Updates to variable-based encoding

FORMAT_ADIOS.md

@@ -33,27 +33,27 @@ Output from `bpls -A` for a boolean attribute `pybool` stored in the location of
 There is no convention yet for a unique representation of ADIOS2 variables with boolean type.
 Thus, implementations should cast the data to and from `unsigned char` instead.
 
-## `stepBased` Encoding of Iterations
+## `variableBased` Encoding of Iterations
 
-The `iterationEncoding` mode `stepBased` must be implemented via ADIOS steps.
+The `iterationEncoding` mode `variableBased` must be implemented via ADIOS steps.
 
 ## Datasets
 
-An openPMD **data set** is represented by an group prefix that contains an ADIOS variable `__data__`.
+An openPMD **data set** is represented by an ADIOS `Variable` at the location where it would usually be stored.
 
 **attributes** are defined further below and can also appear at the dataset's **group** prefix level.
 
 ## Attributes
 
-openPMD **attributes** stored as ADIOS `Variables` at the location where they would usually be stored.
+openPMD **attributes** stored as ADIOS `Attributes` at the location where they would usually be stored.
 
 Example for a mesh record `E` with record component `x` and attributes `unitDimension` and `unitSI`:
 ```
-  double    /data/meshes/E/unitDimension     10*{7}
-  double    /data/meshes/E/x/__data__        10*{1000}
-  double    /data/meshes/E/x/position        10*{1}
-  double    /data/meshes/E/x/unitSI          10*scalar
+  double    /data/meshes/E/unitDimension          attr   = {1, 1, -3, -1, 0, 0, 0}
+  double    /data/meshes/E/x                      {128, 2048, 128}
+  double    /data/meshes/E/x/position             attr   = {0.5, 0.5, 0.5}
+  double    /data/meshes/E/x/unitSI               attr   = 1.22627e+13
 ```
 
-This example uses `stepBased` iteration encoding, but other iteration encodings would work similarly with their respective `basePath` prefix.
+This example uses `variableBased` iteration encoding, but other iteration encodings would work similarly with their respective `basePath` prefix.
 

STANDARD.md

@@ -90,7 +90,7 @@ Each file's *root* group (path `/`) must at least contain the attributes:
     - allowed values:
       - see *Iterations and Time Series* below
       - for `fileBased` and `groupBased`, this is fixed to `/data/%T/`
-      - for `stepBased` this is fixed to `/data/`
+      - for `variableBased` this is fixed to `/data/`
     - note: all the data that is formatted according to the present
       standard (i.e. both the meshes and the particles) is to be
       stored within a path of the form given by `basePath` (e.g. in
@@ -214,9 +214,9 @@ Each file's *root* group (path `/`) must further define the attributes:
                    is an other `open/close` call necessary to access other
                    iterations
     - allowed values:
-      - `fileBased` (multiple files)
-      - `groupBased` (one file)
-      - `stepBased` (one file with internal encoding for iterations, if supported by the data format)
+      - `fileBased` (multiple files; one iteration per file)
+      - `groupBased` (one file; iterations use groups in that file)
+      - `variableBased` (one file; if the data format supports to store multiple iterations in the same variables and attributes)
 
   - `iterationFormat`
     - type: *(string)*
@@ -233,13 +233,13 @@ Each file's *root* group (path `/`) must further define the attributes:
         - `filename_%T.h5` (without file system directories)
       - for `groupBased`: (fixed value)
         - `/data/%T/` (must be equal to and encoded in the `basePath`)
-      - for `stepBased`: (fixed value)
+      - for `variableBased`: (fixed value)
         - data-format internal convention
         - *slowest varying index* of data
 
-### `stepBased` Encoding of Iterations
+### `variableBased` Encoding of Iterations
 
-In order to correlate openPMD iterations with an index of data-format internal updates/steps or an index in the slowest varying dimension of an array, the *root* group (path `/`) must contain an additional variable once `stepBased` is chosen for `iterationEncoding`:
+In order to correlate openPMD iterations with an index of data-format internal updates/steps or an index in the slowest varying dimension of an array, the iteration base path (default: path `/data`) must contain an additional variable once `variableBased` is chosen for `iterationEncoding`:
 
   - `snapshot`
     - type: 1-dimensional array containing N *(int)* elements, where N is the number of updates/steps in the data format
@@ -248,11 +248,19 @@ In order to correlate openPMD iterations with an index of data-format internal u
     - advice to implementers: an openPMD iteration might be spread over multiple updates/steps, but not vice versa.
                               In such a scenario, an individual openPMD record's update/step must appear exactly once per iteration.
 
+Notes:
+
+* In implementations without support for storing multiple versions of datasets/attributes, the variable-based encoding of iterations may still be used for storage of a single iteration.
+  In that case, the `snapshot` attribute is optional and defaults to zero (0).
+* In implementations with support for storing multiple versions of datasets/attributes, the `snapshot` attribute may optionally be used in group-based encoding to associate openPMD iterations with IO steps.
+  In group-based encoding, there is still only one instance of this attribute globally (`/data/snapshot`).
+  In consequence, the attribute shall only be written if modifiable attributes are supported by the implementation.
+
 
 Required Attributes for the `basePath`
 --------------------------------------
 
-In addition to holding information about the iteration, each series of files  (`fileBased`), series of groups (`groupBased`) or internally encoded iterations (`stepBased`) should have attributes that describe the current time and the last time step.
+In addition to holding information about the iteration, each series of files  (`fileBased`), series of groups (`groupBased`) or internally encoded iterations (`variableBased`) should have attributes that describe the current time and the last step.
 
  - `time`
    - type: *(floatX)*