Metadata

LEGEND metadata is stored in JSON. Formatting guidelines:

• In general, field names should be interpretable as valid variable names in common programming languages (e.g. use underscores (_) instead of dashes (-))

Physical units

Physical units should be specified as part of a field name by adding _in_<units> at the end. For example:

{
  "radius_in_mm": 11,
  "temperature_in_K": 7
}

Specifying metadata validity in time (and system)

LEGEND adopts a custom file format to specify the validity of metadata (for example a data production configuration that varies in time or according to the data taking mode), called JSONL (JSON + Legend).

A JSONL file is essentially a collection of JSON-formatted records. Each record is formatted as follows:

{"valid_from": "TIMESTAMP", "category": "DATATYPE", "apply": ["FILE1", "FILE2", ...]}

where:

• TIMESTAMP is a LEGEND-style timestamp yyymmddThhmmssZ (in UTC time), also used to label data cycles, specifying the start of validity
• DATATYPE is the data type (all, phy, cal, lar, etc.) to which the metadata applies
• apply takes an array of metadata files, to be combined "in cascade" (precedence order right to left) into the final metadata object

The record above translates to:

Combine FILE1, FILE2 etc. into a single metadata object. Fields in FILE2 override fields in FILE1. This metadata applies only to DATATYPE data and is valid from TIMESTAMP on.

Records are stored in JSONL files one per line, without special delimiters:

{"valid_from": "TIMESTAMP1", "category": "DATATYPE1", "apply": ["FILE1", "FILE2", ...]}
{"valid_from": "TIMESTAMP2", "category": "DATATYPE2", "apply": ["FILE3", "FILE4", ...]}
...