# 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.

```mermaid
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:

1. Repo on an instance receives a star (regardless of whether via UI or federation)
2. 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:

```mermaid
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
```

1. 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.   
2. User stars repo on instance A via UI.
3. Instance A sends Like-Activity to repo on instance B.
4. Instance B creates local FederatedUser, stars the repo and sends Like-Activity to repo on instance C.
5. Instance C creates local FederatedUser, stars the repo and sends Like-Activity to repo on instance A.
6. 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.
7. The circular distribution of Like-Activities continues, since the actor is always the local FederatedUser of the sending instance.

### 2. Direct Federation

```mermaid
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)
```

In this case the star federation process would look like:

1. Case: Repo on an instance receives a star by an authenticated user via UI/API:
    1. Repository gets starred by the authenticated User.
    2. Instance federates star to all repos that are set as federated repositories.
2. Case: Repo on an instance receives a star via a Like-Activity:
    1. Instance creates FederatedUser and stars the repository.
    2. No further star federation to federated repos is triggered.

## See also