What is ActivityPub?

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ActivityPub is an open, decentralized (federated) social networking protocol—the main standard that powers the Fediverse, including apps like Mastodon, Pixelfed, PeerTube, Lemmy, and many others. It’s a W3C Recommendation (standard) published in January 2018. w3.org+2GitHub+2

In simple terms: ActivityPub is the “language” different social servers use to talk to each other so people on different platforms can follow, reply, like, and share across servers.

✅ What ActivityPub enables

ActivityPub lets social networks work like email:

  • Your account lives on one server (like gmail.com)
  • Someone on another server (like outlook.com) can still follow and message you
  • Servers exchange updates (posts, likes, follows) automatically

This is called federation. w3.org+1

🧩 Key building blocks (the “inbox/outbox” model)

ActivityPub is built around an Actor (a user or bot) that sends and receives Activities (actions like “Post”, “Like”, “Follow”). Each Actor has:

Outbox

Where your activities go when you post/like/follow.

Inbox

Where activities from others arrive (replies, likes, follows, boosts, etc.).

Servers deliver activities by sending HTTP requests (usually POST) to inbox URLs. w3.org+1

🛠️ Two parts of the ActivityPub standard

ActivityPub defines two related APIs:

1) Client-to-Server (C2S)

How your app (client) posts content to your home server.

2) Server-to-Server (S2S)

How servers federate with each other to deliver posts, follows, and notifications. w3.org+1

🧠 How a post travels (example)

When you post on Mastodon:

  1. Your client sends a Create activity to your server.
  2. Your server stores it, then delivers it to followers on other servers.
  3. Those servers put it in their users’ timelines.

This is defined in the spec and implemented (with extensions) by Mastodon. w3.org+1

🌍 Where ActivityPub is used

ActivityPub is the core protocol behind the Fediverse, including:

  • Mastodon (microblogging)
  • Pixelfed (photos)
  • PeerTube (video)
  • Lemmy / Kbin (community forums)

Even newer apps (like Meta’s Threads federation efforts) are based around ActivityPub compatibility. Wikipedia+1

🔍 ActivityPub vs AT Protocol (quick contrast)

Since you asked about AT Protocol earlier, here’s a helpful comparison:

  • ActivityPub: many servers federate with each other; identity is usually tied to a server, and migrations exist but aren’t “native” in the spec. Wikipedia
  • AT Protocol: designed from the start for stronger account portability and algorithm marketplace concepts.