An organization wants to improve the reliability of a search service which architecture would ensure 99.9% availability for both read and write operations?

The chosen answer involves creating an Azure Cognitive Search service with any Standard service tier and at least three replicas, which is the optimal choice for ensuring a high level of availability for search operations.

In this context, having at least three replicas allows the service to balance load and continue functioning even if one of the replicas becomes unavailable. This redundancy is critical for achieving a 99.9% availability target, as it ensures that there are sufficient operational instances to handle requests consistently, both for reading and writing operations.

The Standard service tier provides the necessary features and scalability while also allowing for multiple replicas without the higher cost associated with premium tiers. This flexibility enables organizations to optimize their costs while maintaining the necessary uptime.

In contrast, other options may not meet the same availability criteria. For instance, a High-density service tier with only one replica lacks redundancy, significantly jeopardizing availability. Similarly, the Storage Optimized service tier with two replicas might not sufficiently meet the 99.9% availability requirement due to fewer instances available to handle requests. While using a Premium tier with two replicas would enhance capabilities, requiring at least three replicas at a Standard tier provides a more balanced and cost-effective approach to reliability.