What is the architectural difference between a cold flow and a hot flow in Kotlin?

Short answer

A cold flow (Flow) does not execute its code until there is a consumer (collector), while a hot flow (StateFlow/SharedFlow) broadcasts data in memory at all times even if there is no listener.

Explanation

A cold flow is like a CD that makes no sound until a specific listener plays it, and each new consumer starts the sequence from scratch (such as a database query). A hot flow is like a live radio broadcast; it lives independently in memory and shares the same data with several listeners at once (multicasting) and keeps the last value (StateFlow) or a replay cache (SharedFlow).

Code example

// Cold Flow: Starts from scratch with each collect()
fun getNumbers(): Flow<Int> = flow {
    emit(1); emit(2)
}

// Hot Flow: Retains state in memory and emits it to current observers
val stateFlow = MutableStateFlow("Initial Value")

Common mistakes

• Converting a cold flow to a hot flow using shareIn or stateIn without specifying a CoroutineScope and a suitable SharingStarted policy, causing a leak in data consumption.

Follow-up questions

• When do you use Channel and when do you prefer Flow in the architecture of Android apps?

Sources

• Kotlin Docs — Asynchronous Flow