If some built-in Virtuoso/PL or XPATH function can invoke XML parser then it is probably have a special argument to pass configuration options to the XML parser. The most typical value of such an argument is "configuration string" that is a sequence of pairs parameter=value, delimited by spaces. Instead of single string, it can be passed as a vector of strings of even length of sort

vector (parameter1, value1, parameter2, value2 ...)

. It can be NULL indicating that the parser can use default values. No errors are reported if a parameter is specified twice, in which case the last specified value will be used. The only exception is the 'Validation' parameter which sets typical values for all parameters; if it is specified then it should be the first parameter in the list. Both parameter names and values are case-insensitive.

Many parameters are used to specify the importance of a particular error. For a particular application some validity constraints may be much more important than others. Because less than perfectly valid XML is common in practice it is important to configure the validator to report only those errors which are relevant to the application. Using configuration parameters, one may specify "importance levels" for every group of problems. There are 5 "importance levels":

Table 15.1. Error Catch Levels

Error Level Description
FATAL violations must be reported and further processing of XML source must be terminated right after the term where the violation was detected, without checking for errors in the rest of the document
ERROR violations must be reported and validation should continue to find further errors, but no XML document will be built if validation is invoked from such function as xtree_doc() or xper_doc()
WARNING violations must be reported but XML processing will continue
IGNORE validator must try to locate violations of given sort but only severe (possibly fatal) violations must be reported
DISABLE the specified check must be turned off fully, saving time and maybe memory

Some parameters are just switches, with only two values available: 'ENABLE' and 'DISABLE'.

AttrCompletion (ENABLE/DISABLE, default is DISABLE).  This is useful when DTD validator is invoked from XML parser. When enabled, the XML document built will contain default values of 'IMPLIED' attributes as if they present in source text. It may be useful if application should perform free-text search on all attribute values including defaults or if XML should be converted in form suitable for external non-validating XML processor or if given XML data should be stored later as part of composite document and composite document will have another DTD with other default values.

AttrMisFormat (FATAL/ERROR/WARNING/IGNORE/DISABLE, default is DISABLE).  This describes how to report errors in syntax of values of attributes.

AttrUnknown (FATAL/ERROR/WARNING/IGNORE/DISABLE).  This describes how to report attributes whose names are not listed in the DTD.

BadRecursion (FATAL/ERROR/WARNING/IGNORE/DISABLE).  This describes how to report circular references, when replacement text of an entity contains reference to this entity again, either directly (e.g. '<!ENTITY bad "some &bad; replacement">) or through other entities (e.g. '<!ENTITY a "&b;"> '<!ENTITY b "&a;">).

BuildStandalone (ENABLE/DISABLE, default is DISABLE).  When set to ENABLE, replacement texts of external entities will be inserted instead of references to these entities, thus all data from a composite document will be gathered together into one large XML. This is useful for checking the element content model of the whole document without breaks on references or if parsed XML will be passed to external application as a standalone document.

Fsa (FATAL/ERROR/WARNING/IGNORE/DISABLE).  This describes how to report violations of specified element-content model. Virtuoso's DTD validator contains a finite state automaton which can detect the first error in the content of some element, but remaining errors in the same element become "obscured" by the first one and will not be reported. Moreover, if element-content model is not SGML-compatible, some errors may remain undiscovered: it is possible to write a complex rule, so ambiguous that full check of all its interpretations will take prohibitively much time and memory. The validator will simplify such rules to make check faster, thus some errors will not be reported.

FsaBadWs (FATAL/ERROR/WARNING/IGNORE/DISABLE).  This describes how to report the most frequent violation of element-content model specified by DTD: the use of whitespace characters in positions where only elements are allowed, not PCDATA. It usually happens when XML is indented for readability. You may wish to specify 'FsaBadWs=IGNORE' to eliminate redundant messages about this violation. Note that if you will specify 'FsaBadWs=DISABLE' then you will disable the check of illegal PCDATA tokens for this particular case, so common rule for 'Fsa' violations will be applied and you will see messages.

FsaSgml (FATAL/ERROR/WARNING/IGNORE/DISABLE).  This describes how to report violations of SGML compatibility in element-content model. Some complex DTD rules for elements are not supported by SGML processors and the validator may report the use of such rules.

GeRedef (FATAL/ERROR/WARNING/IGNORE/DISABLE).  This describes how to report redundant definitions of generic entities. There redefinitions are errors in SGML but they may be ignored in XML processing. The first definition will be used and others will be ignored.

