An enterprise architect is designing an Instana self-hosted deployment and needs to understand data storage responsibilities across backend components. Which backend component is primarily responsible for storing long-term infrastructure metrics data in the Instana platform?

Apache Cassandra serves as the primary time-series metrics datastore in the Instana backend, designed to handle the high write throughput generated by infrastructure metrics from monitored hosts. Kafka handles message streaming between components, Elasticsearch manages the dynamic graph and events index, and ClickHouse is used for trace and span analytics rather than metrics storage. The correct answer is 'Apache Cassandra, which stores time-series metrics data with configurable retention periods'. The incorrect options — 'Apache Kafka, which buffers incoming metrics before forwarding them to permanent storage', 'Elasticsearch, which indexes all metrics for fast ad-hoc search queries', 'ClickHouse, which stores all metrics in columnar format for analytics' — are wrong because they do not align with IBM Instana's architecture or recommended practices for this scenario. Understanding this concept is essential for the Domain 5: Planning domain of the IBM Instana Observability certification.