Actor dispatcher

In ActivityPub, actors are entities that can perform activities. You can register an actor dispatcher so that Fedify can dispatch an appropriate actor by its identifier. Since the actor dispatcher is the most significant part of the Fedify, it is the first thing you need to do to make Fedify work.

An actor dispatcher is a callback function that takes a Context object and an identifier, and returns an actor object. The actor object can be one of the following:

The below example shows how to register an actor dispatcher:

typescript

import { createFederation
, Person
 } from "@fedify/fedify";

const federation
 = createFederation
({
  // Omitted for brevity; see the related section for details.
});

federation
.setActorDispatcher
("/users/{identifier}", async (ctx
, identifier
) => {
  // Work with the database to find the actor by the identifier.
  if (user
 == null) return null;  // Return null if the actor is not found.
  return new Person
({
    id
: ctx
.getActorUri
(identifier
),
    preferredUsername
: identifier
,
    // Many more properties; see the next section for details.
  });
});

In the above example, the setActorDispatcher() method registers an actor dispatcher for the /users/{identifier} path. This pattern syntax follows the URI Template specification.

TIP

By registering the actor dispatcher, Federation.fetch() automatically deals with WebFinger requests for the actor.

TIP

By default, Fedify assumes that the actor's identifier is the WebFinger username. If you want to decouple the WebFinger username from the actor's identifier, you can register an actor handle mapper through the mapHandle() method.

See the next section for details.

Actor identifier and WebFinger username

An actor identifier is a unique string that identifies the actor. It can be a username, a UUID, or any other unique string. The actor identifier is used as a URL parameter in the actor dispatcher and other dispatchers. It's usually used to find the actor in your database, i.e., primary key.

A WebFinger username is a string that comes before the domain part of the fediverse handle, e.g., hongminhee in @hongminhee@fosstodon.org. The WebFinger username is also used as the preferredUsername property of the actor. It's usually displayed in the user interface, and used to find the actor by the WebFinger protocol, i.e., looking up the fediverse handle in the search box. It's also called the bare handle.

By default, Fedify assumes that the actor's identifier is the WebFinger username, but you can decouple the WebFinger username from the actor's identifier if you want. You can think of the difference between these two approaches as analogous to natural key vs. surrogate key in the database design.

There are pros and cons to using the WebFinger username as the actor's identifier (which is Fedify's default):

Pros

The actor URI is more predictable and human-readable, which makes debugging easier.
The internal ID of the actor can be hidden from the public.

Cons

Changing the WebFinger username may break the existing network. Hence, the fediverse handle is immutable in practice.
It's usually treated as an anti-pattern in the fediverse.

You need to choose the best approach for you before implementing the actor dispatcher. If you decided to use the WebFinger username as the actor's identifier, there's nothing to do—Fedify assumes it by default.

If you decided to decouple the WebFinger username from the actor's identifier, see the next section for details.

Key properties of an `Actor`

Despite ActivityPub declares every property of an actor as optional, in practice, you need to set some of them to make the actor work properly with the existing ActivityPub implementations. The following shows the key properties of an Actor object:

`id`

The id property is the URI of the actor. It is a required property in ActivityPub. You can use the Context.getActorUri() method to generate the dereferenceable URI of the actor by its identifier.

`preferredUsername`

The preferredUsername property is the WebFinger username of the actor. Unless you decouple the WebFinger username from the actor's identifier, it is okay to set the preferredUsername property to the actor's identifier.

`name`

The name property is the full name of the actor.

`summary`

The summary property is usually a short biography of the actor.

`url`

The url property usually refers to the actor's profile page.

`published`

The published property is the date and time when the actor was created. Note that Fedify represents the date and time in the Temporal.Instant value.

`inbox`

The inbox property is the URI of the actor's inbox. You can use the Context.getInboxUri() method to generate the URI of the actor's inbox.

See the Inbox listeners section for details.

`outbox`

The outbox property is the URI of the actor's outbox. You can use the Context.getOutboxUri() method to generate the URI of the actor's outbox.

`followers`

The followers property is the URI of the actor's followers collection. You can use the Context.getFollowersUri() method to generate the URI of the actor's followers collection.

`following`

The following property is the URI of the actor's following collection. You can use the Context.getFollowingUri() method to generate the URI of the actor's following collection.

`endpoints`

The endpoints property is an Endpoints instance, an object that contains the URIs of the actor's endpoints. The most important endpoint is the sharedInbox. You can use the Context.getInboxUri() method with no arguments to generate the URI of the actor's shared inbox:

typescript

new Endpoints
({ sharedInbox
: ctx
.getInboxUri
() })

`publicKey`

The publicKey property contains the public key of the actor. It is a CryptographicKey instance. This property is usually used for verifying HTTP Signatures.

See the next section for details.

