What is idempotency?

Idempotency is a property of an operation where multiple executions of the same operation with the same input data produce the same result as if the operation was performed once.

In the context of web services or APIs, this means that a repeated request will not alter the system's state further.

An example of an idempotent HTTP method is PUT, which updates a resource. Sending the same PUT request multiple times will result in the same outcome.

In Go, idempotency can be ensured, for example, by checking for the existence of a record before inserting:

func CreateUser(id string, data UserData) error {
    if userExists(id) {
        return nil // already created, operation is idempotent
    }
    return insertUser(id, data)
}

Thus, calling CreateUser repeatedly with the same id will not lead to an error or duplication.