Fix cache metrics not being updated when not using the legacy exposition module.

synapse/metrics/_legacy_exposition.py

@@ -34,8 +34,6 @@
 from twisted.web.resource import Resource
 from twisted.web.server import Request
 
-from synapse.util import caches
-
 CONTENT_TYPE_LATEST = "text/plain; version=0.0.4; charset=utf-8"
 
 
@@ -102,11 +100,6 @@ def generate_latest(registry: CollectorRegistry, emit_help: bool = False) -> byt
     by prometheus-client.
     """
 
-    # Trigger the cache metrics to be rescraped, which updates the common
-    # metrics but do not produce metrics themselves
-    for collector in caches.collectors_by_name.values():
-        collector.collect()
-
     output = []
 
     for metric in registry.collect():

synapse/util/caches/__init__.py

@@ -20,37 +20,74 @@
 from typing import Any, Callable, Dict, List, Optional, Sized, TypeVar
 
 import attr
+from prometheus_client import REGISTRY
 from prometheus_client.core import Gauge
 
 from synapse.config.cache import add_resizable_cache
+from synapse.util.metrics import DynamicCollectorRegistry
 
 logger = logging.getLogger(__name__)
 
 
 # Whether to track estimated memory usage of the LruCaches.
 TRACK_MEMORY_USAGE = False
 
+# We track cache metrics in a special registry that lets us update the metrics
+# just before they are returned from the scrape endpoint.
+CACHE_METRIC_REGISTRY = DynamicCollectorRegistry()
 
 caches_by_name: Dict[str, Sized] = {}
-collectors_by_name: Dict[str, "CacheMetric"] = {}
 
-cache_size = Gauge("synapse_util_caches_cache_size", "", ["name"])
-cache_hits = Gauge("synapse_util_caches_cache_hits", "", ["name"])
-cache_evicted = Gauge("synapse_util_caches_cache_evicted_size", "", ["name", "reason"])
-cache_total = Gauge("synapse_util_caches_cache", "", ["name"])
-cache_max_size = Gauge("synapse_util_caches_cache_max_size", "", ["name"])
+cache_size = Gauge(
+    "synapse_util_caches_cache_size", "", ["name"], registry=CACHE_METRIC_REGISTRY
+)
+cache_hits = Gauge(
+    "synapse_util_caches_cache_hits", "", ["name"], registry=CACHE_METRIC_REGISTRY
+)
+cache_evicted = Gauge(
+    "synapse_util_caches_cache_evicted_size",
+    "",
+    ["name", "reason"],
+    registry=CACHE_METRIC_REGISTRY,
+)
+cache_total = Gauge(
+    "synapse_util_caches_cache", "", ["name"], registry=CACHE_METRIC_REGISTRY
+)
+cache_max_size = Gauge(
+    "synapse_util_caches_cache_max_size", "", ["name"], registry=CACHE_METRIC_REGISTRY
+)
 cache_memory_usage = Gauge(
     "synapse_util_caches_cache_size_bytes",
     "Estimated memory usage of the caches",
     ["name"],
+    registry=CACHE_METRIC_REGISTRY,
 )
 
-response_cache_size = Gauge("synapse_util_caches_response_cache_size", "", ["name"])
-response_cache_hits = Gauge("synapse_util_caches_response_cache_hits", "", ["name"])
+response_cache_size = Gauge(
+    "synapse_util_caches_response_cache_size",
+    "",
+    ["name"],
+    registry=CACHE_METRIC_REGISTRY,
+)
+response_cache_hits = Gauge(
+    "synapse_util_caches_response_cache_hits",
+    "",
+    ["name"],
+    registry=CACHE_METRIC_REGISTRY,
+)
 response_cache_evicted = Gauge(
-    "synapse_util_caches_response_cache_evicted_size", "", ["name", "reason"]
+    "synapse_util_caches_response_cache_evicted_size",
+    "",
+    ["name", "reason"],
+    registry=CACHE_METRIC_REGISTRY,
 )
-response_cache_total = Gauge("synapse_util_caches_response_cache", "", ["name"])
+response_cache_total = Gauge(
+    "synapse_util_caches_response_cache", "", ["name"], registry=CACHE_METRIC_REGISTRY
+)
+
+
+# Register our custom cache metrics registry with the global registry
+REGISTRY.register(CACHE_METRIC_REGISTRY)
 
 
 class EvictionReason(Enum):
@@ -168,9 +205,9 @@ def register_cache(
         add_resizable_cache(cache_name, resize_callback)
 
     metric = CacheMetric(cache, cache_type, cache_name, collect_callback)
-    metric_name = "cache_%s_%s" % (cache_type, cache_name)
+    # TODO evil ??metric_name = "cache_%s_%s" % (cache_type, cache_name)
     caches_by_name[cache_name] = cache
-    collectors_by_name[metric_name] = metric
+    CACHE_METRIC_REGISTRY.register_hook(metric.collect)
     return metric
 
 

synapse/util/metrics.py

@@ -15,9 +15,9 @@
 import logging
 from functools import wraps
 from types import TracebackType
-from typing import Awaitable, Callable, Optional, Type, TypeVar
+from typing import Awaitable, Callable, Generator, List, Optional, Type, TypeVar
 
-from prometheus_client import Counter
+from prometheus_client import CollectorRegistry, Counter, Metric
 from typing_extensions import Concatenate, ParamSpec, Protocol
 
 from synapse.logging.context import (
@@ -208,3 +208,33 @@ def _update_in_flight(self, metrics: _InFlightMetric) -> None:
         metrics.real_time_sum += duration
 
         # TODO: Add other in flight metrics.
+
+
+class DynamicCollectorRegistry(CollectorRegistry):
+    """
+    Custom Prometheus Collector registry that calls a hook first, allowing you
+    to update metrics on-demand.
+
+    Don't forget to register this registry with the main registry!
+    """
+
+    def __init__(self) -> None:
+        super().__init__()
+        self._pre_update_hooks: List[Callable[[], None]] = []
+
+    def collect(self) -> Generator[Metric]:
+        """
+        Collects metrics, calling pre-update hooks first.
+        """
+
+        for pre_update_hook in self._pre_update_hooks:
+            pre_update_hook()
+
+        yield from super().collect()
+
+    def register_hook(self, hook: Callable[[], None]) -> None:
+        """
+        Registers a hook that is called before metric collection.
+        """
+
+        self._pre_update_hooks.append(hook)