From e13af5c4d4857af50e88f01d320bd219f39a3675 Mon Sep 17 00:00:00 2001
From: dklimpel <5740567+dklimpel@users.noreply.github.com>
Date: Mon, 22 Nov 2021 17:17:43 +0100
Subject: [PATCH 01/11] Add admin API to get some information about federation
 status with remote servers.

---
 docs/SUMMARY.md                               |   1 +
 .../administration/admin_api/federation.md    |  81 ++++
 synapse/rest/admin/__init__.py                |   2 +
 synapse/rest/admin/federation.py              |  96 ++++
 .../storage/databases/main/transactions.py    |  68 +++
 tests/rest/admin/test_federation.py           | 419 ++++++++++++++++++
 6 files changed, 667 insertions(+)
 create mode 100644 docs/usage/administration/admin_api/federation.md
 create mode 100644 synapse/rest/admin/federation.py
 create mode 100644 tests/rest/admin/test_federation.py

diff --git a/docs/SUMMARY.md b/docs/SUMMARY.md
index cdedf8bccc28..d2573a80e9b6 100644
--- a/docs/SUMMARY.md
+++ b/docs/SUMMARY.md
@@ -64,6 +64,7 @@
       - [Statistics](admin_api/statistics.md)
       - [Users](admin_api/user_admin_api.md)
       - [Server Version](admin_api/version_api.md)
+      - [Federation](usage/administration/admin_api/federation.md)
     - [Manhole](manhole.md)
     - [Monitoring](metrics-howto.md)
     - [Request log format](usage/administration/request_log.md)
diff --git a/docs/usage/administration/admin_api/federation.md b/docs/usage/administration/admin_api/federation.md
new file mode 100644
index 000000000000..2636f2feb95b
--- /dev/null
+++ b/docs/usage/administration/admin_api/federation.md
@@ -0,0 +1,81 @@
+# Federation API
+
+This API allows a server administrator to manage the federation.
+
+**Note**: This API is new, experimental and "subject to change".
+
+## List of destinations
+
+This API gets the current destination retry timing info for a remote server.
+
+The API is:
+
+A standard request with no filtering:
+
+```
+GET /_synapse/admin/v1/federation/destinations
+```
+
+A response body like the following is returned:
+
+```json
+{
+   "destinations":[
+      {
+         "destination": "matrix.org",
+         "retry_last_ts": 1557332397936,
+         "retry_interval": 3000000,
+         "failure_ts": 1557329397936,
+         "last_successful_stream_ordering": null
+      }
+   ],
+   "total": 1
+}
+```
+
+To paginate, check for `next_token` and if present, call the endpoint again
+with `from` set to the value of `next_token`. This will return a new page.
+
+If the endpoint does not return a `next_token` then there are no more destinations
+to paginate through.
+
+**Parameters**
+
+The following query parameters are available:
+
+- `from` - Offset in the returned list. Defaults to `0`.
+- `limit` - Maximum amount of destinations to return. Defaults to `100`.
+- `order_by` - The method in which to sort the returned list of destinations.
+  Valid values are:
+  - `destination` - Destinations are ordered alphabetically by remote server name.
+    This is the default.
+  - `retry_last_ts` - Destinations are ordered by time of last retry attempt in ms.
+  - `retry_interval` - Destinations are ordered by how long until next retry in ms.
+  - `failure_ts` - Destinations are ordered by when the server started failing in ms.
+  - `last_successful_stream_ordering` - Destinations are ordered by the stream ordering
+    of the most recent successfully-sent PDU.
+- `dir` - Direction of room order. Either `f` for forwards or `b` for backwards. Setting
+  this value to `b` will reverse the above sort order. Defaults to `f`.
+
+*Caution:* The database only has an index on the column `destination`.
+This means that if a different sort order is used,
+this can cause a large load on the database, especially for large environments.
+
+**Response**
+
+The following fields are returned in the JSON response body:
+
+- `destinations` - An array of objects, each containing information about a destination.
+  Destinations objects contain the following fields:
+  - `destination` - string - Name of the remote server to federate.
+  - `retry_last_ts` - integer - The last time Synapse tried and failed to reach the
+    remote server, in ms
+  - `retry_interval` - integer - How long since the last time Synapse tried to reach
+    the remote server before trying again, in ms.
+  - `failure_ts` - integer - The first time Synapse tried and failed to reach the
+    remote server, in ms.
+  - `last_successful_stream_ordering` - integer - The stream ordering of the most
+    recent successfully-sent [PDU](Understanding-Synapse-Performance-Issues-Through-Grafana-Graphs.md#federation)
+    to this destination or `null` if this information has not been tracked yet.
+- `next_token`: string representing a positive integer - Indication for pagination. See above.
+- `total` - integer - Total number of destinations.
diff --git a/synapse/rest/admin/__init__.py b/synapse/rest/admin/__init__.py
index ee4a5e481bee..dd25e3afa65e 100644
--- a/synapse/rest/admin/__init__.py
+++ b/synapse/rest/admin/__init__.py
@@ -39,6 +39,7 @@
     EventReportDetailRestServlet,
     EventReportsRestServlet,
 )
+from synapse.rest.admin.federation import FederationDestinationRestServlet
 from synapse.rest.admin.groups import DeleteGroupAdminRestServlet
 from synapse.rest.admin.media import ListMediaInRoom, register_servlets_for_media_repo
 from synapse.rest.admin.registration_tokens import (
@@ -256,6 +257,7 @@ def register_servlets(hs: "HomeServer", http_server: HttpServer) -> None:
     ListRegistrationTokensRestServlet(hs).register(http_server)
     NewRegistrationTokenRestServlet(hs).register(http_server)
     RegistrationTokenRestServlet(hs).register(http_server)
+    FederationDestinationRestServlet(hs).register(http_server)
 