IdDuplicates (FATAL/ERROR/WARNING/IGNORE/DISABLE).  This describes how to report non-unique values of ID attributes. It is a data integrity error, because IDs are usually parts of some primary keys, and are expected to be unique.

IdrefIntegrity (FATAL/ERROR/WARNING/IGNORE/DISABLE).  This describes how to report "dangling references". Any value of IDREF attribute and any name from value of IDREFS attribute should appear in the same XML document as value of some ID attribute. You can think that an ID attribute specifies a hyperlink anchor and IDREF is a hyperlink, so it's a data integrity error if a hyperlink points to unknown location.

Include (FATAL/ERROR/WARNING/IGNORE/DISABLE).  This configures reading of external sub-documents into "main" document you validate (and maybe load in database). If 'DISABLE', no additional documents will be read, otherwise external parameter-entities, external generic-entities and external DTD will be located, using their SYSTEM names. External documents may reside in file system, in database or in the Web. Absolute SYSTEM names (of form 'protocol://server/resource') will be used without any modifications, relative SYSTEM names should be "resolved", i.e. converted to absolute by adding a prefix from the base_uri argument of SQL function.

MaxErrors (a string that represents an integer from 1 to 10000, default is '25').  This specifies how many errors may be logged before the "Too many error messages" fatal error will be reported.

MaxWarnings (a string that represents an integer from 0 to 10000, default is '100').  This specifies how many warnings may be logged before the "Too many warning messages" event will stop their logging.

NamesUnknown (FATAL/ERROR/WARNING/IGNORE/DISABLE).  This describes how to report if the document contains element names which are not the declared in DTD. They may be typos in element names or signal that DTD is incomplete or obsolete. In addition, unknown names may be reported as element-content model violations.

NamesUnordered (FATAL/ERROR/WARNING/IGNORE/DISABLE).  This describes how to report element names not declared before use in DTD. Proper order ("declare element name before use it") is important solely for compatibility with SGML standard.

NamesUnresolved (FATAL/ERROR/WARNING/IGNORE/DISABLE).  This describes how to report if an element name used in the DTD is not declared at all. This may occur if DTD is incomplete or if some declaration in it are ignored conditional sections. Unresolved names cause no data integrity errors while remain unused in data section of the XML document, NamesUnknown parameter defines what happens if they're actually used.

PeRedef (FATAL/ERROR/WARNING/IGNORE/DISABLE).  This describes how to report redundant definitions of parameter entities. Similarly to redefinitions of generic entities, there redefinitions are errors in SGML but they may be ignored in XML processing. The first definition will be used and others will be ignored.

Sgml (FATAL/ERROR/WARNING/IGNORE/DISABLE).  This describes how to report violations of SGML compatibility. In fact, not all such violations are detected by the Virtuoso Server, because known SGML readers are insensitive to some sorts of violations.

TooManyWarns (FATAL/ERROR/WARNING/IGNORE/DISABLE).  This describes how to report "Too many warning messages" event. While "Too many errors" is fatal error and terminates XML processing, "Too many warning messages" may have arbitrary "importance levels".

TraceLoading (ENABLE/DISABLE, default is DISABLE).  If set to 'ENABLE', the validator will log every reading of any resource, for easier tracking of URI resolving problems. It's possible that some readings of sub-documents will not be reported: there's a limit for number of records in the log returned by the validator. In addition, sub-documents may be cached inside validator, so only first references to some sub-document will require reading procedure.

Validation (SGML/RIGOROUS/QUICK/DISABLE, default is DISABLE).  This loads one of four "preset configurations". It must be the first parameter in configuration string, if used. DISABLE means "do not check for any type of error", QUICK is to check only for violation of "local" validity constraints, with disabled FsaBadWs, IdDuplicates and IdrefIntegrity, RIGOROUS enables these three groups, too, SGML enables all checks including all checks for SGML compatibility.

VcData (ENABLE/DISABLE, default is DISABLE).  This describes how to report violations of generic validity constraints in data section of XML document. If constraint is not configured by other parameters listed here, it will be configured by this parameter (or by VcDtd if relates to the text of DTD section).

VcDtd (ENABLE/DISABLE, default is DISABLE).  This describes how to report violations of generic validity constraints in DTD section of XML document. If constraint is not configured by other parameters listed here, it will be configured by this parameter (or by VcData if relates to the text of data section).