Using PL/SQL object types to get nested or repeating XML structures efficiently to the database with the DbAdapter

There are different methods for getting nested or repeating XML structures to the database. For example, loops on elements of an XML structure can be implemented in BPEL, calling the DbAdapter for every element. See for example http://javaoraclesoa.blogspot.nl/2012/03/loops-in-bpel-11-and-20.html. The method mentioned requires some work and one can question it's efficiency in terms of performance. For every item to be processed, the DbAdapter is called and a transformation is required. An alternative is using PL/SQL object types. PL/SQL object types can contain repeating and nested structures and can be send to the database in a single DbAdapter call. This reduces the overhead caused by calling the DbAdapter (faster) and the complexity of BPEL code (easier). Because no complex (technical) logic is required in the code calling the DbAdapter, this method also allows for easier implementation in for example Oracle BPM. In this post I'll describe how this method can be implemented using a BPEL code example. There are some considerations though when using PL/SQL object types.

Implementation

Database code

The below code shows how a repeating structure can be implemented using a PL/SQL object type;

CREATE OR REPLACE TYPE ITEM
IS
  OBJECT
  (
    NAME  VARCHAR2(255) ,
    VALUE VARCHAR2(255) );

CREATE OR REPLACE
TYPE         ITEM_ARR AS TABLE OF ITEM

The PL/SQL type ITEM_ARR contains an array of PL/SQL objects; ITEM. The object ITEM contains two fields; NAME and VALUE.

To illustrate the usage of the ITEM_ARR type, a sequence, table, trigger are created and a small TAPI (table API) package. The trigger is optional for this example.

CREATE OR REPLACE TRIGGER ITEMS_TAB_BI 
BEFORE INSERT ON ITEMS_TAB
FOR EACH ROW
BEGIN
  IF (:NEW.ID IS NULL) THEN
    SELECT ITEMS_TAB_SEQ.NEXTVAL INTO :NEW.ID FROM DUAL;
  END IF;
END;

CREATE OR REPLACE
PACKAGE BODY ITEMS_TAB_UTILS AS
  PROCEDURE ADD_ITEMS(P_ITEMS ITEM_ARR) AS
  BEGIN
    FORALL x in P_ITEMS.First..P_ITEMS.Last
     INSERT INTO ITEMS_TAB (ID,NAME,VALUE) VALUES (ITEMS_TAB_SEQ.NEXTVAL,P_ITEMS(x).NAME,P_ITEMS(x).VALUE);
  END ADD_ITEMS;
END ITEMS_TAB_UTILS;

To use an object type from another schema, the connecting schema needs to have execute permission on the object type.

BPEL code

In BPEL the ADD_ITEMS procedure can be called. This procedure will bulk insert all received items in one action. From BPEL, the DbAdapter only has to be called once. This is a good thing considering calling the DbAdapter is relatively expensive. In BPEL only one simple transformation is required and no ForEach or While activities have to be used.

The below image shows a short overview of the process and simple exception 'handling'.

I've used the message type of the itemCollection (see also  http://javaoraclesoa.blogspot.nl/2012/03/loops-in-bpel-11-and-20.html). An itemCollection is as the name might suggest, an array of items. An item contains a name and a value. The below image shows the used transformation from the input message to the message used to call the DbAdapter. As can be seen, the repeating elements are present in the target message.

When calling the service from the webservice test page in the Enterprise Manager, it looks as followed;

The result from this call is shown below;

The code can be downloaded here; https://dl.dropbox.com/u/6693935/blog/ItemUtils.zip

Considerations

PL/SQL object types can be used to get repeated or nested structures to the database in an easy and performing way. There are some things to mind though when using PL/SQL object types.

Datamodel changes

The datamodel is defined in XML schema's in the BPEL process and also in the database definition of the object type. When adding a field or changing the order of fields, the database code and the BPEL code need changing. You won't get errors in every situation when you don't do it right, since object type fields are passed in order and not by name. This is a thing to mind when making changes.

Object types in tables and queues

Object types can be used as a type in a database table or a queue message type. My suggestion is not to use object types this way. The below error message says much.

ORA-02303: cannot drop or replace a type with type or table dependents
02303. 00000 -  "cannot drop or replace a type with type or table dependents"
*Cause:    An attempt was made to drop or replace a type that has dependents.
           There could be a substitutable column of a supertype of the type
           being dropped.
*Action:   Drop all type(s) and table(s) depending on the type, then retry
           the operation using the VALIDATE option, or use the FORCE option.

If the object type changes, you will have to consider migration strategies of existing tables and queues. This can be troublesome. Sometimes tables need to be dropped and recreated because an alter type might not be sufficient to achieve the desired change. In a production environment, if the PL/SQL code has not been written in a modular way, making sure the queue is empty and will remain empty during the installation, can be a bother.

