlets,
+    ]
+
+    def prepare(self, reactor: MemoryReactor, clock: Clock, hs: HomeServer) -> None:
+        self.metrics_manager = hs.get_common_usage_metrics_manager()
+        self.get_success(self.metrics_manager.setup())
+
+    def test_dau(self) -> None:
+        """Tests that the daily active users count is correctly updated."""
+        self._assert_metric_value("daily_active_users", 0)
+
+        self.register_user("user", "password")
+        tok = self.login("user", "password")
+        self.make_request("GET", "/sync", access_token=tok)
+
+        self.pump(1)
+
+        self._assert_metric_value("daily_active_users", 1)
+
+    def _assert_metric_value(self, metric_name: str, expected: int) -> None:
+        """Compare the given value to the current value of the common usage metric with
+        the given name.
+
+        Args:
+            metric_name: The metric to look up.
+            expected: Expected value for this metric.
+        """
+        metrics = self.get_success(self.metrics_manager.get_metrics())
+        value = getattr(metrics, metric_name)
+        self.assertEqual(value, expected)