This document defines a complete, unambiguous, standard specification that can be used to serialize and de-serialize instances of Data Model types. It was designed to be simple and straightforward to implement by Data Providers and by different kind of clients. We can classify clients in terms of how they parse the VOTable in order to harvest its content. Of course, in the real word such distinction is somewhat fuzzy, but this section tries and describe the different levels of usage of this specification.
We say that a client is naïve if:
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it does not parse the VO-DML description file
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it assumes the a priori knowledge of one or more Data Models
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it discovers information by looking for a set of predefined vodml-refs in the VOTable
In other terms, a naïve client has knowledge of the Data Model it is
sensitive to, and simply discovers information useful to its own use
cases by traversing the VODML element.
Examples of such clients are the DAL service clients that allow users to discover and fetch datasets. They will just inspect the response of a service and present the user with a subset of its metadata. They do not reason on the content, and they are not interested in the structure of the serialized objects.
If such clients allow users to download the files that they load into memory, they should make sure to preserve the structure of the metadata, so to be interoperable with other applications that might ingest the same file at a later stage.
We say that a client is advanced if:
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it does not parse the VO-DML description file
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it is interested in the structure of the serialized instances
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can follow the mapping patterns defined in this specification, for example collections, references, and inheritance
Examples of such clients are science applications that display information to the user in a structured way (e.g. by plotting it, or by displaying its metadata in a user-friendly format), that reason on the serialized instances, perform operations on those instances, and possibly allow the users to save the manipulated version of the serialization.
Notice that the fact that an application does not directly use some elements that are out of the scope of its requirements does not mean that the application cannot provide them to the user in a useful way. For example, an application might allow users to build Boolean filters on a table, using a user-friendly tree representing the whole metadata. This exposes all the metadata provided by the Data Provider in a way that might not be meaningful for the application, but that may be meaningful for the user.
Notice, also, that advanced clients may be DM-agnostic: for instance, an Advanced Data Discovery application may allow the user to filter the results of a query by using a structured view of its metadata, even though it does not possess any knowledge of Data Models.
We say that a client falls into this category if:
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it parses the VO-DML descriptions
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it does not assume any a priori knowledge of any Data Models.
Such applications can, for example, dynamically allow users and Data Providers to map their files or databases to the IVOA Data Models in order to make them compliant, or display the content of any file annotated according to this standard.
This specification allows the creation of universal validators equivalent to the XML/XSD ones.
It also allows the creation of VO-enabled frameworks and universal libraries. For instance, a Python universal I/O library can parse any VOTable according to the Data Models it uses, and dynamically build objects on the fly, so that users can directly interact with those objects or use them in their scripts or in science applications, and then save the results in a VO-compliant format.
Java and Python guru clients could automatically generate interfaces for representing Data Models and dynamically implement those interfaces at runtime, maybe building different views of the same file in different contexts.
Notice that Guru frameworks and libraries can be used to build Advanced or even Naïve applications in a user-friendly way, abstracting the developers from the technical details of the standards and using scientific concepts as first class citizens instead.