SimpleX-Chat/docs/rfcs/2023-07-25-contact-groups.md

Contact groups

Problem

Bad UX when communicating from multiple devices:

• Lack of data synchronization between devices.
• Participating in the same conversation from multiple devices requires manually adding each device to a group.
• For a direct (1-to-1) conversation, it has to be initially created as a group - a contact does not support adding devices to conversation. Not obvious use of groups.

Solution

Full device synchronization is a complex feature and will not be discussed in this rfc. Instead a proposed solution is to support adding devices to a 1-to-1 conversation, using existing group technology.

Pros:

• Fully decentralized, no leading and following devices.
• No complex full state synchronization.
• More obvious UX than using groups for 1-to-1 conversations.
• Can be automated to include user devices into all [new] conversations.

Cons:

• Same downsides groups have - increased traffic, diverging group state, possible failures to establish connections (requires out-of-band "connection doctor"?).
• Muddies existing abstracts of contacts and groups. Adds new orthogonal complexity to many existing features - preferences, incognito, multi-profile, calls, etc.
• Requires cooperation of contact's client - to support new protocol message for contact-to-group upgrade / contact-group invitation link.
• Still not full data synchronization. Devices have different profiles and likely different lists of conversations.

Possible approaches

1. Add device to contact conversation - upgrade contact to group. Optionally automate adding user devices to other conversations.
2. Managed list of user devices - non optionally automate adding to other conversations.

Upgrade contact to group

Pros:

• Allows to migrate existing contacts to multi-device conversation.

Cons:

• Button would be hidden in chat info.
• Requires upgrading contact to group on both sides of the conversation:
  • Replace contact with group, update entities - chat items, files, etc. How will files in progress migrate?
  • New protocol message for contact to perform same upgrade + introduction of new device.
• Implicit list of "my devices", unclear whether to automate adding devices for other contacts? Not as useful if it's not automated?

Managed list of user devices

Pros:

• New button in Settings - easier to discover than chat info button.
• User devices are automatically added to all new conversations.
• Start each conversation as a group? -> Doesn't require upgrading contact to group.
  • New invitation link type - contact-group.
  • Expanded info in confirmation (when invitation link is for regular contact but joining client has other devices).
• Can automatically add own devices to existing groups.
• Doesn't require tricky contact to group upgrade.

Cons:

• Cannot migrate existing contacts. Create all contacts going forward as contact-groups?