第4章 Creating DRL rules in Business Central

Procedure

1. In Business Central, go to Menu Design Projects and click the project name.
2. Click Add Asset DRL file.

3. Enter an informative DRL file name and select the appropriate Package. The package that you specify must be the same package where the required data objects have been assigned or will be assigned.

   You can also select Show declared DSL sentences if any domain specific language (DSL) assets have been defined in your project. These DSL assets will then become usable objects for conditions and actions that you define in the DRL designer.

4. Click Ok to create the rule asset.

   The new DRL file is now listed in the DRL panel of the Project Explorer, or in the DSLR panel if you selected the Show declared DSL sentences option. The package to which you assigned this DRL file is listed at the top of the file.

5. In the Fact types list in the left panel of the DRL designer, confirm that all data objects and data object fields (expand each) required for your rules are listed. If not, you can either import relevant data objects from other packages by using import statements in the DRL file, or create data objects within your package.

6. After all data objects are in place, return to the Model tab of the DRL designer and define the DRL file with any of the following components:

   Components of a DRL file

   package  //automatic
   
   import
   
   function  //optional
   
   query  //optional
   
   declare   //optional
   
   rule
   
   rule
   
   ...

   • package: (automatic) This was defined for you when you created the DRL file and selected the package.

   • import: Use this to identify the data objects from either this package or another package that you want to use in the DRL file. Specify the package and data object in the format package.name.object.name, one import per line.

     Importing data objects

     import mortgages.mortgages.LoanApplication;

   • function: (optional) Use this to include a function to be used by rules in the DRL file. Functions put semantic code in your rule source file. Functions are especially useful if an action (then) part of a rule is used repeatedly and only the parameters differ for each rule. Above the rules in the DRL file, you can declare the function or import a static method as a function, and then use the function by name in an action (then) part of the rule.

     Declaring and using a function with a rule (option 1)

     function String hello(String applicantName) {
         return "Hello " + applicantName + "!";
     }
     
     rule "Using a function"
       when
         eval( true )
       then
         System.out.println( hello( "James" ) );
     end

     Importing and using the function with a rule (option 2)

     import function my.package.applicant.hello;
     
     rule "Using a function"
       when
         eval( true )
       then
         System.out.println( hello( "James" ) );
     end

   • query: (optional) Use this to search the decision engine for facts related to the rules in the DRL file. Queries search for a set of defined conditions and do not require when or then specifications. Query names are global to the KIE base and therefore must be unique among all other rule queries in the project. To return the results of a query, construct a traditional QueryResults definition using ksession.getQueryResults("name"), where "name" is the query name. This returns a list of query results, which enable you to retrieve the objects that matched the query. Define the query and query results parameters above the rules in the DRL file.

     Query and query results for people under the age of 21, with a rule

     query "people under the age of 21"
         person : Person( age < 21 )
     end
     
     QueryResults results = ksession.getQueryResults( "people under the age of 21" );
     System.out.println( "we have " + results.size() + " people under the age  of 21" );
     
     rule "Underage"
       when
         application : LoanApplication( )
         Applicant( age < 21 )
       then
         application.setApproved( false );
         application.setExplanation( "Underage" );
     end

   • declare: (optional) Use this to declare a new fact type to be used by rules in the DRL file. The default fact type in the java.lang package of Red Hat Decision Manager is Object, but you can declare other types in DRL files as needed. Declaring fact types in DRL files enables you to define a new fact model directly in the decision engine, without creating models in a lower-level language like Java.

     Declaring and using a new fact type

     declare Person
       name : String
       dateOfBirth : java.util.Date
       address : Address
     end
     
     rule "Using a declared type"
       when
         $p : Person( name == "James" )
       then   // Insert Mark, who is a customer of James.
         Person mark = new Person();
         mark.setName( "Mark" );
         insert( mark );
     end

   • rule: Use this to define each rule in the DRL file. Rules consist of a rule name in the format rule "name", followed by optional attributes that define rule behavior (such as salience or no-loop), followed by when and then definitions. The same rule name cannot be used more than once in the same package. The when part of the rule contains the conditions that must be met to execute an action. For example, if a bank requires loan applicants to have over 21 years of age, then the when condition for an Underage rule would be Applicant( age < 21 ). The then part of the rule contains the actions to be performed when the conditional part of the rule has been met. For example, when the loan applicant is under 21 years old, the then action would be setApproved( false ), declining the loan because the applicant is under age. Conditions (when) and actions (then) consist of a series of stated fact patterns with optional constraints, bindings, and other supported DRL elements, based on the available data objects in the package. These patterns determine how defined objects are affected by the rule.

     Rule for loan application age limit

     rule "Underage"
       salience 15
       dialect "mvel"
       when
         application : LoanApplication( )
         Applicant( age < 21 )
       then
         application.setApproved( false );
         application.setExplanation( "Underage" );
     end

     At minimum, each DRL file must specify the package, import, and rule components. All other components are optional.

     図4.1 Sample DRL file with required components and optional rule attributes

     
7. After you define all components of the rule, click Validate in the upper-right toolbar of the DRL designer to validate the DRL file. If the file validation fails, address any problems described in the error message, review all syntax and components in the DRL file, and try again to validate the file until the file passes.
8. Click Save in the DRL designer to save your work.