How to Trigger Activities

Status

Proposal

Context

While implementing the trigger of federated stars we have to handle the distribution of corresponding Like-Activities to the federated repositories.

This must be done consistently and without circularity, such that a repository starring by a user leads to exactly one added star in every federated repository.

flowchart TD
    U(User) -->|Press Star on UI| A(A: repository - follow C by incident) 
    A ~~~ B(B: follow A)
    B ~~~ C(C: follow B)
    C ~~~ A

Decision

Choices

1. Transient Federation

In this case the star federation process would look like:

Repo on an instance receives a star (regardless of whether via UI or federation)
Instance federates star to all repos that are set as federated repositories.

Problem - Circularity And Inconsistency

Circular federation would lead to a infinite circular distribution of Like-Activities:

flowchart TD
    U(User) -->|Press Star on UI| A(A: repository - follow C by incident) 
    A -->|federate Like Activity| B(B: follow A)
    B -->|federate Like Activity| C(C: follow B)
    C -->|federate Like Activity| A

Given a repo on the 3 instances A, B, C.
Repo on instance A has set repo on instance B as federation repo.
Repo on instance B has set repo on instance C as federation repo.
Repo on instance C has set repo on instance A as federation repo.
User stars repo on instance A via UI.
Instance A sends Like-Activity to repo on instance B.
Instance B creates local FederatedUser, stars the repo and sends Like-Activity to repo on instance C.
Instance C creates local FederatedUser, stars the repo and sends Like-Activity to repo on instance A.
Instance A creates local FederatedUser, since the Actor of the Like-Activity is the local FederatedUser created on instance C.
Thus, the repo on instance A gets another star by this user and sends Like-Activity to the repo on instance C.
The circular distribution of Like-Activities continues, since the actor is always the local FederatedUser of the sending instance.

2. Direct Federation

flowchart TD
    U(User) -->|Press Star on UI| A(A: repository - follow C not allowed) 
    A -->|federate Like Activity| B(B: follow A)
    A -->|federate Like Activity| C(C: follow B not allowed, has to follow A)