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Jetbrains backs off on Kotlin/Native memory model, Android devs too stupid to learn new tricks

Now that Google IO 2021 is behind our backs and it essentially boiled down to refining existing libs, fresh coat of paint, and copying Apple Privacy policies, the most shocking revelation to me was Jetbrains update on the Kotlin/Native memory model which dropped like a bomb:

TL;DR: The original Kotlin/Native memory management approach was very easy to implement, but it created a host of problems for developers trying to share their Kotlin code between different platforms.

Of course this reversal is hidden among piles of marketing speech, but is happily pointed out by the comments, where Roman Elizarov can't say much other than, "yeah, we will make that work".

This needed change comes a bit too late to the party, because most Android developers with an interest in Kotlin/Native have probably already touched the big pile of poo that Kotlin/Native memory model is and decided to avoid it, or are waiting for somebody else to do the hard work. In related articles the change is mentioned briefly, but more damning is the continuous highlight of KMM success cases (Kotlin Multiplatform Mobile, implicitly Kotlin/Native). Why? Well, if you go into advocacy content, you will see on Google's own blog that Kotlin is the most used primary language by professional Android developers and is included in 80% of the top 1000 apps.

But when I go to Youtube, a Google product, it suggests me videos like Is Kotlin Multiplatform Mobile Ready for Production?. Gosh, it does not suggest me videos like "Is Kotlin Android ready for production?", simply because nobody is doing videos like that, everybody knows Kotlin is perfectly fine, so much that you get videos titled Why Kotlin is taking over the world. But KMM? Not so much. I see frequent "I made a KMM app" articles, but again, nobody is writing "I made a Kotlin Android app". Nobody is interested in something that works, but everybody advocates things that maybe are not popular for good reasons, or is writing themselves some CV item for a job hire.

Mutable state between threads is evil

Mutate shared memory? Disgusting. What are you, a caveman?

So what happened here? What did we miss? What was wrong and made KMM unpopular among the masses? Well, point 4 of Nine Highlights form the Kotlin Roadmap explains it briefly:

"Prepare to say goodbye to your old buddy InvalidMutabilityException, as you’ll stop seeing it every time you work with Kotlin/Native!"

When the first Kotlin/Native documentation was published you had to carefully look for it but it stated that the garbage collector would disallow sharing memory between threads. At some point after 1.x the documentation started to mention this more up front, but it doesn't change the fact that it's a pretty horrible feature. And the biggest reason for this being horrible is that Kotlin/Native attempts (without success) to make Android code shareable with iOS and fails. It fails not because it won't work, but because existing code won't work.

A happy Android Kotlin programmer will have their own pile of battle tested code that works perfectly fine on Android. They follow the guides that tell them to move separate logic into a Kotlin/Native module, recompile, and yep, everything works. Now they try to run this on iOS and nothing works, everything crashes. Why? The Android version does not actually use Kotlin/Native, because Kotlin already runs on the JVM. Making Android use Kotlin/Native under the hood would be potentially worse in terms of performance because the jumps between JVM and native would decrease the performance. But at least the Android developer would experience first hand if what they are writing is actually good, because the compiler or IDE won't help. Of course nobody has time to care about shit like that.

This unfortunate state of affairs means that Android programmers can't actually test their Kotlin/Native code until they reach out for the iOS version, and the experience of doing so is terribly more inconvenient and slow than simply running the Android version. So much, that the usual behaviour is to implement first the Android version, then try on iOS, see surprise crashes and pull out your hair trying to figure out why. The better alternative would be to write code step by step and test it incrementally on iOS, but as mentioned the jump is so cumbersome compared to testing the Android version that nobody cares (until it's too late).

Another consequence of Kotlin/Native memory model is that most likely no code you already have will work, because chances are high you are mutating some class from a different thread that this class was created from. If this sounds familiar, it's because I've already mentioned this in the context of Apple's Core Data or the Realm mobile database and how it can fuck up your program if you are not careful enough. Being careful means that you should not happily pass around classes as you are used to, and instead make immutable copies that can be shared without danger. But immutable copies means that if you want to change something now you have to go through hoops to refresh that same data being shared somewhere else.

