Basic Model Interface specification for Julia
Author Deltares
3 Stars
Updated Last
10 Months Ago
Started In
February 2020


Build Status

Basic Model Interface (BMI) specification for the Julia programming language. This package contains all 41 functions that are part of the BMI 2.0 specification. These functions are declared without any methods, like so: function initialize end. They do have documentation that shows how they should be used.

It is up to model specific implementations to extend the functions defined here, adding methods for their own model type, such as:

using BasicModelInterface
const BMI = BasicModelInterface

# any type you created to represent your model
struct MyModel end

function BMI.update(model::MyModel)
    # write MyModel update implementation here

This package is not yet registered, and is currently being developed independently of Community Surface Dynamics Modeling System(CSDMS). After it has been proven successful with several Julia BMI implementations, we should consider proposing CSDMS adoption as well, to join the existing C, C++, Fortran and Python specifications.

Summary of BMI functions

Table below taken from

Function Description
initialize Perform startup tasks for the model.
update Advance model state by one time step.
update_until Advance model state until the given time.
finalize Perform tear-down tasks for the model.
get_component_name Name of the model.
get_input_item_count Count of a model's input variables.
get_output_item_count Count of a model's output variables.
get_input_var_names List of a model's input variables.
get_output_var_names List of a model's output variables.
get_var_grid Get the grid identifier for a variable.
get_var_type Get the data type of a variable.
get_var_units Get the units of a variable.
get_var_itemsize Get the size (in bytes) of one element of a variable.
get_var_nbytes Get the total size (in bytes) of a variable.
get_var_location Get the grid element type of a variable.
get_current_time Current time of the model.
get_start_time Start time of the model.
get_end_time End time of the model.
get_time_units Time units used in the model.
get_time_step Time step used in the model.
get_value Get a copy of values of a given variable.
get_value_ptr Get a reference to the values of a given variable.
get_value_at_indices Get variable values at specific locations.
set_value Set the values of a given variable.
set_value_at_indices Set the values of a variable at specific locations.
get_grid_rank Get the number of dimensions of a computational grid.
get_grid_size Get the total number of elements of a computational grid.
get_grid_type Get the grid type as a string.
get_grid_shape Get the dimensions of a computational grid.
get_grid_spacing Get the spacing between grid nodes.
get_grid_origin Get the origin of a grid.
get_grid_x Get the locations of a grid's nodes in dimension 1.
get_grid_y Get the locations of a grid's nodes in dimension 2.
get_grid_z Get the locations of a grid's nodes in dimension 3.
get_grid_node_count Get the number of nodes in the grid.
get_grid_edge_count Get the number of edges in the grid.
get_grid_face_count Get the number of faces in the grid.
get_grid_edge_nodes Get the edge-node connectivity.
get_grid_face_edges Get the face-edge connectivity.
get_grid_face_nodes Get the face-node connectivity.
get_grid_nodes_per_face Get the number of nodes for each face.

Notes and open questions

This specification is adopted from the BMI 2.0 Python specification. Instead of Python's class methods, the Julia specification expects a model parameter as the first argument. Julia will dispatch to the right implementation based on the type of the model parameter.

We do not apply the Julia convention to put a ! after mutating function names, to keep the function names consistent with the other languages.

  • Time must be a float.
  • Grid is an integer grid identifier.
  • Units should be string.

See if we can instead use richer types that keep more information, yet still convert to the right Int64, Float64, String, etc.

get_grid_shape: Instead of passing in a Vector to fill, do not require a shape argument, and return a Tuple, like the size function.

Constructs and features that are natural for the language should be used when implementing a BMI. BMI strives to be developer-friendly.

BMI functions always use flattened, one-dimensional arrays. This avoids any issues stemming from row/column-major indexing when coupling models written in different languages. It’s the developer's responsibility to ensure that array information is flattened/redimensionalized in the correct order.

From the above and the get_grid_shape docs it is not quite clear yet what the correct order is. Flattening a row-array and a column-major array will result in the same size but different order array.

“ij” indexing (as opposed to “xy”)

Does i here stand for the first dimension, regardless of row/column-major indexing?

the length of the z-dimension, nz, would be listed first.

If the z-dimension needs to go first, that means different z at the same xy location will be close in memory for column-major, and far in memory for row-major.

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