Repository: matrix-org/synapse

Issue:
Problems with `BatchingQueue`
#10017 introduced a `BatchingQueue` implementation. While reviewing #10035 I noticed a few oddities that could do with cleaning up:

* The params to `__init__` could do with docstrings, especially `name` - what is it used for?
* The prometheus metrics (`synapse_util_batching_queue_number_queued` etc) don't appear to include the `name` label in their result.
* If the key is already being processed when `_process_queue` starts, then `_process_queue` immediately returns (https://github.com/matrix-org/synapse/blob/v1.35.0rc1/synapse/util/batching_queue.py#L118-L119). However, it will pass through the `finally` block which will [remove the key from `self._processing_keys`](https://github.com/matrix-org/synapse/blob/v1.35.0rc1/synapse/util/batching_queue.py#L153). This looks incorrect to me.
* `PreserveLoggingContext` isn't free. consider moving it outside the loops?
* It would be nice to document the behaviour on exceptions.


Discussion:

Resolving patch:
diff --git a/synapse/util/batching_queue.py b/synapse/util/batching_queue.py
--- a/synapse/util/batching_queue.py
+++ b/synapse/util/batching_queue.py
@@ -25,10 +25,11 @@
     TypeVar,
 )
 
+from prometheus_client import Gauge
+
 from twisted.internet import defer
 
 from synapse.logging.context import PreserveLoggingContext, make_deferred_yieldable
-from synapse.metrics import LaterGauge
 from synapse.metrics.background_process_metrics import run_as_background_process
 from synapse.util import Clock
 
@@ -38,6 +39,24 @@
 V = TypeVar("V")
 R = TypeVar("R")
 
+number_queued = Gauge(
+    "synapse_util_batching_queue_number_queued",
+    "The number of items waiting in the queue across all keys",
+    labelnames=("name",),
+)
+
+number_in_flight = Gauge(
+    "synapse_util_batching_queue_number_pending",
+    "The number of items across all keys either being processed or waiting in a queue",
+    labelnames=("name",),
+)
+
+number_of_keys = Gauge(
+    "synapse_util_batching_queue_number_of_keys",
+    "The number of distinct keys that have items queued",
+    labelnames=("name",),
+)
+
 
 class BatchingQueue(Generic[V, R]):
     """A queue that batches up work, calling the provided processing function
@@ -48,10 +67,20 @@ class BatchingQueue(Generic[V, R]):
     called, and will keep being called until the queue has been drained (for the
     given key).
 
+    If the processing function raises an exception then the exception is proxied
+    through to the callers waiting on that batch of work.
+
     Note that the return value of `add_to_queue` will be the return value of the
     processing function that processed the given item. This means that the
     returned value will likely include data for other items that were in the
     batch.
+
+    Args:
+        name: A name for the queue, used for logging contexts and metrics.
+            This must be unique, otherwise the metrics will be wrong.
+        clock: The clock to use to schedule work.
+        process_batch_callback: The callback to to be run to process a batch of
+            work.
     """
 
     def __init__(
@@ -73,19 +102,15 @@ def __init__(
         # The function to call with batches of values.
         self._process_batch_callback = process_batch_callback
 
-        LaterGauge(
-            "synapse_util_batching_queue_number_queued",
-            "The number of items waiting in the queue across all keys",
-            labels=("name",),
-            caller=lambda: sum(len(v) for v in self._next_values.values()),
+        number_queued.labels(self._name).set_function(
+            lambda: sum(len(q) for q in self._next_values.values())
         )
 
-        LaterGauge(
-            "synapse_util_batching_queue_number_of_keys",
-            "The number of distinct keys that have items queued",
-            labels=("name",),
-            caller=lambda: len(self._next_values),
-        )
+        number_of_keys.labels(self._name).set_function(lambda: len(self._next_values))
+
+        self._number_in_flight_metric = number_in_flight.labels(
+            self._name
+        )  # type: Gauge
 
     async def add_to_queue(self, value: V, key: Hashable = ()) -> R:
         """Adds the value to the queue with the given key, returning the result
@@ -107,17 +132,18 @@ async def add_to_queue(self, value: V, key: Hashable = ()) -> R:
         if key not in self._processing_keys:
             run_as_background_process(self._name, self._process_queue, key)
 
-        return await make_deferred_yieldable(d)
+        with self._number_in_flight_metric.track_inprogress():
+            return await make_deferred_yieldable(d)
 
     async def _process_queue(self, key: Hashable) -> None:
         """A background task to repeatedly pull things off the queue for the
         given key and call the `self._process_batch_callback` with the values.
         """
 
-        try:
-            if key in self._processing_keys:
-                return
+        if key in self._processing_keys:
+            return
 
