Websocket Wire Format¶
All encoding is done using msgpack with lists as the base structure that have a string type as the first element. Websockets with a binary message payload a re used. Message delivery follows a pub/sub pattern.
Motivation and Goals¶
- Keep local data local if possible (e.g., glass -> phone shouldn’t need to go through a server if they are connected)
- Pub/Sub over Point-to-Point: Focusing on channels which represent data types naturally handles 0-to-many devices reacting to it.
- Minimize data transfer, if nothing is listening on a channel then sending to it is a null-op
- Support a directed-acyclic-graph topology, where local devices can act as hubs (e.g., a phone is a hub for a watch, Glass, and arduino) and connect to other hubs (e.g., a remote server)
- Instead of having strict guarantees about delivery, provide a set of constraints that can be met given the fickle nature of mobile connectivity to eliminate edge cases
- Messages are delivered with “best effort” but not guaranteed: devices can pop in and out of existance and buffers are not infinite
- If a sender’s messages are delivered they are in order
- A message SHOULD only be sent to a client that is subscribed to it or is connected to clients that are
- A message sent to a client that neither it or its clients are subscribed to MUST ignore it
- The “subscriptions” channel is special in that it MUST be sent to all connected clients
- When a client connects to a server it MUST be send the subscriptions for itself and all other clients
- If multiple channels match for a client the most specific MUST be called and no others
- When a client subscribes to a channel a single callback MUST be provided
The following table gives examples for when data will be sent given that there is a listener for the specified channel.