Exchange of messages between broker and gateway via message channels

[danielwiehl]

Is your feature request related to a problem? Please describe.

Consider using web channels provided by the MessageChannel Web API for the message exchange between the gateway and broker. This allows for a dedicated channel per message class and thus eliminates the manual mapping of messages to handlers in the broker/gateway, which massively reduces the load on the UI thread.

A PoC has illustrated that the message throughput can be more than doubled compared to the current solution.

Describe the solution you'd like

Instead of exchanging messages on a single channel (the window channel), the platform uses a dedicated MessageChannel per message class. The gateway connects to the broker using window.postMessage and transfers message ports for subsequent communications (i.e., for publishing a message, publishing an intent, subscribing to a topic, subscribing to an intent, ...). In addition, a dedicated channel is established for each interaction with the broker, i.e., for receiving the ACK message, the response(s) to a request-response communication, and messages for a subscription.

[danielwiehl]

See branch issue/message-channel for inspiration.

[danielwiehl]

Note that this change must be backward compatible and that we need to migrate the scion-rcp/ch.sbb.scion.rcp.microfrontend library.