Extra Data / Extensions

Messages in all of the operations, both in the request and in the response, have a field in which additional information may be provided. This is a built in extension mechanism where profiles may specify a schema for what to include in this section without requiring the developers to change the basic messages and thus render their implementation uninteroperable with other servers and clients. It is expected that if there is sufficient demand for a particular piece of additional information, that piece of information will be migrated into the protocol in a later version. In this way, only implemented and useful features will be added in future versions, rather than features which just seem like a good idea.

Rules and Semantics

If the server does not understand a piece of information in an extraRequestData field, it may silently ignore it.
This is unlike many other request parameters, where if the server does not implement that particular feature it should respond with a diagnostic. If the particular request requires some confirmation that it has been carried out rather than ignored, then the profile designer should include a field in the extraResponseData.
The semantics of parameters in the request MAY NOT be modified by extraRequestData.
For example, a x-qt-queryType parameter could not change query to be an SQL query, as a server that does not understand the x-qt-queryType extension would expect the query to be in CQL, and thus be unable to parse it. Instead, the extraData should create a new element for the information.
The semantics of parts of the response MAY be modified by extensions.
The response semantics may be changed in this way only if the client specifically requests the change. Clients should also expect to receive the regular semantics, as servers are at liberty to ignore extraRequestData fields.
ExtraResponseData may be sent that is not connected to the search in the request.
For example it may contain cost information regarding the query or information on the server or database supplying the results. (This data must, however, have been requested. See next rule.)
ExtraResponseData MAY NOT be sent unless explicitly requested in the request.

SRU Parameter

Request
For SRU the name for an extra-data parameter must begin with 'x-' -- lower case x followed by hyphen. (The SRU protocol will never include an official parameter with a name beginning with 'x-'.) It is suggested, but not required, that the parameter name be 'x-' followed by an identifier for the extension (again followed by hyphen), followed by the name of the element within the XML namespace associated with the extension. For example 'x-info-4-onSearchFail' for <theo:onSearchFail>.

http://lx2.loc.gov:210/LCDB?...&x-info4-onSearchFail=scan

(In this example 'info-4' identifies the namespace; see next paragraph.). Note that this convention does not guarantee uniqueness since (in contrast to SRW - see SRW Parameter below.) the parameter name will not include a URI. The extension owner should try to make the name as unique as possible.

If the namespace is identified by an 'info:srw' URI (and note that there is no such requirement, the namespace may be identified by a URI of a different scheme, for example 'http'), then a convention that may be used (as in the example above) is to name the parameter "x-info-<nnn>-<element name>" where <nnn> is the 'srw:info' authority string. This convention (suggested but not required) should guarantee uniqueness. It is strongly suggested that an extension name never be assigned with this form except by the proper authority for the given 'info' namespace.

Response
Every response has an extraResponseData section. This section can include any well-formed XML, and hence servers can include namespaced XML fragments within it in order to convey information back to the client. The extension MUST supply a namespace and the element names with which to do this, if feedback to the client is necessary. For example:

<sru:extraResponseData>
<auth:token xmlns:auth="info:srw/extension/2/auth-1.0">
277c6d19-3e5d-4f2d-9659-86a77fb2b7c8
</auth:token>
</sru:extraResponseData>

As the request may be echoed in SRU, the server must be able to transform the parameters into their XML form. If it encounters an unrecognised parameter, the server may either make its best guess as to how to transform the parameter, or simply not return it at all. It should not, however, add an undefined namespace to the element as this would invalidate the response.

If the contents of the parameter is an XML structure, then the profile designer should also specify how to encode this structure for SRU. This may simply be to escape all of the special characters, but the designer could also create a string encoding form with rules as to how to generate the XML in much the same fashion as the relationship between CQL and XCQL.

SRW Parameter

For SRW, The extra data fields is an XML structure. Even if there is only one piece of additional information supplied, it must be within a namespaced XML element. This is in order to ensure that servers can distinguish a field from one profile from another. Examples:

Extra data fields are identified by the root XML element qualified by the supplied namespace. In the above examples, for the extra request data the extension's identifier is 'onSearchFail' from namespace info:srw/extension/4/searchExtensions. The extra response data has identifier 'relevancyAlgorithm' from namespace info:srw/extension/2/relevancy. Below there is another example, where the extension's identifier is 'rank', also from the latter namespace.

Extra Data Example

For an example of how to create an extension, see the Record Schema Negotiation Extension whose purpose is to allow the client to propose multiple record schemas that it will accept in response and let the server select one, rather than the client giving the server an ultimatum as to which record schema to return. This extension modifies the base semantics of the protocol, as the record schema URI in the response might not be the one in the recordSchema parameter of the request.

Record, Term and Query Extensions

In addition to extra information being included at the top level of the response, there are three further elements in which information can be returned.

These are:

extraRecordData: Extra information relating to the record in the searchRetrieve responses. For example, extraRecordData might include a field with the relevance associated with the record in a relevance ranked search.
extraTermData: Extra information relating to the term in the scan response.
extraOperandData: Extra information relation to the query clause echoed back to the client in XCQL.

extraRecordData example (highlighted in orange):

<record> <recordSchema>info:srw/schema/1/dc-v1.1</recordSchema>
     <recordPacking>xml</recordPacking>
    <recordData>
        <srw_dc:dc>
            <dc:title>Thisis a Sample Record</dc:title>
        </srw_dc:dc>
    </recordData>
    <recordPosition>1</recordPosition>
    <extraRecordData>
     <rel:rank xmlns:rel="xmlns:rob="info:srw/extension/2/relevancy">0.965</rel:rank>
   </extraRecordData>
</record>

extraTermData

And in addition "extra data" can occur within the term element for any term in a scanResponse message containing profiled information about the term. This data can include (but is not limited to) metadata about the term. For example, extraTermData might include a link to the term within a thesaurus.

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