Add ReliableWebsocketProvider to JavaScript module

[eliias]

Problem

A reliable channel ensures at-least-once guarantees via acknolwedge messages sent by both, client and server. Therefore, the client, and the server must reliably store messages that weren't acknowledged by all recipients yet. In case of the client, this means the server must respond with a last-ack-id response when messages are sent, and the client must always send the full message queue to workaround missing “in-between” updates.

Implementation

The default WebsocketProvider does not support message buffers and acknowledgment of messages. The ReliableWebsocketProvider is therefore a drop-in replacement that takes on the additional responsibilities of transparently managing a send buffer, and acknowledging message retrieval.

Due to Y.js supporting staging of messages (messages are not integrated until all necessary messages are available), and applying updates being idempotent, this isn't an issue.

sequenceDiagram
Client->>+Server: Send message buffer to server
alt no response
  loop Retry (w/ exponential backoff)
        Client-->>Server: Send message buffer
  end
else
  Server-->>-Client: Acknowledge message retrieval
end

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Caveats

We aim for eventual consistency (at-least-once delivery), and similar transport latency as with the regular WebsocketProvider on the happy path (exactly-once delivery). There is a chance of transmitting messages twice, when an “older” message wasn't transmitted, but the client is producing “new” messages.