# The Record Model

The Zebra system is designed to support a wide range of data management applications. The system can be configured to handle virtually any kind of structured data. Each record in the system is associated with a record schema which lends context to the data elements of the record. Any number of record schemas can coexist in the system. Although it may be wise to use only a single schema within one database, the system poses no such restrictions.

The record model described in this chapter applies to the fundamental, structured record type `grs`, introduced in .

Records pass through three different states during processing in the system.

- When records are accessed by the system, they are represented in their local, or native format. This might be SGML or HTML files, News or Mail archives, MARC records. If the system doesn't already know how to read the type of data you need to store, you can set up an input filter by preparing conversion rules based on regular expressions and possibly augmented by a flexible scripting language (Tcl). The input filter produces as output an internal representation, a tree structure.

- When records are processed by the system, they are represented in a tree-structure, constructed by tagged data elements hanging off a root node. The tagged elements may contain data or yet more tagged elements in a recursive structure. The system performs various actions on this tree structure (indexing, element selection, schema mapping, etc.),

- Before transmitting records to the client, they are first converted from the internal structure to a form suitable for exchange over the network - according to the Z39.50 standard.

## Local Representation

As mentioned earlier, Zebra places few restrictions on the type of data that you can index and manage. Generally, whatever the form of the data, it is parsed by an input filter specific to that format, and turned into an internal structure that Zebra knows how to handle. This process takes place whenever the record is accessed - for indexing and retrieval.

The RecordType parameter in the `zebra.cfg` file, or the `-t` option to the indexer tells Zebra how to process input records. Two basic types of processing are available - raw text and structured data. Raw text is just that, and it is selected by providing the argument text to Zebra. Structured records are all handled internally using the basic mechanisms described in the subsequent sections. Zebra can read structured records in many different formats. How this is done is governed by additional parameters after the "grs" keyword, separated by "." characters.

Four basic subtypes to the grs type are currently available:

grs.sgml

  This is the canonical input format — described below. It is a simple SGML-like syntax.

grs.regx.filter

  This enables a user-supplied input filter. The mechanisms of these filters are described below.

grs.tcl.filter

  Similar to grs.regx but using Tcl for rules.

grs.marc.abstract syntax

  This allows Zebra to read records in the ISO2709 (MARC) encoding standard. In this case, the last parameter abstract syntax names the `.abs` file (see below) which describes the specific MARC structure of the input record as well as the indexing rules.

grs.xml

  This filter reads XML records. Only one record per file is supported. The filter is only available if Zebra/YAZ is compiled with EXPAT support.

### Canonical Input Format

Although input data can take any form, it is sometimes useful to describe the record processing capabilities of the system in terms of a single, canonical input format that gives access to the full spectrum of structure and flexibility in the system. In Zebra, this canonical format is an "SGML-like" syntax.

To use the canonical format specify `grs.sgml` as the record type.

Consider a record describing an information resource (such a record is sometimes known as a locator record). It might contain a field describing the distributor of the information resource, which might in turn be partitioned into various fields providing details about the distributor, like this:

          <Distributor>
            <Name> USGS/WRD </Name>
            <Organization> USGS/WRD </Organization>
            <Street-Address>
              U.S. GEOLOGICAL SURVEY, 505 MARQUETTE, NW
            </Street-Address>
            <City> ALBUQUERQUE </City>
            <State> NM </State>
            <Zip-Code> 87102 </Zip-Code>
            <Country> USA </Country>
            <Telephone> (505) 766-5560 </Telephone>
          </Distributor>

The keywords surrounded by <...> are tags, while the sections of text in between are the data elements. A data element is characterized by its location in the tree that is made up by the nested elements. Each element is terminated by a closing tag - beginning with `<`/, and containing the same symbolic tag-name as the corresponding opening tag. The general closing tag - `</>` - terminates the element started by the last opening tag. The structuring of elements is significant. The element Telephone, for instance, may be indexed and presented to the client differently, depending on whether it appears inside the Distributor element, or some other, structured data element such a Supplier element.

