Revert "Replace global data documentation with workaround"

This reverts commit dd6cabb.

pages/docs/couple-your-code/couple-your-code-global-data.md

@@ -2,33 +2,73 @@
 title: Global data
 permalink: couple-your-code-global-data.html
 keywords: api, adapter, data, global
-summary: "Define and exchange global data not accociated to a mesh."
+summary: "Define and exchange global data (data not associated to a mesh) by using specific optional API functions."
 ---
 
-Global data is not natively supported, but there is a simple workaround.
+{% warning %}
+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.
+{% endwarning %}
 
-Every solver defines a mesh with a single vertex at `(0,0,0)` and uses it to read/write data.
+{% note %}
+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.
+{% endnote %}
+
+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).
+
+## Configuration
+
+Configuration of global data objects is described below with examples.
+
+In order to use this feature, global data needs to be explicitly defined in the configuration file using the `global-data` tag.
 
 ```xml
-<data:scalar name="GlobalData" />
-
-<mesh name="GlobalA" dimensions="3">
-  <use-data name="GlobalData" />
-</mesh>
-
-<mesh name="GlobalB" dimensions="3">
-  <use-data name="GlobalData" />
-</mesh>
-
-<participant name="A" >
-  <provide-mesh name="GlobalMeshA" />
-  <write-data   mesh="GlobalMeshA" name="GlobalData" />
-</participant>
-
-<participant name="B" >
-  <provide-mesh name="GlobalMeshB" />
-  <receive-mesh name="GlobalMeshA" />
-  <read-data    mesh="GlobalMeshB" name="GlobalData" />
-  <mapping:nearest-neighbor constraint="consistent" direction="read" from="GlobalMeshA" to="GlobalMeshB"/>
-</participant>
+<precice-configuration dimensions="3" experimental="true">
+    <global-data:vector name="angles" />
+...
+```
+
+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).
+
+```xml
+...
+<participant name="SolverOne">
+    <write-data name="angles" />
+...
+<participant name="SolverTwo">
+    <read-data name="angles">
+...
+<coupling-scheme:serial-explicit>
+    <exchange data="angles" from="SolverOne" to="SolverTwo" />
+...
+```
+
+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.
+
+## API
+
+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.
+
+For SolverOne:
+
+```C++
+...
+participant.writeGlobalData("angles", writeAngles);
+...
 ```
+
+For SolverTwo:
+
+```C++
+...
+participant.readGlobalData("angles", dt, readAngles);
+...
+
+```
+
+## Supported functionality
+
+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).
+
+Within implicit coupling, convergence measures and acceleration are not yet supported.
+
+[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.