In this code will the fist collect finish before the second

Question

In this code will the fist collect finish before the second collect is started or is possible that the execution will interleave ```val singleThread = newSingleThreadContext SingleThread val flow = flow emit a blocking call delay 1000 emit b flowOn singleThread launch flow collect println it launch flow collect println it ```

octylFractal · Answer

interleaving is impossible here because the collects are linear one must finish before the other can even be called

octylFractal · Answer

if you were to launch them separately then I would say that they could interleave

Luis Munoz · Answer

this is what makes coroutines so difficult to work with I can t be sure if they will interleave or not if I lauch them separately is this documented anywhere or how am I to verify this

octylFractal · Answer

why is it difficult to work with if launched separately it s like asking if I start two programs will one finish before the other obviously you can t know the answer they re executed concurrently not in any ordered way

octylFractal · Answer

if you want them to be ordered you must place some sort of ordering guarantee otherwise you cannot assume they are ordered as you asked for concurrency by using two separate coroutines

Luis Munoz · Answer

it adds another dimension to shared mutable state here I have 1 thread so I know I can share mutable state between coroutines I just can t tell when they overlap because the scheduling of the coroutines is not documented or obvious in anyway

octylFractal · Answer

I don t think they should document the scheduling I don t even know if they can deterministic ally say when one will switch to the other That s partially up to the OS you re running on to pick which threads to run

Luis Munoz · Answer

my execution is on 1 thread

Luis Munoz · Answer

nothing to do with os

octylFractal · Answer

no you re launching on the IO dispatchers

octylFractal · Answer

`collect` s block will be running on the IO threads

Luis Munoz · Answer

oh true so it seems that it must switch between the collects if they suspend

octylFractal · Answer

basically yes if one isn t doing work the other can run this even applies to that `delay` you have in the single thread section that can allow the other flow to start processing because the thread is available for executing work

Luis Munoz · Answer

``` flowOn singleThread ```

Luis Munoz · Answer

code that runs can only be in 1 thread though

Luis Munoz · Answer

so I should be able to share state

octylFractal · Answer

`flowOn` only affects code prior to that call you can use e g HashMap safely but you may not assume that the block runs to completion before another one starts

octylFractal · Answer

if you want that you should probably use `Mutex` to ensure that it can t execute simultaneously it s basically a suspending lock

Rechee Jozil · Answer

You are correct that there s no guarantee of execution order of the blocks within those launch calls They can each be executed on a different thread within <http Dispatchers IO|Dispatchers IO> thread pool

Luis Munoz · Answer

< Rechee Jozil> the order is not the problem the problem is the interleaving