#### Record Root

The first tag in a record describes the root node of the tree that makes up the total record. In the canonical input format, the root tag should contain the name of the schema that lends context to the elements of the record (see ). The following is a GILS record that contains only a single element (strictly speaking, that makes it an illegal GILS record, since the GILS profile includes several mandatory elements - Zebra does not validate the contents of a record against the Z39.50 profile, however - it merely attempts to match up elements of a local representation with the given schema):

           <gils>
              <title>Zen and the Art of Motorcycle Maintenance</title>
           </gils>

#### Variants

Zebra allows you to provide individual data elements in a number of variant forms. Examples of variant forms are textual data elements which might appear in different languages, and images which may appear in different formats or layouts. The variant system in Zebra is essentially a representation of the variant mechanism of Z39.50-1995.

The following is an example of a title element which occurs in two different languages.

           <title>
           <var lang lang "eng">
           Zen and the Art of Motorcycle Maintenance</>
           <var lang lang "dan">
           Zen og Kunsten at Vedligeholde en Motorcykel</>
           </title>

The syntax of the variant element is `<var class type value>`. The available values for the class and type fields are given by the variant set that is associated with the current schema (see ).

Variant elements are terminated by the general end-tag </>, by the variant end-tag </var>, by the appearance of another variant tag with the same class and value settings, or by the appearance of another, normal tag. In other words, the end-tags for the variants used in the example above could have been omitted.

Variant elements can be nested. The element

           <title>
           <var lang lang "eng"><var body iana "text/plain">
           Zen and the Art of Motorcycle Maintenance
           </title>

Associates two variant components to the variant list for the title element.

Given the nesting rules described above, we could write

           <title>
           <var body iana "text/plain>
           <var lang lang "eng">
           Zen and the Art of Motorcycle Maintenance
           <var lang lang "dan">
           Zen og Kunsten at Vedligeholde en Motorcykel
           </title>

The title element above comes in two variants. Both have the IANA body type "text/plain", but one is in English, and the other in Danish. The client, using the element selection mechanism of Z39.50, can retrieve information about the available variant forms of data elements, or it can select specific variants based on the requirements of the end-user.

### Input Filters

In order to handle general input formats, Zebra allows the operator to define filters which read individual records in their native format and produce an internal representation that the system can work with.

Input filters are ASCII files, generally with the suffix `.flt`. The system looks for the files in the directories given in the profilePath setting in the `zebra.cfg` files. The record type for the filter is `grs.regx.`filter-filename (fundamental type `grs`, file read type `regx`, argument filter-filename).

Generally, an input filter consists of a sequence of rules, where each rule consists of a sequence of expressions, followed by an action. The expressions are evaluated against the contents of the input record, and the actions normally contribute to the generation of an internal representation of the record.

An expression can be either of the following:

INIT

  The action associated with this expression is evaluated exactly once in the lifetime of the application, before any records are read. It can be used in conjunction with an action that initializes tables or other resources that are used in the processing of input records.

BEGIN

  Matches the beginning of the record. It can be used to initialize variables, etc. Typically, the BEGIN rule is also used to establish the root node of the record.

END

  Matches the end of the record - when all of the contents of the record has been processed.

/pattern/

  Matches a string of characters from the input record.

BODY

  This keyword may only be used between two patterns. It matches everything between (not including) those patterns.

FINISH

  The expression associated with this pattern is evaluated once, before the application terminates. It can be used to release system resources - typically ones allocated in the INIT step.

An action is surrounded by curly braces ({...}), and consists of a sequence of statements. Statements may be separated by newlines or semicolons (;). Within actions, the strings that matched the expressions immediately preceding the action can be referred to as $0, $1, $2, etc.

