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Data Views in Liquid Data
By: Kunal Mittal
Oct. 3, 2003 12:00 AM
BEA Liquid Data for WebLogic provides a unified view of data aggregated from multiple resources such as databases, XML files, Web services, EJBs, or Java 2 Connector Architecture (J2CA) adapters. At a very high level, Liquid Data can be thought of as an enterprise-wide JOIN tool that provides an XML view of data from any source. This data can be accessed using evolving technologies such as XQuery. Liquid Data provides several different mechanisms through which Data Views in Liquid Data can be leveraged by the rest of the BEA Platform. In this article I'll introduce several ways of accessing Data Views outside of the Liquid Data for WebLogic Data View Builder itself.
Quick Introduction to Data Views Data Views can aggregate data from various sources such as relational databases (using standard SQL or stored procedures), XML, or delimited files stored in a file system; Web services; and legacy EIS. Using the Data View Builder, one or more of these sources' schemas are aggregated to form the target schema. Users can define JOIN conditions, and other data aggregation or selection logic that you are used to doing while accessing relational data using SQL. Users also have the ability to design custom functions to access data in different way. This article assumes that you have already created some Data Views. I'll focus on the various mechanisms available to access them.
Accessing Data Views
A stateless session EJB exposes the query API, which delegates queries through a query processor to the underlying data stores. Any Java program that can access EJBs can use this EJB-based API. Stand-alone Java applications, Javabeans, and other EJBs all use this API, and collectively are called EJB clients. These programs can use the EJB remote or local interfaces directly. EJB clients are any applications that invoke queries on the Liquid Data server using the Liquid Data EJB API. All Java clients can leverage the flexibility and data integration properties offered by XQuery to meet their data access needs. All of these clients access the EJB remote interfaces directly; therefore, they can be collectively characterized as EJB clients. The Data View builder that is part of the core Liquid Data product is a special kind of EJB client.
The four basic steps to access Data Views through this mechanism are: JNDI is used to look up the EJBs and standard EJB calls are used to connect to a Liquid Data server. Once connected, queries can be executed anonymously or through authentication itself by passing a security context to the corresponding JNDI environment properties SECURITY_PRINCIPAL and SECURITY_CREDENTIALS.
In most cases users would want to execute queries based on certain conditions. XQuery-based parameters are used to pass search conditions to a Liquid Data server. The following code snippet is an excerpt of how to pass search parameters to a data view.
import com.bea.ldi.server.common.QueryParameters;
execute (queryString, [qp]) command If the query is a stored, named query, you would use executeStored (queryName, [qp]) command The "qp" parameter is optional based on whether you are passing query parameters.
When the query is executed, the Liquid Data server returns an unformatted XML string. You can use the toXML() or getDocument() methods to receive an XML or DOM format with the results of the query.
Access to Data Views Through JSP The "query" tag is used to connect to a Liquid Data server and execute a query. It takes the name of the query, the connection to the server, and the username and password needed to authenticate with the server The "param" tag is used to pass parameters for the query using name/value pairs. Listing 1 shows some sample code on how to use the "query" and "param" tags. You can use these tag libraries to execute both ad hoc and stored queries. The resulting XML can be displayed on the JSP using XLST translations. WebLogic server comes with an XSLT JSP Tag Library that can be used for an XSL transformation. Alternatively, you could use a third-party or open source library for this functionality.
Access to Data Views Through Web Services http://host:port/liquiddata/query_name/webservice?WSDL Once you have exposed a Liquid Data query as a Web service, you would use standard Web services mechanisms to look up the WSDL and execute the Web service. BEA Liquid Data for WebLogic 8.1 also provides a control in WebLogic Workshop that can be used to invoke this query. WebLogic Workshop can also be used to test the Liquid Data Web service-based query.
Access to Data Views Through WebLogic Integration BPM Components Liquid Data helps BEA WebLogic Integration integrate data from various relational, non-relational, and Web sources. The Liquid Data views are read-only. WebLogic Integration components such as workflows are used to initiate two-way transactions from the rich set of data that is provided using the Data Views in BEA Liquid Data for WebLogic.
Access to Data Views Through Portlets Liquid Data thus becomes a rich data feed and WebLogic Portal becomes the layer that handles the presentation of this rich data.
Summary
References
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