Also when a complex datamodel is implemented using PL/SQL object types, the dependencies between the types can become difficult to manage in case of changes. This is illustrated in my example by the following;  I can't CREATE OR REPLACE the ITEM type because it is used in the ITEM_ARR type.

Conclusion

Object types can be used to increase performance and decrease complexity, however changes are more difficult to implement. My suggestion on this is to only use them in the following cases;

- when using them allows you to avoid complex BPEL code
- when performance is important and you want to minimize DbAdapter calls and transformations
- when changes to the data structure implemented in PL/SQL object types, are rare

Don't use them as a column in a table or queue. You are more flexible by using other methods such as using primitive types and spreading the data structure over several tables or by using XMLType or LOB data types.

Loops in BPEL 1.1 and 2.0

Introduction

When programming in BPEL, an often used construction is the following;
- the input of a process contains a collection of elements
- the elements need to be processed one at a time

There are several solutions to implement this. A couple of these solutions will be discussed here.

SOA Suite 10g and SOA Suite 11g have some differences in usage of extension functions such as the one used for transformations. SOA Suite 11g (of course) provides several improvements over 10g. Also BPEL 1.1 and BPEL 2.0 standards provide different activities for handling loops.

An alternative for using loop constructions in BPEL when XML needs to be send to the database is by using XML object types. See http://javaoraclesoa.blogspot.nl/2013/03/using-plsql-object-types-to-get-nested.html for more information.

Example processes

The complete BPEL 1.1 and BPEL 2.0 example (including XSD and XSLT) can be downloaded here; http://dl.dropbox.com/u/6693935/blog/TestXSLT.zip

The example used will use the following input and output schema definitions;


<?xml version="1.0" encoding="UTF-8"?>
<schema attributeFormDefault="unqualified" elementFormDefault="qualified"
        targetNamespace="http://test.ms/itemcollections"
        xmlns="http://www.w3.org/2001/XMLSchema"
        xmlns:me="http://test.ms/itemcollections">

 <element name="item" type="me:itemType"/>
<complexType name="itemType">
  <sequence>
   <element name="name" type="string"/>
   <element name="value" type="string"/>
  </sequence>
 </complexType>

 <element name="itemsCollection" type="me:itemsCollectionType"/>
 <complexType name="itemsCollectionType">
  <sequence>
   <element ref="me:item" minOccurs="0" maxOccurs="unbounded"/>
  </sequence>
 </complexType>

<element name="itemCollectionArray" type="me:itemCollectionArrayType"/>
 <complexType name="itemCollectionArrayType">
  <sequence>
   <element ref="me:itemsCollection" minOccurs="0" maxOccurs="unbounded"/>
  </sequence>
 </complexType>

 <element name="simpleString" type="me:simpleStringType"/>
 <complexType name="simpleStringType">
  <sequence>
   <element name="value" type="string"/>
  </sequence>
 </complexType>

<element name="simpleNumber" type="me:simpleNumberType"/>
 <complexType name="simpleNumberType">
  <sequence>
   <element name="value" type="decimal"/>
  </sequence>
 </complexType>

</schema>

This schema has been created to illustrate how collections can be handled.

It is advisable to define your element and type definitions inside a separate XSD. This promotes re-use and maintainability. The XSD can for example be put in the MDS (as described in http://javaoraclesoa.blogspot.com/2012/02/using-mds.html). It is also very useful to define separate types for every element used. This makes defining variables containing only a part of the message easier.

The workspace contains a BPEL 1.1 and a BPEL 2.0 process which are both exposed as webservices. The BPEL 1.1 process uses a While activity together with Assign activities to process a collection. The BPEL 2.0 process uses the For Each activity and a transformation to achieve the same.

BPEL 1.1 (SOA Suite 10g + 11g)

BPEL 1.1 contains the While activity and the FlowN activity which can both be used to process collections. The While activity allows to loop over a set of activities until a certain condition is met. The FlowN activity provides the option for parallel execution and an index variable to indicate the branche which is processed. FlowN has been described in; http://javaoraclesoa.blogspot.com/2012/02/parallel-execution-and-variable-scoping.html. This part will focus on the While activity. The part describing how to get a parameter inside an XSLT transformation or use it inside an assign activity, can of course also be applied for FlowN.

The below picture shows my BPEL 1.1 sample process. It transforms the input collection on a per item basis to a local variable (which would allow processing of the individual item). Then it adds the result to the output variable.

The While condition

You can create a variable inside the scope of the while activity and increase this. The condition can be set to check whether the counter is smaller then or equal to the count of elements in the message that need processing.

Getting to your element; The element is an XSLT element

There are several ways to obtain the element you want to process. Depending on the circumstances, some options might not be available.

The easiest option is creating a variable of the type of one of the items of the collection. Then use the assign activity to assign the element.

The empty activity in the process symbolizes actions on the l_item variable. This can for example be a transformation followed by a call to a DbAdapter. The result can be transformed to a different format which can then be used to generate the output. The example is meant as a bare-bones illustration of the functionality.