+        try:
             self._processing_keys.add(key)
 
             while True:
@@ -137,16 +163,16 @@ async def _process_queue(self, key: Hashable) -> None:
                     values = [value for value, _ in next_values]
                     results = await self._process_batch_callback(values)
 
-                    for _, deferred in next_values:
-                        with PreserveLoggingContext():
+                    with PreserveLoggingContext():
+                        for _, deferred in next_values:
                             deferred.callback(results)
 
                 except Exception as e:
-                    for _, deferred in next_values:
-                        if deferred.called:
-                            continue
+                    with PreserveLoggingContext():
+                        for _, deferred in next_values:
+                            if deferred.called:
+                                continue
 
-                        with PreserveLoggingContext():
                             deferred.errback(e)
 
         finally:


Test patch:
diff --git a/tests/util/test_batching_queue.py b/tests/util/test_batching_queue.py
--- a/tests/util/test_batching_queue.py
+++ b/tests/util/test_batching_queue.py
@@ -14,7 +14,12 @@
 from twisted.internet import defer
 
 from synapse.logging.context import make_deferred_yieldable
-from synapse.util.batching_queue import BatchingQueue
+from synapse.util.batching_queue import (
+    BatchingQueue,
+    number_in_flight,
+    number_of_keys,
+    number_queued,
+)
 
 from tests.server import get_clock
 from tests.unittest import TestCase
@@ -24,6 +29,14 @@ class BatchingQueueTestCase(TestCase):
     def setUp(self):
         self.clock, hs_clock = get_clock()
 
+        # We ensure that we remove any existing metrics for "test_queue".
+        try:
+            number_queued.remove("test_queue")
+            number_of_keys.remove("test_queue")
+            number_in_flight.remove("test_queue")
+        except KeyError:
+            pass
+
         self._pending_calls = []
         self.queue = BatchingQueue("test_queue", hs_clock, self._process_queue)
 
@@ -32,6 +45,41 @@ async def _process_queue(self, values):
         self._pending_calls.append((values, d))
         return await make_deferred_yieldable(d)
 
+    def _assert_metrics(self, queued, keys, in_flight):
+        """Assert that the metrics are correct"""
+
+        self.assertEqual(len(number_queued.collect()), 1)
+        self.assertEqual(len(number_queued.collect()[0].samples), 1)
+        self.assertEqual(
+            number_queued.collect()[0].samples[0].labels,
+            {"name": self.queue._name},
+        )
+        self.assertEqual(
+            number_queued.collect()[0].samples[0].value,
+            queued,
+            "number_queued",
+        )
+
+        self.assertEqual(len(number_of_keys.collect()), 1)
+        self.assertEqual(len(number_of_keys.collect()[0].samples), 1)
+        self.assertEqual(
+            number_queued.collect()[0].samples[0].labels, {"name": self.queue._name}
+        )
+        self.assertEqual(
+            number_of_keys.collect()[0].samples[0].value, keys, "number_of_keys"
+        )
+
+        self.assertEqual(len(number_in_flight.collect()), 1)
+        self.assertEqual(len(number_in_flight.collect()[0].samples), 1)
+        self.assertEqual(
+            number_queued.collect()[0].samples[0].labels, {"name": self.queue._name}
+        )
+        self.assertEqual(
+            number_in_flight.collect()[0].samples[0].value,
+            in_flight,
+            "number_in_flight",
+        )
+
     def test_simple(self):
         """Tests the basic case of calling `add_to_queue` once and having
         `_process_queue` return.
@@ -41,6 +89,8 @@ def test_simple(self):
 
         queue_d = defer.ensureDeferred(self.queue.add_to_queue("foo"))
 
+        self._assert_metrics(queued=1, keys=1, in_flight=1)
+
         # The queue should wait a reactor tick before calling the processing
         # function.
         self.assertFalse(self._pending_calls)
@@ -52,12 +102,15 @@ def test_simple(self):
         self.assertEqual(len(self._pending_calls), 1)
         self.assertEqual(self._pending_calls[0][0], ["foo"])
         self.assertFalse(queue_d.called)
+        self._assert_metrics(queued=0, keys=0, in_flight=1)
 
         # Return value of the `_process_queue` should be propagated back.
         self._pending_calls.pop()[1].callback("bar")
 
         self.assertEqual(self.successResultOf(queue_d), "bar")
 
+        self._assert_metrics(queued=0, keys=0, in_flight=0)
+
     def test_batching(self):
         """Test that multiple calls at the same time get batched up into one
         call to `_process_queue`.
@@ -68,6 +121,8 @@ def test_batching(self):
         queue_d1 = defer.ensureDeferred(self.queue.add_to_queue("foo1"))
         queue_d2 = defer.ensureDeferred(self.queue.add_to_queue("foo2"))
 
