1 files changed, 27 insertions, 10 deletions
diff --git a/synapse/config/workers.py b/synapse/config/workers.py
index c784a71508..f23e42cdf9 100644
--- a/synapse/config/workers.py
+++ b/synapse/config/workers.py
@@ -13,12 +13,24 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from typing import List, Union
+
 import attr
 
 from ._base import Config, ConfigError, ShardedWorkerHandlingConfig
 from .server import ListenerConfig, parse_listener_def
 
 
+def _instance_to_list_converter(obj: Union[str, List[str]]) -> List[str]:
+    """Helper for allowing parsing a string or list of strings to a config
+    option expecting a list of strings.
+    """
+
+    if isinstance(obj, str):
+        return [obj]
+    return obj
+
+
 @attr.s
 class InstanceLocationConfig:
     """The host and port to talk to an instance via HTTP replication.
@@ -33,11 +45,13 @@ class WriterLocations:
     """Specifies the instances that write various streams.
 
     Attributes:
-        events: The instance that writes to the event and backfill streams.
-        events: The instance that writes to the typing stream.
+        events: The instances that write to the event and backfill streams.
+        typing: The instance that writes to the typing stream.
     """
 
-    events = attr.ib(default="master", type=str)
+    events = attr.ib(
+        default=["master"], type=List[str], converter=_instance_to_list_converter
+    )
     typing = attr.ib(default="master", type=str)
 
 
@@ -105,15 +119,18 @@ class WorkerConfig(Config):
         writers = config.get("stream_writers") or {}
         self.writers = WriterLocations(**writers)
 
-        # Check that the configured writer for events and typing also appears in
+        # Check that the configured writers for events and typing also appears in
         # `instance_map`.
         for stream in ("events", "typing"):
-            instance = getattr(self.writers, stream)
-            if instance != "master" and instance not in self.instance_map:
-                raise ConfigError(
-                    "Instance %r is configured to write %s but does not appear in `instance_map` config."
-                    % (instance, stream)
-                )
+            instances = _instance_to_list_converter(getattr(self.writers, stream))
+            for instance in instances:
+                if instance != "master" and instance not in self.instance_map:
+                    raise ConfigError(
+                        "Instance %r is configured to write %s but does not appear in `instance_map` config."
+                        % (instance, stream)
+                    )
+
+        self.events_shard_config = ShardedWorkerHandlingConfig(self.writers.events)
 
     def generate_config_section(self, config_dir_path, server_name, **kwargs):
         return """\