Inbox listeners

In ActivityPub, an inbox is where an actor receives incoming activities from other actors. Fedify provides a way to register inbox listeners so that you can handle incoming activities from other actors.

Registering an inbox listener

An inbox is basically an HTTP endpoint that receives webhook requests from other servers. There are two types of inboxes in ActivityPub: the shared inbox and the personal inbox. The shared inbox is a single inbox that receives activities for all actors in the server, while the personal inbox is an inbox for a specific actor.

With Fedify, you can register an inbox listener for both types of inboxes at a time. The following shows how to register an inbox listener:

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

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

federation
  .setInboxListeners("/users/{handle}/inbox", "/inbox")
  .on(Follow, async (ctx, follow) => {
    const parsed = ctx.parseUri(follow.objectId);
    if (parsed?.type !== "actor") return;
    const recipient = await follow.getActor(ctx);
    await ctx.sendActivity(
      { handle: parsed.handle },
      recipient,
      new Accept({ actor: follow.objectId, object: follow }),
    );
  });

In the above example, the setInboxListeners() method registers path patterns for the personal inbox and the shared inbox, and the following on() method registers an inbox listener for the Follow activity. The on() method takes a class of the activity and a callback function that takes a Context object and the activity object.

Note that the on() method can be chained to register multiple inbox listeners for different activity types.

WARNING

Activities of any type that are not registered with the on() method are silently ignored. If you want to catch all types of activities anyway, add a listener for the Activity class.

TIP

You can get a personal or shared inbox URI by calling the getInboxUri() method. It takes an optional parameter handle to get the personal inbox URI for the actor with the bare handle. If the handle parameter is not provided, the method returns the shared inbox URI.

Context.documentLoader on an inbox listener

The Context.documentLoader property carries a DocumentLoader object that you can use to fetch a remote document. If a request is made to a shared inbox, the Context.documentLoader property is set to the default documentLoader that is specified in the createFederation() function. However, if a request is made to a personal inbox, the Context.documentLoader property is set to an authenticated DocumentLoader object that is identified by the inbox owner's key.

This means that you can pass the Context object to dereferencing accessors^[1] inside a personal inbox listener so that they can fetch remote documents with the correct authentication.

Shared inbox key dispatcher

This API is available since Fedify 0.11.0.

TIP

We highly recommend configuring the shared inbox key dispatcher to avoid potential incompatibility issues with ActivityPub servers that require authorized fetch (i.e., secure mode).

If you want to use an authenticated DocumentLoader object as the Context.documentLoader for a shared inbox, you can set the identity for the authentication using setSharedKeyDispatcher() method. For example, the following shows how to implement the instance actor pattern:

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

federation
  .setInboxListeners("/users/{handle}/inbox", "/inbox")
  // The following line assumes that there is an instance actor named `~actor`
  // for the server.  The leading tilde (`~`) is just for avoiding conflicts
  // with regular actor handles, but you don't have to necessarily follow this
  // convention:
  .setSharedKeyDispatcher((_ctx) => ({ handle: "~actor" }));

federation
  .setActorDispatcher("/users/{handle}", async (ctx, handle) => {
    if (handle === "~actor") {
      // Returns an Application object for the instance actor:
      return new Application({
        // ...
      });
    }

    // Fetches the regular actor from the database and returns a Person object:
    return new Person({
      // ...
    });
  });

Or you can manually configure the key pair instead of referring to an actor by its handle:

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

interface InstanceActor {
  privateKey: JsonWebKey;
  publicKeyUri: string;
}

federation
  .setInboxListeners("/users/{handle}/inbox", "/inbox")
  .setSharedKeyDispatcher(async (_ctx) => {
    // The following getInstanceActor() is just a hypothetical function that
    // fetches information about the instance actor from a database or some
    // other storage:
    const instanceActor: InstanceActor = await getInstanceActor();
    return {
      privateKey: await importJwk(instanceActor.privateKey, "private"),
      keyId: new URL(instanceActor.publicKeyUri),
    };
  });

NOTE

If a shared inbox key dispatcher returns null, the default documentLoader, which is not authenticated, is used for the shared inbox.

Making inbox listeners non-blocking

This API is available since Fedify 0.12.0.

Usually, processes inside an inbox listener should be non-blocking because they may involve long-running tasks. Fortunately, you can easily turn inbox listeners into non-blocking by providing a queue option to createFederation() function:

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

const federation = createFederation({
  // Omitted for brevity; see the related section for details.
  queue: new InProcessMessageQueue(),  
});

NOTE

The InProcessMessageQueue is a simple in-memory message queue that is suitable for development and testing. For production use, you should consider using a more robust message queue, such as DenoKvMessageQueue from @fedify/fedify/x/deno module or RedisMessageQueue from @fedify/redis package.

If it is not present, incoming activities are processed immediately and block the response to the sender until the processing is done.

While the queue option is not mandatory, it is highly recommended to use it in production environments to prevent the server from being overwhelmed by incoming activities.

With the queue enabled, the failed activities are automatically retried after a certain period of time. The default retry strategy is exponential backoff with a maximum of 10 retries, but you can customize it by providing an inboxRetryPolicy option to the createFederation() function.

NOTE

Activities with invalid signatures/proofs are silently ignored and not queued.

Error handling

Since an incoming activity can be malformed or invalid, you may want to handle such cases. Also, your listener itself may throw an error. The onError() method registers a callback function that takes a Context object and an error object. The following shows an example of handling errors:

federation
  .setInboxListeners("/users/{handle}/inbox", "/inbox")
  .on(Follow, async (ctx, follow) => {
    // Omitted for brevity
  })
  .onError(async (ctx, error) => {
    console.error(error);
  });

NOTE

Activities with invalid signatures/proofs are silently ignored and not passed to the error handler.

Constructing inbox URIs

To construct an inbox URI, you can use the getInboxUri() method. This method optionally takes a handle of an actor and returns a dereferenceable URI of the inbox of the actor. If no argument is provided, the method returns the shared inbox URI.

The following shows how to construct an inbox URI of an actor named "alice":

ctx.getInboxUri("alice")

NOTE

The getInboxUri() method does not guarantee that the inbox actually exists. It only constructs a URI based on the given handle, which may respond with 404 Not Found. Make sure to check if the handle is valid before calling the method.

The following shows how to construct a shared inbox URI:

ctx.getInboxUri()

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1. See the Object IDs and remote objects section if you are not familiar with dereferencing accessors. ↩︎