+        self._assert_metrics(queued=2, keys=1, in_flight=2)
+
         self.clock.pump([0])
 
         # We should see only *one* call to `_process_queue`
@@ -75,12 +130,14 @@ def test_batching(self):
         self.assertEqual(self._pending_calls[0][0], ["foo1", "foo2"])
         self.assertFalse(queue_d1.called)
         self.assertFalse(queue_d2.called)
+        self._assert_metrics(queued=0, keys=0, in_flight=2)
 
         # Return value of the `_process_queue` should be propagated back to both.
         self._pending_calls.pop()[1].callback("bar")
 
         self.assertEqual(self.successResultOf(queue_d1), "bar")
         self.assertEqual(self.successResultOf(queue_d2), "bar")
+        self._assert_metrics(queued=0, keys=0, in_flight=0)
 
     def test_queuing(self):
         """Test that we queue up requests while a `_process_queue` is being
@@ -92,13 +149,20 @@ def test_queuing(self):
         queue_d1 = defer.ensureDeferred(self.queue.add_to_queue("foo1"))
         self.clock.pump([0])
 
+        self.assertEqual(len(self._pending_calls), 1)
+
+        # We queue up work after the process function has been called, testing
+        # that they get correctly queued up.
         queue_d2 = defer.ensureDeferred(self.queue.add_to_queue("foo2"))
+        queue_d3 = defer.ensureDeferred(self.queue.add_to_queue("foo3"))
 
         # We should see only *one* call to `_process_queue`
         self.assertEqual(len(self._pending_calls), 1)
         self.assertEqual(self._pending_calls[0][0], ["foo1"])
         self.assertFalse(queue_d1.called)
         self.assertFalse(queue_d2.called)
+        self.assertFalse(queue_d3.called)
+        self._assert_metrics(queued=2, keys=1, in_flight=3)
 
         # Return value of the `_process_queue` should be propagated back to the
         # first.
@@ -106,18 +170,24 @@ def test_queuing(self):
 
         self.assertEqual(self.successResultOf(queue_d1), "bar1")
         self.assertFalse(queue_d2.called)
+        self.assertFalse(queue_d3.called)
+        self._assert_metrics(queued=2, keys=1, in_flight=2)
 
         # We should now see a second call to `_process_queue`
         self.clock.pump([0])
         self.assertEqual(len(self._pending_calls), 1)
-        self.assertEqual(self._pending_calls[0][0], ["foo2"])
+        self.assertEqual(self._pending_calls[0][0], ["foo2", "foo3"])
         self.assertFalse(queue_d2.called)
+        self.assertFalse(queue_d3.called)
+        self._assert_metrics(queued=0, keys=0, in_flight=2)
 
         # Return value of the `_process_queue` should be propagated back to the
         # second.
         self._pending_calls.pop()[1].callback("bar2")
 
         self.assertEqual(self.successResultOf(queue_d2), "bar2")
+        self.assertEqual(self.successResultOf(queue_d3), "bar2")
+        self._assert_metrics(queued=0, keys=0, in_flight=0)
 
     def test_different_keys(self):
         """Test that calls to different keys get processed in parallel."""
@@ -140,6 +210,7 @@ def test_different_keys(self):
         self.assertFalse(queue_d1.called)
         self.assertFalse(queue_d2.called)
         self.assertFalse(queue_d3.called)
+        self._assert_metrics(queued=1, keys=1, in_flight=3)
 
         # Return value of the `_process_queue` should be propagated back to the
         # first.
@@ -148,6 +219,7 @@ def test_different_keys(self):
         self.assertEqual(self.successResultOf(queue_d1), "bar1")
         self.assertFalse(queue_d2.called)
         self.assertFalse(queue_d3.called)
+        self._assert_metrics(queued=1, keys=1, in_flight=2)
 
         # Return value of the `_process_queue` should be propagated back to the
         # second.
@@ -161,9 +233,11 @@ def test_different_keys(self):
         self.assertEqual(len(self._pending_calls), 1)
         self.assertEqual(self._pending_calls[0][0], ["foo3"])
         self.assertFalse(queue_d3.called)
+        self._assert_metrics(queued=0, keys=0, in_flight=1)
 
         # Return value of the `_process_queue` should be propagated back to the
         # third deferred.
         self._pending_calls.pop()[1].callback("bar4")
 
         self.assertEqual(self.successResultOf(queue_d3), "bar4")
+        self._assert_metrics(queued=0, keys=0, in_flight=0)