What are the main differences between synchronous and asynchronous queue structures in data processing?

A synchronous queue processes tasks sequentially: each task is fully completed before the next begins, which can block the execution thread.

An asynchronous queue allows tasks to be queued and execution to continue without waiting for their completion. Processing occurs in the background, and results can be obtained via callbacks, promises, or async/await.

In iOS, for example, a synchronous queue (DispatchQueue) blocks the current thread until the task is finished, while an asynchronous queue starts the task and immediately returns control, allowing the UI to remain responsive.

Example:

let queue = DispatchQueue(label: "com.example.queue")

// Synchronous
queue.sync {
    print("Executing task")
}

// Asynchronous
queue.async {
    print("Executing task")
}
print("This code runs immediately after async")