explain load balancing for federation_sender_instances (#17776)

Adding information on how the load is distributed for
`federation_sender_instances`.

Thx to @devonh for the information.

causal source:
c2e5e9e67c/synapse/config/_base.py (L946-L989)

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---------

Co-authored-by: Devon Hudson <devon.dmytro@gmail.com>
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Dirk Klimpel 2024-10-04 00:01:33 +02:00 committed by GitHub
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Explain how load balancing works for `federation_sender_instances`.

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@ -4368,7 +4368,13 @@ It is possible to scale the processes that handle sending outbound federation re
by running a [`generic_worker`](../../workers.md#synapseappgeneric_worker) and adding it's [`worker_name`](#worker_name) to
a `federation_sender_instances` map. Doing so will remove handling of this function from
the main process. Multiple workers can be added to this map, in which case the work is
balanced across them.
balanced across them.
The way that the load balancing works is any outbound federation request will be assigned
to a federation sender worker based on the hash of the destination server name. This
means that all requests being sent to the same destination will be processed by the same
worker instance. Multiple `federation_sender_instances` are useful if there is a federation
with multiple servers.
This configuration setting must be shared between all workers handling federation
sending, and if changed all federation sender workers must be stopped at the same time