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 and
IntroductionJanuary 23, 2018; 7 years ago (2018-01-23)
Based onActivityStreams, JSON-LD
InfluencedAT Protocol[1]
Websiteactivitypub.rocks Edit this at Wikidata

ActivityPub is a protocol and open standard for decentralized social networking. It provides a client-to-server (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 is the defining standard of the Fediverse, a decentralised social network of various social interaction models, and content types, which consists of independently managed instances of software such as Mastodon, Pixelfed and PeerTube, among others.[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 account for the newly received activity and update 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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The SocialCG previously organized a yearly free conference called ActivityPub Conf about the future of ActivityPub.[8][9] Triages are held regularly to review issues pertaining to the ActivityPub and ActivityStreams 2.0 specifications as part of the SocialCG.[10]

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.[11]

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.[12] 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.[13] Various major social media platforms and corporations have since pledged to implement ActivityPub support, including Tumblr,[14] Flipboard[15] and Meta Platforms' Threads.[16] Threads introduced crossposting to ActivityPub in 2024 for users outside of the European Economic Area,[17] however full 2-way compatibility remains incomplete as of 2025.[18]

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 (DDOS) 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.[19][20]

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.[21] 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.[22]

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.[23][24] However, this isn't a problem with the ActivityPub protocol itself, but with implementations not refreshing their content for updated data when needed.[25][26][citation needed]

Software using ActivityPub

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Software name Total users[27][28] Initial ActivityPub-compatible release Type of software Fork of
Akkoma 9,530 2022[29] Blogging Pleroma
BookWyrm[30] 49,600 2021[31] Book cataloging
Castopod 818 podcasts[32] 2020[33] Audio hosting
Discourse ? 2025[34] Internet forum
dokieli ? 2018[35] Authoring system, Blogging, Notification system, Semantic publishing, Web annotation
Epicyon 3 2019[36] Blogging
Firefish 4,560 2022[37] Blogging Misskey
Flipboard[38] 145,000,000[39] 2023[40][41] Social news
Friendica[42] 19,800 2019 Blogging, event management, groups, image gallery
Funkwhale 8,850 2018[43] Audio hosting
Gancio[44] 1,900 2020[45] Calendar, event planner
Ghost[46][47] ? 2025 (in beta)[48] Blogging
GNU social 93 2018[49] Blogging
Guppe[50] not relevant 2021[51] Groups
GoToSocial 38 2021[52] Blogging
Hollo 64 2024[53] Blogging
Honk 6 2019[54] Blogging
Hubzilla[55] 6,740 2017 Blogging, event planner, file hosting, image gallery, wiki
Iceshrimp[56] ? 2023 Blogging Firefish
Iceshrimp.NET[57] ? 2024 Blogging
Inventaire.io[58] ? 2021 Book cataloging
kbin[59] 507 2023 Social news
Lemmy[60] 485,000 2019 Social news
Libervia[61] ? 2022 (in beta) Blogging, event management, file sharing, instant messaging
lotide[62] 462 2020[63] Social news
Mastodon 7,960,000 2017[64] Blogging
Mobilizon 76,900 2020 Event management, groups
mbin[65] 6,950 2023 Social news kbin
Micro.blog 198,000 2021[66] Microblogging
microblog.pub 39 2022[67] Blogging
Misskey 1,052,511 2018[68] Blogging
Nextcloud Social 50 (approx.) 2018[69] Blogging
NodeBB 2,750,000 2025[70] Internet forum
Owncast[71] 191 2022 Live streaming
PeerTube[72] 473,000 2018 Video sharing
PieFed 1,116[73] 2023[74] Social News
Pixelfed[75] 857,000 2018 Image sharing
Pleroma 52,000 2018[76] Blogging
Plume[77] 24,400 2018[78] Blogging
Postmarks[79] 25 2023[80] Social bookmarking
Sharkey[81] 23,100 2023 Blogging Misskey
Snac[82] 3,070 2022[83] Blogging
Socialhome 3,000 2016[84] Blogging
Streams[85] ? 2022[86] Blogging, image sharing, wiki
Takahē 341 2022[87] Blogging
Threads 130,000,000[88] 2023[89] Blogging
Wafrn[90] 2,480 2023 Blogging
WordPress[91][92] 29,100 in 6,000+ blogs[93] 2023[94] Blogging
WriteFreely 73,200 2018[95] Blogging
Zap[96] 1 2019[97] Blogging, file hosting, image gallery

Future implementations

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Uncertain future implementations

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  • GitLab, a Git forge and development platform which had previously had an open issue discussing the topic, but was later closed due to the development team moving focus to other areas.[99]
  • Tumblr, a microblogging platform. Despite previous statements from Automattic CEO Matt Mullenweg,[100][101][102] ActivityPub integration has been delayed indefinitely. The integration would've been implemented with its WordPress migration, as the first-party plugin for interoperability would've been used for federation.[103]

See also

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References

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