The available statements are:

begin type [parameter ... ]

  Begin a new data element. The type is one of the following:

  record

    Begin a new record. The following parameter should be the name of the schema that describes the structure of the record, eg. `gils` or `wais` (see below). The `begin record` call should precede any other use of the begin statement.

  element

    Begin a new tagged element. The parameter is the name of the tag. If the tag is not matched anywhere in the tagsets referenced by the current schema, it is treated as a local string tag.

  variant

    Begin a new node in a variant tree. The parameters are class type value.

data parameter

  Create a data element. The concatenated arguments make up the value of the data element. The option `-text` signals that the layout (whitespace) of the data should be retained for transmission. The option `-element` tag wraps the data up in the tag. The use of the `-element` option is equivalent to preceding the command with a begin element command, and following it with the end command.

end [type]

  Close a tagged element. If no parameter is given, the last element on the stack is terminated. The first parameter, if any, is a type name, similar to the begin statement. For the element type, a tag name can be provided to terminate a specific tag.

unread no

  Move the input pointer to the offset of first character that match rule given by no. The first rule from left-to-right is numbered zero, the second rule is named 1 and so on.

The following input filter reads a Usenet news file, producing a record in the WAIS schema. Note that the body of a news posting is separated from the list of headers by a blank line (or rather a sequence of two newline characters.

          BEGIN                { begin record wais }

          /^From:/ BODY /$/    { data -element name $1 }
          /^Subject:/ BODY /$/ { data -element title $1 }
          /^Date:/ BODY /$/    { data -element lastModified $1 }
          /\n\n/ BODY END      {
             begin element bodyOfDisplay
             begin variant body iana "text/plain"
             data -text $1
             end record
          }

If Zebra is compiled with support for Tcl enabled, the statements described above are supplemented with a complete scripting environment, including control structures (conditional expressions and loop constructs), and powerful string manipulation mechanisms for modifying the elements of a record.

## Internal Representation

When records are manipulated by the system, they're represented in a tree-structure, with data elements at the leaf nodes, and tags or variant components at the non-leaf nodes. The root-node identifies the schema that lends context to the tagging and structuring of the record. Imagine a simple record, consisting of a 'title' element and an 'author' element:

         ROOT
            TITLE     "Zen and the Art of Motorcycle Maintenance"
            AUTHOR    "Robert Pirsig"

A slightly more complex record would have the author element consist of two elements, a surname and a first name:

         ROOT
            TITLE  "Zen and the Art of Motorcycle Maintenance"
            AUTHOR
               FIRST-NAME "Robert"
               SURNAME    "Pirsig"

The root of the record will refer to the record schema that describes the structuring of this particular record. The schema defines the element tags (TITLE, FIRST-NAME, etc.) that may occur in the record, as well as the structuring (SURNAME should appear below AUTHOR, etc.). In addition, the schema establishes element set names that are used by the client to request a subset of the elements of a given record. The schema may also establish rules for converting the record to a different schema, by stating, for each element, a mapping to a different tag path.

### Tagged Elements

A data element is characterized by its tag, and its position in the structure of the record. For instance, while the tag "telephone number" may be used different places in a record, we may need to distinguish between these occurrences, both for searching and presentation purposes. For instance, while the phone numbers for the "customer" and the "service provider" are both representatives for the same type of resource (a telephone number), it is essential that they be kept separate. The record schema provides the structure of the record, and names each data element (defined by the sequence of tags - the tag path - by which the element can be reached from the root of the record).

### Variants

The children of a tag node may be either more tag nodes, a data node (possibly accompanied by tag nodes), or a tree of variant nodes. The children of variant nodes are either more variant nodes or a data node (possibly accompanied by more variant nodes). Each leaf node, which is normally a data node, corresponds to a variant form of the tagged element identified by the tag which parents the variant tree. The following title element occurs in two different languages:

          VARIANT LANG=ENG  "War and Peace"
          TITLE
          VARIANT LANG=DAN  "Krig og Fred"

Which of the two elements are transmitted to the client by the server depends on the specifications provided by the client, if any.

In practice, each variant node is associated with a triple of class, type, value, corresponding to the variant mechanism of Z39.50.

### Data Elements

Data nodes have no children (they are always leaf nodes in the record tree).

