2. Concept of Context
Context is the main part in a business rule. Together with restriction and query they form a business rule. It could be described as an address or a starting point for the rule. It defines which part of the XML message is being checked by the restriction.
Context is always given as an element type as it is defined by a schema.
Below is a screenshot of a reference schema viewed in XSD editor (XMLspy):
Each box depicts an XML element, first row stating the name and second for stating the type. Type is not mandatory to be defined, and in the above schema "Message" element only has children and name.
It is crucial to note that a single type may be used in multiple places within the schema. Id, for example, is present under Header and Transaction and both of their types is Max35Text. Debtor and Creditor also share the type PartyIdentification.
Additionally, one type may be present multiple times in an XML file, even though it is defined only once in schema. Transaction, for example does not have maximum occurrences defined.
Usage of context in OCL business rules
The restriction defined in an OCL rule is always executed when its type is found in an XML file in the validation process.
This means that a rule with simpleType Max35Text as context will be executed every time a Max35Text element is found in the XML file. Similarly, context TransactionType1 is executed every time a Transaction is present in the file.
Consequences of this is that it can be used as a quick way to limit values for certain types when it is known that an exact business rule applies in many places. For example, if we wanted to limit Nm for Cdtr and Dbtr, we could write a rule using PartyIdentification as context. However, if we wanted to only limit Dbtr/Nm length and keep Cdtr/Nm length as it is, using PartyIdentification as context would lead to invalid consequences, as it would apply for Cdtr as well. In this case, context would have to be TransactionType1 and/or HeaderType1.
Debtor mandatory in header
The screenshot from myXML in the bottom left corner of this slide shows structure of one XML message. If we would like to create a rule which makes the debtor mandatory in the header we should set the context to be the Header.
- The OCL rule then ”sees” the part which is highlighted in the picture in the bottom right corner
- If context would be set to be the Debtor itself then the rule would not be run at all if debtor is missing.
Context: HeaderType1 OCL: self.Debtor->size() = 1
Limiting values in Max140Text
Max140Text type can be seen in three different places in the schema. Amount of Max140Text instances in a XML file conforming to this schema can however be more or less. In the below XML example four instances are found.
Rule having Max140Text as a context is executed against each instance found from the file
Below rule states that Max140Text element has to be shorter than or equal of 70 characters.
Context: Max140Text OCL: self.size() <= 70