BIND 10 #2907: add ConfigurableClientList::getZoneTableAccessor method

[BIND 10 Development]

#2907: add ConfigurableClientList::getZoneTableAccessor method
-------------------------------------+-------------------------------------
            Reporter:  jinmei        |                        Owner:
                Type:  task          |  pselkirk
            Priority:  medium        |                       Status:
           Component:  data source   |  accepted
            Keywords:                |                    Milestone:
           Sensitive:  0             |  Sprint-20130514
         Sub-Project:  DNS           |                   Resolution:
Estimated Difficulty:  3             |                 CVSS Scoring:
         Total Hours:  0             |              Defect Severity:  N/A
                                     |  Feature Depending on Ticket:
                                     |  shared memory data source
                                     |          Add Hours to Ticket:  0
                                     |                    Internal?:  0
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Changes (by pselkirk):

 * milestone:  Sprint-20130528 => Sprint-20130514


Comment:

 I have to admit I'm stuck here.

 1) ConfigurableClientList says:
 {{{
 /// This is the implementation which is expected to be used in the
 servers.
 /// However, it is expected most of the code will use it as the
 ClientList,
 /// only the creation is expected to be direct.
 }}}

 But if getZoneTableAccessor is only in ConfigurableClientList, then either
 this comment should change, or we should add getZoneTableAccessor as an
 abstract method to ClientList.

 2) getZoneTableAccessor is supposed to create a ZoneTableAccessor object,
 but ZoneTableAccessor is an abstract class, so we can't instantiate it; we
 have to instantiate a concrete derived class, of which we've only defined
 ZoneTableAccessorCache.

 3) ZoneTableAccessorCache is constructed with a CacheConfig reference, but
 ConfigurableClientList is configured (not constructed) with a
 ConstElementPtr reference.

 The ZoneTableAccessorTest constructor is an example for creating
 CacheConfig from isc::data::ConstElementPtr, and using that to construct
 ZoneTableAccessorCache. But I'm confused about what to use for
 datasrc_type (the unit test uses "MasterFiles"). And I'm confused about
 the relationship between the use_cache argument and the cache-enable
 configuration member.

-- 
Ticket URL: <http://bind10.isc.org/ticket/2907#comment:7>
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