Sunday, April 03, 2016

Working with the Interface Segregation Principle

Obviously Interface Segregation is crucial for implementing restricted access. For any given set of data there are three broad categories of access:

  • reporting 
  • manipulation 
  • administration and governance 

So we need to define at least one interface - packages - for each category in order that we can grant the appropriate access to different groups of users: read-only users, regular users, power users.

But there's more to Interface Segregation. This example is based on a procedure posted on a programming forum. Its purpose is to maintain medical records relating to a patient's drug treatments. The procedure has some business logic (which I've redacted) but its overall structure is defined by the split between the Verification task and the De-verification task, and flow is controlled by the value of the p_verify_mode parameter.
 
procedure rx_verification
     (p_drh_id in number,
       p_patient_name in varchar2,
       p_verify_mode in varchar2)
as
    new_rxh_id number;
    rxh_count number;
    rxl_count number;
    drh_rec drug_admin_history%rowtype;
begin
    select * into drh_rec ....;
    select count(*) into rxh_count ....;

    if p_verify_mode = 'VERIFY' then

        update drug_admin_history ....;
        if drh_rec.pp_id <> 0 then
            update patient_prescription ....;
        end if;
        if rxh_count = 0 then
            insert into prescription_header ....;
        else
            select rxh_id into new_rxh_id ....;
        end if;
        insert into prescription_line ....;
        if drh_rec.threshhold > 0
            insert into prescription_line ....;
        end if;

    elsif p_verify_mode = 'DEVERIFY' then

        update drug_admin_history ....;
        if drh_rec.pp_id <> 0 then
            update patient_prescription ....;
        end if;
        select rxl_rxh_id into new_rxh_id ....;
        delete prescription_line ....;
        delete prescription_header ....;

    end if;
end;

Does this procedure have a Single Responsibility?  Hmmm. It conforms to Common Reuse - users who can verify can also de-verify. It doesn't break Common Closure, because both tasks work with the same tables. But there is a nagging doubt. It appears to be doing two things: Verification and De-verification.

So, how does this does this procedure work as an interface? There is a definite problem when it comes to calling the procedure: how do I as a developer know what value to pass to p_verify_mode?

  rx_management.rx_verification
     (p_drh_id => 1234,
       p_patient_name => 'John Yaya',
       p_verify_mode => ???);

The only way to know is to inspect the source code of the procedure. That breaks the Information Hiding principle, and it might not be viable (if the procedure is owned by a different schema). Clearly the interface could benefit from a redesign. One approach would be to declare constants for the acceptable values; while we're at it, why not define a PL/SQL subtype for verification mode and tweak the procedure's signature to make it clear that's what's expected:        


create or replace package rx_management is
 
  subtype verification_mode_subt is varchar2(10);
  c_verify constant verification_mode_subt := 'VERIFY'; 
  c_deverify constant verification_mode_subt := 'DEVERIFY'; 
 
  procedure rx_verification
     (p_drh_id in number,
       p_patient_name in varchar2,
       p_verify_mode in verification_mode_subt);

end rx_management;

Nevertheless it is still possible for a caller program to pass a wrong value:


  rx_management.rx_verification
     (p_drh_id => 1234,
       p_patient_name => 'John Yaya',
       p_verify_mode => 'Verify');

What happens then? Literally nothing. The value drops through the control structure without satisfying any condition. It's an unsatisfactory outcome. We could change the implementation of rx_verification() to validate the parameter value and raise and exception. Or we could add an ELSE branch and raise an exception. But those are runtime exceptions. It would be better to mistake-proof the interface so that it is not possible to pass an invalid value in the first place.

Which leads us to to a Segregated Interface :

create or replace package rx_management is
 
  procedure rx_verification
     (p_drh_id in number,
       p_patient_name in varchar2);
 
  procedure rx_deverification
     (p_drh_id in number);
     
end rx_management;

Suddenly it becomes clear that the original procedure was poorly named (I call rx_verification() to issue an RX de-verification?!)  We have two procedures but their usage is now straightforward and the signatures are cleaner (the p_patient_name is only used in the Verification branch so there's no need to pass it when issuing a De-verification).

Summary

Interface Segregation creates simpler and safer controls but more of them. This is a general effect of the Information Hiding principle. It is a trade-off. We need to be sensible. Also, this is not a proscription against flags. There will always be times when we need to pass instructions to called procedures to modify their behaviour. In those cases it is important that the interface includes a definition of acceptable values.

Part of the Designing PL/SQL Programs series

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