Musings

As well as providing vendor-independent means of creating, retrieving, modifying and deleting XSets, the SNIA XAM v1.0 specification also defines a query language (XAM QL), based on a subset of the SQL language, for selecting and retrieving the XUIDs of XSets based on content-defined criteria.

The set of reserved words for this query language is quite small: select, where, and, or, not, like, exists, binding, readonly, typeof, length, date, TRUE, FALSE, before, after, contains, and within. By design, XAM queries look like an SQL select statement. The query language is case insensitive and uses the ASCII character set.

Here is an example of a simple XAM query: select “.xset.xuid” where “com.example.name” = ’Tuckers Plantation’

Version 1.0 of the XAM specification defines two levels of query language support, i.e. Level 1 and Level 2. Level 1 defines queries on properties and field attributes in XSets and is mandatory. Any XSet property value that is accessible to an application program via the XAM library can be queried. Level 2 extends Level 1 to support queries on XStreams and is optional. Both levels of query are accessed through a single, defined job type that all XAM providers must support. Since no vendor that I am aware of has actually implemented level 2 queries, the remainder of this post focuses on level 1 queries.

A XAM query statement consists of a mandatory select clause followed by an optional where clause. For XAM v1.0 the only valid select clause is select ‘.xset.xuid’. This specifies that the application is requesting a list of XUID values. For example select “.xset.xuid ”

will return a list of every XSet that is readable at the time of the query.

The where clause is used to specify a subset of XSets to be matched. For level 1 queries it is restricted to comparisons between XSet properties and literal values and/or field attributes and literal values. select “.xset.xuid” where “.xam.time.xuid” > date(’2009-02-01T00:00:00.0’)

will return the list of all Xsets which were created on or after February 1st 2009.

The following table shows which field and literal types can be validly compared. xam_intxam_doublexam_stringxam_datetimexam_xuid int** double** string* datetime* XUID* The XAM library validates that strings and strings liberals are conforming UTF-8 strings. Non-conforming UTF-8 string literals generate a XAM non-fatal query syntax error. Issues such as single versus multiple glyph characters and non printable characters are unspecified and are