Activities

Activities are the verbs of ActivityPub — they represent actions that actors perform. When a user posts, follows someone, or likes content, those actions are represented as Activity objects.

Activity Structure

All activities share a common structure:

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "type": "Create",
  "id": "https://example.com/activities/1",
  "actor": "https://example.com/users/alice",
  "object": "https://example.com/notes/1",
  "to": ["https://www.w3.org/ns/activitystreams#Public"],
  "cc": ["https://example.com/users/alice/followers"],
  "published": "2024-01-15T10:30:00Z"
}

Core Properties

Property	Required	Description
`type`	Yes	Activity type
`id`	Yes	Unique identifier
`actor`	Yes	Who performed the activity
`object`	Usually	Target of the activity
`to`	No	Primary recipients
`cc`	No	Secondary recipients
`published`	No	When performed

Content Activities

Create

Creates new content:

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "type": "Create",
  "id": "https://example.com/activities/create-1",
  "actor": "https://example.com/users/alice",
  "object": {
    "type": "Note",
    "id": "https://example.com/notes/1",
    "attributedTo": "https://example.com/users/alice",
    "content": "Hello, world!",
    "to": ["https://www.w3.org/ns/activitystreams#Public"]
  },
  "to": ["https://www.w3.org/ns/activitystreams#Public"],
  "cc": ["https://example.com/users/alice/followers"]
}

note

When receiving a Create, verify that activity.actor matches object.attributedTo.

Update

Modifies existing content:

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "type": "Update",
  "id": "https://example.com/activities/update-1",
  "actor": "https://example.com/users/alice",
  "object": {
    "type": "Note",
    "id": "https://example.com/notes/1",
    "content": "Hello, world! (edited)",
    "updated": "2024-01-15T11:00:00Z"
  }
}

Only the original author (or server) should be able to Update objects.

Delete

Removes content:

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "type": "Delete",
  "id": "https://example.com/activities/delete-1",
  "actor": "https://example.com/users/alice",
  "object": "https://example.com/notes/1",
  "to": ["https://www.w3.org/ns/activitystreams#Public"]
}

When receiving Delete:

Verify the actor owns the object
Remove or mark as deleted
Return 410 Gone for future fetches

Follow

Request to follow an actor:

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "type": "Follow",
  "id": "https://example.com/activities/follow-1",
  "actor": "https://example.com/users/alice",
  "object": "https://other.example/users/bob"
}

The receiving server should respond with Accept or Reject.

Accept

Accepts a Follow (or other request):

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "type": "Accept",
  "id": "https://other.example/activities/accept-1",
  "actor": "https://other.example/users/bob",
  "object": {
    "type": "Follow",
    "id": "https://example.com/activities/follow-1",
    "actor": "https://example.com/users/alice",
    "object": "https://other.example/users/bob"
  }
}

Reject

Rejects a Follow request:

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "type": "Reject",
  "id": "https://other.example/activities/reject-1",
  "actor": "https://other.example/users/bob",
  "object": {
    "type": "Follow",
    "id": "https://example.com/activities/follow-1",
    "actor": "https://example.com/users/alice",
    "object": "https://other.example/users/bob"
  }
}

Reaction Activities

Like

Express appreciation:

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "type": "Like",
  "id": "https://example.com/activities/like-1",
  "actor": "https://example.com/users/alice",
  "object": "https://other.example/notes/1"
}

Announce (Boost/Reblog)

Share content with your followers:

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "type": "Announce",
  "id": "https://example.com/activities/announce-1",
  "actor": "https://example.com/users/alice",
  "object": "https://other.example/notes/1",
  "to": ["https://www.w3.org/ns/activitystreams#Public"],
  "cc": [
    "https://other.example/users/bob",
    "https://example.com/users/alice/followers"
  ]
}

Negative Activities

Block

Block an actor:

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "type": "Block",
  "id": "https://example.com/activities/block-1",
  "actor": "https://example.com/users/alice",
  "object": "https://other.example/users/spammer"
}

note

Blocks are typically not federated — they stay on your server. Only send Block if you want the blocked actor's server to know.

