EclipseLink/UserGuide/DBWS/Overview

EclipseLink DBWS Overview

The goal of EclipseLink DBWS is to enable simple and efficient access to relational database artifacts via a Web service, providing Java EE-compliant client-neutral access to the database without having to write Java code. EclipseLink DBWS extends EclipseLink's core capabilities while leveraging existing ORM and OXM components.

EclipseLink DBWS has two parts: a design-time component (DBWSBuilder) and a runtime provider component that takes a service descriptor (along with related deployment artifacts) and realizes it as a JAX-WS 2.0 Web service. The runtime provider uses EclipseLink to bridge between the database and the XML SOAP Messages used by Web service clients.

An EclipseLink DBWS service may be comprised of any number of operations of which there are 4 types:

1. insert - inserts into the database persistent entities described by an XML document.
2. update - updates database persistent entities described by an XML document.
3. delete - removes from the database persistent entities described by an XML document.
4. query - retrieves from the database persistent entities described by an XML document.
   Selection criteria for Query operations can be specified by:
   • custom SQL SELECT statement
   • Stored Procedure invocation
   • EclipseLink Named Query (that can use the complete range of EclipseLink ORM Expression Framework APIs)
   • (future) JP-QL

The XML documents used by an operation conform to an XML Schema Definition (.xsd file).

XML-to-Relational Mapping (XRM)

EclipseLink's ORM and OXM features provides the basis for a powerful bridge between a database's relational structure(s) and XML's hierarchical structure.

Configuration

A typical EclipseLink DBWS service is packaged in an archive (.jar or .war file) with a service descriptor file eclipselink-dbws.xml in the META-INF directory (or WEB-INF/classes/META-INF when packaged in a .war file). To bridge the relational database and XML worlds, an EclipseLink sessions.xml eclipselink-dbws-sessions.xml points to two Eclipse projects - one for the ORM side, the other for the OXM side. The service also requires an XML Schema Definition file eclipselink-dbws-schema.xsd which in conjunction with the OXM project, specifies how information from the database is to be 'shaped' into XML documents.

 root of archive
 {not all files displayed ...}
   \---META-INF
     |   eclipselink-dbws.xml
     |   eclipselink-dbws-sessions.xml -- name can be overriden by <sessions-file> entry in eclipselink-dbws.xml
     |   eclipselink-dbws-or.xml
     |   eclipselink-dbws-ox.xml
     |   eclipselink-dbws-schema.xsd -- when deployed in a .war file, located in a different directory

The EclipseLink DBWS service descriptor file eclipselink-dbws.xml is easy to read, with minimum required information and simple defaults for omitted fields, allowing for both auto-generation by a utility or manual editing:

• <name> of EclipseLink DBWS service
• <sessions-file> name of sessions.xml file - if not present, then eclipselink-dbws-sessions.xml will be used
• operation definitions (see the four types mentioned above)

Example DBWS Service descriptor file

<?xml version="1.0" encoding="UTF-8"?>
<dbws
  xmlns:xsd="http://www.w3.org/2001/XMLSchema"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  >
  <name>example</name>
  <sessions-file>example-dbws-sessions.xml</sessions-file>
  <query>
    <name>countEmployees
    <result>
      <type>xsd:int</type>
    </result>
    <sql><![CDATA[select count(*) from EMP]]></sql>
  </query>
</dbws>

The EclipseLink DBWS service descriptor file is described in full in the User Guide.

XML Schema Definition

The EclipseLink DBWS service schema file eclipselink-dbws-schema.xsd can be created by hand, or auto-generated by the design-time DBWSBuilder utility that derives XML element-tag names from Database metadata (column names, types, nullable, etc).

Example Simple XML Format (SXF) document

The DBWSBuilder utility will not generate an XML Schema Definition when the information returned by a query operation has no pre-determined structure, such as:

• a resultSet from a custom SQL query operation
• the results from a Stored Procedure query operation
• the row-count from an update operation

In these cases, the EclipseLink DBWS runtime provider uses information only available at the time of query execution to build the XML document:

{Element tag names are direct copies of table's column names}
<?xml version = '1.0' encoding = 'UTF-8'?>
<simple-xml-format>
  <simple-xml>
    <EMPNO>7788</EMPNO>
    <ENAME>SCOTT</ENAME>
    <JOB>ANALYST</JOB>
    <MGR>7566</MGR>
    <HIREDATE>1987-04-19T00:00:00.000-0400</HIREDATE>
    <SAL>3000</SAL>
    <DEPTNO>20</DEPTNO>
  </simple-xml>
  <simple-xml>
    <EMPNO>7369</EMPNO>
    <ENAME>SMITH</ENAME>
    <JOB>CLERK</JOB>
    <MGR>7902</MGR>
    <HIREDATE>1980-12-17T00:00:00.000-0400</HIREDATE>
    <SAL>800</SAL>
    <DEPTNO>20</DEPTNO>
  </simple-xml>
</simple-xml-format>

These XML documents are 'dumb' as they cannot be validated against any pre-determined schema - or more accurately, only the following very permissive 'sequence-of-any' schema can validate such documents:

<?xml version="1.0" encoding="UTF-8"?>
<xsd:schema
  xmlns:xsd="http://www.w3.org/2001/XMLSchema"
  >
  <xsd:complexType name="simple-xml-format">
    <xsd:sequence>
      <xsd:any minOccurs="0"/>
    </xsd:sequence>
  </xsd:complexType>
</xsd:schema>

The element tags simple-xml-format and simple-xml can be customized by setting the appropriate properties on an operation.

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