[Koha-devel] Koha Core anyone?

[Benjamin Rokseth]

Community hackers,

on hackfest I got introvertly enthusiastic about the concept of a Koha Core, and
about time I shared some thoughts.

Background: Deichman (Oslo Public Library) is heavily leaning on bleeding edge Koha
development (REST, Objects, Auth, NCIP and such) and, like at least some others, maintain
a lot of local patches to tweak Koha into our users needs. Some are probably interesting to
Community, others not. Now to keep everything in sync with Community would be amazing,
but not likely to happen anytime soon.

Great work has been done on refactoring Koha (new namespace, Koha Objects and REST api, etc.),
but we'd like to suggest one more - a Koha core.
The idea is simple: borrow from object oriented languages, java, or actually more ruby, since
we're dealing with a dynamic language, use class/module inheritance and method overrides.
Perl has the "use parent" concept which simplifies inheritance/subclassing and allows for
nested overrides.

As an example we refactored the current circulation in Koha, since this for us is the core
functionality that we depend on and need to hook our local quirks on top of.
An attempt to illustrate:

+------------+
| Core::Main |
+--^---------+
   |
+--+----------------+
| Core::Prefs       |
| Core::Exceptions  |                +-----------------------+
| Core::Circulation <-----+------+---| Deichman::Circulation |
| ...               |     |      |   +---^-------------------+
+-------------------+     |      |       |
                          |      |       |
       +------------------+------+       +--------------------------+
       | Core::Circulation::SIP  |       |Deichman::Circulation::SIP|
       +------------------------------------------------------------+
                                 |        use parent qw(
                                 |          Deichman::Circulation
          +----------------------+          Core::Circulation::SIP
          | Core::Circulation::UI|        )
          +----------------------+
                                 |
                                 ~

* Core::Main is simply an empty class that act as a parent for any child, including Core::Circulation.
* Core::Circulation has a constructor that takes koha objects item and library, optionally patron
  and sysprefs overrides. It can have accessors such as checkout, messages and other things needed for
  intra, SIP or whatever. It has methods Checkin, Checkout and Renew, amongst others.
* then: Deichman::Circulation::SIP in this example is a local override that inherits from parents
  Deichman::Circulation and Core::Circulation::SIP

now the beauty of this is that Deichman::Circulation::SIP can override anything (even the constructor)
without touching any of the core code, and perl will traverse the inheritance tree until it finds the
first matching constructor and method.

Pros:
  - simpler, more readable and more reusable code.
  - local adaptations are easy to hande, and reusable for others
  - the slight overhead of using blessed objects and inheritance is easily gained by the fact that any
    operation will only need fetching Koha objects once (item,library,patron etc) instead of refetching
    them numerous times spread across methods calls and loops
  - way less db calls if done right, faster Koha
  - no more C4::Context, hopefully
  - systempreferences can be dramatically reduced, since most of them are about overrides anyways
  - can be done incrementally, replacing one functionality at a time

cons:
  - refactoring doesnt make end users happy (but needs to be done in any case)
  - a bit of work to keep templates happy
  - requires a basic understanding of oop

So to sum up: We already have a working example for circulation (though not in production)
that we can demonstrate. It reimplements basically the entire C4::Circulation, just some small
parts missing. So it can be done.

But we'd love to hear second opinions from the community! We know the fear for breaking changes, but
its neither scary or complicated to implement!

Benjamin Rokseth
Oslo Public Library