     # Some servlets only get registered for the main process.
     if hs.config.worker.worker_app is None:
diff --git a/synapse/rest/admin/federation.py b/synapse/rest/admin/federation.py
new file mode 100644
index 000000000000..d040558fda2a
--- /dev/null
+++ b/synapse/rest/admin/federation.py
@@ -0,0 +1,96 @@
+# Copyright 2021 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.
+import logging
+from http import HTTPStatus
+from typing import TYPE_CHECKING, Tuple
+
+from synapse.api.errors import Codes, SynapseError
+from synapse.http.servlet import RestServlet, parse_integer, parse_string
+from synapse.http.site import SynapseRequest
+from synapse.rest.admin._base import admin_patterns, assert_requester_is_admin
+from synapse.storage.databases.main.transactions import DestinationSortOrder
+from synapse.types import JsonDict
+
+if TYPE_CHECKING:
+    from synapse.server import HomeServer
+
+logger = logging.getLogger(__name__)
+
+
+class FederationDestinationRestServlet(RestServlet):
+    """Get request to list all destinations.
+    This needs user to have administrator access in Synapse.
+
+    GET /_synapse/admin/v1/federation/destinations?from=0&limit=10
+
+    returns:
+        200 OK with list of destinations if success otherwise an error.
+
+    The parameters `from` and `limit` are required only for pagination.
+    By default, a `limit` of 100 is used.
+    The parameter `destination` can be used to filter by destination.
+    The parameter `order_by` can be used to order the result.
+    """
+
+    PATTERNS = admin_patterns("/federation/destinations$")
+
+    def __init__(self, hs: "HomeServer"):
+        self._auth = hs.get_auth()
+        self._store = hs.get_datastore()
+
+    async def on_GET(self, request: SynapseRequest) -> Tuple[int, JsonDict]:
+        await assert_requester_is_admin(self._auth, request)
+
+        start = parse_integer(request, "from", default=0)
+        limit = parse_integer(request, "limit", default=100)
+
+        if start < 0:
+            raise SynapseError(
+                400,
+                "Query parameter from must be a string representing a positive integer.",
+                errcode=Codes.INVALID_PARAM,
+            )
+
+        if limit < 0:
+            raise SynapseError(
+                400,
+                "Query parameter limit must be a string representing a positive integer.",
+                errcode=Codes.INVALID_PARAM,
+            )
+
+        destination = parse_string(request, "destination")
+
+        order_by = parse_string(
+            request,
+            "order_by",
+            default=DestinationSortOrder.DESTINATION.value,
+            allowed_values=(
+                DestinationSortOrder.DESTINATION.value,
+                DestinationSortOrder.RETRY_LAST_TS.value,
+                DestinationSortOrder.RETTRY_INTERVAL.value,
+                DestinationSortOrder.FAILURE_TS.value,
+                DestinationSortOrder.LAST_SUCCESSFUL_STREAM_ORDERING.value,
+            ),
+        )
+
+        direction = parse_string(request, "dir", default="f", allowed_values=("f", "b"))
+
+        destinations, total = await self._store.get_destinations_paginate(
+            start, limit, destination, order_by, direction
+        )
+        response = {"destinations": destinations, "total": total}
+        if (start + limit) < total:
+            response["next_token"] = str(start + len(destinations))
+
+        return HTTPStatus.OK, response
diff --git a/synapse/storage/databases/main/transactions.py b/synapse/storage/databases/main/transactions.py
index d7dc1f73ac16..5e651eb77eb8 100644
--- a/synapse/storage/databases/main/transactions.py
+++ b/synapse/storage/databases/main/transactions.py
@@ -14,6 +14,7 @@
 
 import logging
 from collections import namedtuple
+from enum import Enum
 from typing import TYPE_CHECKING, Iterable, List, Optional, Tuple
 
 import attr
@@ -44,6 +45,16 @@
 )
 
 
+class DestinationSortOrder(Enum):
+    """Enum to define the sorting method used when returning destinations."""
+
+    DESTINATION = "destination"
+    RETRY_LAST_TS = "retry_last_ts"
+    RETTRY_INTERVAL = "retry_interval"
+    FAILURE_TS = "failure_ts"
+    LAST_SUCCESSFUL_STREAM_ORDERING = "last_successful_stream_ordering"
+
+
 @attr.s(slots=True, frozen=True, auto_attribs=True)
 class DestinationRetryTimings:
     """The current destination retry timing info for a remote server."""
@@ -480,3 +491,60 @@ def _get_catch_up_outstanding_destinations_txn(
 
