yDoc must be saved with each change.

[naporin0624]

Problems with existing implementation

In the Hibernation API, instances of DurableObjects go dormant as soon as no data is sent from the client, so yDocs stored in-memory are quickly volatile.

Therefore, the existing implementation of storing the in-memory state in transaction storage when the number of connections drops below a certain number is not appropriate.

Reason why it seems to work in the demo site

However, https://yjs.napochaan.dev, which is now available on the demo site, seems to work fine. This is because the client implementation of y-websocket, WebSocketProvider, wrote a constructor that periodically sends local yDoc when the connection becomes Open, so it does not go into hibernation state even if Hibernation API is used, It is assumed that the server-side yDoc remained unvolatilized until the client communication was disconnected.

https://github.com/yjs/y-websocket/blob/master/src/y-websocket.js#L302-L311

What should be changed

• Store changes received via webSocketMessage in transaction storage on a per-snapshot basis
  • Introduce state with the following key to guarantee order.
  • snapshot_size.
  • snapshot:${count}
    • count: the total number of currently stored snapshots +1
• Join snapshots with constructor and restore yDoc.
• Control when to merge snapshot.
  • idea1: when the number of concurrent connections reaches 0
  • idea2: when size exceeds a certain number
  • cf: https://docs.partykit.io/reference/y-partykit-api/#:~:text=%7D-,Persistence,-By%20default%2C%20PartyKit

[naporin0624]

The resync defined in the constructor of y-websocket's Provider was not firing and lexical was periodically updating its awareness on the client side, which led to the periodic transmission.

https://github.com/yjs/y-websocket/blob/master/src/y-websocket.js#L344-L353

[naporin0624]

Yjs Update Document

https://docs.yjs.dev/api/document-updates#alternative-update-api

External Storage Implements

https://github.com/yjs/y-leveldb/blob/master/src/y-leveldb.js#L322

https://github.com/partykit/partykit/blob/main/packages/y-partykit/src/storage.ts