Expose statistics on extrems to prometheus (#5384)

synapse/storage/events.py

@@ -17,7 +17,7 @@
 
 import itertools
 import logging
-from collections import OrderedDict, deque, namedtuple
+from collections import Counter as c_counter, OrderedDict, deque, namedtuple
 from functools import wraps
 
 from six import iteritems, text_type
@@ -33,6 +33,7 @@ from synapse.api.constants import EventTypes
 from synapse.api.errors import SynapseError
 from synapse.events import EventBase  # noqa: F401
 from synapse.events.snapshot import EventContext  # noqa: F401
+from synapse.metrics import BucketCollector
 from synapse.metrics.background_process_metrics import run_as_background_process
 from synapse.state import StateResolutionStore
 from synapse.storage.background_updates import BackgroundUpdateStore
@@ -220,13 +221,38 @@ class EventsStore(
     EventsWorkerStore,
     BackgroundUpdateStore,
 ):
-
     def __init__(self, db_conn, hs):
         super(EventsStore, self).__init__(db_conn, hs)
 
         self._event_persist_queue = _EventPeristenceQueue()
         self._state_resolution_handler = hs.get_state_resolution_handler()
 
+        # Collect metrics on the number of forward extremities that exist.
+        self._current_forward_extremities_amount = {}
+
+        BucketCollector(
+            "synapse_forward_extremities",
+            lambda: self._current_forward_extremities_amount,
+            buckets=[1, 2, 3, 5, 7, 10, 15, 20, 50, 100, 200, 500, "+Inf"]
+        )
+
+        # Read the extrems every 60 minutes
+        hs.get_clock().looping_call(self._read_forward_extremities, 60 * 60 * 1000)
+
+    @defer.inlineCallbacks
+    def _read_forward_extremities(self):
+        def fetch(txn):
+            txn.execute(
+                """
+                select count(*) c from event_forward_extremities
+                group by room_id
+                """
+            )
+            return txn.fetchall()
+
+        res = yield self.runInteraction("read_forward_extremities", fetch)
+        self._current_forward_extremities_amount = c_counter(list(x[0] for x in res))
+
     @defer.inlineCallbacks
     def persist_events(self, events_and_contexts, backfilled=False):
         """
@@ -568,17 +594,11 @@ class EventsStore(
             )
 
             txn.execute(sql, batch)
-            results.extend(
-                r[0]
-                for r in txn
-                if not json.loads(r[1]).get("soft_failed")
-            )
+            results.extend(r[0] for r in txn if not json.loads(r[1]).get("soft_failed"))
 
         for chunk in batch_iter(event_ids, 100):
             yield self.runInteraction(
-                "_get_events_which_are_prevs",
-                _get_events_which_are_prevs_txn,
-                chunk,
+                "_get_events_which_are_prevs", _get_events_which_are_prevs_txn, chunk
             )
 
         defer.returnValue(results)
@@ -640,9 +660,7 @@ class EventsStore(
 
         for chunk in batch_iter(event_ids, 100):
             yield self.runInteraction(
-                "_get_prevs_before_rejected",
-                _get_prevs_before_rejected_txn,
-                chunk,
+                "_get_prevs_before_rejected", _get_prevs_before_rejected_txn, chunk
             )
 
         defer.returnValue(existing_prevs)