XML design: don't use duplicate keys in XML objects

Hi Folks,

An XML object is an element whose content is a collection of heterogeneous data items. For example, this Book element contains a collection of heterogeneous data items and therefore is an XML object:

<Book>
    <Title>The First and Last Freedom</Title>
    <Author>J. Krishnamurti</Author>
    <Date>1954</Date>
    <ISBN>0-06-064831-7</ISBN>
    <Publisher>Harper & Row</Publisher>
</Book>

The fact that it is an object becomes more apparent with this graphic:

Recommendation: Don’t create XML objects with duplicate keys. For example, this Book object has duplicate Author keys:

<Book>
    <Title>Parsing Techniques</Title>
    <Author>Dick Grune</Author>
    <Author>Ceriel Jacobs</Author>
    <Date>2007</Date>
    <ISBN>978-0387202488</ISBN>
    <Publisher>Springer</Publisher>
</Book>

Instead, create a single Authors element whose content is an array:

<Book>
    <Title>Parsing Techniques</Title>
    <Authors><Author>Dick Grune</Author><Author>Ceriel Jacobs</Author></Authors>
    <Date>2007</Date>
    <ISBN>978-0387202488</ISBN>
    <Publisher>Springer</Publisher>
</Book>

Now the Book object contains a collection of unique keys. The value of the Authors key is an array of Author elements.

Why follow this recommendation? Here’s why:

1. It enables XML objects to evolve, using the Must-Ignore policy, as I described in yesterday’s message.

2. It enables conversion between XML and programming language objects and databases.

3. It is consistent with rfc 7493 (http://www.rfc-editor.org/rfc/rfc7493.txt) which recommends that no JSON object have duplicate keys.

/Roger