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Defining Data Models

In this section, we explain how to define data models, an important component of the OEF Search & Discovery. It allows agents to describe themselves and to discover the services/resources they are interested in.

In a sentence, a DataModel is a set of attributes, and a Description of a service/resource is an assignment of those attributes.

All you need to specify data models and descriptions (that is, instances of the data model) can be found in the module.


At the lowest level of our data model language, we have the Attribute. An attribute is an abstract definition of a property.

It is identified by a name, that must be unique in a given data model (that is, we can't have two attributes that share the same name).

Every attribute has a type, that specifies the domain of the property, that is, the possible values that the attribute can assume. At the moment, we support five types of attributes:

  • strings
  • integers
  • booleans
  • floats
  • locations, i.e. instances of Location (pairs of (latitude, longitude))

An attribute can be optional, in the sense that instantiation of the attribute is not mandatory by the instances of the data model.

Finally, every attribute might have a description that explains the purpose of the attribute.

Example: suppose we have a bookshop, and we want to describe the books we sell. Presumably, we would like to include: the following properties of our books:

  • The title
  • The author
  • The genre (e.g. science fiction, horror)
  • The year of publication
  • The average rating (average of the ratings between 0 and 5)
  • The ISBN code
  • If it can be sold as an e-book.

For each of these fields, we can define an attribute by using Attribute:

from import Attribute, Location
attr_title   = Attribute("title", str, True, "The title of the book.")
attr_author  = Attribute("author", str, True, "The author of the book.")
attr_genre   = Attribute("genre", str, True, "The genre of the book.")
attr_year    = Attribute("year", int, True, "The year of publication of the book.")
attr_avg_rat = Attribute("average_rating",  float,    False, "The average rating of the book.")
attr_isbn    = Attribute("ISBN", str, True, "The ISBN.")
attr_ebook   = Attribute("ebook_available", bool, False, "If the book can be sold as an e-book.")
attr_bookshop = Attribute("bookshop_pos", Location, False, "The location of the bookshop where you can find the book")

Let's focus on the parameters of the Attribute constructor:

  1. the first one is the name of the attribute. It is needed to instantiate a data model and to define queries over it.
  2. the second one is the type of the attribute. It specifies the domain of the possible values the attribute can assume. E.g. the attribute year can only be an integer, whereas the average_rating can only be a floating-point number. The supported types are: str, int, bool, float and Location.
  3. the third one is a boolean that specifies whether the attribute is always required or it can be omitted. For example, we might not be able to specify the ebook_available attribute, maybe because it's not applicable to some kind of books.
  4. the fourth parameter is the description, that is a short description of the purpose of the attribute.

Data Models

A data model is just a set of attributes. The class that implements the data model is DataModel.

Example: let's continue with the example of the bookshop. Once we've defined the attributes, we'd like to group them in the same structure. We can do it in the following way:

from import DataModel

book_model = DataModel("book", [
], "A data model to describe books.")

A DataModel requires:

  1. a name (in the example the name is "book") used to refer to the data model.
  2. a list of attributes, that constitutes the abstract data model.
  3. an (optional) description about the purpose of the data model.


A Description is just an instantiation of a data model. That is, we specify a value to every attribute belonging to the data model we are interested in.

The class that implements the description is Description.

Example: now we have all we need to create a little catalogue about our books:

from import Description

It = Description({
    "title" :           "It",
    "author":           "Stephen King",
    "genre":            "horror",
    "year":             1986,
    "average_rating":   4.5,
    "ISBN":             "0-670-81302-8",
    "ebook_available":  True,
    "bookshop_pos":     Location(52.2057092, 0.1183431)
}, book_model)

_1984 = Description({
    "title" :           "1984",
    "author":           "George Orwell",
    "genre":            "novel",
    "year":             1949,
    "ISBN":             "978-0451524935",
    "ebook_available":  False
}, book_model)

We defined the descriptions for two books, namely It and _1984, that refers to a data model.

The attributes are instantiated with a dictionary that has:

  • as keys, the name of the attributes.
  • as values, the values associated with the attributes.

Notice that in the latter book we omitted the average_rating field. We are allowed to do that because of the average_rating attribute is not mandatory.