Issues with Assign activity

Question

I am trying to assign a value(string) from an element of one variable, to an attribute of an element in another.
I then test the process and get an error on this activity. The message says:
XPath query string returns zero node.
According to BPEL4WS spec 1.1 section 14.3, The assign activity <to> part query should not return zero node.
Please check the BPEL source at line number "162" and verify the <to> part xpath query.
Any thoughts or suggestions?
Thanks,
Matthew

Answer 1

Matthew,
I don't know if this is the problem you're having, but I can take a guess at one possible cause: you are using a variable, or part of a variable, before it has been instantiated.
I apologise in advance for this long-winded explanation. :)
I hope you know Java because I'll use an example from that to illustrate my point. Take this (psuedocode) example where we have some code that wants to use a complex type of class Person:
class MyClass
  private class Person
    public String nameOfPerson;
    public int    age;
  private void someMethod()
    Person ppp;
    ppp.nameOfPerson = "Homer Simpson";
    ppp.age  = 38;
}Hopefully you realise that this will fail. That's because even though ppp is of type Person, you can't start using the data structures it contains because they haven't been created yet. That is, it hasn't been instantiated. Instead, the variable creation line can look like this:
Person ppp = new Person();Now memory has been set aside and the data structures are created. There are empty "slots" for name and age and you can start populating those fields.
If that makes sense to you then you should understand how that pertains to BPEL and your problem.
The thing that's going on with BPEL variables is much the same, except that they are based on XML, which adds some difficulty. In Java, a variable has just a value. e.g. ppp.nameOfPerson = "Homer Simpson". But in XML, you have an element name and an element value. The XML (again, psuedocode) equivalent might be myVar = "<nameOfPerson>Homer Simpson</nameOfPerson>". And you can't put in the "Homer Simpson" until the <nameOfPerson> slot exists.
In our BPEL solution, we do something nice for you which most people don't even realise. We initialise your variables for you. But -- and here's the catch -- not in all circumstances.
Imagine our schema is this (everything in this post is psuedocode and not meant to be exact):
<element name="Person">
  <complexType>
    <sequence>
      <element name="nameOfPerson" type="string"/>
      <element name="age" type="int"/>
    </sequence>
  </complexType>
</element>Suppose our BPEL variable is called pppB (to differentiate it from the Java example, above). In your BPEL process you essentially declare your variable of that type:
<variable name="pppB" type="Person"/>Just like in the Java example above, you can't then simply do an <assign> because the variable hasn't been initialised. Except that you can because we automatically initialise the variable for you! In every programming language a complex variable has to be instantiated or initialised before it can be used. Usually you have to do it explicitly yourself. Some, like in our case, do it for you automatically.
For example, if you want to do XML operations in Java, you have to do something like the following (and this is very psuedocode!):
Document doc = createNewDocument();
Element pppE = createNewElement();
pppE.setNodeName = "Person";
Element nameE = createNewElement();
nameE.setNodeName = "nameOfPerson";
nameE.setNodeValue = "Homer Simpson";
Element ageE = createNewElement();
ageE.setNodeName = "age";
ageE.setNodeValue = "38";
pppE.addChild(nameE);
pppE.addChild(ageE);Note that you cannot assign a value to a node until is created. Also, in general, you can't work with a node unless the parent nodes also exist. The following won't work:
Document doc = createNewDocument();
Element pppE = createNewElement();
pppE.setNodeName = "Person";
Element nameE = pppE.getChild("nameOfPerson")
nameE.setNodeValue("Homer Simpson");That's because even though the Person node was created, no child nodes were. The following would work:
Document doc = createNewDocument();
Element pppE = createNewElement();
pppE.setNodeName = "Person";
Element nameE = createNewElement();
nameE.setNodeName = "nameOfPerson";
nameE.setNodeValue = "Homer Simpson";
pppE.addChild(nameE);
Element newName = pppE.getChild("nameOfPerson");
newName.setNodeValue("Homer J. Simpson");Now returning back to BPEL, you have your variable:
<variable name="pppB" type="Person"/>Normally you could not do such an assign because the variable hasn't been initialised:
<assign name="addName">
  <copy>
    <from expression="string('Homer Simpson')"/>
    <to   variable="pppB" query="/ns1:Person/ns1:nameOfPerson"/>
  </copy>
</assign>Except that with our solution you can because we are nice and do the initialisation for you automatically. People don't realise this and take it for granted, which leads to the problem you are having.
However we don't always instantiate the variable for you. And one time we don't is when a variable is passed in. Suppose the BPEL process took a Person element as it's input:
<variable name="inputVariable" messageType="ns1:PersonMessage"/>and PersonMessage resolved to a Person element.
Now suppose you invoke the BPEL process with the following:
<Person>
  <nameOfPerson>Homer Simpson</nameOfPerson>
</Person>(Assuming that the <age> element is optional, as defined in the schema.)
Now your BPEL process is started and inputVariable looks like this, as expected:
<Person>
  <nameOfPerson>Homer Simpson</nameOfPerson>
</Person>So if you then try to do an <assign> and set the age like this:
<assign name="addName">
  <copy>
    <from expression="string('38')"/>
    <to   variable="inputVariable" query="/ns1:Person/ns1:age"/>
  </copy>
</assign>you're going to get an error because that element does not exist. There is nowhere to put the value for age because the node for age doesn't exist! To be more precise, the query "/ns1:Person/ns1:age" returns null because there is no such element. This results in your error:
The assign activity <to> part query should not return zero node.
A zero node is like saying null here.
This is what I meant above when I said XML is different from "normal" variables that we're used to. You have to have both a node and a value, whereas a variable in Java, say, only has a value.
The solution to this is add the node first, then assign the value. We have a nice way that lets you do that with an append. In the BPEL designer you create an <assign>, then instead of a copy rule, which is what we use most of the time, you use an append. The code will look like this:
<assign name="addAgeElement">
  <bpelx:append>
    <bpelx:from>
      <ns1:age></ns1:age>
    </bpelx:from>
  <bpelx:to variable="inputVariable" part="payload" query="/ns1:Person"/>
</assign>The part between <bpelx:from> and </bpelx:from> is the XML you want to append. The query attribute in <bpelx:to> is where you want that XML to go. Once you've done that, you can now go and do your normal assign and set the age:
<assign name="addName">
  <copy>
    <from expression="string('38')"/>
    <to   variable="inputVariable" query="/ns1:Person/ns1:age"/>
  </copy>
</assign>I hope that makes sense. The code in this post is not accurate, just pseudocode to make the point.
Regards,
Robin.