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Comment by SebastianKra | Hacker News Reader

The discussion around async await always focuses on asynchronous use-cases, but I see the biggest benefits when writing synchronous code. In JS, not having await in front of a statement means that nothing will interfere with your computation. This simplifies access to shared state without race conditions.

The other advantage is a rough classification in the type system. Not marking a function as async means that the author believes it can be run in a reasonable amount of time and is safe to run eg. on a UI main thread. In that sense, the propagation through the call hierarchy is a feature, not a bug.

I can see that maintaining multiple versions of a function is annoying for library authors, but on the other hand, functions like fs.readSync shouldn’t even exist. Other code could be running on this thread, so it's not acceptable to just freeze it arbitrarily.

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Maybe I am missing something. But the function coloring problem is basically the tension that async can dominate call hierarchies and the sync code in between looses it's beneficial properties to a degree. It's at least awkward to design a system that smoothly tries to blend sync that executes fast and async code that actually requires it.

Saying that fs.readSync shouldn't exist is really weird. Not all code written benefits from async nor even requires it. Running single threaded, sync programs is totally valid.

> Not all code written benefits from async nor even requires it. Running single threaded, sync programs is totally valid.

Maybe, but is it useful to have sync options?

You can still write single threaded programs

I mean single threaded + sync.

Sync options are useful. If everything is on the net probably less so. But if you have a couple of 1ms io ops that you want to get done asap, it's better to get them done asap.

> But if you have a couple of 1ms io ops that you want to get done asap, it's better to get them done asap.

and async prevents this how?

my statement was in response to "fs.readSync shouldn't exist". that is how.

if it didn't exist, the async version would still exist, which you could use to get it done asap

stop

'readSync' does two different things - tells the OS we want to read some data and then waits for the data to be ready.

In a good API design, you should exposed functions that each do one thing and can easily be composed together. The 'readSync' function doesn't meet that requirement, so it's arguably not necessary - it would be better to expose two separate functions.

This was not a big issue when computers only had a single processor or if the OS relied on cooperative multi-threading to perform I/O. But these days the OS and disk can both run in parallel to your program so the requirement to block when you read is a design wart we shouldn't have to live with.

> tells the OS we want to read some data and then waits for the data to be ready

No, it tells the OS "schedule the current thread to wake up when the data read task is completed".

Having to implement that with other OS primitives is a) complex and error-prone, and b) not atomic.

The application in question is frozen for that period though, that's the wait they're referring to.

Even websites had this problem with freezing the browser in the early AJAX days, when people would do a synchronous XMLHttpRequest without understanding it.

he was referring to fs.readSync (node) which has also has fs.read, which is async. there is also no parallelism in node.

i don't see it as very useful or elegant to integrate any form for parallelism or concurrency into every imaginable api. depends on context of course. but generalized, just no. if a kind of io takes a microsecond, why bother.

The function coloring problem represents multiple complaints. I disagree that the propagation of async makes the sync case irrelevant. In the frontend, receiving a promise has completely different implications on loading states. In the backend, I usually try to separate side-effects from pure functions, so the pure functions are usually sync.

Because JS is single threaded, fs.readSync will freeze the entire app. The only case where I would find that acceptable is in cli-scripts. But that could also be achieved with nodejs’ support for top-level await. There's perhaps a slight overhead from the Promise being created, but JS-Engines have so many optimizations that I don't even know if that matters. If nothing else is scheduled, awaiting a promise is functionally the same as blocking. Even in rare cases where you do want to block other scheduled events from running, you could achieve that with an explicit locking mechanism instead.

You could argue that filesystem access is fast so blocking everything is fine, but what if the file happens to be on a NAS somewhere?

> This simplifies access to shared state without race conditions

But in ordinary JS there just can't be a race condition, everything is single threaded.

And it doesn't actually prevent concurrency.

Sure, but concurrent != parallel. You can't have data races with a single thread of execution - a while loop writing i=0 or i=1 on each iteration is not a data race.

Two async functions doing so is not a data race either.

Data race != Race condition

Data races are a specific race condition - they may be safe or cause tearing.

Serially, completely synchronously overwriting values is none of these categories though.

You're mixing up quite a few somewhat related but different concepts: data races, race conditions, concurrency and parallelism.

Concurrency is needed for race conditions, parallelism is needed for data races. Many single threaded runtimes including JS have concurrency, and hence the potential for race conditions, but don't have parallelism and hence no data races.

Concurrency with a single thread of execution runs with complete mutual exclusion, so no "pure" single threaded concurrency is definitely race condition free.

What we may argue over (and it becomes more of a what definition to use): IO/external event loop/signal handlers. These can cause race conditions even in a single threaded program, but one may argue (this is sort of where I am) that these are then not single threaded. The kernel IO operation is most definitely not running on the same thread of execution as JS.

I think I have been fairly consistent in the definition of a data race as a type of race condition, where a specific shared memory is written to while other(s) read it with no synchronization mechanism. This can be safe (most notably OpenJDK's implementation is tear-free, no primitive or reference pointer may ever be observed as a value not explicitly set by a writer), or unsafe (c/c++/rust with unsafe, surprisingly go) where you have tearing and e.g. a pointer data race can cause the pointer to appear as a value that was never set by anyone, causing a segfault or worse.

You can implement your own event loop within a single thread

You should really look up the definition of race condition; it has nothing to do with parallel processing. Parallel processing just makes it harder to deal with.

You can definitely have a race condition in JS. Being single-threaded means you don't have parallelism, but you still have concurrency, and that's enough to have race conditions. For example you might have some code that behaves differently depending on which promise resolves first.

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