Variables and Data

The variables are organized in an abstract way, independently of the way they are persisted.

The following diagram presents a ‘traditional’ view of what is a table:

  • the ‘columns’ are the variables,
  • the ‘rows’ are the value sets for each entity,
  • the ‘cells’ are the variable entity values.

The following diagram shows the relationships between the different concepts:



Variables and Categories

A variable describes a set of values. The values of a variable are all of the same type. Possible value types are:

  • integer
  • decimal
  • text
  • binary
  • locale
  • boolean
  • datetime
  • date
  • point
  • linestring
  • polygon

A variable is about an entity, i.e. all the values for a variable are from the same entity type. Possible entity types are:

  • Participant
  • Instrument

A category describe some of the possible values of a variable. A category is associated to one and only one variable.

Datasources and Tables

A variable is in one and only one table.

A table has several variables and is in one and only one datasource.

A datasource has several tables. A datasource is not a database: it can be persisted in a database, using different schema. It can also be persisted in a file in xml or Excel formats. It is important to understand that Opal separates the formal description of the variables from the way they are persisted. This gives to Opal a lot of versatility.


Datasources, variables and categories have attributes. These attributes provide additional meta-information. An attribute is made of:

  • a namespace (optional),
  • a name (required),
  • a locale (optional), that specifies in which language is the attribute value,
  • a value (required even if null).

Example of a variable asked_age which has the following attributes:

Name Locale Value
label en What is your age ?
label fr Quel est votre age ?
questionnaire   IdentificationQuestionnaire
page   P1

The variable asked_age has also some categories (with their attributes):

Name Attributes
label:en=Don’t know
label:fr=Ne sait pas
label:en=Prefer not to answer
label:en=Préfère ne pas répondre

Fully Qualified Names

Each of these elements has a short name. A fully qualified name will identify them uniquely:

  • Datasource fully qualified name: <datasource_name>
  • Table fully qualified name: <datasource_name>.<table_name>
  • Variable fully qualified name: <datasource_name>.<table_name>:<variable_name>

The fully qualified name is useful for disambiguation.

Following the example of the asked_age variable, its fully qualified name could be: opal-data.IdentificationQuestionnaire:asked_age

Derived Variables

A derived variable is a variable which values are computed using a script. This script is expressed using the Magma Javascript API.


Opal deals with variables and values in tables. Views are here to:

  • define a subset of a table, both in terms of variables and values,
  • define a subset of many tables in terms of variables and values,
  • define #Derived Variables that are to be resolved against ‘real’ ones.

These virtual tables are then manipulated just like standard tables (for instance they can be copied to a datasource).

Given table Table1:

ID Var1 Var2 Var3
1 Value 1.1 Value 2.1 Value 3.1
2 Value 1.2 Value 2.2 Value 3.2
3 Value 1.3 Value 2.3 Value 3.3

A view can be defined so that the resulting ‘table’ may be View1:

ID Var1 Var3
1 Value 1.1 Value 3.1
3 Value 1.3 Value 3.3

or View2:

ID DerivedVar = function(Var1, Var2)
1 function(Value1.1, Value2.1)
3 function(Value1.3, Value2.3)

Given Table1 above and the following table Table2:

ID VarA VarB VarC
100 Value A.100 Value B.100 Value C.100
200 Value A.200 Value B.200 Value C.200
300 Value A.300 Value B.300 Value C.300

A view can also be a combination or a ‘join’ of both tables, as in View3:

ID Var1 Var3 VarA VarC
1 Value 1.1 Value 3.1    
3 Value 1.3 Value 3.3    
100     Value A.100 Value C.100
300     Value A.300 Value C.300



The entities can be of different types:

  • Participant (most common)
  • Instrument (provided by Onyx)
  • Workstation (provided by Onyx)
  • … (any that might fit your needs)

Each entity has a unique identifier. An entity can have several value sets, but only one value set for a particular table.

Value Types

The following table gives more information about the textual representation of a value, given a value type:

Value Type Value as a String
integer The string value must all be decimal digits, except that the first character may be an ASCII minus sign ‘-‘ to indicate a negative value. The resulting integer has radix 10 and the supported range is [-2 63, 2 63-1].
decimal As described by Java Double documentation.
text As-is.
binary Base64 encoded.
String representation of a locale is <language>[_<country>[_<variant>]] (for instance en, en_CA etc.) where:

language: lowercase two-letter ISO-639 code.
country: uppercase two-letter ISO-3166 code.
variant: vendor specific code, see Java Locale.
boolean True value if is equal, ignoring case, to the string “true”.
Date times are represented in ISO_8601 format: “yyyy-MM-dd’T’HH:mm:ss.SSSZ”
Supported input formats are (four digits year is required):

yyyy-MM-dd HH:mm:ss
yyyy/MM/dd HH:mm:ss
yyyy.MM.dd HH:mm:ss
yyyy MM dd HH:mm:ss
yyyy-MM-dd HH:mm
yyyy/MM/dd HH:mm
yyyy.MM.dd HH:mm
yyyy MM dd HH:mm
Dates are represented in ISO_8601 format: “yyyy-MM-dd”
Supported input formats are (four digits year is required):

yyyy MM dd
dd MM yyyy
Point coordinates (longitude, latitude).
Supported input formats are:

Google Map

{type: “Point”, coordinates: [-71.34, 41.12]}

# GeoJSON coordinates only: [lon,lat]
[-71.34, 41.12]

# JSON (different flavours of keys)
{“lat” : 41.12,”lon” : -71.34}
{“lat” : 41.12,”lng” : -71.34}
{“latitude” : 41.12,”longitude” : -71.34}
{“lt” : 41.12,”lg” : -71.34}

# String, comma separated latitude and longitude (Google map like): lat,lon
Array of point coordinates.
Supported input format is GeoJSON:

{type: “LineString”, coordinates: [[22.2,44.1],[33.4,55.3],[32.12,44]]}

# GeoJSON coordinates only
Array of shapes. A shape is a list of points. The last point must be equal to the first point.
Supported input format is GeoJSON:

{type: “Polygon”, coordinates: [[ [100.0, 0.0], [101.0, 0.0], [101.0, 1.0], [100.0, 1.0], [100.0, 0.0] ]]}

# GeoJSON coordinates only: one shape polygon
[ [100.0, 0.0], [101.0, 0.0], [101.0, 1.0], [100.0, 1.0], [100.0, 0.0] ]

# GeoJSON coordinates only: polygon with outter and inner shapes
[ [100.0, 0.0], [101.0, 0.0], [101.0, 1.0], [100.0, 1.0], [100.0, 0.0] ],
[ [100.2, 0.2], [100.8, 0.2], [100.8, 0.8], [100.2, 0.8], [100.2, 0.2] ]

Value Sets and Values

A value is associated to a variable and is part of a value set. Each value set is for a particular entity and a particular table. An entity has a maximum of one value set in one table.

A value is always associated with a type and a data (or a sequence of data if the variable is repeatable).