Things to mind;
- it is advisable to create a scope inside the While activity and use local variables inside this scope
- in the Assign activity where you assign the item to be processed, cast the local counter variable to a number like in the following example (else the condition which selects which item to be processed, doesn't work correctly);

bpws:getVariableData('inputVariable','payload','/ns1:itemsCollection/ns1:item')[number(bpws:getVariableData('l_counter'))]

- Use the Assign append rule type and append an item to the collection for creating output.

            <assign name="AssignOutputItemProcessToOutputProcess">
              <bpelx:append>
                <bpelx:from expression="bpws:getVariableData('l_item')"/>
                <bpelx:to variable="outputVariable" part="payload"
                          query="/ns1:itemsCollection"/>
              </bpelx:append>
            </assign>

- use >= and not => in the While activity condition!

Getting to your element; The element is not an XSLT element type. BPEL 1.1 SOA Suite 10g.

BPEL 1.1 in 10g uses a different transformation function as BPEL 1.1 in 11g. The transformation activity is an Oracle extension and not part of the BPEL specification (actually it's implemented as an Assign which is part of the specification and made easier to use with a wizard). The below part is for using the BPEL 10g version.

Sometimes the element of the collection is not available as a type. This makes processing less straightforward, since a variable of the item can not readily be created in BPEL. A solution for this is using an XSLT transformation with a parameter. It is important to think about to what type you are going to transform to, since the type of the element is not available. You can create a custom element type for this., for example the name/value pair items (as specified in the introduction), can be used for that.

Using a parameter inside an XSLT transformation can for example be found on;  http://rigterink.blogspot.com/2009/09/passing-xml-as-parameter-to-xslt.html

The parameter can be assigned the counter value of the While loop and be passed to the XSLT transformation. This XSLT parameter can then be used to select the correct element of the source XML to be used.

Selecting the correct element from the source XML inside an XSLT transformation, will be illustrated in the BPEL 2.0 part.

BPEL 2.0

BPEL 2.0 is only available in SOA Suite 11g and not in 10g. It has several new features which make it more easy to deal with loops. Also the bpws:getVariableData does not need to be used anymore; the syntax for accessing variables has been simplified. A dot notation can be used, for example an assign activity;

          <assign name="AssignCounter">
            <copy>
              <from>$ForEach1Counter</from>
              <to>$l_counter/ns1:value</to>
            </copy>
          </assign>

For Each activity

The For Each activity can be compared with the While activity, however it is more specific.
- It provides a scope for a loop (in a While activity you have to create the scope yourself)
- It provides a start and endpoint for the counter variable and the counter variable can be created in the For Each activity wizard.
- It provides an option for an additional completion expression
- It provides the option of parallel execution of the individual branches (similar to the FlowN activity in BPEL 1.1)
- it provides a variable which can be used in Assign activities, however not in XSLT transformations without some additions!


The below picture shows the BPEL 2.0 process using the For Each activity. Noticable is that I have to use less activities to achieve the same as with a While activity in BPEL 1.1.

XSLT transformations allow more input variables

The function which is used in the transformation for SOA Suite 10g BPEL 1.1 is different from the transformation function used in SOA Suite 11g BPEL 1.1 and BPEL 2.0. In SOA Suite 11g it is easier to add multiple input parameters to a single transformation in the wizard and the input parameters do not need to confer to a specific schema.

The source types for the transformation all need to be simple or complex types. it is thus easiest to use a complex type. A complex types can be selected when using the wizard to define the type of BPEL variable and by choosing one from the 'Element' or 'Message type' category. you can see examples in the schema shown at the start of this post; simpleNumberType and simpleStringType. These types are complex type wrappers for the simple types string and decimal. They allow the simple types to be used as complex types inside transformations.

Using the counter variable of a For Each activity inside an XSLT transformation, can be done as followed;
- assign the For Each counter variable to a complextype (for example the simpleNumberType)
- use the variable inside a for-each XSLT construction with a selection on the passed simpleNumberType by using the position() function.

The transformation without namespaces used, is the following (see the example ZIP for the complete XSLT);

  <xsl:param name="l_counter"/>
  <xsl:template match="/">
    <xsl:for-each select="/ns1:itemsCollection/ns1:item[position()=$l_counter/ns1:simpleNumber/ns1:value]">
      <ns1:item>
        <ns1:name>
          <xsl:value-of select="ns1:name"/>
        </ns1:name>
        <ns1:value>
          <xsl:value-of select="ns1:value"/>
        </ns1:value>
      </ns1:item>
    </xsl:for-each>
  </xsl:template>
</xsl:stylesheet>

Note on Assigns

Changing the rule type for the assign is in SOA Suite 11g in BPEL 1.1 different then in BPEL 2.0.

BPEL 1.1
Select the rule type from the drop-down box

BPEL 2.0
Right click the From / To rule, Change rule type