## Configuring Your Data Model

The following sections describe the configuration files that govern the internal management of data records. The system searches for the files in the directories specified by the profilePath setting in the `zebra.cfg` file.

### The Abstract Syntax

The abstract syntax definition (also known as an Abstract Record Structure, or ARS) is the focal point of the record schema description. For a given schema, the ABS file may state any or all of the following:

- The object identifier of the Z39.50 schema associated with the ARS, so that it can be referred to by the client.

- The attribute set (which can possibly be a compound of multiple sets) which applies in the profile. This is used when indexing and searching the records belonging to the given profile.

- The tag set (again, this can consist of several different sets). This is used when reading the records from a file, to recognize the different tags, and when transmitting the record to the client - mapping the tags to their numerical representation, if they are known.

- The variant set which is used in the profile. This provides a vocabulary for specifying the forms of data that appear inside the records.

- Element set names, which are a shorthand way for the client to ask for a subset of the data elements contained in a record. Element set names, in the retrieval module, are mapped to element specifications, which contain information equivalent to the Espec-1 syntax of Z39.50.

- Map tables, which may specify mappings to other database profiles, if desired.

- Possibly, a set of rules describing the mapping of elements to a MARC representation.

- A list of element descriptions (this is the actual ARS of the schema, in Z39.50 terms), which lists the ways in which the various tags can be used and organized hierarchically.

Several of the entries above simply refer to other files, which describe the given objects.

### The Configuration Files

This section describes the syntax and use of the various tables which are used by the retrieval module.

The number of different file types may appear daunting at first, but each type corresponds fairly clearly to a single aspect of the Z39.50 retrieval facilities. Further, the average database administrator, who is simply reusing an existing profile for which tables already exist, shouldn't have to worry too much about the contents of these tables.

Generally, the files are simple ASCII files, which can be maintained using any text editor. Blank lines, and lines beginning with a (#) are ignored. Any characters on a line followed by a (#) are also ignored. All other lines contain directives, which provide some setting or value to the system. Generally, settings are characterized by a single keyword, identifying the setting, followed by a number of parameters. Some settings are repeatable (r), while others may occur only once in a file. Some settings are optional (o), while others again are mandatory (m).

### The Abstract Syntax (.abs) Files

The name of this file type is slightly misleading in Z39.50 terms, since, apart from the actual abstract syntax of the profile, it also includes most of the other definitions that go into a database profile.

When a record in the canonical, SGML-like format is read from a file or from the database, the first tag of the file should reference the profile that governs the layout of the record. If the first tag of the record is, say, `<gils>`, the system will look for the profile definition in the file `gils.abs`. Profile definitions are cached, so they only have to be read once during the lifespan of the current process.

When writing your own input filters, the record-begin command introduces the profile, and should always be called first thing when introducing a new record.

The file may contain the following directives:

name symbolic-name

  (m) This provides a shorthand name or description for the profile. Mostly useful for diagnostic purposes.

reference OID-name

  (m) The reference name of the OID for the profile. The reference names can be found in the util module of YAZ.

attset filename

  (m) The attribute set that is used for indexing and searching records belonging to this profile.

tagset filename

  (o) The tag set (if any) that describe that fields of the records.

varset filename

  (o) The variant set used in the profile.

maptab filename

  (o,r) This points to a conversion table that might be used if the client asks for the record in a different schema from the native one.

marc filename

  (o) Points to a file containing parameters for representing the record contents in the ISO2709 syntax. Read the description of the MARC representation facility below.

esetname name filename

  (o,r) Associates the given element set name with an element selection file. If an (@) is given in place of the filename, this corresponds to a null mapping for the given element set name.

any tags

  (o) This directive specifies a list of attributes which should be appended to the attribute list given for each element. The effect is to make every single element in the abstract syntax searchable by way of the given attributes. This directive provides an efficient way of supporting free-text searching across all elements. However, it does increase the size of the index significantly. The attributes can be qualified with a structure, as in the elm directive below.