Flag (Report)

Report content or actors:

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "type": "Flag",
  "id": "https://example.com/activities/flag-1",
  "actor": "https://example.com/users/alice",
  "object": [
    "https://other.example/users/abuser",
    "https://other.example/notes/offensive"
  ],
  "content": "This user is posting abusive content"
}

Send to the remote server's inbox (actor or shared inbox).

Undo

Reverses a previous activity:

Undo Follow (Unfollow)

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "type": "Undo",
  "id": "https://example.com/activities/undo-1",
  "actor": "https://example.com/users/alice",
  "object": {
    "type": "Follow",
    "id": "https://example.com/activities/follow-1",
    "actor": "https://example.com/users/alice",
    "object": "https://other.example/users/bob"
  }
}

Undo Like

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "type": "Undo",
  "id": "https://example.com/activities/undo-2",
  "actor": "https://example.com/users/alice",
  "object": {
    "type": "Like",
    "id": "https://example.com/activities/like-1",
    "actor": "https://example.com/users/alice",
    "object": "https://other.example/notes/1"
  }
}

Undo Announce

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "type": "Undo",
  "id": "https://example.com/activities/undo-3",
  "actor": "https://example.com/users/alice",
  "object": {
    "type": "Announce",
    "id": "https://example.com/activities/announce-1"
  }
}

Collection Activities

Add

Add an object to a collection:

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "type": "Add",
  "id": "https://example.com/activities/add-1",
  "actor": "https://example.com/users/alice",
  "object": "https://example.com/notes/1",
  "target": "https://example.com/users/alice/collections/featured"
}

Used for pinning posts to profile.

Remove

Remove an object from a collection:

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "type": "Remove",
  "id": "https://example.com/activities/remove-1",
  "actor": "https://example.com/users/alice",
  "object": "https://example.com/notes/1",
  "target": "https://example.com/users/alice/collections/featured"
}

Move

For account migration:

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "type": "Move",
  "id": "https://old.example/activities/move-1",
  "actor": "https://old.example/users/alice",
  "object": "https://old.example/users/alice",
  "target": "https://new.example/users/alice"
}

Activity Processing

Handling Incoming Activities

async function handleActivity(activity) {
  // Verify signature
  await verifySignature(request);

  // Verify actor
  await verifyActor(activity);

  switch (activity.type) {
    case 'Create':
      return handleCreate(activity);
    case 'Update':
      return handleUpdate(activity);
    case 'Delete':
      return handleDelete(activity);
    case 'Follow':
      return handleFollow(activity);
    case 'Accept':
      return handleAccept(activity);
    case 'Reject':
      return handleReject(activity);
    case 'Like':
      return handleLike(activity);
    case 'Announce':
      return handleAnnounce(activity);
    case 'Undo':
      return handleUndo(activity);
    default:
      console.log('Unknown activity type:', activity.type);
      return { status: 202 };
  }
}

Side Effects

Activities trigger side effects:

Activity	Side Effects
Create	Store object, notify mentioned users
Update	Update stored object, notify
Delete	Remove object, cascade to local copies
Follow	Add pending follow or auto-accept
Accept	Add to following, start delivery
Like	Increment counter, notify author
Announce	Add boost, notify author
Undo	Reverse the original activity's effects

Idempotency

Activities should be idempotent — processing the same activity twice should have the same result as processing it once.

async function handleLike(activity) {
  const existing = await db.likes.findOne({
    activityId: activity.id
  });

  if (existing) {
    // Already processed, ignore
    return { status: 200 };
  }

  await db.likes.insert({
    activityId: activity.id,
    actor: activity.actor,
    object: activity.object,
    createdAt: new Date()
  });

  return { status: 201 };
}

Next Steps

Collections - Organizing activities
Delivery - Sending activities
Handling Activities - Implementation

Activity Structure​

Core Properties​

Content Activities​

Create​

Update​

Delete​

Social Activities​

Follow​

Accept​

Reject​

Reaction Activities​

Like​

Announce (Boost/Reblog)​

Negative Activities​

Block​

Flag (Report)​

Undo​

Undo Follow (Unfollow)​

Undo Like​

Undo Announce​

Collection Activities​

Add​

Remove​

Move​

Activity Processing​

Handling Incoming Activities​

Side Effects​

Idempotency​

Next Steps​