Consider a typical case of app where the user opens a screen with a list and the client fetches data from the server to show to the user. Usual pseudo steps would be:

  1. UI layer wants data, starts spinning some visual indicator that the app is working, then spawns a thread to actually fetch the data.
  2. Your background thread actually performs the call to the server (synchronous or asynchronous doesn't really matter). The fetched result is likely to be parsed from JSON or some other wire representation into your business domain objects.
  3. Once your background thread finishes and stores the results somewhere, calls back the UI thread with the fresh data.
  4. The UI thread stops spinning the visual indicator and displays the new data.

The user now touches some item on the list and sees the detailed view and decides to change something, which requires performing changes on the backend.

  1. UI layer requests modification, starts spinning some visual indicator that the app is working, then spawns a thread to actually perform a network request that changes the data and possibly fetches a new copy of it, just in case.
  2. A background thread performs the request, and presuming everything went OK, receives new wire representation of your business domain object with the updated changes.
  3. Once your background thread finishes, it calls the UI thread with the fresh data.
  4. The UI thread stops spinning the visual indicator and displays the updated data.

This would all be great if not for the fact that if the user goes back to the previous list they will see the old data, because we didn't update it. And here is where different programmers with different views on architecture clash. Since the detail view is only updating a single element, and this element is already viewed in a different screen, one way to handle the situation is to keep track of the first collection somewhere in memory (oh no, a scary global variable!), and whenever your detail view wants to modify the data the business logic code updates the global array with the new entry and notifies all observers that the collection was updated, requiring them to refresh it. But since the collection pointer is the same, the observers only need to redraw the data, since they already have access to the globally shared collection.

This wont work in Kotlin/Native, and now the code has to be updated. The correct way to handle this situation is to not let the different observers have a pointer/reference to the original data, and instead observers should get a copy of the global list. Now, when the user wants to change any object, the global list will be updated, and a refresh event will be signalled, but each observer will have to request a new copy of the list containing the updated objects. Nothing really hard, but, you know, it's the kind of thing that it's pretty difficult to implement incrementally when your already existing app already shares all of its data everywhere. You can't just stop the sharing at some arbitrary boundary and still have it work were you want it, the whole way you update data across the app requires changing.

And this is only in the ideal cases where you have structured your data in a separate business logic layer with separate threads handling the mutability of the data. I'm not afraid to say that I have to maintain apps where these clean boundaries are frequently broken. Say you want to change the data, well, the first thing the UI layer does is of course modify the data itself, then start the spinner to notify the backend in a thread with the already mutated object as input, and if the backend fails, the local version is mutated again so as to revert the initial local change. This UI transgression doing stuff that should be part of the business logic is unfortunate, but if your employer cuts corners in the time you have to implement features, sometimes you are stuck with such code and ordered to maintain it (refactor? We don't spend dollars on that shit here!). For such bad luck Brian developers adopting Kotlin/Native probably means discarding most of its codebase.

Essentially, all Kotlin/Native code runs fine on JVM, but only a subset of JVM valid code will work at all on Kotlin/Native. And this is a hard rock to swallow compared to the incredible Java interoperability that has allowed Kotlin to replace it in many places incrementally. After all, if your old code doesn't work and you have to write new code, why not use a different tool other than Kotlin anyway?

Mutating business objects from the UI layer? I've heard enough. You better start running now or I'm going to solve all your memory model problems once and for all.

Who cares about garbage collection with separate heaps?

Ironically the people caring the most about garbage collectors are probably the fewest of its actual users: language compiler writers. See, in the examples above there is no actual performance bottleneck, most of the time your app will spend waiting seconds to get something from the network. Seconds which are like ages in terms of CPU time. I've yet to see an UI developer say: "Oh, bollocks, I wish the garbage collector was faster because it's the bottleneck of my app".