         destinations = [row[0] for row in txn]
         return destinations
+
+    async def get_destinations_paginate(
+        self,
+        start: int,
+        limit: int,
+        destination: Optional[str] = None,
+        order_by: str = DestinationSortOrder.DESTINATION.value,
+        direction: str = "f",
+    ) -> Tuple[List[JsonDict], int]:
+        """Function to retrieve a paginated list of destinations.
+        This will return a json list of destinations and the
+        total number of destinations matching the filter criteria.
+
+        Args:
+            start: start number to begin the query from
+            limit: number of rows to retrieve
+            destination: search string in destination
+            order_by: the sort order of the returned list
+            direction: sort ascending or descending
+        Returns:
+            A tuple of a list of mappings from destination to information
+            and a count of total destinations.
+        """
+
+        def get_destinations_paginate_txn(txn):
+            order_by_column = DestinationSortOrder(order_by).value
+
+            if direction == "b":
+                order = "DESC"
+            else:
+                order = "ASC"
+
+            args = []
+            where_statement = ""
+            if destination:
+                args.extend(["%" + destination.lower() + "%"])
+                where_statement = "WHERE LOWER(destination) LIKE ?"
+
+            sql_base = f"FROM destinations {where_statement} "
+            sql = f"SELECT COUNT(*) as total_destinations {sql_base}"
+            txn.execute(sql, args)
+            count = txn.fetchone()[0]
+
+            sql = f"""
+                SELECT destination, retry_last_ts, retry_interval, failure_ts,
+                last_successful_stream_ordering
+                {sql_base}
+                ORDER BY {order_by_column} {order}, destination ASC
+                LIMIT ? OFFSET ?
+            """
+            txn.execute(sql, args + [limit, start])
+            destinations = self.db_pool.cursor_to_dict(txn)
+            return destinations, count
+
+        return await self.db_pool.runInteraction(
+            "get_destinations_paginate_txn", get_destinations_paginate_txn
+        )
diff --git a/tests/rest/admin/test_federation.py b/tests/rest/admin/test_federation.py
new file mode 100644
index 000000000000..0809cbce6685
--- /dev/null
+++ b/tests/rest/admin/test_federation.py
@@ -0,0 +1,419 @@
+# Copyright 2021 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 http import HTTPStatus
+from typing import List, Optional
+
+import synapse.rest.admin
+from synapse.api.errors import Codes
+from synapse.rest.client import login
+from synapse.server import HomeServer
+from synapse.types import JsonDict
+
+from tests import unittest
+
+
+class FederationTestCase(unittest.HomeserverTestCase):
+    servlets = [
+        synapse.rest.admin.register_servlets,
+        login.register_servlets,
+    ]
+
+    def prepare(self, reactor, clock, hs: HomeServer):
+        self.store = hs.get_datastore()
+        self.register_user("admin", "pass", admin=True)
+        self.admin_user_tok = self.login("admin", "pass")
+
+        self.url = "/_synapse/admin/v1/federation/destinations"
+
+    def test_requester_is_no_admin(self):
+        """
+        If the user is not a server admin, an error 403 is returned.
+        """
+
+        self.register_user("user", "pass", admin=False)
+        other_user_tok = self.login("user", "pass")
+
+        channel = self.make_request(
+            "GET",
+            self.url,
+            content={},
+            access_token=other_user_tok,
+        )
+
+        self.assertEqual(HTTPStatus.FORBIDDEN, channel.code, msg=channel.json_body)
+        self.assertEqual(Codes.FORBIDDEN, channel.json_body["errcode"])
+
+    def test_invalid_parameter(self):
+        """
+        If parameters are invalid, an error is returned.
+        """
+
+        # negative limit
+        channel = self.make_request(
+            "GET",
+            self.url + "?limit=-5",
+            access_token=self.admin_user_tok,
+        )
+
+        self.assertEqual(HTTPStatus.BAD_REQUEST, channel.code, msg=channel.json_body)
+        self.assertEqual(Codes.INVALID_PARAM, channel.json_body["errcode"])
+
+        # negative from
+        channel = self.make_request(
+            "GET",
+            self.url + "?from=-5",
+            access_token=self.admin_user_tok,
+        )
+
+        self.assertEqual(HTTPStatus.BAD_REQUEST, channel.code, msg=channel.json_body)
+        self.assertEqual(Codes.INVALID_PARAM, channel.json_body["errcode"])
+
+        # unkown order_by
+        channel = self.make_request(
+            "GET",
+            self.url + "?order_by=bar",
+            access_token=self.admin_user_tok,
+        )
+
+        self.assertEqual(HTTPStatus.BAD_REQUEST, channel.code, msg=channel.json_body)
+        self.assertEqual(Codes.UNKNOWN, channel.json_body["errcode"])
+
+        # invalid search order
+        channel = self.make_request(
+            "GET",
+            self.url + "?dir=bar",
+            access_token=self.admin_user_tok,
+        )
+
+        self.assertEqual(HTTPStatus.BAD_REQUEST, channel.code, msg=channel.json_body)
+        self.assertEqual(Codes.UNKNOWN, channel.json_body["errcode"])
+
+    def test_limit(self):
+        """
+        Testing list of destinations with limit
+        """
+
+        number_destinations = 20
+        self._create_destinations(number_destinations)
+
+        channel = self.make_request(
+            "GET",
+            self.url + "?limit=5",
+            access_token=self.admin_user_tok,
+        )
+
+        self.assertEqual(HTTPStatus.OK, channel.code, msg=channel.json_body)
+        self.assertEqual(channel.json_body["total"], number_destinations)
+        self.assertEqual(len(channel.json_body["destinations"]), 5)
+        self.assertEqual(channel.json_body["next_token"], "5")
+        self._check_fields(channel.json_body["destinations"])
+
+    def test_from(self):
+        """
+        Testing list of destinations with a defined starting point (from)
+        """
+
+        number_destinations = 20
+        self._create_destinations(number_destinations)
+
+        channel = self.make_request(
+            "GET",
+            self.url + "?from=5",
+            access_token=self.admin_user_tok,
+        )
+
+        self.assertEqual(HTTPStatus.OK, channel.code, msg=channel.json_body)
+        self.assertEqual(channel.json_body["total"], number_destinations)
+        self.assertEqual(len(channel.json_body["destinations"]), 15)
+        self.assertNotIn("next_token", channel.json_body)
+        self._check_fields(channel.json_body["destinations"])
+
+    def test_limit_and_from(self):
+        """
+        Testing list of destinations with a defined starting point and limit
+        """
+
+        number_destinations = 20
+        self._create_destinations(number_destinations)
+
+        channel = self.make_request(
+            "GET",
+            self.url + "?from=5&limit=10",
+            access_token=self.admin_user_tok,
+        )
+
+        self.assertEqual(HTTPStatus.OK, channel.code, msg=channel.json_body)
+        self.assertEqual(channel.json_body["total"], number_destinations)
+        self.assertEqual(channel.json_body["next_token"], "15")
+        self.assertEqual(len(channel.json_body["destinations"]), 10)
+        self._check_fields(channel.json_body["destinations"])
+
+    def test_next_token(self):
+        """
+        Testing that `next_token` appears at the right place
+        """
+
+        number_destinations = 20
+        self._create_destinations(number_destinations)
+
+        #  `next_token` does not appear
+        # Number of results is the number of entries
+        channel = self.make_request(
+            "GET",
+            self.url + "?limit=20",
+            access_token=self.admin_user_tok,
+        )
+
+        self.assertEqual(HTTPStatus.OK, channel.code, msg=channel.json_body)
+        self.assertEqual(channel.json_body["total"], number_destinations)
+        self.assertEqual(len(channel.json_body["destinations"]), number_destinations)
+        self.assertNotIn("next_token", channel.json_body)
+
+        #  `next_token` does not appear
+        # Number of max results is larger than the number of entries
+        channel = self.make_request(
+            "GET",
+            self.url + "?limit=21",
+            access_token=self.admin_user_tok,
+        )
+
+        self.assertEqual(HTTPStatus.OK, channel.code, msg=channel.json_body)
+        self.assertEqual(channel.json_body["total"], number_destinations)
+        self.assertEqual(len(channel.json_body["destinations"]), number_destinations)
+        self.assertNotIn("next_token", channel.json_body)
+
+        #  `next_token` does appear
+        # Number of max results is smaller than the number of entries
