FHIR Search Consistency Levels: Handling Data in AWS HealthLake

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This guide explains the concept of consistency levels for searching FHIR data within AWS HealthLake. The search index operates based on POST operations and eventual consistency models. Developers can choose between 'strong' or 'eventual' consistency when querying updated resources. By default, eventual consistency is used, which only includes resources whose search index updates have completed. However, if the most current data is required immediately, specifying 'strong' consistency allows the inclusion of N-1 version resources even while the search index update is pending. This behavior is controlled using the `x-amz-fhir-history-consistency-level` request header. While this feature supports use cases requiring immediate visibility, developers must be aware that using strong consistency doubles the write capacity consumption for each resource. It is recommended to use 'strong' consistency for immediate search of recently updated resources and 'eventual' consistency for general queries where real-time visibility is not critical.