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Arcane
4.2.2.0
User documentation
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Arcane
4.2.2.0
User documentation
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This page defines the main concepts of the Arcane platform. The terms are grouped into the core concepts (subdomain, mesh, Cartesian mesh, mesh item, item family, item group, parallel manager, variable, variable manager, module and service) and the materials-system concepts (material manager, material, constituent, environment and block). Each term links to the class that implements it.
A subdomain (ISubDomain) is the basic unit of execution of an Arcane application. It represents the part of the computation executed by one thread (or by one group of threads) in the hybrid (MPI + threads) execution of Arcane. A subdomain owns its own meshes, variables, modules and services, and is the object from which all these resources are accessed. In a sequential execution there is a single subdomain; in a parallel execution there is one subdomain per MPI rank (and possibly more when threads are used). A subdomain is associated with a sub-partition of a parallel manager (IParallelMng).
A mesh (IMesh) is a set of mesh entities (nodes, edges, faces and cells) together with their connectivity. A mesh is owned by a subdomain and, when created with a non-sequential IParallelMng, is distributed across the ranks: each rank holds a local part of the mesh plus one or more ghost layers of neighbouring entities. A mesh has a spatial dimension (2 or 3) and exposes one family per entity kind through its node, edge, face and cell families. A mesh can be created empty and then filled, or read directly from a file.
A Cartesian mesh (ICartesianMesh) is a special kind of mesh whose cells are arranged on a regular structured grid. It is associated with a standard IMesh (retrieved or created from it via ICartesianMesh::getReference()) and adds, on top of the regular mesh facilities, directional access to the entities (cells, faces and nodes in a given direction) and the operations needed for adaptive mesh refinement (refining or coarsening blocks of the mesh). A Cartesian mesh is typically used by codes that solve the equations on a structured grid.
A mesh item (or simply an item, Item) is a single entity of a mesh: one node, one edge, one face or one cell. The Item class is the base class of the typed entities (Node, Edge, Face, Cell and Particle). An item has a local id (unique within the local part of its family) and a unique id (globally unique, used to identify the entity across ranks), and it belongs to an item family. An item is a lightweight handle: the associated data (coordinates, variable values) are stored in the family.
An item family (IItemFamily) is the collection of all the items of a given kind (nodes, edges, faces or cells) of a mesh. A mesh exposes its families through IMeshBase::nodeFamily(), IMeshBase::edgeFamily(), IMeshBase::faceFamily() and IMeshBase::cellFamily(). A family stores the items, their connectivity and the variables defined on them, and is the primary object used to iterate over and to modify mesh entities. Particles form a separate kind of item family (IParticleFamily).
An item group (ItemGroup) is a subset (possibly empty) of the items of a given item family. Unlike an item family, which is the whole set of the items of a kind in a mesh, a group is an arbitrary subset that can be created, combined (union, intersection, difference) and iterated independently of the family. A group is created through its family using IItemFamily::createGroup() or IItemFamily::findGroup(). Groups are used to select specific sets of items (for example the cells of a material or of a region) on which to perform operations.
A parallel manager (IParallelMng) is the object that manages the parallel decomposition (MPI ranks and/or threads) of an Arcane application and provides the collective communication operations (barriers, reductions, exchanges) used by the framework and by the application. Each subdomain is associated with a sub-partition of a parallel manager. The default parallel manager of the application covers all the ranks, and sub-managers can be created for subsets of ranks. A sequential parallel manager (a single rank) is used for non-parallel meshes.
A variable (IVariable) is a named piece of data associated with a subdomain. There are two main kinds of variables: common variables, which hold a single value per subdomain, and item variables, which hold one value per mesh item (for example one value per cell). A variable is declared either in the AXL descriptor of a module or of a service, or directly in the code, and is accessed in the code through a variable reference (VariableRef and its derived classes, the typed variable references).
The variable manager (IVariableMng) is the object that maintains the list of the variables declared in a subdomain. It allows these variables to be retrieved and to be read or written (for example for checkpointing or for post-processing). It is obtained from the subdomain via ISubDomain::variableMng().
A module (IModule) is a basic unit of application code in Arcane. A module groups a set of entry points (the points from which the Arcane time loop calls the module), variables and configuration options. A module is represented by a class and by an XML file called the module descriptor (extension .axl) that describes its variables, entry points and options. Module instances are created and configured by the Arcane runtime from the simulation case file.
A service (IService) has the same characteristics as a module, except that it has no entry point. A service is used to factorize code across several modules (for example a numerical scheme or an equation of state shared by several physics modules) or to parameterize a module with different algorithms. Like a module, a service is represented by a class and by an XML service descriptor (extension .axl) that describes the interface it implements, its options and its variables. A service instance is created by the Arcane runtime when a module references it.
A material manager (IMeshMaterialMng) is the object that manages all the materials, environments and blocks of a mesh, as well as their associated material and environment variables. There is one material manager per mesh, retrieved or created via the static method IMeshMaterialMng::getReference(). The materials and environments are registered and created during the initialization of the computation and cannot be modified afterward; only the list of cells of a material or an environment can change during the run (via a MeshMaterialModifier).
A material (IMeshMaterial) is a named set of mesh cells that represents a physical substance present in those cells. It belongs to the materials system (namespace Arcane::Materials). A material is associated only with cells, and its list of cells is dynamic: it can change during the computation. A material belongs to one or more environments. The materials of a mesh are managed by an IMeshMaterialMng instance (one per mesh).
A constituent (ConstituentItem) is the occurrence of a component (a material or an environment) inside a single cell. A cell can contain several materials and several environments, and the constituents of a cell form a hierarchy: AllEnvCell (the cell together with all its environments), EnvCell (one environment of the cell) and MatCell (one material of the cell). Constituent items are used to access the per-cell data and variables of the materials and environments.
An environment (IMeshEnvironment) is a named grouping of one or more materials of a mesh. It belongs to the materials system (namespace Arcane::Materials) and forms an intermediate level between the (optional) blocks and the materials. The list of cells of an environment is the union of the lists of cells of its materials and is recomputed automatically when the materials change. As for materials, an environment is associated only with cells and can be empty.
A block (IMeshBlock) is an optional named grouping, located at a level above the environments, that groups a set of environments together with a group of cells. It belongs to the materials system (namespace Arcane::Materials) and is managed by the IMeshMaterialMng instance along with the materials and the environments.
1.16.1