+        channel = self.make_request(
+            "GET",
+            self.url + "?limit=19",
+            access_token=self.admin_user_tok,
+        )
+
+        self.assertEqual(HTTPStatus.OK, channel.code, msg=channel.json_body)
+        self.assertEqual(channel.json_body["total"], number_destinations)
+        self.assertEqual(len(channel.json_body["destinations"]), 19)
+        self.assertEqual(channel.json_body["next_token"], "19")
+
+        # Check
+        # Set `from` to value of `next_token` for request remaining entries
+        #  `next_token` does not appear
+        channel = self.make_request(
+            "GET",
+            self.url + "?from=19",
+            access_token=self.admin_user_tok,
+        )
+
+        self.assertEqual(HTTPStatus.OK, channel.code, msg=channel.json_body)
+        self.assertEqual(channel.json_body["total"], number_destinations)
+        self.assertEqual(len(channel.json_body["destinations"]), 1)
+        self.assertNotIn("next_token", channel.json_body)
+
+    def test_list_all_destinations(self):
+        """
+        List all destinations.
+        """
+        number_destinations = 5
+        self._create_destinations(number_destinations)
+
+        channel = self.make_request(
+            "GET",
+            self.url,
+            {},
+            access_token=self.admin_user_tok,
+        )
+
+        self.assertEqual(HTTPStatus.OK, channel.code, msg=channel.json_body)
+        self.assertEqual(number_destinations, len(channel.json_body["destinations"]))
+        self.assertEqual(number_destinations, channel.json_body["total"])
+
+        # Check that all fields are available
+        self._check_fields(channel.json_body["destinations"])
+
+    def test_order_by(self):
+        """
+        Testing order list with parameter `order_by`
+        """
+
+        def _order_test(
+            expected_destination_list: List[str],
+            order_by: Optional[str],
+            dir: Optional[str] = None,
+        ):
+            """Request the list of destinations in a certain order.
+            Assert that order is what we expect
+
+            Args:
+                expected_destination_list: The list of user_id in the order
+                    we expect to get back from the server
+                order_by: The type of ordering to give the server
+                dir: The direction of ordering to give the server
+            """
+
+            url = f"{self.url}?"
+            if order_by is not None:
+                url += f"order_by={order_by}&"
+            if dir is not None and dir in ("b", "f"):
+                url += f"dir={dir}"
+            channel = self.make_request(
+                "GET",
+                url,
+                access_token=self.admin_user_tok,
+            )
+            self.assertEqual(HTTPStatus.OK, channel.code, msg=channel.json_body)
+            self.assertEqual(channel.json_body["total"], len(expected_destination_list))
+
+            returned_order = [
+                row["destination"] for row in channel.json_body["destinations"]
+            ]
+            self.assertEqual(expected_destination_list, returned_order)
+            self._check_fields(channel.json_body["destinations"])
+
+        # create destinations
+        dest = [
+            ("sub-a.example.com", 100, 300, 200, 300),
+            ("sub-b.example.com", 200, 200, 100, 100),
+            ("sub-c.example.com", 300, 100, 300, 200),
+        ]
+        for (
+            destination,
+            failure_ts,
+            retry_last_ts,
+            retry_interval,
+            last_successful_stream_ordering,
+        ) in dest:
+            self.get_success(
+                self.store.set_destination_retry_timings(
+                    destination, failure_ts, retry_last_ts, retry_interval
+                )
+            )
+            self.get_success(
+                self.store.set_destination_last_successful_stream_ordering(
+                    destination, last_successful_stream_ordering
+                )
+            )
+
+        # order by default (destination)
+        _order_test([dest[0][0], dest[1][0], dest[2][0]], None)
+        _order_test([dest[0][0], dest[1][0], dest[2][0]], None, "f")
+        _order_test([dest[2][0], dest[1][0], dest[0][0]], None, "b")
+
+        # order by destination
+        _order_test([dest[0][0], dest[1][0], dest[2][0]], "destination")
+        _order_test([dest[0][0], dest[1][0], dest[2][0]], "destination", "f")
+        _order_test([dest[2][0], dest[1][0], dest[0][0]], "destination", "b")
+
+        # order by failure_ts
+        _order_test([dest[0][0], dest[1][0], dest[2][0]], "failure_ts")
+        _order_test([dest[0][0], dest[1][0], dest[2][0]], "failure_ts", "f")
+        _order_test([dest[2][0], dest[1][0], dest[0][0]], "failure_ts", "b")
+
+        # order by retry_last_ts
+        _order_test([dest[2][0], dest[1][0], dest[0][0]], "retry_last_ts")
+        _order_test([dest[2][0], dest[1][0], dest[0][0]], "retry_last_ts", "f")
+        _order_test([dest[0][0], dest[1][0], dest[2][0]], "retry_last_ts", "b")
+
+        # order by retry_interval
+        _order_test([dest[1][0], dest[0][0], dest[2][0]], "retry_interval")
+        _order_test([dest[1][0], dest[0][0], dest[2][0]], "retry_interval", "f")
+        _order_test([dest[2][0], dest[0][0], dest[1][0]], "retry_interval", "b")
+
+        # order by last_successful_stream_ordering
+        _order_test(
+            [dest[1][0], dest[2][0], dest[0][0]], "last_successful_stream_ordering"
+        )
+        _order_test(
+            [dest[1][0], dest[2][0], dest[0][0]], "last_successful_stream_ordering", "f"
+        )
+        _order_test(
+            [dest[0][0], dest[2][0], dest[1][0]], "last_successful_stream_ordering", "b"
+        )
+
+    def test_search_term(self):
+        """Test that searching for a destination works correctly"""
+
+        def _search_test(
+            expected_destination: Optional[str],
+            search_term: str,
+        ):
+            """Search for a destination and check that the returned destinationis a match
+
+            Args:
+                expected_destination: The room_id expected to be returned by the API.
+                    Set to None to expect zero results for the search
+                search_term: The term to search for room names with
+            """
+            url = f"{self.url}?destination={search_term}"
+            channel = self.make_request(
+                "GET",
+                url.encode("ascii"),
+                access_token=self.admin_user_tok,
+            )
+            self.assertEqual(HTTPStatus.OK, channel.code, msg=channel.json_body)
+
+            # Check that destinations were returned
+            self.assertTrue("destinations" in channel.json_body)
+            self._check_fields(channel.json_body["destinations"])
+            destinations = channel.json_body["destinations"]
+
+            # Check that the expected number of destinations were returned
+            expected_destination_count = 1 if expected_destination else 0
+            self.assertEqual(len(destinations), expected_destination_count)
+            self.assertEqual(channel.json_body["total"], expected_destination_count)
+
+            if expected_destination:
+                # Check that the first returned destination is correct
+                self.assertEqual(expected_destination, destinations[0]["destination"])
+
+        number_destinations = 3
+        self._create_destinations(number_destinations)
+
+        # Test searching
+        _search_test("sub0.example.com", "0")
+        _search_test("sub0.example.com", "sub0")
+
+        _search_test("sub1.example.com", "1")
+        _search_test("sub1.example.com", "1.")
+
+        # Test case insensitive
+        _search_test("sub0.example.com", "SUB0")
+
+        _search_test(None, "foo")
+        _search_test(None, "bar")
+
+    def _create_destinations(self, number_destinations: int):
+        """Create a number of destinations
+
+        Args:
+            number_destinations: Number of destinations to be created
+        """
+        for i in range(0, number_destinations):
+            dest = f"sub{i}.example.com"
+            self.get_success(self.store.set_destination_retry_timings(dest, 50, 50, 50))
+            self.get_success(
+                self.store.set_destination_last_successful_stream_ordering(dest, 100)
+            )
+
+    def _check_fields(self, content: JsonDict):
+        """Checks that the expected destination attributes are present in content
+
+        Args:
+            content: List that is checked for content
+        """
+        for c in content:
+            self.assertIn("destination", c)
+            self.assertIn("retry_last_ts", c)
+            self.assertIn("retry_interval", c)
+            self.assertIn("failure_ts", c)
+            self.assertIn("last_successful_stream_ordering", c)

