What s the best option for controlling shared mutable state

Question

What s the best option for controlling shared mutable state in common code I have a cache that can be accessed and updated by multiple coroutines on JVM anyway Actors are unavailable on native so should I go with a Mutex approach or make the cache an AtomicReference

Arkadii Ivanov · Answer

Mutex won t help as you can t share mutable state across threads even if access is properly synchronized Go with AtomicReferenc + freeze + immutable structures

basher · Answer

stately has frozen mutable structures

kpgalligan · Answer

kpgalligan · Answer

You can also keep all of your data in a single thread by way of a worker and communicate through messages It feels like a weird way to do it but it ll work

kpgalligan · Answer

Stately collections work but pending the outcome of relaxed mode they need a rewrite Performance is so so

serebit · Answer

Could I have the cache be thread local and contain mutable data and have other threads communicate with it via messages with immutable data I m not very familiar with the memory model of native

kpgalligan · Answer

Yeah that s essentially what I m proposing You can keep frozen data in a non frozen container That s not going to be an issue Just don t freeze the container

serebit · Answer

I also know that threading behavior of coroutines in K N is going to be revamped significantly in the coming months so I m more concerned with race conditions on JVM than with threading on Native at least for now

serebit · Answer

That reminds me is data frozen by default when crossing threads or do I have to freeze it myself Because having to freeze it myself would make writing this system in common code a lot more difficult

kpgalligan · Answer

Valid concern

basher · Answer

you have to freeze it yourself ` freeze `

Arkadii Ivanov · Answer

But you can freeze it inside your cache class So clients won t care about it

kpgalligan · Answer

That reminds me is data frozen by default when crossing threads or do I have to freeze it myself Because having to freeze it myself would make writing this system in common code a lot more difficult Not with stately There s a common definition for freeze that is a noop on jvm and js

serebit · Answer

Cool no pun intended Thanks for the info

Kris Wong · Answer

have you seen calling `freeze` on an object deadlock

kpgalligan · Answer

No

kpgalligan · Answer

Crash or just stop

Kris Wong · Answer

i guess that makes me special stuck out tongue

kpgalligan · Answer

Example

Kris Wong · Answer

well in unit tests it appears to be crashing from an iOS app unit test it s getting stuck

Kris Wong · Answer

so both

kpgalligan · Answer

Do you have a public example I ve been doing this for a while and have never seen that I think a lot of people would be very interested in seeing that happen

Kris Wong · Answer

unfortunately I do not

kpgalligan · Answer

Can you write an example of a block of code you think is deadlocking I will try it now Is it always or just sometimes

Kris Wong · Answer

always

basher · Answer

are you calling it on a background thread not managed by K N

basher · Answer

i could see something weird happening if you forgot to `initRuntimeIfNeeded `

kpgalligan · Answer

I mean I ve never seen this and have done quite a bit of both testing and calling freeze

kpgalligan · Answer

An example piece of code would be helpful

Kris Wong · Answer

could be XCTest is managing the threads let me explore a little more in the debugger

Kris Wong · Answer

no it s the main thread

Kris Wong · Answer

I added a method to a class `makeImmutable` which just calls freeze unfortunately I can t step into that

Kris Wong · Answer

let me try App Code

kpgalligan · Answer

kpgalligan · Answer

Kotlin xcode plugin

serebit · Answer

This isn t really related to my original post can you guys move this to a new thread

basher · Answer

Can just DM Kevin and I ll find out what happened later slightly smiling face

Kris Wong · Answer

hah