A look at Bluesky's claims to being decentralised, written by Christine Lemmer-Webber, who helped create ActivityPub (the protocol that lies underneath Mastodon, Lemmy, Pixelfed, PeerTube and others).
The best way to understand the reason for this difference in hosting requirements is to understand the underlying architecture of these systems. ActivityPub follows an message passing architecture (utilizing publish-subscribe architecture prominently for most "subscription" oriented uses), the same as email, XMPP, and so on. A message is addressed, and then delivered to recipients. (Actually a more fully peer-to-peer system would deliver more directly; all of email, XMPP, ActivityPub and so on use a client-server architecture, so there is a particular server which tends to operate on behalf of a particular user. See comments on the fediverse later in this article for how things can be moved more peer-to-peer.) This turns out to be pretty efficient; if only users on five servers need to know about a message, out of tens of thousands of servers, only those five servers will be contacted. Until recently, every system I knew of described as federated used a message passing architecture, to the degree where I and others assumed that federation implied a message passing architecture, because achieving the architectural goal of many independent nodes cooperating to produce a unified whole seemed to imply this was necessary for efficiency of a substantially sized network. If Alyssa wants to write a piece of mail to Ben, she can send it directly to Ben, and it can arrive at Ben's house. If Ben wants to reply, Ben can reply directly to Alyssa. Your intuitions about email apply exactly here, because that's effectively what this design is.
Bluesky does not utilize message passing, and instead operates in what I call a shared heap architecture. In a shared heap architecture, instead of delivering mail to someone's house (or, in a client-to-server architecture as most non p2p mailing lists are, at least their apartment's mail room), letters which may be interesting all are dumped at a post office (called a "relay") directly. From there it's the responsibility of interested parties to show up and filter through the mail to see what's interesting to them. This means there is no directed delivery; if you want to see replies which are relevant to your messages, you (or someone operating on behalf of you) had better sort through and know about every possible message to find out what messages could be a reply.
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The answer is: Bluesky solves this problem via centralization. Since there is really just one very large relay which everyone is expected to participate in, this relay has a god's-eye knowledge base. Entities which sort through mail and relevant replies for users are AppViews, which pull from the relay and also have a god's-eye knowledge base, and also do filtering. So too do any other number of services which participate in the network: they must operate at the level of gods rather than mortals.
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I'm not sure this behavior is consistent after all with how blocking works on X-Twitter; it was not my understanding that blocking someone would be public information. But blocks are indeed public information on Bluesky, and anyone can query who is blocking or being blocked by anyone. It is true that looking at a blocking account from a blocked account on most social media systems or observing the results of interactions can reveal information about who is blocked, but this is not the same as this being openly queryable information. There is a big difference between "you can look at someone's post and see who is being blocked" to "you can query the network for every person who is blocking or is blocked by JK Rowling".
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The reason for this is very simple: we have seen people who utilize blocklists be retaliated against for blocking someone who is angry about being blocked. It was our opinion that sharing such information could result in harassment. (Last I checked, Mastodon provides the user with the choice of whether or not to send a "report" about a block to the offending instance so that moderators of that server can notice a problematic user and take action, but delivering such information is not required.)
That said, to Bluesky's credit, this is an issue that is being openly considered. There is an open issue to consider whether or not private blocks are possible. Which does lead to a point, despite my many critiques here: it is true that even many of the things I have talked about could be changed and evaluated in the future. But nonetheless, in many ways I consider the decision to have blocks be publicly queryable to be an example of emergent behavior from initial decisions... early architectural decisions can have long-standing architectural results, and while many things can be changed, some things are particularly difficult to change form an initial starting point.
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I've analyzed previously in the document the challenges Bluesky has in achieving meaningful decentralization or federation. Bluesky now has much bigger pressures than decentralization, namely to satisfy the massive scale of users who wish to flock to the platform now, to satisfy investors which will increasingly be interested in whether or not they can see a return, and to achieve enough income to keep their staff and servers going. Rearchitecting towards meaningful decentralization will be a big pivot and will likely introduce many of the problems that Bluesky has touted their platform as not having that other decentralized platforms have.
There are early signs that Bluesky the company is already considering or exploring features that only make sense in a centralized context. Direct messages were discussed previously in this document, but with the announcement of premium accounts, it will be interesting to see what happens. Premium accounts would be possible to handle in a fully decentralized system: higher quality video uploads makes sense. What becomes more uncertain is what happens when a self-hosted PDS user uploads their own higher quality videos, will those be mirrored onto Bluesky's CDN in higher quality as well? Likewise, ads seem likely to be coming to Bluesky
A common way to make premium accounts more valuable is to make them ad-free. But if Bluesky is sufficiently decentralized and its filtering and labeling tools work as described, it will be trivial for users to set up filters which remove ads from the stream. Traditionally when investors realize users are doing this and removing a revenue stream, that is the point at which they start pressuring hard on enshittification and removing things like public access to APIs, etc. What will happen in Bluesky's case?
Here is where "credible exit" really is the right term for Bluesky's architectural goals. Rearchitecting towards meaningful decentralization and federation is a massive overhaul of Bluesky's infrastructure, but providing "credible exit" is not. It is my opinion that leaning into "credible exit" is the best thing that Bluesky can do: perhaps a large corporation or two always have to sit at the center of Bluesky, but perhaps also it will be possible for people to leave.