Functions are event driven by nature, and thus provide an unlimited amount of vertical scaling, otherwise max bounded by the FaaS providers. It makes high availability of your backend FaaS services a breeze.

AWS, Azure, and a number of others provide a set of triggers, which let you invoke your functions. Additionally, now providers also support possible invoking of your functions from the API gateways, whereby your rich client app can invoke a service on the API gateway, and that triggers the execution of your functions. However, this does entail a heavy reliance on the API gateway when it ultimately mimics to act as a microservice, which is mostly not considered as a good practice. Additionally, the chances of vendor lock-in are also increased.

For message-oriented communication to functions and between different functions, a message queuing system as a service is facilitated by the vendor, for example, SNS by AWS and Queue Storage by Azure. The following is the serverless architecture transitions (an Indicative view) diagram: