Using Async Iterators in Node.js

This talk demonstrates how to use async iterators and the new for-await-of syntax in Node.js 10. It includes some real-world examples, like iterating over the contents of a file, handling a streaming database query, and using SSE to publish the values yielded by an async iterator. The talk also dives into cancellation patterns for promises and async iterators.

Transcript

1. Async Iterators
   REAL WORLD USAGE in NODE.JS 10
   

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2. Ryan Paul
   http://seg.phault.net
   

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3. Iterators & Async Functions
   Bringing Together
   

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4. Availability
   Supported Not Yet
   Chrome 63 Safari (Preview)
   Node.js 10 Edge
   Firefox 55
   

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5. Async Iterators in Theory
   •Write generator functions that yield promises
   •Loop over async iterators with the new for-
   await-of syntax
   •A useful abstraction for representing streams
   

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6. Async Iterators in Practice
   const timer = time =>
   new Promise(resolve => setTimeout(resolve, time));
   async function* counter() {
   let counter = 0;
   while (true) {
   await timer(5000);
   yield counter++;
   }
   }
   

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7. Consuming an Async Iterator
   let iter = counter();
   for await (let item of iter)
   console.log(item)
   Using the for-await-of syntax
   

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8. Underneath the Sugar
   •An iterator is an object with a next() function
   •The next() function returns an object with value
   and done properties
   •The value is the yielded value and done is a
   boolean indicating whether the iterator is complete
   

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9. Async Iteration Protocol
   •Use the async iteration protocol to turn any object
   into an async iterator
   •Objects that conform with the protocol can be
   consumed with for-await-of
   •The protocol uses Symbol.asyncIterator
   

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10. Async Iteration Protocol
    class Example {
    async *[Symbol.asyncIterator]() {
    let counter = 0;
    while (true) {
    await timer(5000);
    yield counter++;
    }
    }
    }
    let example = new Example();
    for await (let item of example)
    console.log(item);
    

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11. Some Practical Examples
    Real-World Async Iterators
    

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12. Using Async Iterators in the Real World
    •You can use async iterators today in node.js 10
    •Every ReadableStream instance supports the
    async iteration protocol
    •Useful for I/O and many other everyday tasks
    

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13. Reading Files with Async Iterators
    let file = fs.createReadStream("file.txt");
    let content = "";
    for await (let chunk of file)
    content += chunk;
    console.log(content);
    

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14. Streaming Database Query
    • RethinkDB is an open source database for building real-
    time web applications
    • RethinkDB changefeed lets you stream updates from a
    table or query
    • You can wrap a changefeed cursor with an async iterator
    so that it can be consumed with a for-await-of loop
    

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15. Streaming Database Query
    const r = require("rethinkdbdash")();
    function* stream(cursor) {
    while (true)
    yield cursor.next();
    }
    let query = await r.db("rethinkdb").table("stats").changes();
    for await (let item of stream(query))
    console.log(item);
    

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16. Serving as an SSE Event Stream
    • Server Sent Events (SSE) is a standard for building real-
    time web applications
    • Broadcast a stream of events in response to an HTTP
    request
    • Easy to consume in the browser with the EventSource
    client API
    

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17. Serving as an SSE Event Stream
    async function iterStream(res, iter) {
    let header = {"Content-Type": "text/event-stream"};
    res.writeHead(200, header);
    for await (let item of iter)
    res.write(`data: ${item}\n\n`);
    }
    http.createServer((req, res) =>
    iterStream(res, counter())).listen(8000);
    

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18. Serving as an SSE Event Stream
    let events = new EventSource("/");
    events.onmessage = ev => console.log(ev);
    Create an EventSource instance in the browser and
    attach a function to onmessage to see events
    

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19. But what happens when the
    client disconnects?
    

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20. Serving as an SSE Event Stream
    • The generator continues to run when the client
    disconnects
    • We shouldn’t leave a dangling generator every time a
    user loads the page
    • Naive solution: what if we invoke iter.return() when
    the connection closes?
    

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21. Serving as an SSE Event Stream
    async function iterStream(res, iter) {
    res.connection.on("close", () => iter.return());
    let header = {"Content-Type": "text/event-stream"};
    res.writeHead(200, header);
    for await (let item of iter)
    res.write(`data: ${item}\n\n`);
    }
    Watch for the connection to close and call the
    iter.return() function
    

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22. Serving as an SSE Event Stream
    •Calling iter.return() doesn’t terminate the
    pending promise
    •The pending promise continues until it rejects or
    resolves
    •We need a way to cancel the pending promise
    

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23. Promise Cancellation
    Introducing
    

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24. Promise Cancellation
    function timer(time, cancelled) {
    let handle;
    return new Promise((resolve, reject) => {
    handle = setTimeout(resolve, time);
    cancelled.then(() => clearTimeout(handle));
    });
    }
    let cancel, cancelled = new Promise(resolve => cancel = resolve);
    timer(5000, cancelled);
    cancel();
    

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25. Promise Cancellation
    let cancel;
    let cancelled = new Promise(resolve => cancel = resolve);
    timer(5000, cancelled);
    cancel();
    Create a promise and assign its resolve function to a
    variable in the parent scope
    

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26. Promise Cancellation
    function timer(time, cancelled) {
    let handle;
    return new Promise((resolve, reject) => {
    handle = setTimeout(resolve, time);
    cancelled.then(() => clearTimeout(handle));
    });
    }
    Pass the cancelled promise into the function and
    terminate the timer when it resolves
    

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27. Promise Cancellation
    function timer(time, cancelled) {
    let handle;
    return new Promise((resolve, reject) => {
    handle = setTimeout(resolve, time);
    cancelled.then(() => {
    reject(new Error("Cancelled"));
    clearTimeout(handle);
    });
    });
    }
    Reject the promise so that all downstream subscribers
    know that it was cancelled
    

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28. SSE Example with Cancellation
    Putting it all Together
    

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29. SSE Example with Cancellation
    async function iterStream(res, iter, cancel) {
    let header = {"Content-Type": "text/event-stream"};
    res.writeHead(200, header);
    res.connection.on("close", cancel);
    for await (let item of iter)
    res.write(`data: ${item}\n\n`);
    }
    Add an extra parameter for the cancel() function and
    invoke it when the user closes their connection
    

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30. SSE Example with Cancellation
    async function* counter(cancelled) {
    let counter = 0;
    while (true) {
    try {
    await timer(5000, cancelled);
    yield counter++;
    } catch (err) {
    if (err.message === "Cancelled") break;
    }
    }
    }
    Pass the cancelled promise into the timer and force
    the loop to break when the exception is thrown
    

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31. SSE Example with Cancellation
    let cancel, cancelled = new Promise(resolve => cancel = resolve);
    http.createServer((req, res) =>
    iterStream(res, counter(cancelled), cancel)).listen(8000);
    Create the cancellation promise and pass it into the
    iterStream() function
    

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32. Resources
    • My Blog post about cancellation and async iterators:
    http://seg.phault.net/blog/2018/03/async-iterators-
    cancellation/
    • Async Iterators TC39 proposal:
    https://github.com/tc39/proposal-async-iteration
    • Follow me on Twitter: @segphault
    

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