TIP

In theory, an actor has multiple publicKeys, but in practice, the most implementations have trouble with multiple keys. Therefore, it is recommended to set only one key in the publicKey property. Usually, it contains the first RSA-PKCS#1-v1.5 public key of the actor.

If you need to set multiple keys, you can use the assertionMethods property instead.

`assertionMethods`

This API is available since Fedify 0.10.0.

The assertionMethods property contains the public keys of the actor. It is an array of Multikey instances. This property is usually used for verifying Object Integrity Proofs.

TIP

Usually, the assertionMethods property contains the Ed25519 public keys of the actor. Although it is okay to include RSA-PKCS#1-v1.5 public keys too, those RSA-PKCS#1-v1.5 keys are not used for verifying Object Integrity Proofs.

Public keys of an `Actor`

In order to sign and verify the activities, you need to set the publicKey and assertionMethods property of the actor. The publicKey property contains a CryptographicKey instance, and the assertionMethods property contains an array of Multikey instances. Usually you don't have to create them manually. Instead, you can register a key pairs dispatcher through the setKeyPairsDispatcher() method so that Fedify can dispatch appropriate key pairs by the actor's identifier:

typescript

federation
.setActorDispatcher
("/users/{identifier}", async (ctx
, identifier
) => {
  // Work with the database to find the actor by the identifier.
  if (user
 == null) return null;  // Return null if the actor is not found.
  // Context.getActorKeyPairs() method dispatches the key pairs of an actor
  // by the identifier, and returns an array of key pairs in various formats:
  const keys
 = await ctx
.getActorKeyPairs
(identifier
);
  return new Person
({
    id
: ctx
.getActorUri
(identifier
),
    preferredUsername
: identifier
,
    // For the publicKey property, we only use first CryptographicKey:
    publicKey
: keys
[0].cryptographicKey
,
    // For the assertionMethods property, we use all Multikey instances:
    assertionMethods
: keys
.map
((key
) => key
.multikey
),
    // Many more properties; see the previous section for details.
  });
})
  .setKeyPairsDispatcher
(async (ctx
, identifier
) => {
    // Work with the database to find the key pair by the identifier.
    if (user
 == null) return [];  // Return null if the key pair is not found.
    // Return the loaded key pair.  See the below example for details.
    return [
      { publicKey
: publicKey1
, privateKey
: privateKey1
 },
      { publicKey
: publicKey2
, privateKey
: privateKey2
 },
      // ...
    ];
  });

In the above example, the setKeyPairsDispatcher() method registers a key pairs dispatcher. The key pairs dispatcher is a callback function that takes context data and an identifier, and returns an array of CryptoKeyPair object which is defined in the Web Cryptography API.

Usually, you need to generate key pairs for each actor when the actor is created (i.e., when a new user is signed up), and securely store actor's key pairs in the database. The key pairs dispatcher should load the key pairs from the database and return them.

How to generate key pairs and store them in the database is out of the scope of this document, but here's a simple example of how to generate key pairs and store them in a Deno KV database in form of JWK:

typescript

import { generateCryptoKeyPair
, exportJwk
 } from "@fedify/fedify";

const kv
 = await Deno.openKv
();
const rsaPair
 = await generateCryptoKeyPair
("RSASSA-PKCS1-v1_5");
const ed25519Pair
 = await generateCryptoKeyPair
("Ed25519");
await kv
.set
(["keypair", "rsa", identifier
], {
  privateKey
: await exportJwk
(rsaPair
.privateKey
),
  publicKey
: await exportJwk
(rsaPair
.publicKey
),
});
await kv
.set
(["keypair", "ed25519", identifier
], {
  privateKey
: await exportJwk
(ed25519Pair
.privateKey
),
  publicKey
: await exportJwk
(ed25519Pair
.publicKey
),
});

TIP

Fedify currently supports two key types:

RSA-PKCS#1-v1.5 ("RSASSA-PKCS1-v1_5") is used for HTTP Signatures and Linked Data Signatures.
Ed25519 ("Ed25519") is used for Object Integrity Proofs.

HTTP Signatures and Linked Data Signatures are de facto standards for signing ActivityPub activities, and Object Integrity Proofs is a new standard for verifying the integrity of the objects in the fediverse. While HTTP Signatures and Linked Data Signatures are widely supported in the fediverse, it's limited to the RSA-PKCS#1-v1.5 algorithm.

If your federated app needs to support HTTP Signatures, Linked Data Signatures, and Object Integrity Proofs at the same time, you need to generate both RSA-PKCS#1-v1.5 and Ed25519 key pairs for each actor, and store them in the database—and we recommend you to support both key types.