From 3c9cd1048eeb954d5b80f535dc69aec8b363cfb3 Mon Sep 17 00:00:00 2001
From: dklimpel <5740567+dklimpel@users.noreply.github.com>
Date: Mon, 22 Nov 2021 18:50:52 +0100
Subject: [PATCH 02/11] newsfile

---
 changelog.d/11407.feature | 1 +
 1 file changed, 1 insertion(+)
 create mode 100644 changelog.d/11407.feature

diff --git a/changelog.d/11407.feature b/changelog.d/11407.feature
new file mode 100644
index 000000000000..1d21bde98f64
--- /dev/null
+++ b/changelog.d/11407.feature
@@ -0,0 +1 @@
+Add admin API to get some information about federation status with remote servers.
\ No newline at end of file

From 003028c7c9ceb01ca121ed39ccbab6456808990c Mon Sep 17 00:00:00 2001
From: Dirk Klimpel <5740567+dklimpel@users.noreply.github.com>
Date: Mon, 29 Nov 2021 09:48:20 +0100
Subject: [PATCH 03/11] Apply suggestions from code review

Co-authored-by: David Robertson <david.m.robertson1@gmail.com>
---
 docs/usage/administration/admin_api/federation.md | 4 ++--
 synapse/rest/admin/federation.py                  | 8 +-------
 2 files changed, 3 insertions(+), 9 deletions(-)

diff --git a/docs/usage/administration/admin_api/federation.md b/docs/usage/administration/admin_api/federation.md
index 2636f2feb95b..7453b68fd110 100644
--- a/docs/usage/administration/admin_api/federation.md
+++ b/docs/usage/administration/admin_api/federation.md
@@ -1,6 +1,6 @@
 # Federation API
 
-This API allows a server administrator to manage the federation.
+This API allows a server administrator to manage Synapse's federation with other homeservers.
 
 **Note**: This API is new, experimental and "subject to change".
 
@@ -69,7 +69,7 @@ The following fields are returned in the JSON response body:
   Destinations objects contain the following fields:
   - `destination` - string - Name of the remote server to federate.
   - `retry_last_ts` - integer - The last time Synapse tried and failed to reach the
-    remote server, in ms
+    remote server, in ms.
   - `retry_interval` - integer - How long since the last time Synapse tried to reach
     the remote server before trying again, in ms.
   - `failure_ts` - integer - The first time Synapse tried and failed to reach the
diff --git a/synapse/rest/admin/federation.py b/synapse/rest/admin/federation.py
index d040558fda2a..ab403f972f50 100644
--- a/synapse/rest/admin/federation.py
+++ b/synapse/rest/admin/federation.py
@@ -75,13 +75,7 @@ async def on_GET(self, request: SynapseRequest) -> Tuple[int, JsonDict]:
             request,
             "order_by",
             default=DestinationSortOrder.DESTINATION.value,
-            allowed_values=(
-                DestinationSortOrder.DESTINATION.value,
-                DestinationSortOrder.RETRY_LAST_TS.value,
-                DestinationSortOrder.RETTRY_INTERVAL.value,
-                DestinationSortOrder.FAILURE_TS.value,
-                DestinationSortOrder.LAST_SUCCESSFUL_STREAM_ORDERING.value,
-            ),
+            allowed_values=[dest.value for dest in DestinationSortOrder],
         )
 
         direction = parse_string(request, "dir", default="f", allowed_values=("f", "b"))

From d9d469eab351130cecb5529b12968e697d894de9 Mon Sep 17 00:00:00 2001
From: dklimpel <5740567+dklimpel@users.noreply.github.com>
Date: Mon, 29 Nov 2021 11:35:56 +0100
Subject: [PATCH 04/11] small updates

- type hints
- format in docs
- replace response code in API
---
 docs/usage/administration/admin_api/federation.md | 2 +-
 synapse/rest/admin/federation.py                  | 4 ++--
 synapse/storage/databases/main/transactions.py    | 4 +++-
 3 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/docs/usage/administration/admin_api/federation.md b/docs/usage/administration/admin_api/federation.md
index 7453b68fd110..523b8cd86634 100644
--- a/docs/usage/administration/admin_api/federation.md
+++ b/docs/usage/administration/admin_api/federation.md
@@ -2,7 +2,7 @@
 
 This API allows a server administrator to manage Synapse's federation with other homeservers.
 
-**Note**: This API is new, experimental and "subject to change".
+Note: This API is new, experimental and "subject to change".
 