elm path name attributes

  (o,r) Adds an element to the abstract record syntax of the schema. The path follows the syntax which is suggested by the Z39.50 document - that is, a sequence of tags separated by slashes (/). Each tag is given as a comma-separated pair of tag type and -value surrounded by parenthesis. The name is the name of the element, and the attributes specifies which attributes to use when indexing the element in a comma-separated list. A ! in place of the attribute name is equivalent to specifying an attribute name identical to the element name. A - in place of the attribute name specifies that no indexing is to take place for the given element. The attributes can be qualified with field types to specify which character set should govern the indexing procedure for that field. The same data element may be indexed into several different fields, using different character set definitions. See the . The default field type is `w` for word.

xelm xpath attributes

  Specifies indexing for record nodes given by xpath. Unlike directive elm, this directive allows you to index attribute contents. The xpath uses a syntax similar to XPath. The attributes have same syntax and meaning as directive elm, except that operator ! refers to the nodes selected by xpath.

encoding encodingname

  This directive specifies character encoding for external records. For records such as XML that specifies encoding within the file via a header this directive is ignored. If neither this directive is given, nor an encoding is set within external records, ISO-8859-1 encoding is assumed.

xpath `enable`/`disable`

  If this directive is followed by `enable`, then extra indexing is performed to allow for XPath-like queries. If this directive is not specified - equivalent to `disable` - no extra XPath-indexing is performed.

systag systemTag actualTag

  Specifies what information, if any, Zebra should automatically include in retrieval records for the ``system fields'' that it supports. systemTag may be any of the following:

  `rank`

    An integer indicating the relevance-ranking score assigned to the record.

  `sysno`

    An automatically generated identifier for the record, unique within this database. It is represented by the `<localControlNumber>` element in XML and the `(1,14)` tag in GRS-1.

  `size`

    The size, in bytes, of the retrieved record.

  The actualTag parameter may be `none` to indicate that the named element should be omitted from retrieval records.

The mechanism for controlling indexing is not adequate for complex databases, and will probably be moved into a separate configuration table eventually.

The following is an excerpt from the abstract syntax file for the GILS profile.

          name gils
          reference GILS-schema
          attset gils.att
          tagset gils.tag
          varset var1.var

          maptab gils-usmarc.map

          # Element set names

          esetname VARIANT gils-variant.est  # for WAIS-compliance
          esetname B gils-b.est
          esetname G gils-g.est
          esetname F @

          elm (1,10)              rank                        -
          elm (1,12)              url                         -
          elm (1,14)              localControlNumber     Local-number
          elm (1,16)              dateOfLastModification Date/time-last-modified
          elm (2,1)               title                       w:!,p:!
          elm (4,1)               controlIdentifier      Identifier-standard
          elm (2,6)               abstract               Abstract
          elm (4,51)              purpose                     !
          elm (4,52)              originator                  -
          elm (4,53)              accessConstraints           !
          elm (4,54)              useConstraints              !
          elm (4,70)              availability                -
          elm (4,70)/(4,90)       distributor                 -
          elm (4,70)/(4,90)/(2,7) distributorName             !
          elm (4,70)/(4,90)/(2,10 distributorOrganization     !
          elm (4,70)/(4,90)/(4,2) distributorStreetAddress    !
          elm (4,70)/(4,90)/(4,3) distributorCity             !

### The Attribute Set (.att) Files

This file type describes the Use elements of an attribute set. It contains the following directives.

name symbolic-name

  (m) This provides a shorthand name or description for the attribute set. Mostly useful for diagnostic purposes.

reference OID-name

  (m) The reference name of the OID for the attribute set. The reference names can be found in the util module of YAZ.

include filename

  (o,r) This directive is used to include another attribute set as a part of the current one. This is used when a new attribute set is defined as an extension to another set. For instance, many new attribute sets are defined as extensions to the bib-1 set. This is an important feature of the retrieval system of Z39.50, as it ensures the highest possible level of interoperability, as those access points of your database which are derived from the external set (say, bib-1) can be used even by clients who are unaware of the new set.

att att-value att-name [local-value]

  (o,r) This repeatable directive introduces a new attribute to the set. The attribute value is stored in the index (unless a local-value is given, in which case this is stored). The name is used to refer to the attribute from the abstract syntax.

