Calculating Solution Service Level Agreement

The requirement for the web application is that the combined service level agreement for all components in the hot path is greater than 99.9%. The components in the hot path comprise of any service that is used in fulfilling a web request from a user.

Development

With a development environment, network isolation is not used. The following services are considered:

Service	Azure SLA
Azure Front Door	99.990%
Entra ID	99.990%
Azure App Service	99.950%
Redis Cache	99.900%
Azure SQL	99.995%
Azure Storage	99.900%
Key Vault	99.990%
App Configuration	99.900%
Combined SLA	99.616%

Production - Single Region

When operating in production, network isolation is used. We do not consider the availability of the hub resources or VNET peering.

Service	Azure SLA
Azure Front Door	99.990%
Entra ID	99.990%
Private DNS Zone	100.00%
AFD Private Link	99.990%
Azure App Service	99.950%
- Private Link	99.990%
Redis Cache	99.900%
- Private Link	99.990%
Azure SQL	99.995%
- Private Link	99.990%
Azure Storage	99.900%
- Private Link	99.990%
Key Vault	99.990%
- Private Link	99.990%
App Configuration	99.900%
- Private Link	99.990%
Combined SLA	99.546%

Production - Two Regions

Since the single region SLA is less than the requested 99.9% availability, we have to deploy to two regions. Azure Front Door, Entra ID, and Private DNS Zones are shared resources. However, the rest of the services can be doubled up for more reliability.

Service	Azure SLA
Shared Services
Azure Front Door	99.990%
Entra ID	99.990%
Private DNS Zone	100.00%
Regional Services
AFD Private Link	99.990%
Azure App Service	99.950%
- Private Link	99.990%
Redis Cache	99.900%
- Private Link	99.990%
Azure SQL	99.995%
- Private Link	99.990%
Azure Storage	99.900%
- Private Link	99.990%
Key Vault	99.990%
- Private Link	99.990%
App Configuration	99.900%
- Private Link	99.990%
Shared Services	99.980%
Regional Services	99.546%
Combined SLA	99.9779%

Using dual regions will help us achieve the requested service level agreement.

For more information on how to calculate effective SLO, please refer to the Well Architected Framework.

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