Basically, on ATProto, everyone stores their own data on a PDS (Personal Data Server). This includes every post, image, video, follow, like, block, etc. The relay crawls the open web for PDSs to consolidate into a single stream of data.

From here, services can build off of that single data stream without needing to do any crawling of their own.

Of course, this does give the operator of the relay a lot of power. If they were to block your PDS, then only services relying on a different relay can access your data. This is especially why it is important that there exist independent third-party relays, but no one has taken up the mantle yet.

ATProto is not nearly as mature as ActivityPub. Businesses are only just now launching ATProto services (Graze being among the first notable ones).

There is a blog post from a dev about running a relay.

Fun fact: Bluesky was originally started at Twitter.

But can ActivityPub scale?

There are many types of Bluesky servers. This post is about ATProto Relays, which consolidate all the data across the network into a single location. They are necessary for efficiency, but they are extremely expensive to run. Currently, there is only one ATProto relay, but there is an initiative to launch a third-party relay.

I want to make you a mod. Can you leave a comment?