GCD (Grand Central Dispatch)

GCD is the most commonly used API to manage concurrent code and execute operations asynchronously at the Unix level of the system. GCD provides and manages queues of tasks.

Dispatch Queues:

Dispatch queues are an easy way to perform tasks asynchronously and concurrently in your application. They are queues where tasks are being submitted by your app in form of blocks (Blocks of codes). There are two varieties of dispatch queues: (1) serial queues, & (2) concurrent queues. Before talking about the differences, you need to know that tasks assigned to both queues are being executed in separate threads than the thread they were created on. In other words, you create blocks of code and submit it to dispatch queues in the main thread. But all these tasks (Blocks of codes) will run in separate threads instead of the main thread.

1. Serial Queues:

When you choose to create a queue as serial queue, the queue can only execute one task at a time. All tasks in the same serial queue will respect each other and execute serially.

However, they don’t care about tasks in separate queues which means that you can still execute tasks concurrently by using multiple serial queues. For example, you can create two serial queues, each queue executes only one task at a time but up to two tasks could still execute concurrently.

Serial queues are awesome for managing a shared resource. It provides guaranteed serialized access to the shared resource and prevents race conditions.

2. Concurrent Queues:

As the name suggests, concurrent queues allows you to execute multiple tasks in parallel. The tasks (blocks of codes) starts in the order in which they are added in the queue. But their execution all occur concurrently and they don’t have to wait for each other to start. Concurrent queues guarantee that tasks start in same order but you will not know the order of execution, execution time or the number of tasks being executed at a given point.

For example, you submit three tasks (task #1, #2 and #3) to a concurrent queue. The tasks are executed concurrently and are started in the order in which they were added to the queue. However, the execution time and finish time vary. Even it may take some time for task #2 and task #3 to start, they both can complete before task #1. It’s up to the system to decide the execution of the tasks.