TransactionsRepository.kt

class TransactionsRepository(
  private val defaultDispatcher: CoroutineDispatcher = Dispatchers.Default
) {

  // Mutex protecting the cache mutable state
  private val cacheMutex = Mutex()
  private val transactionsCache = mutableMapOf<User, List<Transaction>()

  private suspend fun addTransaction(user: User, transaction: Transaction) =
    withContext(defaultDispatcher) {
      // Mutex makes the read&write cache operation thread safe
      cacheMutex.withLock {
        if (transactionsCache.contains(user)) {
          val oldList = transactionsCache[user]
          val newList = oldList!!.toMutableList()
          newList.add(transaction)
          transactionsCache.put(user, newList)
        } else {
          transactionsCache.put(user, listOf(transaction))
        }
      }
    }
}