[bheadmaster]

Even in Node, if you perform asynchronous operations on a shared resource, you need synchronization mechanisms to prevent interleaving of async functions.

There has been more than one occasion when I "fixed" a system in NodeJS just by wrapping some complex async function up in a mutex.

[nurple]

This lacks quite a bit of nuance. In node you are guaranteed that synchronous code between two awaits will run to completion before another task(that could access your state) from the event loop gets a turn; with multi-threaded concurrency you could be preempted between any two machine instructions. So while you _do_ have to serialize access to shared IO resources, you do _not_ have to serialize access to memory(just add the connection to the hashset, no locks).

What you usually see with JS for concurrency of shared IO resources in practice is that they are "owned" by the closure of a flow of async execution and rarely available to other flows. This architecture often obviates the need to lock on the shared resource at all as the natural serialization orchestrated by the string of state machines already naturally accomplishes this. This pattern was even quite common in the CPS style before async/await.

For example, one of the first things an app needs do before talking to a DB is to get a connection which is often retrieved by pulling from a pool; acquiring the reservation requires no lock, and by virtue of the connection being exclusively closed over in the async query code, it also needs no locking. When the query is done, the connection can be replaced to the pool sans locking.

The place where I found synchronization most useful was in acquiring resources that are unavailable. Interestingly, an async flow waiting on a signal for a shared resource resembles a channel in golang in how it shifts the state and execution to the other flow when a pooled resource is available.

All this to say, yeah I'm one of the huge fans of node that finds rust's take on default concurrency painfully over complicated. I really wish there was an event-loop async/await that was able to eschew most of the sync, send, lifetime insanity. While I am very comfortable with locks-required multithreaded concurrency as well, I honestly find little use for it and would much prefer to scale by process than thread to preserve the simplicity of single-threaded IO-bound concurrency.

[mike_hearn]

> So while you _do_ have to serialize access to shared IO resources, you do _not_ have to serialize access to memory(just add the connection to the hashset, no locks).

No, this can still be required. Nothing stops a developer setting up a partially completed data structure and then suspending in the middle, allowing arbitrary re-entrancy that will then see the half-finished change exposed in the heap.

This sort of bug is especially nasty exactly because developers often think it can't happen and don't plan ahead for it. Then one day someone comes along and decides they need to do an async call in the middle of code that was previously entirely synchronous, adds it and suddenly you've lost data integrity guarantees without realizing it. Race conditions appear and devs don't understand it because they've been taught that it can't happen if you don't have threads!

[bheadmaster]

> So while you _do_ have to serialize access to shared IO resources, you do _not_ have to serialize access to memory

Yes, in Node you don't get the usual data races like in C++, but data-structure races can be just as dangerous. E.g. modifying the same array/object from two interleaved async functions was a common source of bugs in the systems I've referred to.

Of course, you can always rely on your code being synchronous and thus not needing a lock, but if you're doing anything asynchronous and you want a guarantee that your data will not be mutated from another async function, you need a lock, just like in ordinary threads.

One thing I deeply dislike about Node is how it convinces programmers that async/await is special, different from threading, and doesn't need any synchronisation mechanisms because of some Node-specific implementation details. This is fundamentally wrong and teaches wrong practices when it comes to concurrency.

[nurple]

But single-threaded async/await _is_ special and different from multi-threaded concurrency. Placing it in the same basket and prescribing the same method of use is fundamentally wrong and fails to teach the magic of idiomatic lock free async javascript.

I'm honestly having a difficult time creating a steel man js sample that exhibits data races unless I write weird C-like constructs and ignore closures and async flows to pass and mutate multi-element variables by reference deep into the call stack. This just isn't how js is written.

When you think about async/await in terms of shepherding data flows it becomes pretty easy to do lock free async/await with guaranteed serialization sans locks.

[bheadmaster]

> I'm honestly having a difficult time creating a steel man js sample that exhibits data races

I can give you a real-life example I've encountered:

    const CACHE_EXPIRY = 1000; // Cache expiry time in milliseconds

    let cache = {}; // Shared cache object

    function getFromCache(key) {
      const cachedData = cache[key];
      if (cachedData && Date.now() - cachedData.timestamp < CACHE_EXPIRY) {
        return cachedData.data;
      }
      return null; // Cache entry expired or not found
    }

    function updateCache(key, data) {
      cache[key] = {
        data,
        timestamp: Date.now(),
      };
    }

    var mockFetchCount = 0;

    // simulate web request shorter than cache time
    async function mockFetch(url) {
      await new Promise(resolve => setTimeout(resolve, 100));
      mockFetchCount += 1;
      return `result from ${url}`;
    }

    async function fetchDataAndUpdateCache(key) {
      const cachedData = getFromCache(key);
      if (cachedData) {
        return cachedData;
      }

      // Simulate fetching data from an external source
      const newData = await mockFetch(`https://example.com/data/${key}`); // Placeholder fetch

      updateCache(key, newData);
      return newData;
    }

    // Race condition:
    (async () => {
      const key = 'myData';

      // Fetch data twice in a sequence - OK
      await fetchDataAndUpdateCache(key);
      await fetchDataAndUpdateCache(key);
      console.log('mockFetchCount should be 1:', mockFetchCount);

      // Reset counter and wait cache expiry
      mockFetchCount = 0;
      await new Promise(resolve => setTimeout(resolve, CACHE_EXPIRY));

      // Fetch data twice concurrently - we executed fetch twice!
      await Promise.all([fetchDataAndUpdateCache(key), fetchDataAndUpdateCache(key)]);
      console.log('mockFetchCount should be 1:', mockFetchCount);
    })();

This is what happens when you convince programmers that concurrency is not a problem in JavaScript. Even though this cache works for sequential fetching and will pass trivial testing, as soon as you have concurrent fetching, the program will execute multiple fetches in parallel. If server implements some rate-limiting, or is simply not capable of handling too many parallel connections, you're going to have a really bad time.

