7 files changed, 90 insertions, 3 deletions
diff --git a/changelog.d/8048.feature b/changelog.d/8048.feature
new file mode 100644
index 0000000000..8521d1920e
--- /dev/null
+++ b/changelog.d/8048.feature
@@ -0,0 +1 @@
+Add a `/health` endpoint to every configured HTTP listener that can be used as a health check endpoint by load balancers.
diff --git a/docs/reverse_proxy.md b/docs/reverse_proxy.md
index 7bfb96eff6..fd48ba0874 100644
--- a/docs/reverse_proxy.md
+++ b/docs/reverse_proxy.md
@@ -139,3 +139,10 @@ client IP addresses are recorded correctly.
 Having done so, you can then use `https://matrix.example.com` (instead
 of `https://matrix.example.com:8448`) as the "Custom server" when
 connecting to Synapse from a client.
+
+
+## Health check endpoint
+
+Synapse exposes a health check endpoint for use by reverse proxies.
+Each configured HTTP listener has a `/health` endpoint which always returns
+200 OK (and doesn't get logged).
diff --git a/synapse/app/generic_worker.py b/synapse/app/generic_worker.py
index 1a16d0b9f8..7957586d69 100644
--- a/synapse/app/generic_worker.py
+++ b/synapse/app/generic_worker.py
@@ -123,6 +123,7 @@ from synapse.rest.client.v2_alpha.account_data import (
 from synapse.rest.client.v2_alpha.keys import KeyChangesServlet, KeyQueryServlet
 from synapse.rest.client.v2_alpha.register import RegisterRestServlet
 from synapse.rest.client.versions import VersionsRestServlet
+from synapse.rest.health import HealthResource
 from synapse.rest.key.v2 import KeyApiV2Resource
 from synapse.server import HomeServer
 from synapse.storage.databases.main.censor_events import CensorEventsStore
@@ -493,7 +494,10 @@ class GenericWorkerServer(HomeServer):
         site_tag = listener_config.http_options.tag
         if site_tag is None:
             site_tag = port
-        resources = {}
+
+        # We always include a health resource.
+        resources = {"/health": HealthResource()}
+
         for res in listener_config.http_options.resources:
             for name in res.names:
                 if name == "metrics":
diff --git a/synapse/app/homeserver.py b/synapse/app/homeserver.py
index d87a77718e..98d0d14a12 100644
--- a/synapse/app/homeserver.py
+++ b/synapse/app/homeserver.py
@@ -68,6 +68,7 @@ from synapse.replication.http import REPLICATION_PREFIX, ReplicationRestResource
 from synapse.replication.tcp.resource import ReplicationStreamProtocolFactory
 from synapse.rest import ClientRestResource
 from synapse.rest.admin import AdminRestResource
+from synapse.rest.health import HealthResource
 from synapse.rest.key.v2 import KeyApiV2Resource
 from synapse.rest.well_known import WellKnownResource
 from synapse.server import HomeServer
@@ -98,7 +99,9 @@ class SynapseHomeServer(HomeServer):
         if site_tag is None:
             site_tag = port
 
-        resources = {}
+        # We always include a health resource.
+        resources = {"/health": HealthResource()}
+
         for res in listener_config.http_options.resources:
             for name in res.names:
                 if name == "openid" and "federation" in res.names:
diff --git a/synapse/http/site.py b/synapse/http/site.py
index f506152fea..79a9229a26 100644
--- a/synapse/http/site.py
+++ b/synapse/http/site.py
@@ -286,7 +286,9 @@ class SynapseRequest(Request):
             # the connection dropped)
             code += "!"
 
-        self.site.access_logger.info(
+        log_level = logging.INFO if self._should_log_request() else logging.DEBUG
+        self.site.access_logger.log(
+            log_level,
             "%s - %s - {%s}"
             " Processed request: %.3fsec/%.3fsec (%.3fsec, %.3fsec) (%.3fsec/%.3fsec/%d)"
             ' %sB %s "%s %s %s" "%s" [%d dbevts]',
@@ -314,6 +316,11 @@ class SynapseRequest(Request):
         except Exception as e:
             logger.warning("Failed to stop metrics: %r", e)
 
+    def _should_log_request(self) -> bool:
+        """Whether we should log at INFO that we processed the request.
+        """
+        return self.path != b"/health"
+
 
 class XForwardedForRequest(SynapseRequest):
     def __init__(self, *args, **kw):
diff --git a/synapse/rest/health.py b/synapse/rest/health.py
new file mode 100644
index 0000000000..0170950bf3
--- /dev/null
+++ b/synapse/rest/health.py
@@ -0,0 +1,31 @@
+# -*- coding: utf-8 -*-
+# Copyright 2020 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 twisted.web.resource import Resource
+
+
+class HealthResource(Resource):
+    """A resource that does nothing except return a 200 with a body of `OK`,
+    which can be used as a health check.
+
+    Note: `SynapseRequest._should_log_request` ensures that requests to
+    `/health` do not get logged at INFO.
+    """
+
+    isLeaf = 1
+
+    def render_GET(self, request):
+        request.setHeader(b"Content-Type", b"text/plain")
+        return b"OK"
diff --git a/tests/rest/test_health.py b/tests/rest/test_health.py
new file mode 100644
index 0000000000..2d021f6565
--- /dev/null
+++ b/tests/rest/test_health.py
@@ -0,0 +1,34 @@
+# -*- coding: utf-8 -*-
+# Copyright 2020 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 synapse.rest.health import HealthResource
+
+from tests import unittest
+
+
+class HealthCheckTests(unittest.HomeserverTestCase):
+    def setUp(self):
+        super().setUp()
+
+        # replace the JsonResource with a HealthResource.
+        self.resource = HealthResource()
+
+    def test_health(self):
+        request, channel = self.make_request("GET", "/health", shorthand=False)
+        self.render(request)
+
+        self.assertEqual(request.code, 200)
+        self.assertEqual(channel.result["body"], b"OK")