What is the difference between a serial and a concurrent queue?

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Main Source: 🔗 Ace the iOS Interview

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TL/DR

We can create both types of queues with the help of DispatchQueue which is built on top of Grand Central Dispatch.

In the case of a serial queue, the tasks will complete in the order they are added to the queue (FIFO); the first task in the queue will complete before the next task begins.

Serial queues are often used to provide synchronized access to a shared resource in order to prevent race conditions.

let serialQueue = DispatchQueue(label: "aryamansharda")

serialQueue.async {
    print("This happens first!")
}

serialQueue.async {
    print("This happens second!")
}

As you’d expect, the output will be “This happens first!” and then “This happens second!”.

On the other hand, a concurrent queue allows us to start multiple tasks at the same time. However, while we’re starting all of these tasks at the same time, a concurrent queue makes no guarantee about the order in which the tasks willfinish as these tasks are run in parallel.

If you need to establish dependencies between operations or need to ensure a completion order, consider using an OperationQueue or a serialqueue instead.

let concurrentQueue = DispatchQueue(label: "aryamansharda",
attributes: .concurrent)

concurrentQueue.async {
print("I'm added to the queue first!")
}

concurrentQueue.async {
print("I'm added to the queue second!")
}

Even though we’re adding tasks sequentially, it’s plausible for the second task to finish before the first one completes. It’s simply a matter of how the system dispatches the work to the underlying threads and their respective workloads.