diff --git a/synapse/metrics/metric.py b/synapse/metrics/metric.py
new file mode 100644
index 0000000000..21b37748f6
--- /dev/null
+++ b/synapse/metrics/metric.py
@@ -0,0 +1,155 @@
+# -*- coding: utf-8 -*-
+# Copyright 2015 OpenMarket Ltd
+#
+# 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 itertools import chain
+
+
+# TODO(paul): I can't believe Python doesn't have one of these
+def map_concat(func, items):
+ # flatten a list-of-lists
+ return list(chain.from_iterable(map(func, items)))
+
+
+class BaseMetric(object):
+
+ def __init__(self, name, labels=[]):
+ self.name = name
+ self.labels = labels # OK not to clone as we never write it
+
+ def dimension(self):
+ return len(self.labels)
+
+ def is_scalar(self):
+ return not len(self.labels)
+
+ def _render_labelvalue(self, value):
+ # TODO: some kind of value escape
+ return '"%s"' % (value)
+
+ def _render_key(self, values):
+ if self.is_scalar():
+ return ""
+ return "{%s}" % (
+ ",".join(["%s=%s" % (k, self._render_labelvalue(v))
+ for k, v in zip(self.labels, values)])
+ )
+
+ def render(self):
+ return map_concat(self.render_item, sorted(self.counts.keys()))
+
+
+class CounterMetric(BaseMetric):
+ """The simplest kind of metric; one that stores a monotonically-increasing
+ integer that counts events."""
+
+ def __init__(self, *args, **kwargs):
+ super(CounterMetric, self).__init__(*args, **kwargs)
+
+ self.counts = {}
+
+ # Scalar metrics are never empty
+ if self.is_scalar():
+ self.counts[()] = 0
+
+ def inc_by(self, incr, *values):
+ if len(values) != self.dimension():
+ raise ValueError(
+ "Expected as many values to inc() as labels (%d)" % (self.dimension())
+ )
+
+ # TODO: should assert that the tag values are all strings
+
+ if values not in self.counts:
+ self.counts[values] = incr
+ else:
+ self.counts[values] += incr
+
+ def inc(self, *values):
+ self.inc_by(1, *values)
+
+ def render_item(self, k):
+ return ["%s%s %d" % (self.name, self._render_key(k), self.counts[k])]
+
+
+class CallbackMetric(BaseMetric):
+ """A metric that returns the numeric value returned by a callback whenever
+ it is rendered. Typically this is used to implement gauges that yield the
+ size or other state of some in-memory object by actively querying it."""
+
+ def __init__(self, name, callback, labels=[]):
+ super(CallbackMetric, self).__init__(name, labels=labels)
+
+ self.callback = callback
+
+ def render(self):
+ value = self.callback()
+
+ if self.is_scalar():
+ return ["%s %d" % (self.name, value)]
+
+ return ["%s%s %d" % (self.name, self._render_key(k), value[k])
+ for k in sorted(value.keys())]
+
+
+class DistributionMetric(object):
+ """A combination of an event counter and an accumulator, which counts
+ both the number of events and accumulates the total value. Typically this
+ could be used to keep track of method-running times, or other distributions
+ of values that occur in discrete occurances.
+
+ TODO(paul): Try to export some heatmap-style stats?
+ """
+
+ def __init__(self, name, *args, **kwargs):
+ self.counts = CounterMetric(name + ":count", **kwargs)
+ self.totals = CounterMetric(name + ":total", **kwargs)
+
+ def inc_by(self, inc, *values):
+ self.counts.inc(*values)
+ self.totals.inc_by(inc, *values)
+
+ def render(self):
+ return self.counts.render() + self.totals.render()
+
+
+class CacheMetric(object):
+ """A combination of two CounterMetrics, one to count cache hits and one to
+ count a total, and a callback metric to yield the current size.
+
+ This metric generates standard metric name pairs, so that monitoring rules
+ can easily be applied to measure hit ratio."""
+
+ def __init__(self, name, size_callback, labels=[]):
+ self.name = name
+
+ self.hits = CounterMetric(name + ":hits", labels=labels)
+ self.total = CounterMetric(name + ":total", labels=labels)
+
+ self.size = CallbackMetric(
+ name + ":size",
+ callback=size_callback,
+ labels=labels,
+ )
+
+ def inc_hits(self, *values):
+ self.hits.inc(*values)
+ self.total.inc(*values)
+
+ def inc_misses(self, *values):
+ self.total.inc(*values)
+
+ def render(self):
+ return self.hits.render() + self.total.render() + self.size.render()
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