Modernizing the 4C linear algebra backend: from Epetra to Tpetra

The 4C (Comprehensive Computational Community Code) multiphysics simulation framework has been developed to address complex physical phenomena across various scientific and engineering domains. From its inception, 4C has relied on the Trilinos project, an open-source software library for scalable numerical computations, as its backbone for sparse linear algebra and MPI-parallel computing. This integration enhances 4C's computational capabilities and, more importantly, allows the 4C developers to focus on their core research interest: the numerical modeling of multiphysics systems. At the same time, ongoing developments within Trilinos necessitate continuous adaptation and maintenance of the 4C code base. A major recent change in Trilinos has been the deprecation and removal of the long-standing Epetra linear algebra package. Its successor, Tpetra, introduces templating over scalar and ordinal types and, crucially, integrates tightly with the Kokkos ecosystem to enable erformance portability across diverse and heterogeneous hardware architectures. In this presentation, we report on our experience migrating a large, mature multiphysics code base from Epetra to Tpetra. We describe our strategy of encapsulating Trilinos data structures behind dedicated abstraction layers to limit their scope within the application code. Achievements, encountered challenges, and design trade-offs arising from both 4C- and Trilinos-specific software patterns are discussed, with the aim of providing guidance and practical insights for other Trilinos-based application developers facing similar transitions.