Composite SLAs – Microsoft AZ-900 Exam

14.3. Composite SLAs

Service Level Agreements (SLAs) are foundational to cloud services and critical for customers who need to understand and manage the risks associated with migrating their applications and data to the cloud. However, as systems become more complex and dependent on a myriad of interconnected services, evaluating the overall reliability of a cloud-based solution becomes more challenging. This is where the concept of Composite SLAs comes into play. Composite SLAs combine the SLAs of individual services to provide a more holistic understanding of the service levels that a customer can expect for their complete solution. In this extensive exploration, we delve into what Composite SLAs are, how they are calculated, their importance, and the challenges associated with them.

Understanding Composite SLAs

A Composite SLA is an aggregated SLA that encompasses all the individual SLAs for services that make up a full cloud solution. It’s essentially the “big picture” SLA, derived from the performance and availability commitments of each component service. For example, if an application is hosted on Azure, its Composite SLA would factor in the SLAs of Azure’s compute services, database services, storage services, and any other Azure services that are part of the application’s architecture.

Calculating a Composite SLA is not as simple as averaging the uptime of all the services involved. It requires a thorough understanding of how each service interacts with others and the overall impact on the application’s availability.

Components of a Composite SLA

When constructing a Composite SLA, several components from individual SLAs are considered:

1. Uptime and Availability: The promised percentage of time that the service will be up and running.
2. Performance Targets: The benchmarks for how well a service should perform.
3. Responsiveness: The expected time it takes for the service provider to respond to and resolve issues.
4. Data Integrity: The guarantee that data will not be lost or corrupted.
5. Support Response: How quickly support requests are addressed.

Calculating a Composite SLA

To calculate a Composite SLA, it’s important to understand that service reliability is multiplicative, not additive. This means that the availability of two services in tandem is the product of their individual availabilities, assuming they are independent. Here’s a simplified formula for two services:

Composite SLA=SLA1×SLA2Composite SLA=SLA1×SLA2

So, if you have two services with SLAs of 99.9% each, the Composite SLA is not 99.9% + 99.9%, but rather 99.9% × 99.9%, which equals approximately 99.8%.

The Importance of Composite SLAs

The reliance on cloud infrastructures for critical operations means that the potential impact of any downtime is magnified. Composite SLAs provide a realistic outlook on the reliability of a complete system rather than its individual parts, which is important for several reasons:

1. Risk Assessment: They help organizations assess the risk of system downtime more accurately.
2. Disaster Recovery Planning: Understanding the overall reliability of a system is crucial in creating effective disaster recovery plans.
3. Contractual and Financial Planning: Organizations can plan for potential financial impacts or contractual liabilities associated with downtime.
4. Vendor Selection: Composite SLAs can influence the decision-making process when selecting cloud service providers and services.
5. Service Architecture: Insights from Composite SLAs can lead to architecture decisions that improve overall system reliability.