Usually, a singleton instance is created when the struct is first initialized. To make this happen, we define the getInstance method on the struct. This method will be responsible for creating and returning the singleton instance. Once created, the same singleton instance will be returned every time the getInstance is called.

How about goroutines? The singleton struct must return the same instance whenever multiple goroutines are trying to access that instance. Because of this, it’s very easy to get the singleton design pattern implemented wrong. The example below illustrates the right way to create a singleton.

Some points worth noting:

There is a nil-check at the start for making sure singleInstance is empty first time around. This is to prevent expensive lock operations every time the getinstance method is called. If this check fails, then it means that the singleInstance field is already populated.

The singleInstance struct is created within the lock.

There is another nil-check after the lock is acquired. This is to ensure that if more than one goroutine bypasses the first check, only one goroutine can create the singleton instance. Otherwise, all goroutines will create their own instances of the singleton struct.