4.1 Introduction: Data Federation of European HPC/Cloud Centers4.2 Requirements on the LEXIS DDI4.2.1 Unified Data Access4.2.2 Usage and Federation of Diverse Data Backend Systems4.2.3 Reliability and Redundancy4.2.4 AAI Support4.2.5 APIs4.2.6 State-of-the-art Research Data Management4.3 Federation via a DDI Based on iRODS4.3.1 Relevant Basic Properties of iRODS4.3.2 iRODS HA Setup4.3.3 iRODS Zones Federation across Centers and Data Movement4.3.4 Storage Tiering and Underlying Data Storage4.3.5 Logical Structure of the DDI4.4 Hardware4.4.1 Storage Systems for HPC and Infrastructure-as-a-Service- Cloud Clusters4.4.2 Storage Systems Dedicated to LEXIS4.4.3 HPC–Cloud-Storage Interconnect and Data Node/Burst Buffer Concept4.4.3.1 SBF (Smart Bunch of Flash)4.4.3.2 SBB4.5 Unified Access to the Platform Based on an AAI4.5.1 LEXIS Identity and Access Management (IAM) Solution, SSO, and AAI4.5.2 Platform Services vs. AAI: Separation of Concerns4.5.3 LEXIS DDI and IAM/AAI System4.6 Data Management via APIs4.6.1 Data Search, Upload, and Download APIs4.6.2 Staging API4.6.3 Replication and PID Assignment API4.6.4 Helper APIs4.6.5 Compression/Decompression/Encryption/Decryption API4.7 Integration with EUDAT Services4.7.1 EUDAT B2HANDLE4.7.2 EUDAT B2SAFE4.7.3 EUDAT B2STAGE4.8 ConclusionAcknowledgmentReferences