 ## List of destinations
 
diff --git a/synapse/rest/admin/federation.py b/synapse/rest/admin/federation.py
index ab403f972f50..d7b07f17a74e 100644
--- a/synapse/rest/admin/federation.py
+++ b/synapse/rest/admin/federation.py
@@ -57,14 +57,14 @@ async def on_GET(self, request: SynapseRequest) -> Tuple[int, JsonDict]:
 
         if start < 0:
             raise SynapseError(
-                400,
+                HTTPStatus.BAD_REQUEST,
                 "Query parameter from must be a string representing a positive integer.",
                 errcode=Codes.INVALID_PARAM,
             )
 
         if limit < 0:
             raise SynapseError(
-                400,
+                HTTPStatus.BAD_REQUEST,
                 "Query parameter limit must be a string representing a positive integer.",
                 errcode=Codes.INVALID_PARAM,
             )
diff --git a/synapse/storage/databases/main/transactions.py b/synapse/storage/databases/main/transactions.py
index 5e651eb77eb8..c1c893efa98b 100644
--- a/synapse/storage/databases/main/transactions.py
+++ b/synapse/storage/databases/main/transactions.py
@@ -515,7 +515,9 @@ async def get_destinations_paginate(
             and a count of total destinations.
         """
 
-        def get_destinations_paginate_txn(txn):
+        def get_destinations_paginate_txn(
+                txn: LoggingTransaction
+        ) -> Tuple[List[JsonDict], int]:
             order_by_column = DestinationSortOrder(order_by).value
 
             if direction == "b":

From 19d04597c37a01c60b31e536ef1d867b48e6a1c5 Mon Sep 17 00:00:00 2001
From: dklimpel <5740567+dklimpel@users.noreply.github.com>
Date: Mon, 29 Nov 2021 12:14:58 +0100
Subject: [PATCH 05/11] lint

---
 synapse/storage/databases/main/transactions.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/synapse/storage/databases/main/transactions.py b/synapse/storage/databases/main/transactions.py
index c1c893efa98b..162282255232 100644
--- a/synapse/storage/databases/main/transactions.py
+++ b/synapse/storage/databases/main/transactions.py
@@ -516,7 +516,7 @@ async def get_destinations_paginate(
         """
 
         def get_destinations_paginate_txn(
-                txn: LoggingTransaction
+            txn: LoggingTransaction,
         ) -> Tuple[List[JsonDict], int]:
             order_by_column = DestinationSortOrder(order_by).value
 

From 8175dcc1de8c2c3f4568c5c16a82f315c965d5a6 Mon Sep 17 00:00:00 2001
From: dklimpel <5740567+dklimpel@users.noreply.github.com>
Date: Thu, 2 Dec 2021 18:08:25 +0100
Subject: [PATCH 06/11] add API for one specific destination

---
 .../administration/admin_api/federation.md    | 27 ++++++++++
 synapse/rest/admin/__init__.py                |  8 ++-
 synapse/rest/admin/federation.py              | 49 ++++++++++++++++++-
 tests/rest/admin/test_federation.py           | 43 ++++++++++++++--
 4 files changed, 120 insertions(+), 7 deletions(-)

diff --git a/docs/usage/administration/admin_api/federation.md b/docs/usage/administration/admin_api/federation.md
index 523b8cd86634..769b5b874780 100644
--- a/docs/usage/administration/admin_api/federation.md
+++ b/docs/usage/administration/admin_api/federation.md
@@ -79,3 +79,30 @@ The following fields are returned in the JSON response body:
     to this destination or `null` if this information has not been tracked yet.
 - `next_token`: string representing a positive integer - Indication for pagination. See above.
 - `total` - integer - Total number of destinations.
+
+# Destination Details API
+
+This API gets the retry timing info for a specific remote server.
+
+The API is:
+
+```
+GET /_synapse/admin/v1/federation/destinations/<destination>
+```
+
+A response body like the following is returned:
+
+```json
+{
+   "destination": "matrix.org",
+   "retry_last_ts": 1557332397936,
+   "retry_interval": 3000000,
+   "failure_ts": 1557329397936,
+   "last_successful_stream_ordering": null
+}
+```
+
+**Response**
+
+The response fields are the same like in the `destinations` array in
+[List of destinations](#list-of-destinations) response.
diff --git a/synapse/rest/admin/__init__.py b/synapse/rest/admin/__init__.py
index dd25e3afa65e..fd9709e9d93f 100644
--- a/synapse/rest/admin/__init__.py
+++ b/synapse/rest/admin/__init__.py
@@ -39,7 +39,10 @@
     EventReportDetailRestServlet,
     EventReportsRestServlet,
 )
-from synapse.rest.admin.federation import FederationDestinationRestServlet
+from synapse.rest.admin.federation import (
+    DestinationsRestServlet,
+    ListDestinationsRestServlet,
+)
 from synapse.rest.admin.groups import DeleteGroupAdminRestServlet
 from synapse.rest.admin.media import ListMediaInRoom, register_servlets_for_media_repo
 from synapse.rest.admin.registration_tokens import (
@@ -257,7 +260,8 @@ def register_servlets(hs: "HomeServer", http_server: HttpServer) -> None:
     ListRegistrationTokensRestServlet(hs).register(http_server)
     NewRegistrationTokenRestServlet(hs).register(http_server)
     RegistrationTokenRestServlet(hs).register(http_server)
-    FederationDestinationRestServlet(hs).register(http_server)
+    DestinationsRestServlet(hs).register(http_server)
+    ListDestinationsRestServlet(hs).register(http_server)
 
     # Some servlets only get registered for the main process.
     if hs.config.worker.worker_app is None:
diff --git a/synapse/rest/admin/federation.py b/synapse/rest/admin/federation.py
index d7b07f17a74e..744687be35fc 100644
--- a/synapse/rest/admin/federation.py
+++ b/synapse/rest/admin/federation.py
@@ -15,7 +15,7 @@
 from http import HTTPStatus
 from typing import TYPE_CHECKING, Tuple
 
-from synapse.api.errors import Codes, SynapseError
+from synapse.api.errors import Codes, NotFoundError, SynapseError
 from synapse.http.servlet import RestServlet, parse_integer, parse_string
 from synapse.http.site import SynapseRequest
 from synapse.rest.admin._base import admin_patterns, assert_requester_is_admin
@@ -28,7 +28,7 @@
 logger = logging.getLogger(__name__)
 
 
-class FederationDestinationRestServlet(RestServlet):
+class ListDestinationsRestServlet(RestServlet):
     """Get request to list all destinations.
     This needs user to have administrator access in Synapse.
 
