Which service model is specifically designed to charge clients based on execution time instead of fixed hourly rates?

Function as a Service (FaaS) is designed to charge clients based on execution time rather than fixed hourly rates. This service model allows developers to run individual functions or pieces of code in response to events without the need to manage servers or infrastructure. When a function is invoked, the user is billed for the actual time the function runs, which can result in significant cost savings for workloads that do not require constant availability.

In contrast, Infrastructure as Code (IaC) provides a way to manage and provision cloud infrastructure through code, but it does not itself establish a billing model based on execution time. Software-Defined Networking (SDN) focuses on network management and orchestration but does not deal with application execution or billing. Finally, Software as a Service (SaaS) delivers software applications over the internet but typically involves subscription-based pricing rather than pay-per-execution models. Therefore, FaaS stands out for its specific billing aligned with actual function execution duration.