explain load balancing for federation_sender_instances

[dklimpel]

Adding information on how the load is distributed for federation_sender_instances.

Thx to @devonh for the information.

causal source:

synapse/synapse/config/_base.py

Lines 946 to 989 in c2e5e9e

	class ShardedWorkerHandlingConfig:
	"""Algorithm for choosing which instance is responsible for handling some
	sharded work.
	For example, the federation senders use this to determine which instances
	handles sending stuff to a given destination (which is used as the `key`
	below).
	"""
	instances: List[str]
	def should_handle(self, instance_name: str, key: str) -> bool:
	"""Whether this instance is responsible for handling the given key."""
	# If no instances are defined we assume some other worker is handling
	# this.
	if not self.instances:
	return False
	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 federation sending and pushers the config for which instance
	is sending is known only to the sender instance, so we don't expose this
	method by default.
	"""
	if not self.instances:
	raise Exception("Unknown worker")
	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.
	#
	# (Technically this introduces some bias and is not entirely uniform,
	# but since the hash is so large the bias is ridiculously small).
	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]

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