[Story] 🦀 Event cache storage

[manuroe]

Description

• As a user
• I do not want the app to reload timeline content every time
• So that I enjoy a snappy app with timeline instantly displayed

This project is about to make the existing Event Cache API persistent. The event cache exists but it is a in-memory cache. We want it to be a on-disk cache.

Acceptance criteria

• Live timeline events are permanently cached
• This cache is reset in case of a gap in the timeline
• The room list pre-fills this cache with X last messages so that the timeline instantly displays when the user opens a room
• More to be discussed

Leads

• Tech: @Hywan
• Design:

Size estimate

None

Dependencies

• None

Out of scope

• Preload the cache from the notification service

Open questions

### Questions

Subtasks

### Android

### iOS

### Rust
- [ ] https://github.com/matrix-org/matrix-rust-sdk/issues/3280

### Other

Sign-off

Android

• [ ] Design sign-off on completion
• [ ] QA sign-off on completion
• [ ] Product sign-off on completion

iOS

• [ ] Design sign-off on completion
• [ ] QA sign-off on completion
• [ ] Product sign-off on completion

[Hywan]

This cache is reset in case of a gap in the timeline

Why? We get gaps pretty often, what's the goal of this feature?

The room list pre-fills this cache with X last messages so that the timeline instantly displays when the user opens a room

It has nothing to do with the event cache storage right? It's a sync requirement (which is already present).

[Hywan]

The full issue that describes all the work is https://github.com/matrix-org/matrix-rust-sdk/issues/3280.