Here's an example of how to load key pairs from the database too:

typescript

import { importJwk
 } from "@fedify/fedify";

interface KeyPairEntry {
  privateKey
: JsonWebKey;
  publicKey
: JsonWebKey;
}

federation

  .setActorDispatcher
("/users/{identifier}", async (ctx
, identifier
) => {
    // Omitted for brevity; see the previous example for details.
  })
  .setKeyPairsDispatcher
(async (ctx
, identifier
) => {
    const kv
 = await Deno.openKv
();
    const result
: CryptoKeyPair[] = [];
    const rsaPair
 = await kv
.get
<KeyPairEntry>(
      ["keypair", "rsa", identifier
],
    );
    if (rsaPair
?.value
 != null) {
      result
.push
({
        privateKey
: await importJwk
(rsaPair
.value
.privateKey
, "private"),
        publicKey
: await importJwk
(rsaPair
.value
.publicKey
, "public"),
      });
    }
    const ed25519Pair
 = await kv
.get
<KeyPairEntry>(
      ["keypair", "ed25519", identifier
],
    );
    if (ed25519Pair
?.value
 != null) {
      result
.push
({
        privateKey
: await importJwk
(ed25519Pair
.value
.privateKey
, "private"),
        publicKey
: await importJwk
(ed25519Pair
.value
.publicKey
, "public"),
      });
    }
    return result
;
  });

Constructing actor URIs

To construct an actor URI, you can use the Context.getActorUri() method. This method takes an identifier and returns a dereferenceable URI of the actor.

The below example shows how to construct an actor URI:

typescript

ctx
.getActorUri
("john_doe")

In the above example, the Context.getActorUri() method generates the dereferenceable URI of the actor with the identifier "john_doe".

If you decouple the WebFinger username from the actor's identifier, you should pass the identifier that is used in the actor dispatcher to the Context.getActorUri() method, not the WebFinger username:

typescript

ctx
.getActorUri
("2bd304f9-36b3-44f0-bf0b-29124aafcbb4")

NOTE

The Context.getActorUri() method does not guarantee that the actor URI is always dereferenceable for every argument. Make sure that the argument is a valid identifier before calling the method.

Decoupling actor URIs from WebFinger usernames

This API is available since Fedify 0.15.0.

TIP

The WebFinger username means the username part of the acct: URI or the fediverse handle. For example, the WebFinger username of the acct:fedify@hollo.social URI or the @fedify@hollo.social handle is fedify.

By default, Fedify uses the identifier as the WebFinger username. However, you can decouple the WebFinger username from the identifier by registering an actor handle mapper through the mapHandle() method:

typescript

federation

  .setActorDispatcher
("/users/{identifier}", async (ctx
, identifier
) => {
    // Since we map a WebFinger username to the corresponding user's UUID below,
    // the `identifier` parameter is the user's UUID, not the WebFinger
    // username:
    const user
 = await findUserByUuid
(identifier
);
    // Omitted for brevity; see the previous example for details.
  })
  .mapHandle
(async (ctx
, username
) => {
    // Work with the database to find the user's UUID by the WebFinger username.
    const user
 = await findUserByUsername
(username
);
    if (user
 == null) return null;  // Return null if the actor is not found.
    return user
.uuid
;
  });

Decoupling the WebFinger username from the identifier is useful when you want to let users change their WebFinger username without breaking the existing network, because changing the WebFinger username does not affect the actor URI.

NOTE

We highly recommend you to set the actor's preferredUsername property to the corresponding WebFinger username so that peers can find the actor's fediverse handle by fetching the actor object.

WebFinger links

Some properties of an Actor returned by the actor dispatcher affect responses to WebFinger requests.

`preferredUsername`

This API is available since Fedify 0.15.0.

The preferredUsername property is the bare handle of the actor. It is used as the WebFinger username, used in the acct: URI of the aliases property of the WebFinger response.

`url`

The url property usually refers to the actor's profile page. It is used as the links property of the WebFinger response, with the rel property set to http://webfinger.net/rel/profile-page.

`icon`

This API is available since Fedify 1.0.0.

The icon property is an Image object that represents the actor's icon (i.e., avatar). It is used as the links property of the WebFinger response, with the rel property set to http://webfinger.net/rel/avatar.

Actor dispatcher ​

Actor identifier and WebFinger username ​

Key properties of an Actor ​

id ​

preferredUsername ​

name ​

summary ​

url ​

published ​

inbox ​

outbox ​

followers ​

following ​

endpoints ​

publicKey ​

assertionMethods ​

Public keys of an Actor ​

Constructing actor URIs ​

Decoupling actor URIs from WebFinger usernames ​

WebFinger links ​

preferredUsername ​

url ​

icon ​

Actor dispatcher

Actor identifier and WebFinger username

Key properties of an `Actor`

`id`

`preferredUsername`

`name`

`summary`

`url`

`published`

`inbox`

`outbox`

`followers`

`following`

`endpoints`

`publicKey`

`assertionMethods`

Public keys of an `Actor`

Constructing actor URIs

Decoupling actor URIs from WebFinger usernames

WebFinger links

`preferredUsername`

`url`

`icon`