The serverless compute tier is a specific configuration available for single databases within the Azure SQL Database service. This model is designed for workloads with intermittent and unpredictable usage patterns. It automatically scales compute resources based on demand and can pause the database during periods of inactivity to save costs. You are billed per second only for the vCore compute resources used. This auto-scaling and pause/resume functionality is the core of the serverless offering for Azure SQL.

A. Azure SQL Managed Instance: This is a fully managed PaaS offering that uses a provisioned compute model, not a serverless one. It is designed for instance-level compatibility.

B. SQL Server on Azure Virtual Machines: This is an Infrastructure-as-a-Service (IaaS) model. You provision and manage a virtual machine, which is the opposite of a serverless architecture.

D. an Azure SQL Database elastic pool: Elastic pools allow multiple databases to share a set of provisioned resources (vCores or DTUs). The pool itself has provisioned capacity, not a serverless, auto-scaling configuration.

1. Microsoft Learn, "Serverless compute tier for Azure SQL Database":

Reference: Under the "Serverless compute tier" section, the first paragraph states, "The serverless compute tier for single databases in Azure SQL Database automatically scales compute based on workload demand and bills for the amount of compute used per second."

Location: https://learn.microsoft.com/en-us/azure/azure-sql/database/serverless-tier-overview

2. Microsoft Learn, "What is Azure SQL Database?":

Reference: In the "Deployment models" section, it describes the "Single database" model and its purchasing models. The vCore-based purchasing model for single databases is where the serverless compute tier is available. It contrasts this with Elastic Pools and Managed Instances, which are described with provisioned resources.

Location: https://learn.microsoft.com/en-us/azure/azure-sql/database/sql-database-paas-overview#deployment-models

3. Microsoft Learn, "Compare the serverless and provisioned compute tiers for Azure SQL Database":

Reference: The comparison table in this document explicitly contrasts the "Provisioned" compute tier with the "Serverless" compute tier, highlighting that serverless is for "Single databases with intermittent, unpredictable usage" while provisioned is for "Single databases or multiple databases in elastic pools with more regular usage".

Location: https://learn.microsoft.com/en-us/azure/azure-sql/database/serverless-tier-overview#comparison-with-the-provisioned-compute-tier