What is the difference between serialized and concurrent queues?

Serialized queues process tasks strictly sequentially, one after another. The next task does not start until the current task is completed.

Concurrent queues can execute multiple tasks in parallel. The order of start is not guaranteed, but tasks from the same queue are executed in the order they were added. The order of completion is also not guaranteed.

Attribute	Serialized queues	Concurrent queues
Start order	Strictly sequential	In order of addition
Execution order	Strictly sequential	Parallel
Completion order	Strictly sequential	Not guaranteed
Data state	Easy to manage	Requires synchronization

Example of creation:

// Serialized queue
let serialQueue = DispatchQueue(label: "com.example.mySerialQueue")

// Concurrent queue with QoS .userInitiated
let concurrentQueue = DispatchQueue(label: "com.example.myConcurrentQueue", attributes: .concurrent)
// Built-in global concurrent queue
let globalQueue = DispatchQueue.global(qos: .userInitiated)

Usage:

serialQueue.async {
    // Task 1
}
serialQueue.async {
    // Task 2 (starts after Task 1 completes)
}

concurrentQueue.async {
    // Task 1
}
concurrentQueue.async {
    // Task 2 (may start before Task 1 finishes)
}