But that is the life of language compiler writers, because after all, their pride is in the generated code, and every language programming fanboy is going to take any useless micro benchmark as comparison and laugh and point at you as a programmer failure because somebody else, with different language constraints can generate a few loop iterations more per second on a Raspberry π. Loop iterations you probably won't be spending in any of your actual real life code but which are tremendously important for marketing Medium posts.

It is therefore easy to understand why language compiler developers prefer a garbage collector with separate threads were no memory sharing happens. In fact, it's Python's famously Global Interpreter Lock that makes people drag it into the mud, since every object creation/destruction has to stop the world and see no other thread is accessing the same data. On the other hand, Python marketeers, those who want to sell you a bridge, say its thanks to the Global Interpreter Lock that Python is so popular. You know, like:

Man, imagine if we had some sort of Virtual Machine which allowed different threads to not have a Global Virtual Machine Lock and somebody implemented Python on top of it. In any case, probably the 99% of the code that 99% of the programmers of the world produce today is not performance constrained, yet most of these language programmers will look at the ridiculous micro benchmarks and discard one language over another because of some stupid example. As such, new language programming creators are pressured to improve the performance of silly examples, and a garbage collector with separate heaps is a total win for these benchmarks.

When a language implements such a garbage collector it's trading performance. You suddenly have top performance inside your separate threads, because they indeed don't have to stop the world every time a different threads allocates/frees memory. But to communicate with other threads you now have the slowing down of memory copies, since memory available to one thread is not directly visible in another (unless you play with fire and eventually get an InvalidMutabilityException). So you are trading the individual memory access overhead with memory sharing overhead that forces you to make copies of everything that needs sharing. On the bright side you are also avoiding potential race conditions over shared mutable memory, but since such race conditions are hard to find and debug people just say "oh, reboot and try again", and mentally it's not even part of the discussion.

The programming world at large is not ready for a thread model with immutable data. Mainstream programming languages use for better or worse a totally mutable memory model, and as such most programmers learn multi threading using this model, battling race conditions manually with locks, semaphores, queues, or whatever else they find. Jetbrain has finally found out that all this mental investment does not want to go away, so they are changing their original tune. If they kept pushing their original memory model, chances are Kotlin/Native would never flourish like Kotlin Android has. Essentially, you can't teach an old dog new programming tricks.

On the Internet, nobody knows you're a dog

Conclusion

It has taken Jetbrains only 4 years to realise Android developers don't like being raped by InvalidMutabilityException whenever their valid JVM code tries to run on Kotlin/Native. Personally I don't mind, I've already made peace with my prejudices and accepted that the future is in memory models which disallow sharing mutable data. But it's easy to look at reactions of other developers at the thought of coming back to a mutable world and verify that yes, everybody likes the status quo:

Mango1666: Fantastic news for kotlin native! The memory model made things a bit weird with my even pretty limited scope of KMM projects. Having KN act "normal" without changes is going to be great

diamond: Yeah, this is really good news. I've been working with KMP for a while now, and the K/N memory model is a constant source of irritation. It's not a total showstopper, but definitely a barrier to widespread adoption. I'm looking forward to seeing where they go with this.

Indeed, it's going to be interesting where Roman Elizarov wants to go with this change, since he seems to be the one spearheading Jetbrain's efforts at structured concurrency and the memory model could potentially impact it. After all, once the restriction of immutable memory between threads is lifted, why learn structured concurrency at all when your good'ol unstructured concurrency works™?.

We could also see these news the other way round: developers are too stupid to learn new ways to code, and so we must sacrifice future performance for ease of use. This is why we can't have nice things.

$ nim c -r threaded_code.nim
Error, no programmers found.

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Published on: 30/05/2021 19:28. Last update: 31/05/2021 11:29. rss feed
Copyright 2019 by Grzegorz Adam Hankiewicz.
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