Add experimental support for sharding event persister. (#8170)

This is *not* ready for production yet. Caveats:

1. We should write some tests...
2. The stream token that we use for events can get stalled at the minimum position of all writers. This means that new events may not be processed and e.g. sent down sync streams if a writer isn't writing or is slow.

3 files changed, 46 insertions, 13 deletions

diff --git a/synapse/config/_base.py b/synapse/config/_base.py
index 1417487427..73f0717b0d 100644
--- a/synapse/config/_base.py
+++ b/synapse/config/_base.py
@@ -832,11 +832,26 @@ class ShardedWorkerHandlingConfig:
     def should_handle(self, instance_name: str, key: str) -> bool:
         """Whether this instance is responsible for handling the given key.
         """
-
-        # If multiple instances are not defined we always return true.
+        # If multiple instances are not defined we always return true
         if not self.instances or len(self.instances) == 1:
             return True
 
+        return self.get_instance(key) == instance_name
+
+    def get_instance(self, key: str) -> str:
+        """Get the instance responsible for handling the given key.
+
+        Note: For things like federation sending the config for which instance
+        is sending is known only to the sender instance if there is only one.
+        Therefore `should_handle` should be used where possible.
+        """
+
+        if not self.instances:
+            return "master"
+
+        if len(self.instances) == 1:
+            return self.instances[0]
+
         # We shard by taking the hash, modulo it by the number of instances and
         # then checking whether this instance matches the instance at that
         # index.
@@ -846,7 +861,7 @@ class ShardedWorkerHandlingConfig:
         dest_hash = sha256(key.encode("utf8")).digest()
         dest_int = int.from_bytes(dest_hash, byteorder="little")
         remainder = dest_int % (len(self.instances))
-        return self.instances[remainder] == instance_name
+        return self.instances[remainder]
 
 
 __all__ = ["Config", "RootConfig", "ShardedWorkerHandlingConfig"]
diff --git a/synapse/config/_base.pyi b/synapse/config/_base.pyi
index eb911e8f9f..b8faafa9bd 100644
--- a/synapse/config/_base.pyi
+++ b/synapse/config/_base.pyi
@@ -142,3 +142,4 @@ class ShardedWorkerHandlingConfig:
     instances: List[str]
     def __init__(self, instances: List[str]) -> None: ...
     def should_handle(self, instance_name: str, key: str) -> bool: ...
+    def get_instance(self, key: str) -> str: ...
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 """\