Guide to AWS Lambda: Provisioned Concurrency

Provisioned Concurrency in AWS Lambda is a feature that helps applications to have consistent start-up times, and handle variable workloads efficiently.

It's importance lies in its ability to reserve a set instance for a Lambda function. This eliminates the latency from a 'cold start' that can occur when a function is invoked after not being used for a time period. It allows your serverless applications to respond rapidly and consistently to events, equating to better user experiences.

What is it: In simple words, Provisioned Concurrency is a setting for your Lambda functions where you tell AWS how many function instances you want to remain warm, ready to respond to invocations at a moment's notice.

How it works: Instead of AWS pausing your function's instances during idle times (cold start), instances are left running. You allocate and pay for these 'warm instances' to allow for immediate responsiveness when called upon. AWS manages the warm instances automatically based on your allocated provisioned concurrency.

When it comes to exam questions on Provisioned Concurrency, here are some tips:
1. Understand how Provisioned Concurrency differs from On-Demand Concurrency (standard case). On-Demand allows for auto-scaling, but can have slow start-up times due to 'cold starts'. Provisioned Concurrency prevents 'cold starts' but needs to be managed and paid for 2. Be aware of how this feature can impact your AWS costs – you are billed for the amount of provisioned concurrency and the duration for which it has been provisioned 3. AWS often asks questions about optimizing costs vs performance in cloud systems architecture- consider, when would maintaining readiness of your function offset the costs?

Always remember, in the exam, focus on benefits and drawbacks of Provisioned Concurrency to decide the best use case scenarios. Application characteristics like predictable peak usage times, and need for low latency can signal its use.