You asked:
Can we perform synchronous operations on Concurrent Queue? As I know synchronous means executing tasks one after another but how it is possible with Concurrent Queue which executes tasks in parallel?
OK, let’s consider terminology before answering your question:
What is a “synchronous operation”? It is one that will block its respective thread during that operation. But a concurrent queue can use multiple threads to perform these individual synchronous operations on that same queue at the same time, each running on its own thread.
Let us use a practical example: Consider a synchronous operation that might be an algorithm to process an image (e.g. resize it or convert a color image to black-and-white). When you perform this operation, it will generally tie up the respective thread until the operation is done.
So, given that example, yes, you can certainly can (and we often do) perform multiple concurrent synchronous operations in parallel. Using our prior example, you might have 4 images that you want to process concurrently. So you might instantiate a concurrent queue, and add these four operations to that queue, and they will be processed in parallel, each on its own “worker thread”.
You then ask:
Similarly, how can we perform asynchronous operation on serial queue as serial queue perform tasks one after another so how they can be executed concurrently?
This depends a little upon what you mean by “operation”. Are you talking about a Swift Operation (or Objective-C NSOperation) on an “operation queue”? Or are you using the term “operation” a little more generally as it applies to GCD and dispatch queues?
The reason I ask, is that in the world of GCD (aka “dispatch queues”), you simply do not “perform an asynchronous operation on a serial queue”. You start asynchronous tasks from a serial queue, but the definition of “asynchronous” means that the current thread does not wait for the task to finish (which generally means that, often behind the scenes, another queue/thread is doing the work).
A good example of that would be when you start a series of network requests from a serial queue. Hidden in NSURLSession/URLSession, it has its own queues/threads that are managing these multiple network requests concurrently. If you do not want these requests to run concurrently, some sleight of hand is required to take an API which is designed for concurrent operation and have it behave sequentially, one after the other.
This is where operation queues come into play, as they do have the concept of custom Operation/NSOperation subclasses, in which you can define an operation to wrap an asynchronous task, such that the operation does not “complete” until the asynchronous task is done. It uses KVO to notify the queue when the operation is executing, is finished, etc. In that scenario, you can define a serial operation queue (i.e., one with a maxConcurrentOperationCount of 1), add a series of your own asynchronous operation subclass instances to that queue, and it can run them sequentially, one after the other. But using operation queues with asynchronous operations can be a little complicated. If that’s really what you are trying to do, we can point you to some examples. But, in the interest of full disclosure, this operation queue pattern is used less frequently nowadays, and you will often see other patterns such as Combine, or the new async-await API, to achieve similar results.
So, we can’t answer this latter question without a little more detail of what precisely you mean by “asynchronous operation on serial queue”. Give us a practical example of what you mean (and what API you are using).
