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ActivityPub

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ActivityPub
Communication protocol
The image depicts a infographic of the basic functionality of ActivityPub. It shows a person (known as an Actor in ActivityPub terminology) reading incoming messages ("activities") from an inbox, which receives messages from other Actors (depicted as a cloud labeled "REST OF THE WORLD") via federation. The Actor also sends messages to their outbox, which the rest of the world receives via federation.
An infographic of the core functionality of ActivityPub
AbbreviationAP
PurposeDecentralized social networking
Developer(s)World Wide Web Consortium, Christine Lemmer-Webber, Evan Prodromou, et al.
IntroductionJanuary 23, 2018; 6 years ago (2018-01-23)
Based onActivityStreams, JSON-LD
InfluencedAT Protocol[1]
Websiteactivitypub.rocks

ActivityPub is a protocol and open standard for decentralized social networking. It provides a client-to-server (shortened to C2S) API for creating and modifying content, as well as a federated server-to-server (S2S) protocol for delivering notifications and content to other servers.[2] ActivityPub has become the main standard used in the fediverse, a popular network used for social networking that consists of software such as Mastodon, Pixelfed and PeerTube.[3]

ActivityPub is considered to be an update to the ActivityPump protocol used in pump.io, and the official W3C repository for ActivityPub is identified as a fork of ActivityPump.[4][5] The creation of a new standard for decentralized social networking was prompted by the complexity of OStatus, the most commonly used protocol at the time. OStatus was built using a multitude of technologies (such as Atom, Salmon, WebSub and WebFinger), a product of the infrastructure used in GNU social (the originator and largest user of the OStatus protocol), which made it difficult to implement the protocol into new software. OStatus was also only designed to work with microblogging services, with little flexibility to the types of data that it could hold.

The standard was first published by the World Wide Web Consortium (W3C) as a W3C Recommendation in January 2018 by the Social Web Working Group (SocialWG), a working group chartered to build the protocols and vocabularies needed to create a standard for social functionality.[6] Shortly after, further development was moved to the Social Web Community Group (SocialCG), the successor to the SocialWG.

Design

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ActivityPub uses the ActivityStreams 2.0 format for building its content, which itself uses JSON-LD. The three main data types used in ActivityPub are Objects, Activities and Actors. Objects are the most common data type, and can be images, videos, or more abstract items such as locations or events. Activities are actions that create and modify objects, for example a Create activity creates an object. Actors are representative of an individual, a group, an application or a service, and are the owners of objects.

Every actor type contains an inbox and outbox stream, which sends and receives activities for a user. In order to publish data (for example liking an article), a user creates an activity that declares that they liked an Article object and publishes it to their outbox, where it is then delivered by the ActivityPub server via a POST request to the inboxes listed in the activity's to, bto, cc and bcc fields. The receiving servers then accounts for the newly received activity and updates the article by adding the like action to it.

Example data

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An example actor object that represents a user account:[7] 

{
  "@context": ["https://www.w3.org/ns/activitystreams",
               {"@language": "ja"}],
  "type": "Person",
  "id": "https://kenzoishii.example.com/",
  "following": "https://kenzoishii.example.com/following.json",
  "followers": "https://kenzoishii.example.com/followers.json",
  "liked": "https://kenzoishii.example.com/liked.json",
  "inbox": "https://kenzoishii.example.com/inbox.json",
  "outbox": "https://kenzoishii.example.com/feed.json",
  "preferredUsername": "kenzoishii",
  "name": "石井健蔵",
  "summary": "この方はただの例です",
  "icon": [
    "https://kenzoishii.example.com/image/165987aklre4"
  ]
}

An example activity that likes an article object: 

{
  "@context": ["https://www.w3.org/ns/activitystreams",
               {"@language": "en"}],
  "type": "Like",
  "actor": "https://dustycloud.org/christine/",
  "summary": "Christine liked 'Minimal ActivityPub update client'",
  "object": "https://rhiaro.co.uk/2016/05/minimal-activitypub",
  "to": ["https://rhiaro.co.uk/#amy",
         "https://dustycloud.org/followers",
         "https://rhiaro.co.uk/followers/"],
  "cc": "https://e14n.com/evan"
}

An example article object: 

{
  "@context": ["https://www.w3.org/ns/activitystreams",
               {"@language": "en-GB"}],
  "id": "https://rhiaro.co.uk/2016/05/minimal-activitypub",
  "type": "Article",
  "name": "Minimal ActivityPub update client",
  "content": "Today I finished morph, a client for posting ActivityStreams2...",
  "attributedTo": "https://rhiaro.co.uk/#amy",
  "to": "https://rhiaro.co.uk/followers/",
  "cc": "https://e14n.com/evan"
}

Project status

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Lead author Christine Lemmer-Webber notes that the team predominantly identified as queer, which led to features that help users and administrators protect against "undesired interaction." She also notes that the team authoring ActivityPub had no corporate participation.[8]

The SocialCG previously organized a yearly free conference called ActivityPub Conf about the future of ActivityPub.[9][10] Triages are held regularly to review issues pertaining to the ActivityPub and ActivityStreams 2.0 specifications as part of the SocialCG.[11]

