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Mastodon
Mastodon is a federated Twitter alternative. It is the most popular client using the ActivityPub federation protocol. It started out using OStatus, and switched to ActivityPub.
Each server is called an "instance". The entire constellation of instances that can interoperate is called the “Fediverse”.
Identity
Users create an account on an instance, but can communicate with users on other instances. A full username is a user’s handle plus the name of the instance the user belongs to, for example:
@alice@mastodon.social
Usernames are unique to each instance, not to Mastodon as a whole, so @alice@mastodon.social can coexist with @alice@modern.town.
If a user moves to a new instance, they can redirect or migrate their old account. Redirection sets up a redirect notice on the old profile which tells users to follow the new account. Migration forces all followers to unfollow the old account and follow the new, if the software on their instance supports this functionality. Previous posts will not be moved.
Account credentials are managed by the user’s instance, so if users forget their password, they can ask for a password reset. Whether users can delete their own accounts or not is a setting dependent on the instance admin.
For user verification, there is no central authority to check identity documents, but link-based verification can help cross-reference links associated with a user. For example, a user can link to their Mastodon profile from their personal homepage, and receive a verification checkmark to confirm that they are the owner. In addition, an identity proof framework was added in 2019, which currently only supports Keybase. It allows users to link their Keybase cryptographic identity to their Mastodon account.
Networking/Message passing
Mastodon uses Webfinger to translate user mentions to actor profile URIs. Webfinger specifies the path at which to find a user's profile information provided by the server.
ActivityPub federation defines a common set of APIs and formats for passing messages between servers. ActivityPub messages consist of the message itself, and a wrapper that communicates what's happening with the message, defining the "activity". The activities Mastodon implements are documented here.
Data Storage/Message Persistence
Mastodon is a Ruby on Rails application that uses PostgreSQL and Redis for data storage.
Moderation/Reputation
Moderation takes place at the server level in Mastodon. Each instance sets its own moderation policies, either through the unilateral decisions of an admin, or through collective discussion and agreement. Admins can ban entire instances, cutting off their visibility. If an instance gets banned by many others, its users can still talk with each other, but they will be isolated from the rest of the Fediverse.
Users can also report posts to moderators, submitting it for a moderation decision. Users can also apply content warnings to their posts themselves, to indicate the nature of the content and hide it behind a click.
Some documented challenges with moderation in Mastodon from 2018. One recommended solution is to provide open APIs for third-party tools to help with moderation, which was added in 2019.
Social/Discovery
There is no unified global search in Mastodon, as each server monitors a different set of messages. Searching for the same keyword on different instances yields different results. The federated timeline shows public posts that the user's server knows about. Essentially, users have access to posts of people followed by people on their instance.
Mastodon users are presented with three timelines: a home timeline with posts from accounts the user follows, a local timeline with posts from the local instance, and a federated timeline with all posts that have been retrieved from remote instances.
A 2019 analysis of the Mastodon ecosystem found that the majority of posts are concentrated on a few instances, and outages in 10 instances would remove almost half of all posts from the network.
Mastodon's interface has a "Profile Directory" tab, where users can browse recently active profiles or new arrivals from both their own instance or the fediverse to discover who to follow. Users can select featured hashtags to be displayed on their public profile so people can browse their posts by hashtag.
Privacy and Access Control
Posts can be public, unlisted, private, or direct. Public posts are shown on local, federated, and hashtag timelines. Unlisted posts are not, but can be accessed through the link. Private posts are only visible to followers. Direct posts can only be seen by the people mentioned in them. Users can also hide their network, so the lists of followers and people they're following are not visible. Servers only receive posts that are public or addressed to a person on that server.
Mastodon can be served through Tor as an onion service.
Monetization
Federated social networks require both hosting and development costs to maintain. Each instance is funded by its own administrator and community. Mastodon’s development is funded through a Patreon run by the main developer, Eugen Rochko. It currently brings in about 70k a year, which supports him working on it full time, and covers hosting costs and a moderation team for the popular mastodon.social instance.
User experience
Mastodon's main initial selling point was its familiar interface that behaved like Tweetdeck. Many other federated social applications that work essentially the same way had tried and failed to get mainstream adoption prior to Mastodon, largely due to unfamiliar user interfaces.
Notable design choices in Mastodon that differ from Twitter: Instead of a 280 character limit, there is a 500 character limit. "Likes" are not broadcast to third-parties. "Retweet" and "Like" numbers are not shown until a post is clicked on. If a user with an unlocked account gets a follow request from an account that has been silenced by the server's moderators (either manually, or at their domain), the user will get a follow request instead of automatically allowing the new account to follow.
User-level content controls include: Users can set content warnings on their own posts. Filtering on posts can automatically hide keywords and phrases. "Boosts" (like retweets) from someone can be hidden. Accounts can be muted or blocked. Entire servers, including all of its posts, can be blocked by a user. Following, muting, blocking, and domain-blocking lists can be imported.
Upload options include: Users can choose where the thumbnail of a picture is focused when opening. Users can enter custom alt-text to image uploads. There is an OCR button to help extract text from an image for the visually impaired. Audio files can be uploaded.
Admins can upload custom emojis to their servers
Interoperability
Mastodon is compatible with all federated applications that use ActivityPub. These include Pleroma, another social application, PixelFed, a photo-sharing application, and PeerTube, a video-sharing application. Mastodon users can follow a PixelFed or PeerTube user from Mastodon.
Mastodon users used to be able to find their Twitter friends using bridge.joinmastodon.org, but the service was shut down after the developer lost access to API keys and was not granted another set. A user request for a global directory for importing friends from other networks.
All Mastodon user data is available for export.
Scalability
Mastodon.Social, the instance started by Mastodon's main developer, initially became the server of choice for new users. The server architecture was scaled up several times, until new registrations were closed in 2017. He has now started mastodon.online to support more registrations. Anyone can run a Mastodon instance, but in practice, the majority of users have concentrated within a few.
When a surge of new users join an instance, server admins can run into scaling issues, as any web host who becomes unexpectedly popular does.
Another scalability issue is the resource requirements of Mastodon. Mastodon hosting providers have emerged as a service to help individuals interested in being admins but without sysadmin experience to spin up servers. Pleroma, another federated social app, is advertised as a more minimal implementation good for single user instances that requires fewer resources to run.
Metrics
Mastodon has around 2.9 million accounts and 2785 nodes, as of June 2020, according to fediverse.network. From Mastodon, 3.8 million accounts can be reached within the Fediverse.