Now, out of curiosity, how would you implement this kind of cache in idiomatic, lock-free javascript?

[wolfgang42]

> how would you implement this kind of cache in idiomatic, lock-free javascript?

The simplest way is to cache the Promise<data> instead of waiting until you have the data:

    -async function fetchDataAndUpdateCache(key: string) {
    +function fetchDataAndUpdateCache(key: string) {
       const cachedData = getFromCache(key);
       if (cachedData) {
         return cachedData;
       }

       // Simulate fetching data from an external source
     -const newData = await mockFetch(`https://example.com/data/${key}`); // Placeholder fetch
     +const newData = mockFetch(`https://example.com/data/${key}`); // Placeholder fetch

       updateCache(key, newData);
       return newData;
     }

From this the correct behavior flows naturally; the API of fetchDataAndUpdateCache() is exactly the same (it still returns a Promise<result>), but it’s not itself async so you can tell at a glance that its internal operation is atomic. (This does mildly change the behavior in that the expiry is now from the start of the request instead of the end; if this is critical to you you can put some code in `updateCache()` like `data.then(() => cache[key].timestamp = Date.now()).catch(() => delete cache[key])` or whatever the exact behavior you want is.)

I‘m not even sure what it would mean to “add a lock” to this code; I guess you could add another map of promises that you’ll resolve when the data is fetched and await on those before updating the cache, but unless you’re really exposing the guts of the cache to your callers that’d achieve exactly the same effect but with a lot more code.

[bheadmaster]

Ok, that's pretty neat. Using Promises themselves in the cache instead of values to share the source of data itself.

While that approach has a limitation that you cannot read the data from inside the fetchDataAndUpdateCache (e.g. to perform caching by some property of the data), that goes beyond the scope of my example.

> I‘m not even sure what it would mean to “add a lock” to this code

It means the same as in any other language, just with a different implementation:

    class Mutex {
        locked = false
        next = []

        async lock() {
            if (this.locked) {
                await new Promise(resolve => this.next.push(resolve));
            } else {
                this.locked = true;
            }
        }

        unlock() {
            if (this.next.length > 0) {
                this.next.shift()();
            } else {
                this.locked = false;
            }
        }
    }

I'd have a separate map of keys-to-locks that I'd use to lock the whole fetchDataAndUpdateCache function on each particular key.

[nurple]

Don't forget to fung futures that are fungible for the same key.

ETA: I appreciate the time you took to make the example, also I changed the extension to `mjs` so the async IIFE isn't needed.

  const CACHE_EXPIRY = 1000; // Cache expiry time in milliseconds
  
  let cache = {}; // Shared cache object
  let futurecache = {}; // Shared cache of future values
  
  function getFromCache(key) {
    const cachedData = cache[key];
    if (cachedData && Date.now() - cachedData.timestamp < CACHE_EXPIRY) {
      return cachedData.data;
    }
    return null; // Cache entry expired or not found
  }
  
  function updateCache(key, data) {
    cache[key] = {
      data,
      timestamp: Date.now(),
    };
  }
  
  var mockFetchCount = 0;
  
  // simulate web request shorter than cache time
  async function mockFetch(url) {
    await new Promise(resolve => setTimeout(resolve, 100));
    mockFetchCount += 1;
    return `result from ${url}`;
  }
  
  async function fetchDataAndUpdateCache(key) {
    // maybe its value is cached already
    const cachedData = getFromCache(key);
    if (cachedData) {
      return cachedData;
    }
  
    // maybe its value is already being fetched
    const future = futurecache[key];
    if(future) {
      return future;
    }
  
    // Simulate fetching data from an external source
    const futureData = mockFetch(`https://example.com/data/${key}`); // Placeholder fetch
    futurecache[key] = futureData;
  
    const newData = await futureData;
    delete futurecache[key];
  
    updateCache(key, newData);
    return newData;
  }
  
  const key = 'myData';
  
  // Fetch data twice in a sequence - OK
  await fetchDataAndUpdateCache(key);
  await fetchDataAndUpdateCache(key);
  console.log('mockFetchCount should be 1:', mockFetchCount);
  
  // Reset counter and wait cache expiry
  mockFetchCount = 0;
  await new Promise(resolve => setTimeout(resolve, CACHE_EXPIRY));
  
  // Fetch data twice concurrently - we executed fetch twice!
  await Promise.all([...Array(100)].map(() => fetchDataAndUpdateCache(key)));
  console.log('mockFetchCount should be 1:', mockFetchCount);

[bheadmaster]

I see, this piece of code seems to be crucial:

    // maybe its value is already being fetched
    const future = futurecache[key];
    if(future) {
      return future;
    }

It indeed fixes the problem in a JS lock-free way.

Note that, as wolfgang42 has shown in a sibling comment, the original cache map isn't necessary if you're using a future map, since the futures already contain the result:

    async function fetchDataAndUpdateCache(key) {
        // maybe its value is cached already
        const cachedData = getFromCache(key);
        if (cachedData) {
          return cachedData;
        }

        // Simulate fetching data from an external source
        const newDataFuture = mockFetch(`https://example.com/data/${key}`); // Placeholder fetch

        updateCache(key, newDataFuture);
        return newDataFuture;
    }

---

But note that this kind of problem is much easier to fix than to actually diagnose.

My hypothesis is that the lax attutide of Node programmers towards concurrency is what causes subtle bugs like these to happen in the first place.

Python, for example, also has single-threaded async concurrency like Node, but unlike Node it also has all the standard synchronization primitives also implemented in asyncio: https://docs.python.org/3/library/asyncio-sync.html