SSR via Cloudflare Workers & `renderToWebStream` (3.2 )

[Cherry]

Version

3.2.0-beta.7

Reproduction link

https://github.com/Cherry/workers-vue-ssr-example

The src/index.mjs file contains a really simple Hello World Vue App, attempting to be rendered server-side. It has 3 different solutions in the code, in this order:

1. Native
2. via TransformStream
3. Buffering stream in memory

Each is commented out, with the only one actually working being 3, buffering in memory.

Steps to reproduce

See the attached reproduction repo for more information.

• Setup Cloudflare Wrangler environment. See https://developers.cloudflare.com/workers/cli-wrangler/install-update and https://developers.cloudflare.com/workers/cli-wrangler/authentication
  • Note you will need an account with the new ESM syntax enabled. See https://developers.cloudflare.com/workers/learning/using-durable-objects
• npm ci
• wrangler dev
• Hit http://127.0.0.1:8787/
• Comment in/out each solution in the src/index.mjs to see how it works. The currently enabled code is the "native" variant, which will throw a Failed to construct 'ReadableStream': the constructor is not implemented. error.

What is expected?

When using SSR and renderToWebStream, Options 1 and/or 2 should work without issue. The docs in @vue/server-renderer seem to suggest this is possible, but all attempts have resulted in errors.

What is actually happening?

Options 1 and 2 throw errors, either relating to ReadableStream not being a constructor, or there being no response. More information can be found in the README of the attached reproduction repo.

---

If I can provide any more information, please let me know and I'll be happy to.

[lidlanca]

cloudflare web worker does not implement ReadableStream instead they provide TransformStream
https://developers.cloudflare.com/workers/runtime-apis/streams/transformstream

you can use a ponyfills of ReadableStream, and then
read the stream, and "pipe" the values yourself to the writer of TransformStream

 let { readable, writable } = new TransformStream()

 let reader = renderToWebStream(...)
 pipeReaderToWriter(reader, writable) // async, read stream from 'reader' and write to `writable`, close writer when done.
 return new Response(readable)

[Cherry]

I tried something like that as per example 2, and https://github.com/Cherry/workers-vue-ssr-example/blob/c195fea2404e4cfd3b02c763ab483877ae1f185e/src/index.mjs#L23-L29. Is this not the correct way to do this?

[lidlanca]

The polyfill and the cloudflare native implementation may not play nice.

that is why I suggested pipeReaderToWriter()

async pipeReaderToWriter(r,w) function(){
   for(;;) {
      let {value, done} = await r.read()
      await w.write(value)
      if(done) {
        break
      }
   } 
   w.close()
}

[Cherry]

Thanks for the additional insight. Unfortunately that appears to result in the same error as described in example 2, with The script will never generate a response.. Specifically with:

async function pipeReaderToWriter(reader, writer){
	for(;;){
		const {value, done} = await reader.read();
		console.log('write value', value);
		await writer.write(value);
		if(done){
			break;
		}
	}
	writer.close();
}

And

const {readable, writable} = new TransformStream();
const render = renderToWebStream(app, {}, polyReadableStream);
pipeReaderToWriter(render.getReader(), writable);
return new Response(readable);

The docs at https://github.com/vuejs/vue-next/tree/master/packages/server-renderer#rendertowebstream seem to imply this should be a lot simpler. I'm wondering if there's something really obvious I'm missing.

[lidlanca]

pipeReaderToWriter(render.getReader(), writable.getWriter());

[Cherry]

🤦 Of course, thank you.

I had to tweak the encoding to ensure I was only writing UInt8Arrays as per the error This TransformStream is being used as a byte stream, but received a string on its writable side. If you wish to write a string, you'll probably want to explicitly UTF-8-encode it with TextEncoder., but this ended up looking like:

async function pipeReaderToWriter(reader, writer){
	const encoder = new TextEncoder();
	for(;;){
		const {value, done} = await reader.read();
		const encoded = encoder.encode(value);
		await writer.write(encoded);
		if(done){
			break;
		}
	}
	writer.close();
}

const {readable, writable} = new TransformStream();
const render = renderToWebStream(app, {}, polyReadableStream);
pipeReaderToWriter(render.getReader(), writable.getWriter());
return new Response(readable);

Would a PR to update the docs at https://github.com/vuejs/vue-next/tree/master/packages/server-renderer?rgh-link-date=2021-08-03T23%3A30%3A34Z#rendertowebstream be a good idea?

[lidlanca]

This is very specific to cloudflare web workers and their non smooth integration with the polyfill

in my opinion if this is a common use case for cloudflare web workers, it should be documented somewhere
to save people some time, may be in the readme, or maybe in the v3 docs / guide.
or just this issue.

[Cherry]

Agreed it should be documented somewhere, or the line at https://github.com/vuejs/vue-next/tree/master/packages/server-renderer?rgh-link-date=2021-08-03T23%3A43%3A16Z#rendertowebstream that says // e.g. inside a Cloudflare Worker simply removed, or linked somewhere else for more information, since it's not the simple as the docs currently imply.

[yyx990803]

I adjusted the streaming API in 86d78d1 and added pipeToWebWritable which should be more flexible in such scenarios:

const { readable, writable } = new TransformStream()
pipeToWebWritable(app, {}, writable)

return new Response(readable)

Will release a beta today so you can test it out.

[Cherry]

This is fantastic, thanks!

[Cherry]

Just tested this @yyx990803. Unfortunately it throws the error:
Error: Failed to get the 'ready' property on 'Writer': the property is not implemented.

It seems this is another place where Cloudflare Workers divert from the spec.