dendrite/docs/peeking.md

Peeking

Local peeking is implemented as per MSC2753.

Implementationwise, this means:

• Users call /peek and /unpeek on the clientapi from a given device.
• The clientapi delegates these via HTTP to the roomserver, which coordinates peeking in general for a given room
• The roomserver writes an NewPeek event into the kafka log headed to the syncserver
• The syncserver tracks the existence of the local peek in the syncapi_peeks table in its DB, and then starts waking up the peeking devices for the room in question, putting it in the peek section of the /sync response.

Peeking over federation is implemented as per MSC2444.

For requests to peek our rooms ("inbound peeks"):

• Remote servers call /peek on federationapi
  • The federationapi queries the federationsender to check if this is renewing an inbound peek or not.
  • If not, it hits the PerformInboundPeek on the roomserver to ask it for the current state of the room.
  • The roomserver atomically (in theory) adds a NewInboundPeek to its kafka stream to tell the federationserver to start peeking.
  • The federationsender receives the event, tracks the inbound peek in the federationsender_inbound_peeks table, and starts sending events to the peeking server.
  • The federationsender evicts stale inbound peeks which haven't been renewed.

For peeking into other server's rooms ("outbound peeks"):

• The roomserver will kick the federationsender much as it does for a federated /join in order to trigger a federated outbound /peek
• The federationsender tracks the existence of the outbound peek in in its federationsender_outbound_peeks table.
• The federationsender regularly renews the remote peek as long as there are still peeking devices syncing for it.
• TBD: how do we tell if there are no devices currently syncing for a given peeked room? The syncserver needs to tell the roomserver somehow who then needs to warn the federationsender.