This is an excerpt from the GILS attribute set definition. Notice how the file describing the bib-1 attribute set is referenced.

          name gils
          reference GILS-attset
          include bib1.att

          att 2001		distributorName
          att 2002		indextermsControlled
          att 2003		purpose
          att 2004		accessConstraints
          att 2005		useConstraints

### The Tag Set (.tag) Files

This file type defines the tagset of the profile, possibly by referencing other tag sets (most tag sets, for instance, will include tagsetG and tagsetM from the Z39.50 specification. The file may contain the following directives.

name symbolic-name

  (m) This provides a shorthand name or description for the tag set. Mostly useful for diagnostic purposes.

reference OID-name

  (o) The reference name of the OID for the tag set. The reference names can be found in the util module of YAZ. The directive is optional, since not all tag sets are registered outside of their schema.

type integer

  (m) The type number of the tagset within the schema profile (note: this specification really should belong to the .abs file. This will be fixed in a future release).

include filename

  (o,r) This directive is used to include the definitions of other tag sets into the current one.

tag number names type

  (o,r) Introduces a new tag to the set. The number is the tag number as used in the protocol (there is currently no mechanism for specifying string tags at this point, but this would be quick work to add). The names parameter is a list of names by which the tag should be recognized in the input file format. The names should be separated by slashes (/). The type is the recommended data type of the tag. It should be one of the following:

  - structured

  - string

  - numeric

  - bool

  - oid

  - generalizedtime

  - intunit

  - int

  - octetstring

  - null

The following is an excerpt from the TagsetG definition file.

          name tagsetg
          reference TagsetG
          type 2

          tag	1	title		string
          tag	2	author		string
          tag	3	publicationPlace string
          tag	4	publicationDate	string
          tag	5	documentId	string
          tag	6	abstract	string
          tag	7	name		string
          tag	8	date		generalizedtime
          tag	9	bodyOfDisplay	string
          tag	10	organization	string

### The Variant Set (.var) Files

The variant set file is a straightforward representation of the variant set definitions associated with the protocol. At present, only the Variant-1 set is known.

These are the directives allowed in the file.

name symbolic-name

  (m) This provides a shorthand name or description for the variant set. Mostly useful for diagnostic purposes.

reference OID-name

  (o) The reference name of the OID for the variant set, if one is required. The reference names can be found in the util module of YAZ.

class integer class-name

  (m,r) Introduces a new class to the variant set.

type integer type-name datatype

  (m,r) Addes a new type to the current class (the one introduced by the most recent class directive). The type names belong to the same name space as the one used in the tag set definition file.

The following is an excerpt from the file describing the variant set Variant-1.

          name variant-1
          reference Variant-1

          class 1 variantId

          type	1	variantId		octetstring

          class 2 body

          type	1	iana			string
          type	2	z39.50			string
          type	3	other			string

### The Element Set (.est) Files

The element set specification files describe a selection of a subset of the elements of a database record. The element selection mechanism is equivalent to the one supplied by the Espec-1 syntax of the Z39.50 specification. In fact, the internal representation of an element set specification is identical to the Espec-1 structure, and we'll refer you to the description of that structure for most of the detailed semantics of the directives below.

Not all of the Espec-1 functionality has been implemented yet. The fields that are mentioned below all work as expected, unless otherwise is noted.

