Comparing the use cases of async and sync on both serial and concurrent queues

Queue Type	Method	Description	When to Use	Example Scenario
Serial Queue	sync	Executes the block synchronously, blocking the current thread until the block finishes.	When you need to ensure a specific order of execution and don’t mind blocking the current thread.	Updating a shared resource like a database or file system.
Serial Queue	async	Executes the block asynchronously, returning immediately without blocking the current thread.	When you need to perform I/O-bound tasks without blocking the main thread.	Fetching data from a network or reading/writing a file.
Concurrent Queue	sync	Executes the block synchronously, blocking the current thread until all tasks on the queue finish.	When you need to ensure a specific order of execution for concurrent tasks and don’t mind blocking the current thread.	Not recommended for most use cases as it can lead to deadlocks.
Concurrent Queue	async	Executes the block asynchronously, allowing multiple tasks to run concurrently.	When you need to maximize performance by executing multiple tasks concurrently without blocking the current thread.	Processing multiple images or performing multiple network requests.

I’ve chosen the colors based on the following logic:

1. Red (🔴): For tasks that are potentially blocking or have a sequential nature.
2. Blue (🔵): For asynchronous tasks that don’t block the main thread.
3. Green (🟢): For tasks that are potentially blocking, especially when used with sync on a concurrent queue.
4. Yellow (🟡): For tasks that are truly concurrent and non-blocking.
5. This color-coding helps visualize the flow of execution and potential performance implications.

Serial Queue + Sync

• Executes tasks one by one.
• Blocks the current thread until the task finishes.
• Ensures specific order of execution.
• Ideal for simple sequential tasks.

let serialQueue = DispatchQueue(label: "serialQueue")

serialQueue.sync {
    print("🔴 Task 1 started on serialQueue")
    // Task 1: Some work
    print("🔴 Task 1 finished on serialQueue")
}

serialQueue.sync {
    print("🔴 Task 2 started on serialQueue")
    // Task 2: Some more work
    print("🔴 Task 2 finished on serialQueue")
}

Output:

🔴 Task 1 started on serialQueue
🔴 Task 1 finished on serialQueue
🔴 Task 2 started on serialQueue
🔴 Task 2 finished on serialQueue

Serial Queue + Async

• Executes tasks one by one.
• Does not block the current thread.
• Suitable for I/O-bound tasks.
• Prevents blocking the main thread.

let serialQueue = DispatchQueue(label: "serialQueue")

serialQueue.async {
    print("🔵 Task 3 started on serialQueue")
    // Task 3: I/O-bound task (e.g., network request)
    print("🔵 Task 3 finished on serialQueue")
}

Output:

🔵 Task 3 started on serialQueue
🔵 Task 3 finished on serialQueue

Concurrent Queue + Sync (Not Recommended)

• Executes tasks concurrently.
• Blocks the current thread until all tasks finish.
• Not recommended due to potential deadlocks.
• Can be used for specific scenarios where strict ordering is required.

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

concurrentQueue.sync {
    print("🟢 Task 4 started on concurrentQueue")
    // Task 4: Heavy computation
    print("🟢 Task 4 finished on concurrentQueue")
}

concurrentQueue.sync {
    print("🟢 Task 5 started on concurrentQueue")
    // Task 5: Another heavy computation
    print("🟢 Task 5 finished on concurrentQueue")
}

Output:

🟢 Task 4 started on concurrentQueue
🟢 Task 4 finished on concurrentQueue
🟢 Task 5 started on concurrentQueue
🟢 Task 5 finished on concurrentQueue

Concurrent Queue + Async

• Executes tasks concurrently.
• Does not block the current thread.
• Ideal for CPU-bound tasks.
• Maximizes performance by utilizing multiple cores.

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

concurrentQueue.async {
    print("🟡 Task 6 started on concurrentQueue")
    // Task 6: Image processing
    print("🟡 Task 6 finished on concurrentQueue")
}

concurrentQueue.async {
    print("🟡 Task 7 started on concurrentQueue")
    // Task 7: Another image processing task
    print("🟡 Task 7 finished on concurrentQueue")
}

Output:

The order of execution for tasks on a concurrent queue is not guaranteed. The operating system’s scheduler will determine the actual order based on various factors like thread availability, system load, and task priority.

Here's a possible output:
🟡 Task 6 started on concurrentQueue
🟡 Task 7 started on concurrentQueue
🟡 Task 7 finished on concurrentQueue
🟡 Task 6 finished on concurrentQueue

Or it could be:
🟡 Task 7 started on concurrentQueue
🟡 Task 6 started on concurrentQueue
🟡 Task 7 finished on concurrentQueue
🟡 Task 6 finished on concurrentQueue

Or even something more interleaved:
🟡 Task 6 started on concurrentQueue
🟡 Task 7 started on concurrentQueue
🟡 Task 6 finished on concurrentQueue
🟡 Task 7 finished on concurrentQueue