@@ -88,3 +88,48 @@ async def on_GET(self, request: SynapseRequest) -> Tuple[int, JsonDict]:
             response["next_token"] = str(start + len(destinations))
 
         return HTTPStatus.OK, response
+
+
+class DestinationsRestServlet(RestServlet):
+    """Get details of a destination.
+    This needs user to have administrator access in Synapse.
+
+    GET /_synapse/admin/v1/federation/destinations/<destination>
+
+    returns:
+        200 OK with details of a destination if success otherwise an error.
+    """
+
+    PATTERNS = admin_patterns("/federation/destinations/(?P<destination>[^/]+)$")
+
+    def __init__(self, hs: "HomeServer"):
+        self._auth = hs.get_auth()
+        self._store = hs.get_datastore()
+
+    async def on_GET(
+        self, request: SynapseRequest, destination: str
+    ) -> Tuple[int, JsonDict]:
+        await assert_requester_is_admin(self._auth, request)
+
+        destination_retry_timings = await self._store.get_destination_retry_timings(
+            destination
+        )
+
+        if not destination_retry_timings:
+            raise NotFoundError("Unknown destination")
+
+        last_successful_stream_ordering = (
+            await self._store.get_destination_last_successful_stream_ordering(
+                destination
+            )
+        )
+
+        response = {
+            "destination": destination,
+            "failure_ts": destination_retry_timings.failure_ts,
+            "retry_last_ts": destination_retry_timings.retry_last_ts,
+            "retry_interval": destination_retry_timings.retry_interval,
+            "last_successful_stream_ordering": last_successful_stream_ordering,
+        }
+
+        return HTTPStatus.OK, response
diff --git a/tests/rest/admin/test_federation.py b/tests/rest/admin/test_federation.py
index 0809cbce6685..5188499ef2d6 100644
--- a/tests/rest/admin/test_federation.py
+++ b/tests/rest/admin/test_federation.py
@@ -14,6 +14,8 @@
 from http import HTTPStatus
 from typing import List, Optional
 
+from parameterized import parameterized
+
 import synapse.rest.admin
 from synapse.api.errors import Codes
 from synapse.rest.client import login
@@ -36,7 +38,13 @@ def prepare(self, reactor, clock, hs: HomeServer):
 
         self.url = "/_synapse/admin/v1/federation/destinations"
 
-    def test_requester_is_no_admin(self):
+    @parameterized.expand(
+        [
+            ("/_synapse/admin/v1/federation/destinations",),
+            ("/_synapse/admin/v1/federation/destinations/dummy",),
+        ]
+    )
+    def test_requester_is_no_admin(self, url: str):
         """
         If the user is not a server admin, an error 403 is returned.
         """
@@ -46,7 +54,7 @@ def test_requester_is_no_admin(self):
 
         channel = self.make_request(
             "GET",
-            self.url,
+            url,
             content={},
             access_token=other_user_tok,
         )
@@ -99,6 +107,16 @@ def test_invalid_parameter(self):
         self.assertEqual(HTTPStatus.BAD_REQUEST, channel.code, msg=channel.json_body)
         self.assertEqual(Codes.UNKNOWN, channel.json_body["errcode"])
 
+        # invalid destination
+        channel = self.make_request(
+            "GET",
+            self.url + "/dummy",
+            access_token=self.admin_user_tok,
+        )
+
+        self.assertEqual(HTTPStatus.NOT_FOUND, channel.code, msg=channel.json_body)
+        self.assertEqual(Codes.NOT_FOUND, channel.json_body["errcode"])
+
     def test_limit(self):
         """
         Testing list of destinations with limit
@@ -392,6 +410,25 @@ def _search_test(
         _search_test(None, "foo")
         _search_test(None, "bar")
 
+    def test_get_single_destination(self):
+        """
+        Get one specific destinations.
+        """
+        self._create_destinations(5)
+
+        channel = self.make_request(
+            "GET",
+            self.url + "/sub0.example.com",
+            access_token=self.admin_user_tok,
+        )
+
+        self.assertEqual(HTTPStatus.OK, channel.code, msg=channel.json_body)
+        self.assertEqual("sub0.example.com", channel.json_body["destination"])
+
+        # Check that all fields are available
+        # convert channel.json_body into a List
+        self._check_fields([channel.json_body])
+
     def _create_destinations(self, number_destinations: int):
         """Create a number of destinations
 
@@ -405,7 +442,7 @@ def _create_destinations(self, number_destinations: int):
                 self.store.set_destination_last_successful_stream_ordering(dest, 100)
             )
 
-    def _check_fields(self, content: JsonDict):
+    def _check_fields(self, content: List[JsonDict]):
         """Checks that the expected destination attributes are present in content
 
         Args:

From 4d966e7acb2634fb44383f407f7adb3c6150c4a3 Mon Sep 17 00:00:00 2001
From: dklimpel <5740567+dklimpel@users.noreply.github.com>
Date: Thu, 2 Dec 2021 18:23:45 +0100
Subject: [PATCH 07/11] add some docs

---
 docs/usage/administration/admin_api/federation.md | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/docs/usage/administration/admin_api/federation.md b/docs/usage/administration/admin_api/federation.md
index 769b5b874780..0102ec03082d 100644
--- a/docs/usage/administration/admin_api/federation.md
+++ b/docs/usage/administration/admin_api/federation.md
@@ -6,7 +6,12 @@ Note: This API is new, experimental and "subject to change".
 
 ## List of destinations
 
-This API gets the current destination retry timing info for a remote server.
+This API gets the current destination retry timing info for all remote servers.
+
+The list contains all the servers with which the server federates,
+regardless of whether an error occurred or not.
+If an error occurs, it may take up to 20 minutes for the error to be displayed here,
+as a complete retry must have failed.
 
 The API is:
 
@@ -69,11 +74,11 @@ The following fields are returned in the JSON response body:
   Destinations objects contain the following fields:
   - `destination` - string - Name of the remote server to federate.
   - `retry_last_ts` - integer - The last time Synapse tried and failed to reach the
-    remote server, in ms.
+    remote server, in ms. This is `0` if no error has occurred.
   - `retry_interval` - integer - How long since the last time Synapse tried to reach
     the remote server before trying again, in ms.
   - `failure_ts` - integer - The first time Synapse tried and failed to reach the
-    remote server, in ms.
+    remote server, in ms. This is `0` if no error has occurred.
   - `last_successful_stream_ordering` - integer - The stream ordering of the most
     recent successfully-sent [PDU](Understanding-Synapse-Performance-Issues-Through-Grafana-Graphs.md#federation)
     to this destination or `null` if this information has not been tracked yet.

