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Claire <> DRAFT 2020-10-24

FEP-8fcf: Followers collection synchronization across servers


In ActivityPub, follow relationships are established, updated and removed by sending activities such as Follow, Accept or Reject, which are assumed to be correctly and promptly processed upon receipt.

However, due to incompatible protocol extensions, software bugs, server crashes or database rollbacks, the two ends of a Follow relationship may end up out of sync.

This can be especially damaging when a remote instance has outdated information about follow relationships that should have been revoked, as some implementations may deliver activities addressed to the sender’s followers collection by using the sharedInbox mechanism and letting the recipient use the sender’s followers collection for local delivery and access control.

This proposal describes an optional mechanism for detecting discrepancies in following relationships across instances, with minimal overhead and without loss of privacy.


The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in this specification are to be interpreted as described in [RFC-2119].

The proposed protocol for followers collection synchronization makes a number of assumptions that may not be suitable to every implementation or deployment.

Implementations and deployments MUST NOT implement the mechanisms described in this proposal unless they match the following requirements:

  • actors managed by an instance are required to all share the same exact URI scheme and authority for their id, inbox and sharedInbox URIs
  • such instances are required to manage all actors using the same URI scheme and authority for either their id, inbox or sharedInbox URIs (that is, for instance, two fediverse implementations cannot implement this proposal if they are set up on the same exact same domain name, unless implementing an additional mechanism to share follower information between them, which is out of scope for this proposal).

The reason for those requirements is to prevent the partial followers collection described below from missing legitimate followers, which could result in followers being removed for no reason.

Failing to implement this proposed synchronization mechanism should not impact compatibility with other implementations, as it is completely optional.

Partial follower collection

For efficiency and privacy purposes, we consider a subset of an actor’s followers collection. This subset is the set of an actor’s followers whose id shares an instance’s specific URI scheme and authority.

For instance, if has the following followers:


The partial follower collection of for the instance serving is:


Partial follower collection digest

To enable quick checking of partial followers consistency across instances, a partial follower collection digest is computed.

This digest is created by XORing together the individual SHA256 digests of each follower’s id.

partialCollectionDigest = SHA256(follower1) XOR SHA256(follower2) XOR ... XOR SHA256(followerN)

For instance, the partial follower collection digest of for the instance serving is: 3a06e99569547f444c352ab7f52e4bab207abec5ca6f07b0045cfdc9723f8fa9 XOR f939a1585d4a8f02ee339210dbe7315d7003476663d6095f7d996fc4bc7a49b6 = c33f48cd341ef046a206b8a72ec97af65079f9a3a9b90eef79c5920dce45c61f

The Collection-Synchronization HTTP Header

The Collection-Synchronization HTTP header provides a mechanism for quickly checking whether the sender’s followers collection part that is relevant to the recipient is consistent with the recipient’s knowledge.

The header field name is Collection-Synchronization and its value is a list of parameters and values, formatted according to the signature syntax defined in [HTTP-Signatures], Section 4.1.


Collection-Synchronization: collectionId="", url="", digest="c33f48cd341ef046a206b8a72ec97af65079f9a3a9b90eef79c5920dce45c61f"

Collection Synchronization Header Parameters

The Collection-Synchronization header’s parameters are defined as follows:

  • collectionId: this is URI of the collection that supports synchronization. It must be the sender’s followers collection.
  • url: this is the URL of the partial followers collection intended for the receiving instance. Accessing it should require authentication from the receiving instance.
  • digest: the partial follower collection digest intended for the receiving instance.

Synchronization procedure

On the sender end

When delivering an Activity to an inbox (or sharedInbox), an instance MAY set a Collection-Synchronization header intended for the corresponding instance (determined by the inbox URI scheme and authority).

When exactly to set this header is up to the sender, but it is recommended to at least send it for any Create activity addressed specifically to the sender’s followers collection.

On the receiving end

On the receiving end, upon receiving an Activity delivery with a signed Collection-Synchronization header, the receiver MUST check that:

  • the collectionId attribute matches the sender’s followers collection id
  • the url attribute also matches the same authority (so that the instance cannot get tricked into requesting the followers list of a third-party individual)

If any of those checks fails, the receiver MUST ignore the Collection-Synchronization header.

The receiver SHOULD then compute the partial collection digest for the sender’s followers based on its own knowledge. If the digest does not match the digest attribute of the header, it SHOULD then query the url, authenticating itself to the remote server using [HTTP-Signatures] or another method.

Having fetched the up-to-date partial followers collection from the autoritative server, the receiving end:

  • SHOULD remove from its local copy of the followers collection any local actor not listed in the partial followers collection.
  • MAY consider any pending outgoing follow listed in the partial followers collection as accepted.
  • SHOULD send an Undo Follow for any other local follower listed in the partial followers collection but not known locally.


This proposal is implemented by Mastodon since the following Pull Request:


CC0 1.0 Universal (CC0 1.0) Public Domain Dedication

To the extent possible under law, the authors of this Fediverse Enhancement Proposal have waived all copyright and related or neighboring rights to this work.