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Explainer for the WebSocketStream JavaScript API
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README.md Clarify the mapping to the protocol (#8) Mar 19, 2020

README.md

WebSocketStream Explained

Introduction

The WebSocket API provides a JavaScript interface to the RFC6455 WebSocket protocol. While it has served well, it is awkward from an ergonomics perspective and is missing the important feature of backpressure. In particular,

  • The onmessage event will keep firing until the page becomes completely unresponsive. The user agent will buffer incoming messages until it runs out of memory and crashes.
  • The only way to determine when the network or remote server can’t keep up with your sent messages is to test the bufferedAmount attribute. To find out when it is safe to start sending messages again, it is necessary to poll bufferedAmount.

WebSocketStream aims to solve these deficiencies with a new API.

Here’s a basic example of usage of the new API:

const wss = new WebSocketStream(url);
const { readable } = await wss.connection;
for await (const chunk of readable) {
  await process(chunk);
}
done();

This is the roughly equivalent code with the old API:

const ws = new WebSocket(url);
ws.onmessage = evt => process(evt.data);
ws.onclose => evt => evt.wasClean ? done() : signalErrorSomehow();

The major difference is that the second example won’t wait for asynchronous activity in process() to complete before calling it again; it will keep hammering it as long as messages keep arriving.

Also note that because this API was designed before Promises were added to the language, error-handling is awkward.

The second argument to WebSocketStream is an option bag to allow for future extension. Currently the only option is “protocols”, which behaves the same as the second argument to the WebSocket constructor:

const wss = new WebSocketStream(url, {protocols: ['chat', 'chatv2']});
const { protocol } = await wss.connection;

The selected protocol is part of the dictionary available via the wss.connection promise, along with “extensions”. All the information about the live connection is provided by this promise, since it is not relevant if the connection failed.

const { readable, writable, protocol, extensions } = await wss.connection;

The information that was available from the onclose and onerror events in the old API is now available via the “closed” Promise. This rejects in the event of an unclean close, otherwise it resolves to the code and reason sent by the server.

const { code, reason } = await wss.closed;

An AbortSignal passed to the constructor makes it simple to abort the handshake.

const controller = new AbortController();
const wss = new WebSocketStream(url, { signal: controller.signal });
setTimeout(() => controller.abort(), 1000);

The close method can also be used to abort the handshake, but its main purpose is to permit specifying the code and reason which is sent to the server.

wss.close({code: 4000, reason: 'Game over'});

Mapping to the protocol

There is a 1:1 mapping between WebSocket messages and stream chunks.

Each call to read() returns one WebSocket message. If a message is split into multiple frames on the wire, it won't be returned by read() until the final frame (the one with the FIN flag set) arrives.

When read() is not called, the browser and operating system will still buffer data to some extent, so backpressure will not be detected immediately by the server.

Text messages appear in JavaScript as strings. Binary messages appear as ArrayBuffer objects.

A clean close will result in read() returning an object with done set to true. An unclean close will result in a rejected promise.

Each call to write() (or chunk that is piped into the writable) will be converted to one message. The browser may split the message into multiple frames. BufferSource (ArrayBuffer or TypedArray) objects will be sent as binary WebSocket messages. Any other type will be converted to a string and sent as a text message.

The promise returned by write() will resolve when the message has been buffered (either by the browser or operating system). The size of the buffer is finite but unspecified. It is not a signal that the message has been delivered to the WebSocket server (the browser does not have this information).

The promise returned by write() will reject if the connection is closed or errored.

Goals

  • Provide a WebSocket API that supports backpressure
  • Provide a modern, ergonomic, easy-to-use WebSocket API
  • Allow for future extension

Non-goals

  • Support Blob chunks. The old WebSocket API defaults to receiving messages as Blobs; however, creating and reading Blobs is more costly than creating and reading ArrayBuffers. In practice, even though it requires explicitly setting binaryType, 97% of messages are received as ArrayBuffers. On the send side, sending Blobs adds considerable complexity to the implementation because the contents are not available synchronously. Since less than 4% of sent messages are Blobs it is better to avoid this complexity where we can.
  • Changing, replacing or extending the underlying network protocol. A new API based on QUIC called WebTransport is being discussed, and it is expected that new network capabilities will be added there.
  • Allowing user JavaScript to select WebSocket extensions. Since the server already negotiates the extensions to use, adding additional controls to client JavaScript seems redundant. The existing JavaScript API has never supported this, although some non-browser implementations have added options to the constructor for it.

Non-goals in the first version

  • Bring-your-own-buffer reading
  • Reading or writing individual messages as streams (for example, to handle messages larger than memory)
  • Exposing WebSocket pings and pongs to JavaScript.

Use cases

  • High-bandwidth WebSocket applications that need to retain interactivity, in particular video and screen-sharing.
  • Similarly, video capture and other applications that generate a lot of data in the browser that needs to be uploaded to the server. With backpressure, the client can stop producing data rather than accumulating data in memory.

End-user benefits

Applications written with the new API will automatically be more responsive due to respecting backpressure. High throughput applications will adapt to the capabilities of the client, providing everyone with a smooth experience.

Alternatives

It’s possible to implement backpressure at the application level, but it’s complex and difficult to achieve peak throughput. For example, client JavaScript could send an application-level confirmation message to the server every time it finishes processing a message. The server could keep track of how many messages it has sent that have not yet been confirmed, and stop sending if the number gets above a certain threshhold.

Aside from backpressure, the rest of the API can be emulated by wrapping the existing WebSocket API, but this will not permit future extensions.

Adding new attributes to the existing WebSocket API was considered but not adopted because having two APIs on one object would be confusing and create odd semantics.

WebTransport will also provide backpressure, and may ultimately supercede this work. In the near future the WebSocket protocol has the advantage that it works on networks that block QUIC, and has much existing deployed infrastructure.

Future work

  • Adding bring-your-own-buffer reading is a natural extension with the potential to improve performance.
  • Customisable buffer sizes to allow developers to make the trade-off between throughput and response to backpressure explicitly.

See also

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