Writing recorders

Overview

Sardana macros may produce data and users are usually interested in storing or visualizing it. Sardana delegates this work to the recorders. A good example of the recorder usage are the scan macros developed with the Scan Framework. Recorders are in charge of writing data to its destinations, for example a file, the Spock output or to plot it on a graph.

What is a recorder?

Recorder class is a Sardana element managed by the MacroServer. It is identified by its name, and is located in a recorder library - another Sardana element which is also identified by its name. Recorders are developed as Python classes, and recorder libraries are just Python modules aggregating these classes.

Type of recorders

Sardana defines some standard recorders e.g. the Spock output recorder or the SPEC file recorder. From the other hand users may define their custom recorders. Sardana provides the following standard recorders (grouped by types):

  • file [*]
    • FIO_FileRecorder
    • NXscan_FileRecorder
    • SPEC_FileRecorder
  • shared memory [*]
    • SPSRecorder
    • ShmRecorder
  • output
    • JsonRecorder [*]
    • OutputRecorder

[*] Scan Framework provides mechanisms to enable and select this recorders using the environment variables.

Writing a custom recorder

Todo

document how to write custom recorders

Configuration

Custom recorders may be added to the Sardana system by placing the recorder library module in a directory which is specified by the MacroServer RecorderPath property. RecorderPath property may contain an ordered, colon-separated list of directories. In case of overriding recorders by name or by file extension (in case of the file recorders), recorders located in the first paths are of higher priority than the ones from the last paths.

Three types of overriding may occur:

By recorder library name
If Python modules with the same name are located in different directories, the library located in the the higher priority directory will be loaded.
By recorder name
If two recorder classes with the same name appear in two different modules, only the recorder from the library located in the higher priority module will be loaded. If both modules are located in the same directory, the behavior is undetermined.
By file extension
If two different recorders supporting the same file extension appear in two different modules, the one from the higher priority path will be used when selection is based on the extension (but both will be available for the selection by name). If both of these recorders’ modules are located in the same directory, the system will assign a list of recorders to a given extension. Then the application is in charge of deciding which one to use.

As previously mentioned recorders are selectable by either the recorder name or the extension. During the MacroServer startup the extension to recorder map is generated while loading the recorder libraries. This dynamically created map may be overridden by the custom map defined in the sardanacustomsettings module (SCAN_RECORDER_MAP variable with a dictionary where key is the scan file extension e.g. ”.h5” and value is the recorder name e.g. “MyCustomRecorder”, where both keys and values are of type string). The SCAN_RECORDER_MAP will make an union with the dynamically created map taking precedence in case the extensions repeats in both of them.