From be1f88954b5372f7dc2a143c4fbcd437801729d9 Mon Sep 17 00:00:00 2001
From: dklimpel <5740567+dklimpel@users.noreply.github.com>
Date: Thu, 2 Dec 2021 18:39:17 +0100
Subject: [PATCH 08/11] fix doc

---
 docs/usage/administration/admin_api/federation.md | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/docs/usage/administration/admin_api/federation.md b/docs/usage/administration/admin_api/federation.md
index 0102ec03082d..49b8017b697b 100644
--- a/docs/usage/administration/admin_api/federation.md
+++ b/docs/usage/administration/admin_api/federation.md
@@ -74,11 +74,11 @@ The following fields are returned in the JSON response body:
   Destinations objects contain the following fields:
   - `destination` - string - Name of the remote server to federate.
   - `retry_last_ts` - integer - The last time Synapse tried and failed to reach the
-    remote server, in ms. This is `0` if no error has occurred.
+    remote server, in ms. This is `0` if no further retrying occuring.
   - `retry_interval` - integer - How long since the last time Synapse tried to reach
-    the remote server before trying again, in ms.
+    the remote server before trying again, in ms. This is `0` if no further retrying occuring.
   - `failure_ts` - integer - The first time Synapse tried and failed to reach the
-    remote server, in ms. This is `0` if no error has occurred.
+    remote server, in ms. This is `null` if no error has occurred.
   - `last_successful_stream_ordering` - integer - The stream ordering of the most
     recent successfully-sent [PDU](Understanding-Synapse-Performance-Issues-Through-Grafana-Graphs.md#federation)
     to this destination or `null` if this information has not been tracked yet.

From 7bd0c47b88ad9bec33709152a4c0f7cd5f165791 Mon Sep 17 00:00:00 2001
From: Dirk Klimpel <5740567+dklimpel@users.noreply.github.com>
Date: Fri, 3 Dec 2021 13:20:15 +0100
Subject: [PATCH 09/11] Apply suggestions from code review

Co-authored-by: David Robertson <david.m.robertson1@gmail.com>
---
 docs/usage/administration/admin_api/federation.md | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/docs/usage/administration/admin_api/federation.md b/docs/usage/administration/admin_api/federation.md
index 49b8017b697b..9ad1ae8d539b 100644
--- a/docs/usage/administration/admin_api/federation.md
+++ b/docs/usage/administration/admin_api/federation.md
@@ -71,7 +71,7 @@ this can cause a large load on the database, especially for large environments.
 The following fields are returned in the JSON response body:
 
 - `destinations` - An array of objects, each containing information about a destination.
-  Destinations objects contain the following fields:
+  Destination objects contain the following fields:
   - `destination` - string - Name of the remote server to federate.
   - `retry_last_ts` - integer - The last time Synapse tried and failed to reach the
     remote server, in ms. This is `0` if no further retrying occuring.
@@ -80,8 +80,8 @@ The following fields are returned in the JSON response body:
   - `failure_ts` - integer - The first time Synapse tried and failed to reach the
     remote server, in ms. This is `null` if no error has occurred.
   - `last_successful_stream_ordering` - integer - The stream ordering of the most
-    recent successfully-sent [PDU](Understanding-Synapse-Performance-Issues-Through-Grafana-Graphs.md#federation)
-    to this destination or `null` if this information has not been tracked yet.
+    recent successfully-sent [PDU](understanding_synapse_through_grafana_graphs.md#federation)
+    to this destination, or `null` if this information has not been tracked yet.
 - `next_token`: string representing a positive integer - Indication for pagination. See above.
 - `total` - integer - Total number of destinations.
 

From c77a10f4db2123a78df1331e21fe1cab5cd9b9f0 Mon Sep 17 00:00:00 2001
From: Dirk Klimpel <5740567+dklimpel@users.noreply.github.com>
Date: Fri, 3 Dec 2021 13:46:52 +0100
Subject: [PATCH 10/11] Apply suggestions from code review

---
 docs/usage/administration/admin_api/federation.md | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/docs/usage/administration/admin_api/federation.md b/docs/usage/administration/admin_api/federation.md
index 9ad1ae8d539b..2869fecdabfb 100644
--- a/docs/usage/administration/admin_api/federation.md
+++ b/docs/usage/administration/admin_api/federation.md
@@ -74,11 +74,12 @@ The following fields are returned in the JSON response body:
   Destination objects contain the following fields:
   - `destination` - string - Name of the remote server to federate.
   - `retry_last_ts` - integer - The last time Synapse tried and failed to reach the
-    remote server, in ms. This is `0` if no further retrying occuring.
+    remote server, in ms. This is `0` if the last attempt to communicate with the
+    remote server was successful.
   - `retry_interval` - integer - How long since the last time Synapse tried to reach
     the remote server before trying again, in ms. This is `0` if no further retrying occuring.
-  - `failure_ts` - integer - The first time Synapse tried and failed to reach the
-    remote server, in ms. This is `null` if no error has occurred.
+  - `failure_ts` - nullable integer - The first time Synapse tried and failed to reach the
+    remote server, in ms. This is `null` if communication with the remote server has never failed.
   - `last_successful_stream_ordering` - integer - The stream ordering of the most
     recent successfully-sent [PDU](understanding_synapse_through_grafana_graphs.md#federation)
     to this destination, or `null` if this information has not been tracked yet.

From 6cc6a2648be8149c5f7c458963bbde26e9175fc8 Mon Sep 17 00:00:00 2001
From: Dirk Klimpel <5740567+dklimpel@users.noreply.github.com>
Date: Mon, 6 Dec 2021 17:31:26 +0100
Subject: [PATCH 11/11] Apply suggestions from code review

Co-authored-by: David Robertson <david.m.robertson1@gmail.com>
---
 docs/usage/administration/admin_api/federation.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/docs/usage/administration/admin_api/federation.md b/docs/usage/administration/admin_api/federation.md
index 2869fecdabfb..8f9535f57b09 100644
--- a/docs/usage/administration/admin_api/federation.md
+++ b/docs/usage/administration/admin_api/federation.md
@@ -80,7 +80,7 @@ The following fields are returned in the JSON response body:
     the remote server before trying again, in ms. This is `0` if no further retrying occuring.
   - `failure_ts` - nullable integer - The first time Synapse tried and failed to reach the
     remote server, in ms. This is `null` if communication with the remote server has never failed.
-  - `last_successful_stream_ordering` - integer - The stream ordering of the most
+  - `last_successful_stream_ordering` - nullable integer - The stream ordering of the most
     recent successfully-sent [PDU](understanding_synapse_through_grafana_graphs.md#federation)
     to this destination, or `null` if this information has not been tracked yet.
 - `next_token`: string representing a positive integer - Indication for pagination. See above.