An Azure SQL elastic pool is the optimal solution as it is specifically designed for managing and scaling multiple databases with varying and unpredictable usage demands. It allows the 20 databases to share a set of compute and storage resources at a set price. This model provides dynamic scaling of resources among the databases as needed, which directly addresses the varying usage patterns and minimizes costs by preventing over-provisioning for each database's peak load. The Azure SQL Database service meets the 99.99% uptime SLA requirement (in the General Purpose tier) and supports reserved capacity for the vCore purchasing model, fulfilling all constraints of the scenario.

A. A single Azure virtual machine does not provide the required 99.99% SLA and lacks the built-in dynamic compute scaling capabilities of a PaaS offering like Azure SQL Database.

B. While 20 serverless databases would provide dynamic scaling, an elastic pool is more cost-effective for managing a group of databases with varying usage by pooling resources, which better aligns with the cost minimization requirement.

C. Using an availability set for Azure virtual machines only increases the SLA to 99.95%, which is below the 99.99% requirement, and it still does not provide automatic, dynamic compute scaling.

1. Microsoft Learn

"Elastic pools in Azure SQL Database": This document states

"Elastic pools are a simple

cost-effective solution for managing and scaling multiple databases that have varying and unpredictable usage demands." It highlights how pools are ideal for SaaS applications where databases share resources

leading to cost savings.

Source: https://learn.microsoft.com/en-us/azure/azure-sql/database/elastic-pool-overview (Section: "When to consider a SQL Database elastic pool")

2. Microsoft Azure

"SLA for Azure SQL Database": The official SLA documentation confirms that the General Purpose service tier for Azure SQL Database

which can be used with elastic pools

provides an uptime guarantee of ≥99.99%.

Source: https://azure.microsoft.com/en-us/support/legal/sla/sql-database/v16/ (Section: "Details")

3. Microsoft Learn

"Prepay for Azure SQL Database compute resources with reserved capacity": This document confirms that reserved capacity is available for Azure SQL Database elastic pools using the vCore-based purchasing model

which satisfies the requirement for reserved capacity.

Source: https://learn.microsoft.com/en-us/azure/azure-sql/database/reserved-capacity-overview (Section: "Purchase reservations")

4. Microsoft Azure

"SLA for Virtual Machines": The SLA for two or more VMs in an Availability Set is 99.95%

and for a single VM using Premium SSD

it is 99.9%. Neither meets the 99.99% requirement.

Source: https://azure.microsoft.com/en-us/support/legal/sla/virtual-machines/v19/ (Section: "Details")