# [Greenline] halo construction

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- Shaped by: Magdalena
- Appetite (FTEs, weeks):
- Developers: <!-- Filled in at the betting table unless someone is specifically required here -->

## Problem
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During the last cycle, progress was made towards being able to decompose a icon grid in python with [this project](https://hackmd.io/ophk8gJ2S9KGFgNpuAMzyg).
What has been achieved:
- Starting from a global grid we can
- decompose cells, vertices and edges
- construct halos
- construct node local connectivities arrays
- some simple tests are in place that
- read a field locally after the distribution gather the data from all nodes and compare to the global field.

This is a follow up project that should add the missing features in order to be able to run **global** simulations in multinodes.

## Appetite
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## Solution
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- compute start/end index arrays for the local grid
- refactor grid_manager.py to
- call the decomposition
- return local topologies.
- enlarge test set
- compute geofac_rot, geofac_div, geofac_n2s in distributed way and compare with 1 node serialized results
- use grid_manager to provide the topology for single and multi node diffusion tests
- add CI action for parallel (MPI) tests

## Rabbit holes
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## No-gos
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## Progress
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- [ ] CI action for mpi tests
- [ ] Integrate in grid_manager.py
- [ ] refactor and simplify
- [x] add decompose
- [ ] return local grid for decomposed setup
- [x] construct start, end indices

- [ ] tests for halo access
- [ ] geofac_rot
- [ ] geofac_div
- [ ] geofac_n2s
- [ ] run diffusion with grid manager