The directives available in the element set file are as follows:

defaultVariantSetId OID-name

  (o) If variants are used in the following, this should provide the name of the variantset used (it's not currently possible to specify a different set in the individual variant request). In almost all cases (certainly all profiles known to us), the name `Variant-1` should be given here.

defaultVariantRequest variant-request

  (o) This directive provides a default variant request for use when the individual element requests (see below) do not contain a variant request. Variant requests consist of a blank-separated list of variant components. A variant compont is a comma-separated, parenthesized triple of variant class, type, and value (the two former values being represented as integers). The value can currently only be entered as a string (this will change to depend on the definition of the variant in question). The special value (@) is interpreted as a null value, however.

simpleElement path ['variant' variant-request]

  (o,r) This corresponds to a simple element request in Espec-1. The path consists of a sequence of tag-selectors, where each of these can consist of either:

  - A simple tag, consisting of a comma-separated type-value pair in parenthesis, possibly followed by a colon (:) followed by an occurrences-specification (see below). The tag-value can be a number or a string. If the first character is an apostrophe ('), this forces the value to be interpreted as a string, even if it appears to be numerical.

  - A WildThing, represented as a question mark (?), possibly followed by a colon (:) followed by an occurrences specification (see below).

  - A WildPath, represented as an asterisk (*). Note that the last element of the path should not be a wildPath (wildpaths don't work in this version).

  The occurrences-specification can be either the string `all`, the string `last`, or an explicit value-range. The value-range is represented as an integer (the starting point), possibly followed by a plus (+) and a second integer (the number of elements, default being one).

  The variant-request has the same syntax as the defaultVariantRequest above. Note that it may sometimes be useful to give an empty variant request, simply to disable the default for a specific set of fields (we aren't certain if this is proper Espec-1, but it works in this implementation).

The following is an example of an element specification belonging to the GILS profile.

          simpleelement (1,10)
          simpleelement (1,12)
          simpleelement (2,1)
          simpleelement (1,14)
          simpleelement (4,1)
          simpleelement (4,52)

### The Schema Mapping (.map) Files

Sometimes, the client might want to receive a database record in a schema that differs from the native schema of the record. For instance, a client might only know how to process WAIS records, while the database record is represented in a more specific schema, such as GILS. In this module, a mapping of data to one of the MARC formats is also thought of as a schema mapping (mapping the elements of the record into fields consistent with the given MARC specification, prior to actually converting the data to the ISO2709). This use of the object identifier for USMARC as a schema identifier represents an overloading of the OID which might not be entirely proper. However, it represents the dual role of schema and record syntax which is assumed by the MARC family in Z39.50.

These are the directives of the schema mapping file format:

targetName name

  (m) A symbolic name for the target schema of the table. Useful mostly for diagnostic purposes.

targetRef OID-name

  (m) An OID name for the target schema. This is used, for instance, by a server receiving a request to present a record in a different schema from the native one. The name, again, is found in the oid module of YAZ.

map element-name target-path

  (o,r) Adds an element mapping rule to the table.

### The MARC (ISO2709) Representation (.mar) Files

This file provides rules for representing a record in the ISO2709 format. The rules pertain mostly to the values of the constant-length header of the record.

### Field Structure and Character Sets

In order to provide a flexible approach to national character set handling, Zebra allows the administrator to configure the set up the system to handle any 8-bit character set — including sets that require multi-octet diacritics or other multi-octet characters. The definition of a character set includes a specification of the permissible values, their sort order (this affects the display in the SCAN function), and relationships between upper- and lowercase characters. Finally, the definition includes the specification of space characters for the set.

The operator can define different character sets for different fields, typical examples being standard text fields, numerical fields, and special-purpose fields such as WWW-style linkages (URx).

The field types, and hence character sets, are associated with data elements by the .abs files (see above). The file `default.idx` provides the association between field type codes (as used in the .abs files) and the character map files (with the .chr suffix). The format of the .idx file is as follows

index field type code

  This directive introduces a new search index code. The argument is a one-character code to be used in the .abs files to select this particular index type. An index, roughly, corresponds to a particular structure attribute during search. Refer to .

sort field code type

  This directive introduces a sort index. The argument is a one-character code to be used in the .abs fie to select this particular index type. The corresponding use attribute must be used in the sort request to refer to this particular sort index. The corresponding character map (see below) is used in the sort process.

completeness boolean

  This directive enables or disables complete field indexing. The value of the boolean should be 0 (disable) or 1. If completeness is enabled, the index entry will contain the complete contents of the field (up to a limit), with words (non-space characters) separated by single space characters (normalized to " " on display). When completeness is disabled, each word is indexed as a separate entry. Complete subfield indexing is most useful for fields which are typically browsed (eg. titles, authors, or subjects), or instances where a match on a complete subfield is essential (eg. exact title searching). For fields where completeness is disabled, the search engine will interpret a search containing space characters as a word proximity search.

charmap filename

  This is the filename of the character map to be used for this index for field type.

