Storing or computing totals for complex models

Question

All,

I'm developing an application using Zend Framework to manage tenders for construction work.

The tender will be a rather complex set of models with an architecture resembling the code snippet below.

My question is... should I store the total value of the tender as part of the tender model, or should I compute it each time it is required? The total value of the tender will be a sum of all the components (e.g. plant/labour/materials/overheads/etc).

object(Application_Model_Tender)#75 (4) {
  ["_id":protected] => int(1)
  ["_name":protected] => string(33) "Tender Name"
  ["_due":protected] => string(10) "2010-12-01"
  ["_labour":protected] => array(2) {
    [0] => object(Application_Model_Gang)#81 (3) {
      ["_id":protected] => int(1)
      ["_name":protected] => string(25) "First Gang Name"
      ["_gangMembers":protected] => array(2) {
        [0] => object(Application_Model_GangMember)#93 (5) {
          ["_id":protected] => NULL
          ["_name":protected] => string(11) "Labour Type"
          ["_workingPattern":protected] => string(7) "Default"
          ["_cost":protected] => float(546)
          ["_qty":protected] => int(3)
        }
        [1] => object(Application_Model_GangMember)#91 (5) {
          ["_id":protected] => NULL
          ["_name":protected] => string(11) "Labour Type"
          ["_workingPattern":protected] => string(8) "Custom 1"
          ["_cost":protected] => float(777)
          ["_qty":protected] => int(1)
        }
      }
    }
    [1] => object(Application_Model_Gang)#90 (3) {
      ["_id":protected] => int(2)
      ["_name":protected] => string(15) "Second Gang Name"
      ["_gangMembers":protected] => array(1) {
        [0] => object(Application_Model_GangMember)#92 (5) {
          ["_id":protected] => NULL
          ["_name":protected] => string(11) "Labour Type"
          ["_workingPattern":protected] => string(8) "Custom 1"
          ["_cost":protected] => float(777)
          ["_qty":protected] => int(2)
        }
      }
    }
  }
}

Answer 1

I don't know zend specifically but as a general rule I'd recommend computing as the initial strategy. Store it only if you need to for performance reasons. Rationale:

1. You need to write the caculation function in both cases.
2. If you store (cache) it, you'll need a strategy for detecting changes to the tender structure & updating the cached value. That's additional complexity so only introduce it if/when you find you need it.

You should be able to hide the implementation strategy behind the interface [getTotal()] - so calling clients won't have to change if you decide to cache at a later date.

(There are of course caveats to above. If the structure is immutable then you can safely cache without the need for update detection).