Mirror

This service enables your server to become a mirror of one or more other SemApps servers, and/or to offer the option to other SemApps servers to mirror you.

Only public data can be mirrored. If your server is using the WebAclService, then the anonymous acl:Read permission should be set for the data to be mirrored.

Dependencies

• ActivityPubService
• LdpService
• WebfingerService
• VoidService

Install

$ yarn add @semapps/mirror

Dataset migration

If you were running a previous version of Jena Fuseki (before July 2022), you will need to migrate your dataset(s) configuration(s). First remove the current version of the docker image:

docker image rm -f semapps/jena-fuseki-webacl

Then follow the guide on dataset migration here.

Usage

const { MirrorService } = require('@semapps/mirror');

module.exports = {
  mixins: [MirrorService],
  settings: {
    baseUrl: 'http://localhost:3000/',
    servers: ['https://otherserver.com'], // Optional. Other servers you want to mirror
    acceptFollowers: true // Default value
  }
};

Settings

Property	Type	Default	Description
baseUrl	String	required	Base URL of the server
servers	Array		List of servers URLs you want to mirror (optional)
acceptFollowers	Boolean	true	Set to false if you want to mirror others, but not offer mirroring service yourself

Setting up the Relay bot

An important step is also to make sure that your server has a container that accepts Application type. The mirror service will create an ActivityPub actor there, with the name relay. You could put the actor in the /users container, or in a dedicated /bots container.

const containers = [{
  path: '/users',
  acceptedTypes: ['pair:Person', 'Application'], // The Application type is important
  blankNodes: ['sec:publicKey'],
  excludeFromMirror: true
}];

You also most probably want to use the option excludeFromMirror: true. It will hide this container from the mirroring servers.

If other containers need to be hidden and not available for mirroring, you can use this option too for them.

The mirroring service works with or without the WebAclService on. If you do not have WebAcl activated, the option excludeFromMirror is a simple way to hide containers.

What's going on ?

The service works automatically. At first launch, the public data of all the servers listed for mirroring are downloaded (via the LDP protocol) and inserted into a separate graph named http://semapps.org/mirror.

Then your server will follow the ActivityPub relay actor(s) of the remote server(s) you are mirroring.

Everytime some data changes occur on the remote server(s), your server will receive ActivityPub activities and will update its local mirror of the data accordingly.

The http://semapps.org/mirror graph is not directly editable through LDP, but can be fetched with the ldp.resource.get action or via the SPARQL endpoint.

LDP containers PATCH

When attaching a remote resource to a local LDP container with the PATCH method, if the remote server has not been mirrored yet locally, the mirror service will mirror only that resource.

It will then periodically refresh that resource's data (using a simple fetch on the LDP resource) to keep it in sync with the remote server.

If the remote server is mirrored, then the resource will be kept up-to-date thanks to the ActivityPub activities sent by the relay bots.

This feature is thus independent of the mirror service and is always active when the LDP service is running.