The contents of the character map files are structured as follows:

lowercase value-set

  This directive introduces the basic value set of the field type. The format is an ordered list (without spaces) of the characters which may occur in "words" of the given type. The order of the entries in the list determines the sort order of the index. In addition to single characters, the following combinations are legal:

  - Backslashes may be used to introduce three-digit octal, or two-digit hex representations of single characters (preceded by `x`). In addition, the combinations \\, \\r, \\n, \\t, \\s (space — remember that real space-characters may not occur in the value definition), and \\ are recognized, with their usual interpretation.

  - Curly braces {} may be used to enclose ranges of single characters (possibly using the escape convention described in the preceding point), eg. {a-z} to introduce the standard range of ASCII characters. Note that the interpretation of such a range depends on the concrete representation in your local, physical character set.

  - paranthesises () may be used to enclose multi-byte characters - eg. diacritics or special national combinations (eg. Spanish "ll"). When found in the input stream (or a search term), these characters are viewed and sorted as a single character, with a sorting value depending on the position of the group in the value statement.

uppercase value-set

  This directive introduces the upper-case equivalencis to the value set (if any). The number and order of the entries in the list should be the same as in the `lowercase` directive.

space value-set

  This directive introduces the character which separate words in the input stream. Depending on the completeness mode of the field in question, these characters either terminate an index entry, or delimit individual "words" in the input stream. The order of the elements is not significant — otherwise the representation is the same as for the `uppercase` and `lowercase` directives.

map value-set target

  This directive introduces a mapping between each of the members of the value-set on the left to the character on the right. The character on the right must occur in the value set (the `lowercase` directive) of the character set, but it may be a paranthesis-enclosed multi-octet character. This directive may be used to map diacritics to their base characters, or to map HTML-style character-representations to their natural form, etc.

## Exchange Formats

Converting records from the internal structure to en exchange format is largely an automatic process. Currently, the following exchange formats are supported:

- GRS-1. The internal representation is based on GRS-1/XML, so the conversion here is straightforward. The system will create applied variant and supported variant lists as required, if a record contains variant information.

- XML. The internal representation is based on GRS-1/XML so the mapping is trivial. Note that XML schemas, preprocessing instructions and comments are not part of the internal representation and therefore will never be part of a generated XML record. Future versions of the Zebra will support that.

- SUTRS. Again, the mapping is fairly straightforward. Indentation is used to show the hierarchical structure of the record. All "GRS" type records support both the GRS-1 and SUTRS representations.

- ISO2709-based formats (USMARC, etc.). Only records with a two-level structure (corresponding to fields and subfields) can be directly mapped to ISO2709. For records with a different structuring (eg., GILS), the representation in a structure like USMARC involves a schema-mapping (see ), to an "implied" USMARC schema (implied, because there is no formal schema which specifies the use of the USMARC fields outside of ISO2709). The resultant, two-level record is then mapped directly from the internal representation to ISO2709. See the GILS schema definition files for a detailed example of this approach.

- Explain. This representation is only available for records belonging to the Explain schema.

- Summary. This ASN-1 based structure is only available for records belonging to the Summary schema - or schema which provide a mapping to this schema (see the description of the schema mapping facility above).

- SOIF. Support for this syntax is experimental, and is currently keyed to a private Index Data OID (1.2.840.10003.5.1000.81.2). All abstract syntaxes can be mapped to the SOIF format, although nested elements are represented by concatenation of the tag names at each level.