Now, we will discuss the actual FaaS layers. We will go from left to right:

• The frontier component of the whole architecture is the FaaS Gateway. The gateway in some implementations is optional, but in many implementations, this component helps serve HTTPS and cache some static content, such as UI parts, of the platform. Gateway instances help for making better throughput. It is usually a stateless HTTP-based reverse proxy. So this component is easy to scale-out.
• The Initiator is one of the most important components of FaaS. An initiator is responsible for imitating the real invocation request to the rest of the platform. In OpenWhisk, this component is called the controller, for example. In Fn, the part inside its Fn server acts as the Initiator.
• The Message Bus is the message backbone of a FaaS platform. Some architectures that do not have this component will have a difficulty to properly implement asynchronous calls, or the retry pattern to make the platform robust. The message bus decouples initiators out of executors.
• The Executor is the component that does the real function invocation. It connects to its own container runtime (application-level) to start the real sequence of function execution. All results and logging will be written to the central log storage.
• Log Storage is the platform's single source of truth. It should be designed to store almost everything, ranging from the function activities to the error logs of each invocation.
• Container Runtime (application level) is a component responsible for starting the function container. We simply use Docker and its underlying engine as the runtime component in this book.