6. Retrieval Facility
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## 6. Retrieval Facility

YAZ version 2.1.20 or later includes a Retrieval facility tool which allows a SRU/Z39.50 to describe itself and perform record conversions. The idea is the following:

- An SRU/Z39.50 client sends a retrieval request which includes a combination of the following parameters: syntax (format), schema (or element set name).

- The retrieval facility is invoked with parameters in a server/proxy. The retrieval facility matches the parameters a set of "supported" retrieval types. If there is no match, the retrieval signals an error (syntax and / or schema not supported).

- For a successful match, the backend is invoked with the same or altered retrieval parameters (syntax, schema). If a record is received from the backend, it is converted to the frontend name / syntax.

- The resulting record is sent back the client and tagged with the frontend syntax / schema.

The Retrieval facility is driven by an XML configuration. The configuration is neither Z39.50 ZeeRex or SRU ZeeRex. But it should be easy to generate both of them from the XML configuration. (unfortunately the two versions of ZeeRex differ substantially in this regard).

### 6.1. Retrieval XML format

All elements should be covered by namespace `http://indexdata.com/yaz` . The root element node must be `retrievalinfo`.

The `retrievalinfo` must include one or more `retrieval` elements. Each `retrieval` defines specific combination of syntax, name and identifier supported by this retrieval service.

The `retrieval` element may include any of the following attributes:

`syntax` (REQUIRED)

  Defines the record syntax. Possible values is any of the names defined in YAZ' OID database or a raw OID in (n.n ... n).

`name` (OPTIONAL)

  Defines the name of the retrieval format. This can be any string. For SRU, the value, is equivalent to schema (short-hand); for Z39.50 it's equivalent to simple element set name. For YAZ 3.0.24 and later this name may be specified as a glob expression with operators `*` and `?`.

`identifier` (OPTIONAL)

  Defines the URI schema name of the retrieval format. This can be any string. For SRU, the value, is equivalent to URI schema. For Z39.50, there is no equivalent.

The `retrieval` may include one `backend` element. If a `backend` element is given, it specifies how the records are retrieved by some backend and how the records are converted from the backend to the "frontend".

The attributes, `name` and `syntax` may be specified for the `backend` element. These semantics of these attributes is equivalent to those for the `retrieval`. However, these values are passed to the "backend".

The `backend` element may includes one or more conversion instructions (as children elements). The supported conversions are:

`marc`

  The `marc` element specifies a conversion to - and from ISO2709 encoded MARC and MARCXML/MarcXchange. The following attributes may be specified:

  `inputformat` (REQUIRED)

    Format of input. Supported values are `marc` (for ISO2709); and `xml` for MARCXML/MarcXchange.

  `outputformat` (REQUIRED)

    Format of output. Supported values are `line` (MARC line format); `marcxml` (for MARCXML), `marc` (ISO2709), `marcxhcange` (for MarcXchange).

  `inputcharset` (OPTIONAL)

    Encoding of input. For XML input formats, this need not be given, but for ISO2709 based inputformats, this should be set to the encoding used. For MARC21 records, a common inputcharset value would be `marc-8`.

  `outputcharset` (OPTIONAL)

    Encoding of output. If outputformat is XML based, it is strongly recommened to use `utf-8`.

`xslt`

  The `xslt` element specifies a conversion via XSLT. The following attributes may be specified:

  `stylesheet` (REQUIRED)

    Stylesheet file.

### 6.2. Retrieval Facility Examples

Example 7.19. MARC21 backend

A typical way to use the retrieval facility is to enable XML for servers that only supports ISO2709 encoded MARC21 records.

         <retrievalinfo>
           <retrieval syntax="usmarc" name="F"/>
           <retrieval syntax="usmarc" name="B"/>
           <retrieval syntax="xml" name="marcxml"
    		  identifier="info:srw/schema/1/marcxml-v1.1">
             <backend syntax="usmarc" name="F">
    	   <marc inputformat="marc" outputformat="marcxml"
    		 inputcharset="marc-8"/>
    	 </backend>
           </retrieval>
           <retrieval syntax="xml" name="dc">
             <backend syntax="usmarc" name="F">
    	   <marc inputformat="marc" outputformat="marcxml"
    		 inputcharset="marc-8"/>
               <xslt stylesheet="MARC21slim2DC.xsl"/>
    	 </backend>
           </retrieval>
         </retrievalinfo>

This means that our frontend supports:

- MARC21 F(ull) records.

- MARC21 B(rief) records.

- MARCXML records.

- Dublin core records.

### 6.3. API

It should be easy to use the retrieval systems from applications. Refer to the headers `yaz/retrieval.h` and `yaz/record_conv.h`.

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