Using yield in nested object in Kotlin sequence - spring

Question

I want to stream result objects captured by Spring JDBC RowCallbackHandler using via a Kotlin Sequence.
The code looks basically like this:
fun findManyObjects(): Sequence<Thing> = sequence {
val rowHandler = object : RowCallbackHandler {
override fun processRow(resultSet: ResultSet) {
val thing = // create from resultSet
yield(thing) // ERROR! No coroutine scope
}
}
jdbcTemplate.query("select * from ...", rowHandler)
}
But I get the compilation error:
Suspension functions can be called only within coroutine body.
However, exactly this "coroutine body" should exist, because the whole block is wrapped in a sequence builder. But it doesn't seem to work with a nested object.
Minimal example to show that it doesn't compile with a nested object:
// compiles
sequence {
yield(1)
}
// doesn't compile
sequence {
object {
fun doit() {
yield(1) // Suspension functions can be called only within coroutine body.
}
}
}
How can I pass an object from the ResultSet into the Sequence?

Answer 1

Use Flow for asynchronous data streams
The reason you can't call yield inside your RowCallbackHandler object is twofold.
The processRow function isn't a suspending function (and can't be, because it's declared in and called by Java). A suspending function like yield can only be called by another suspending function.
A sequence always ends when the sequence { ... } builder returns. Even if you and I know that the query method will invoke the RowCallbackHandler before returning from the sequence, the Kotlin compiler has no way of knowing that. Yielding sequence values from functions and objects other than the body of the sequence itself is never allowed, because there's no way of knowing where or when they will run.
To solve this problem, we need to introduce a different kind of coroutine: one that can suspend itself while it waits for the RowCallbackHandler to be invoked.
Unfortunately, because we're talking about JDBC here, there may not be much to gain by introducing full-blown coroutines. Under the hood, calls to the database will always be made in a blocking way, removing a lot of the benefit. It might well be simpler not to try and 'stream' results, and just iterate over them in a boring, old-fashioned way. But let's explore the possibilities all the same.
The problem with sequences
Sequences are designed for on-demand computation, and are not asynchronous. They can't wait for other asynchronous operations, such as callbacks. The sequence builder's yield function simply suspends while waiting for the caller to retrieve the next item, and it's the only suspending function a sequence is ever allowed to call. You can demonstrate this if you try to use a simple suspending call like delay inside a sequence. You'll get a compile error letting you know that you're operating in a restricted coroutine scope.
sequence<String> { delay(1000) } // doesn't compile
Without the ability to call suspending functions, there's no way to wait for a callback to be invoked. Recognising this limitation, Kotlin provides an alternative mechanism for streams of on-demand values that do provide data in an asynchronous way. It's called a Flow.
Callback flows
The mechanism for using Flows to provide values from a callback interface is described very nicely by Roman Elizarov in his Medium article Callbacks and Kotlin Flows.
If you did want to use a callback flow, you'd simply replace sequence with callbackFlow, and replace yield with sendBlocking.
Your code might look something like this:
fun findManyObjects(): Flow<Thing> = callbackFlow {
val rowHandler = object : RowCallbackHandler {
override fun processRow(resultSet: ResultSet) {
val thing = // create from resultSet
sendBlocking(thing)
}
}
jdbcTemplate.query("select * from ...", rowHandler)
close() // the query is finished, so there are no more rows
}
A simpler flow
While that's the idiomatic way to stream values provided by a callback, it might not be the simplest approach to this problem. By avoiding callbacks altogether, you can use the much more common flow builder, passing each value to its emit function. But now that you have asynchrony in the form of coroutines, you can't just return a flow and then allow Spring to immediately close the result set. You need to be able to delay the closing of the result set until the flow has actually been consumed. That means peeling back the abstractions provided by RowCallbackHandler or ResultSetExtractor, which expect to process all the results in a blocking way, and instead providing your own implementation.
fun Connection.findManyObjects(): Flow<Thing> = flow {
prepareStatement("select * from ...").use { statement ->
statement.executeQuery().use { resultSet ->
while (resultSet.next()) {
val thing = // create from resultSet
emit(thing)
}
}
}
}
Note the use blocks, which will deal with closing the statement and result set. Because we don't reach the end of the use blocks until the while loop has completed and all the values have been emitted, the flow is free to suspend while the result set remains open.
So why use a flow at all?
You might notice that if you do it this way, you can actually replace flow and emit with sequence and yield. So have we come full circle? Well, sort of. The difference is that a flow can only be consumed from a coroutine, whereas with sequence, you can iterate over the resulting values without suspending at all. In this particular case, it's a hard call to make, because JDBC operations are always blocking.
If you use a sequence, the calling thread will block as it waits to receive the data. Values in a sequence are always computed by the thing consuming the sequence, so if the sequence invokes a blocking function, the consumer's thread will block waiting for the value. In a non-coroutine application, that might be okay, but if you're using coroutines, you really want to avoid hiding blocking calls inside innocuous-looking sequences.
If you use a flow, you can at least isolate the blocking calls by having the flow run on a particular dispatcher. For example, you could use the built-in IO dispatcher to perform the JDBC call, then switch back to the default dispatcher for any further processing. If you definitely want to stream values, I think this is a better approach than using a sequence.
With all this in mind, you'll need to be careful with your use of coroutines and dispatchers if you do choose one of these solutions. If you'd rather not worry about that, there's nothing wrong with using a regular ResultSetExtractor and forgetting about both sequences and flows for now.