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kotlinlang

Hello all, I am having a difficult time wrapping my head around a concurrency issue when using coroutines.  As a small example I have data coming in from the system that I save locally to a database.  At some frequency I want to push those results in a batch request to a server.  I would like to ensure that only one push “task” is running at time to reduce load on the server.  Will throw some code in this thread…

```private var pushJob: Job?  = null

fun onEvent(event: Event) {
   // Called on Main thread
   lifecycleScope.launch {
      eventRepository.save()  // Executes on IO thread
      pushJob?.join()
      pushJob = push(location)
   }
}

private suspend fun push() = coroutineScope {
   launch {
      Log.v("Test", "start push")
      ...
      locationRepository.pushLocations()  // Executes on IO thread
      ...
      Log.v("Test", "end push")
   }
}```
Attempt a single push function executing at any given time. However I am not seeing start/stop pairs in the logs. Issue with the join and assignment not being synchronized?

`coroutineScope { launch { ... } }` is basically equivalent to `...`, the job is always complete by the time the `suspend fun` returns

Ok, that makes sense, thank you.  So something like this?

```fun onEvent(event: Event) {
   // Called on Main thread
   lifecycleScope.launch {
      eventRepository.save()  // Executes on IO thread
      pushJob?.join()
      pushJob = launch {
         Log.v("Test", "start push")
          ...
          locationRepository.pushLocations()  // Executes on IO thread
          ...
          Log.v("Test", "end push")
      }
   }
}```

that could work but why not just use a `Mutex`?

or a `limitedParallelism` dispatcher, if what you're running is a blocking operation

I would like to allow the save to be non blocking, but yeah I think this would achieve what I am after as the save is inside the coroutine and new calls to onEvent would just launch a new coroutine, save and then block if necessary

```fun onEvent(event: Event) {
   // Called on Main thread
   lifecycleScope.launch {
      eventRepository.save()  // Executes on IO thread
      mutex.withLock {
         Log.v("Test", "start push")
          ...
          locationRepository.pushLocations()  // Executes on IO thread
          ...
          Log.v("Test", "end push")
      }
   }
}```