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Conversation Threads
Due to the nature of decentralization and federation, it is practically impossible for any one server on the fediverse to be aware of every post in a given conversation thread.
With that said, it is possible to do 'best effort' dereferencing of threads, whereby remote replies are fetched from one server onto another, to try to more fully flesh out a conversation.
GoToSocial does this by iterating up and down the thread of a conversation, pulling in remote statuses where possible.
Example
Let's say we have two accounts: local_account on our.server, and remote_1 on remote.1.
In this scenario, local_account follows remote_1, so posts from remote_1 show up in the home timeline of local_account.
Now, remote_1 boosts/reblogs a post from a third account, remote_2, residing on server remote.2.
local_account does not follow remote_2, and neither does anybody else on our.server, which means that our.server has not seen this post by remote_2 before.
What GoToSocial will do now, is 'dereference' the post by remote_2 to check if it is part of a thread and, if so, whether any other parts of the thread can be obtained.
GtS begins by checking the inReplyTo property of the post, which is set when a post is a reply to another post. See here. If inReplyTo is set, GoToSocial derefences the replied-to post. If this post also has an inReplyTo set, then GoToSocial dereferences that too, and so on.
Once all of these ancestors of a status have been retrieved, GtS will begin working down through the descendants of posts.
It does this by checking the replies property of a derefenced post, and working through replies, and replies of replies. See here.
This process of thread dereferencing will likely involve making multiple HTTP calls to different servers, especially if the thread is long and complicated.
The end result of this dereferencing is that, assuming the reblogged post by remote_2 was part of a thread, then local_account should now be able to see posts in the thread when they open the status on their home timeline. In other words, they will see replies from accounts on other servers (who they may not have come across yet), in addition to any previous and next posts in the thread as posted by remote_2.
This gives local_account a more complete view on the conversation, as opposed to just seeing the reblogged post in isolation and out of context. It also gives local_account the opportunity to discover new accounts to follow, based on replies to remote_2.
Privacy and Security
During the dereferencing process, GoToSocial signs outgoing requests using the key of the actor who received the activity that necessitated dereferencing. To use the above example, this means that all dereferencing requests would be signed by local_account. This gives remote servers the ability to refuse these dereferencing requests, assuming that local_account is blocked by one or more participants in the conversation.
From GoToSocial's side, domain blocks will be respected during the dereferencing process, to avoid making calls to servers that our.server has blocked.
Individual account blocks will also be respected, meaning that our.server won't try to dereference posts from accounts blocked by local_account.
Finally, GoToSocial expects that remote servers will only list replies that are marked as public (either to or cc). GtS may try to dereference followers-only posts, but it will assume that remote servers will check whether or not local_account is allowed to view them, and refuse accordingly.
Of course, when local_account opens up the conversation thread in whatever application they are using, GoToSocial will apply the usual post visibility filtering to ensure that they do not see any posts that they shouldn't have access to.