[DO] Update WebSocket Hibernation example

[harshil1712]

Summary

This example adds the serializeAttachment and deserialzieAttachemt APIs to the existing example. It also shows how to send messages with the following condition:

• Send message to the client
• Send message to all the connected clients
• Send message to all the connected clients except sender

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Original Link	Updated Link
https://developers.cloudflare.com/durable-objects/examples/websocket-hibernation-server/	https://update-do-examples.cloudflare-docs-7ou.pages.dev/durable-objects/examples/websocket-hibernation-server/

[MellowYarker]

This is lost every time we hibernate FYI. You would probably want to reconstruct it whenever the constructor() runs.

[joshthoward]

What Milan said, but you probably want something like

	constructor(ctx: DurableObjectState, env: Env) {
    super(ctx, env);
    this.sessions = new Map();
    this.ctx.getWebSockets().forEach((ws) => {
      const { ...session } = ws.deserializeAttachment();
      this.sessions.set(ws, { ...session });
    });
	}

You'll also have to ws.serializeAttachment every time that you update sessions.

[MellowYarker]

I'm confused by this comment? Hibernation means evicting and losing all in-memory state. When the DO "hibernates" and then comes back, this.sessions is set to {}.

[MellowYarker]

Every time we hibernate and then receive a new message, this.sessions.get(sender) will not return the associated session because this.sessions is lost whenever we hibernate.

[MellowYarker]

As per above comment, this doesn't actually happen on a "per connection basis". Since this.sessions is cleared whenever we're evicted, the session.id will always not exist the first time we get a message on a pre-existing connection.

I think what you really want to do is something more like:

1. When we receive a message, check the sessions map. If we have an associated session there, great, use that.
2. If we don't have a session, check if we've already serialized something on the WS, i.e. sender.deserializeAttachment() and check if we have an ID, if we do, create the session and set it on sessions this.sessions.set(sender, sendersID);.

[MellowYarker]

Alternatively, in the DOs constructor() you could just do something like this.ctx.getWebSockets() and then for each load it into sessions.

[MellowYarker]

Just FYI calling getWebSockets() will unhibernate all your websocket connections. In other words, when the DO hibernates, all connections are also "hibernated", and when the DO wakes back up, we only load your connections back into memory when you need them.

This means that if you call getWebSockets() (bc you wanna broadcast), you load all your WS connections back into memory. If you don't broadcast, and instead only reply to the WS that sent the DO a message, then only that DO is brought back into DO memory.

I don't think we actually explain this well in the docs, but yeah, just something you might wanna know!

[MellowYarker]

You probably want to actually set the session.id when you first create the websocket in fetch().

[MellowYarker]

I think we should make the same changes for the non-hibernation example. New devs might look at regular WS example, then see the Hibernation example and be hesitant to use hibernation because it looks significantly more complicated.

If we don't do that, we should at least modify the description of the example, because currently it says "This example is similar to the Build a WebSocket server example, but uses the WebSocket Hibernation API".