This article describes how Alcion, a Microsoft 365 backup-as-a-service platform, migrated from Amazon OpenSearch Service managed domains to OpenSearch Serverless to support their multi-tenant search architecture.

• Alcion initially used OpenSearch managed domains with fine-grained access control for tenant isolation
• Migrated to OpenSearch Serverless to reduce operational overhead and optimize costs for bursty workloads
• Evaluated three tenancy models: silo, pool, and bridge; selected bridge model
• Bridge model: single collection with one index per tenant for optimal isolation and scalability
• Implemented role-based access control using IAM roles and OpenSearch data access policies
• Each tenant gets dedicated index with tenant ID prefix and corresponding IAM role
• Achieved 30% cost reduction and eliminated complex scaling code after migration

Alcion successfully migrated to OpenSearch Serverless, reducing costs by 30% while maintaining strong tenant isolation and simplifying operations through auto-scaling capabilities.