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Replace global data documentation with workaround
Revert once the native feature is ready.
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‎pages/docs/couple-your-code/couple-your-code-global-data.md‎

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title: Global data
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permalink: couple-your-code-global-data.html
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keywords: api, adapter, data, global
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summary: "Define and exchange global data (data not associated to a mesh) by using specific optional API functions."
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summary: "Define and exchange global data not accociated to a mesh."
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---
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{% warning %}
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These API functions are a work in progress, experimental, and are not yet released. The API might change during the ongoing development process. Use with care.
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{% endwarning %}
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Global data is not natively supported, but there is a simple workaround.
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{% note %}
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This feature is only available for coupling between two participants, i.e. does not yet support [multi coupling](https://precice.org/configuration-coupling-multi.html). Further, it does not yet allow [convergence measures](https://precice.org/configuration-coupling.html#implicit-coupling-schemes) and [acceleration](https://precice.org/configuration-acceleration) to be defined for global data. An extension to these features is planned.
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{% endnote %}
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preCICE allows participants to exchange data that is not associated with any mesh. Examples of such data are global background pressure (e.g. fluid-acoustic coupling) and angles between coordinate systems (e.g. fluid-structure coupling for rotor blades).
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## Configuration
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Configuration of global data objects is described below with examples.
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In order to use this feature, global data needs to be explicitly defined in the configuration file using the `global-data` tag.
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Every solver defines a mesh with a single vertex at `(0,0,0)` and uses it to read/write data.
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```xml
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<precice-configuration dimensions="3" experimental="true">
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<global-data:vector name="angles" />
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...
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```
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Rest of the global data configuration steps are similar to the usual *mesh-associated* data as described in [introduction to configuration](https://precice.org/configuration-introduction.html).
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```xml
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...
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<participant name="SolverOne">
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<write-data name="angles" />
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...
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<participant name="SolverTwo">
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<read-data name="angles">
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...
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<coupling-scheme:serial-explicit>
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<exchange data="angles" from="SolverOne" to="SolverTwo" />
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...
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```
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Since global data is not associated with any mesh, it should not be configured with the `use-data` tag under the `mesh` definition. Similarly, tags such as `read-data`, `write-data`, or `exchange` require no `mesh` attribute if used for global data.
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## API
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The API functions `writeGlobalData(...)` and `readGlobalData(...)` enable exchange of global data analogous to the `writeData(...)` and `readData(...)` functions for mesh-associated data. For the above example, the API calls would be as follows.
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For SolverOne:
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```C++
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...
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participant.writeGlobalData("angles", writeAngles);
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...
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<data:scalar name="GlobalData" />
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<mesh name="GlobalA" dimensions="3">
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<use-data name="GlobalData" />
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</mesh>
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<mesh name="GlobalB" dimensions="3">
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<use-data name="GlobalData" />
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</mesh>
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<participant name="A" >
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<provide-mesh name="GlobalMeshA" />
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<write-data mesh="GlobalMeshA" name="GlobalData" />
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</participant>
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<participant name="B" >
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<provide-mesh name="GlobalMeshB" />
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<receive-mesh name="GlobalMeshA" />
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<read-data mesh="GlobalMeshB" name="GlobalData" />
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<mapping:nearest-neighbor constraint="consistent" direction="read" from="GlobalMeshA" to="GlobalMeshB"/>
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</participant>
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```
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For SolverTwo:
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```C++
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...
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participant.readGlobalData("angles", dt, readAngles);
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...
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```
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## Supported functionality
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Global data exchange is supported in both explicit and implicit [coupling schemes](https://precice.org/configuration-coupling.html) with serial as well as parallel [coupling flow](https://precice.org/couple-your-code-coupling-flow.html#parallel-coupling-schemes).
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Within implicit coupling, convergence measures and acceleration are not yet supported.
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[Data initialization](https://precice.org/couple-your-code-initializing-coupling-data.html) and [time interpolation](https://precice.org/couple-your-code-waveform.html) are also supported for global data.

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