Introduces Streaming

[liamgriffiths]

Implements streaming in the SDK!

The streaming interface largely mirrors the non-streaming one. Instead of substrate.run we have substrate.stream which returns a SubstrateStreamingResponse (vs a SubstrateResponse).

This new response object is also an async generator and can also be iterated over to read the stream messages as they are received.

  // simple example of what this looks like
  const stream = await substrate.stream(a);

  for await (let message of stream) {
      console.log(message);
  }

In addition to using the iterator, there are two additional utilities I added in for now, tee and get.

• tee is useful because once a stream is consumed, it cannot be read again - however if the user would like to have multiple consumer/iterators they can use tee to split the stream into however many they like.
• get is the streaming analog to res.get in the non-streaming interface. It filters the stream for messages that belong to a specific node and re-uses some the type inference we have so far.

  // using `tee`
  const stream = await substrate.stream(a);
  const streams = stream.tee(); // defaults to 2, but can split into any number

  const log = async (stream) => {
    for await (let message of stream) {
      console.log(message);
    }
  }

  await Promise.all(streams.map(s => log(s)))

  // using `get`
  const stream = await substrate.stream(a, b, c);

  for await (let message of stream.get(a)) {
      // only messages for node `a`
      console.log(message);
  }

The server will respond with server-sent events when the request is:

• using the special x-substrate-streaming: 1 header
• the client uses the accept: text/event-stream header
• the nodes being requested support streaming

The server-sent event messages come back in a few flavors:

• node.delta - chunks of a nodes result (when possible)
• node.result - the final result for a node
• node.error - any errors encountered trying to run the node
• graph.result - the accumulated result of the entire graph

We might also include other message types, but tbd:

• node.usage - various usage metrics for the node
• graph.error - any graph-level errors we may encounter

[0thernet]

neat