Code interoperability. Deliverable D2.2 of the HORIZON-EUROHPC-JU-2021-COE-01 project MaX (101093374)

This report contains an update on plans for the design and implementation of mechanisms for interoperability of the MaX lighthouse codes in the context of exascale workflows, as pertains to the task T2.4 of WP2.Deliverable D1.1 1 already provided, in the context of the sister task T1.4, some initial ideas and implementation plans, with a focus on work to be carried out in the codes themselves. Here, with the extra perspective afforded by the workflow-design ideas presented in D2.1, we provide general objectives that are seen as desirable for the achievement of interoperability, and suggest further specific implementation plans that should be taken up by T1.4.After a general introduction to frame the subject of interoperability, we discuss the main areas of development towards enhancing it, which are structured I/O and error logging, checkpointing and recovery data, performance models, and data-exchange techniques. In addition, we comment on the special role of the HyperQueue tool and AiiDA framework, and on their ongoing and planned enhancements in the context of exascale workflows.The document then gathers the reports on current work and development plans of the individual flagships codes with regard to interoperability. While there are common themes and techniques, each code has its own interoperability baseline and implementation priorities, which are also linked to the specific kind of exascale workflows, addressing scientific showcases, in which a particular code participates.