Deferred

In Kotlin, Deferred<T> is a type that represents a future result of an asynchronous computation, where T is the type of the result. It's a subtype of Job, which represents a cancellable coroutine. Deferred is used with coroutines, which are Kotlin's way of handling asynchronous programming.

A Deferred object is essentially a promise that the computation will eventually provide a result of type T. You can think of it as a non-blocking future or a lightweight thread.

Here are some key points about Deferred:

1. Creation: A Deferred object is usually created through coroutine builder functions like async or produce. For example, val deferredResult: Deferred<Int> = async { computeInt() }.

2. Awaiting Results: To get the result of a Deferred, you use the await() function. This suspends the coroutine until the result is available, without blocking the thread. For example, val result: Int = deferredResult.await().

3. Cancelling: Since Deferred is a subtype of Job, you can cancel a Deferred object using the cancel() method. This is useful if you no longer need the result of the computation, and you want to free up resources. For example, deferredResult.cancel().

4. Status Checking: You can check the status of a Deferred object (e.g., whether it's active, completed, or cancelled) using properties and functions inherited from Job, like isActive, isCompleted, and isCancelled.

5. Exception Handling: If the coroutine that produces the Deferred result fails with an exception, the await() function will rethrow that exception. You can handle this using standard try-catch blocks.

Here's a simple example to illustrate the usage of Deferred:

import kotlinx.coroutines.Deferred
import kotlinx.coroutines.async
import kotlinx.coroutines.delay
import kotlinx.coroutines.runBlocking
import java.time.Instant

/**
 * output:
 * 2024-03-21T16:07:10.130705Z
 * Waiting for result...
 * Result: 42
 * 2024-03-21T16:07:11.164989Z
 * */
fun main() = runBlocking {
    println(Instant.now())

    val deferredResult: Deferred<Int> = async {
        delay(1000) // Simulate some asynchronous computation
        42 // The result of the computation
    }

    println("Waiting for result...")
    val result: Int = deferredResult.await() // Suspends until the result is available
    println("Result: $result")
    println(Instant.now())

    deferredResult.cancel() // Cancel the deferred if needed
}

In your code, the PendingQueryManagerResponse class contains a responsePayload property of type Deferred<QueryReply>. This means that the result of the query is represented as a Deferred object, which can be awaited to get the QueryReply once it's ready, or cancelled if it's no longer needed.