In 2023, Germany's Sovereign Tech Fund donated €152,000 to socialweb.coop with the goal of building a new suite for testing various ActivityPub implementations and their compliance with the specification.[12]

Adoption

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Further information: Fediverse § Adoption

The initial wave of adoption for ActivityPub (circa 2016-2018) came from software that was already using OStatus as their federation protocol, such as Mastodon, GNU social and Pleroma.[13] Following the acquisition of Twitter by Elon Musk in 2022, many groups of users that were critical of the acquisition migrated to Mastodon, bringing new attention to the ActivityPub protocol with it.[14] Various major social media platforms and corporations have since pledged to implement ActivityPub support, including Tumblr,[15] Flipboard[16] and Meta Platforms' Threads.[17]

Criticism

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Accidental denial-of-service attacks

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Poorly optimized ActivityPub implementations can cause unintentional distributed-denial-of-service attacks on other websites and servers, due to the decentralized nature of the network.[citation needed] An example would be Mastodon's implementation of OpenGraph link previews, wherein every instance that receives a post that contains a link with OpenGraph metadata will download the associated data, such as a thumbnail, in a very short timeframe, which can slow down or crash servers as a result of the sudden burst of requests.[18][19]

Account migration

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ActivityPub has been criticized for not natively supporting moving accounts from one server to another, forcing implementations to build their own solutions.[20] While there has been work on building a standardized system for migrating accounts using the Move activity via the Fediverse Enhancement Proposal organization, the current proposal only allows for basic follower migration, with all other data remaining linked to the original account.[21]

Missing content and data

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ActivityPub implementations have been criticized for missing replies and parts of reply threads from remote posts, and presenting outdated statistics (e.g. likes and reposts) about remote posts.[22][23] However, this isn't a problem with the ActivityPub protocol itself, but with implementations not refreshing their content for updated data when needed.[24][25][citation needed]

Username format

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The username format commonly used by ActivityPub software (acct URIs with the scheme replaced by an at sign, an example being @[email protected]) has been criticized for being too complex for most users to understand. However, ActivityPub itself supports any URI as a username, and it is a limitation of Mastodon and other ActivityPub implementations, not the protocol itself.[3][citation needed]

Software using ActivityPub

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Audio hosting

Software name Total users[26][27] Initial ActivityPub-compatible release
Castopod ? 2020[28]
Funkwhale 11,448 2018[29]

Blogging

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Software name Total users[26][27] Initial ActivityPub-compatible release Fork of
Akkoma 18,108 2022[30] Pleroma
Epicyon 3 2019[31]
Firefish (f. Calckey) 19,695 2022[32] Misskey
GNU social

(f. StatusNet; orig. Laconica)

368 2018[33]
GoToSocial 1,919 2021[34]
Honk 7 2019[35]
Iceshrimp 3,096 2023 Firefish
Mastodon 9,630,383 2017[36]
Micro.blog 168,418 2021[37]
microblog.pub 66 2022[38]
Misskey 849,930 2018
Nextcloud Social ~50 2018[39]
Pleroma 138,294 2018[40]
Plume[41] 25,290 2018[42]
Sharkey 11,061 2023 Misskey
Snac[43] 176 2022[44]
Socialhome 2,325 2016[45]
Takahē 278 2022[46]
Threads 130,000,000 (February 2024)[47] 2023[48]
Wafrn[49] 891 2023
WordPress[50][51] 6,000+ blogs[52] 2023[53]
WriteFreely 160,761 2018[54]

Book cataloging

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Software name Total users[26][27] Initial ActivityPub-compatible release
BookWyrm[55] 27,698 2021[56]
Inventaire.io[57] ? 2021

Social news

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Software name Total users[26][27] Initial ActivityPub-compatible release Fork of
Flipboard[58] 145,000,000 (February 2023)[59] 2023[60][61]
kbin[62] 66,320 2023
Lemmy[63] 392,074 2019
lotide[64] 457 2020[65]
mbin[66] 5,490 2023 kbin

Other/Multi-format

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Software name Total users[26][27] Initial ActivityPub-compatible release Type
Friendica
(f. Friendika; orig. Mistpark)[67] 		20,069 2019 Blogging, event management, groups, image gallery
Gancio[68] 1,273 2020[69] Calendar, event planner
Guppe[70] ? 2021[71] Groups
Hubzilla
(f. RedMatrix; orig. Friendica-Red)[72] 		5,748 2017 Blogging, event planner, file hosting, image gallery, wiki
Libervia[73] ? 2022 (in beta) Blogging, event management, file sharing, instant messaging
Matrix (via bridges)[74] - 2021 Instant messaging
Mobilizon 45,503 2020 Event management, groups
Owncast[75] 240 2022 Live streaming
PeerTube[76] 351,142 2018 Video sharing
Pixelfed[77] 18,733 2018 Image sharing
Postmarks[78] 29 2023[79] Social bookmarking
Streams[80] ? 2022[81] Blogging, image sharing, wiki
Zap[82] 22 2019[83] Blogging, file hosting, image gallery

Future implementations

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See also

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References

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