Lemmy Federation Architecture Change Proposal

HTTP_404_NotFound@lemmyonline.com to Technology@beehaw.org – 48 points – [Architecture] Hub-Spoke model for federation? · Issue #3245 · LemmyNet/lemmy
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https://github.com/LemmyNet/lemmy/issues/3245

I posted far more details on the issue then I am putting here-

But, just to bring some math in- with the current full-mesh federation model, assuming 10,000 instances-

That will require nearly 50 million connections.

Each comment. Each vote. Each post, will have to be sent 50 million seperate times.

In the purposed hub-spoke model, We can reduce that by over 99%, so that each post/vote/comment/etc, only has to be sent 10,000 times (plus n*(n-1)/2 times, where n = number of hub servers).

The current full mesh architecture will not scale. I predict, exponential growth will continue to occur.

Let's work on a solution to this problem together.

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The core of the problem is that if an instance doesn’t process an update (inbound or outbound), it doesn’t ever retry, the instances are just out of sync for that post forever.

With the pub/sub method- that should be able to be minimized.

At least, with my experience of messing with rabbitmq- A message stays in the queue, until I have told rabbitMQ, Hey, I have processed this message.

If I accept a message, an encounter an exception mid-way through, that message returns back to the queue, until It has been processed, or dead-letter logic handles it.

Granted, there is a hard-coded timeout somewhere in lemmy, where, older messages cannot be processed. That would need to be adjusted.

If you ensure that all messages are queued until processed, with retries on failure, what's the point of the hub model? As pointed out elsewhere, the large instances would be acting as hubs already.

Just removing that load from the main instance server, allowing it to just handle serving its local user-base.

In short- splitting the load into multiple components, rather than everything being handled by just the single instance server.

I'm just not seeing a benefit here, I think this is a solution to the wrong problem. Your proposal in theory cuts outbound updates from the big hubs, but in reality they're only updating a subset of other instances for any given update, and it doesn't do anything to help with inbound updates. And to do that, you have to solve a pretty tricky problem.

If my instance gets an update from Beehaw, I can validate that they're allowed to do so, because Beehaw has a TLS certificate that says "Yep, this is actually Beehaw." If you introduce a hub system, I need some way to determine that the hub system that's telling me "Beehaw has an update for you" is allowed to send updates on behalf of Beehaw.