Using coroutines to write to a log file without loosing the order the logs are in

Question

I have created a custom logger to write all of my logs to file as well as to the android log. Since file writing is a blocking operation I now want to make the file-write asynchronous with Kotlin coroutines. Just wrapping the write in a coroutine doesn't work because then some logs get switched around and aren't written to file in the correct order. How can I make sure the logs are written sequentially while not blocking the main thread with file operations, optimally using Kotlin coroutines?

Answer 1

You can try to emit logs to MutableSharedFlow to write logs sequentially to a file:

class Logger {
    // make sure `replay`(in case some logs were emitted before sharedFlow is being collected)
    // and `extraBufferCapacity` is enough to handle all logs
    private val sharedFlow = MutableSharedFlow<String>(replay = 64, extraBufferCapacity = 64)
    private val scope = CoroutineScope(Dispatchers.IO)

    init {
        sharedFlow.onEach { log ->
            println("$log")
            // write to file here
            TimeUnit.MILLISECONDS.sleep(100) // simulate writing to file
        }.launchIn(scope)
    }

    fun log(message: String) {
        sharedFlow.tryEmit(message) 
    }
}

fun test() {

    val logger = Logger()

    repeat(10) { item ->
        logger.log("Log $item")
    }
}

Need to be sure there are enough elements set to replay and extraBufferCapacity parameters of MutableSharedFlow to handle all logs.