BIND 10 #2107: redefine in-memory zone data

[BIND 10 Development]

#2107: redefine in-memory zone data
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            Reporter:  jinmei        |                        Owner:
                Type:  task          |                       Status:  new
            Priority:  medium        |                    Milestone:  Next-
           Component:  data source   |  Sprint-Proposed
           Sensitive:  0             |                     Keywords:
         Sub-Project:  DNS           |              Defect Severity:  N/A
Estimated Difficulty:  0             |  Feature Depending on Ticket:
         Total Hours:  0             |  scalable inmemory
                                     |          Add Hours to Ticket:  0
                                     |                    Internal?:  0
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 We'll need a memory encoded form of zone data.  My suggestion
 is to revise the current `ZoneData` structure so that it will be
 simpler data-only structure, and allocated/deallocated using
 static method and a memory segment, e.g.:

 {{{#!cpp
 template <typename TreeType, typename TreeNodeType>
 struct ZoneData {
     static ZoneData* allocate(MemorySegment& segment, RRClass zone_class,;
                               const Name& zone_name);
     static void deallocate(MemorySegment& segment);

     // These will be initialized on allocation and won't change
     const RRClass zone_class;
     // some other attributes
     offset_ptr<TreeType> const tree;
     offset_ptr<TreeNodeType> const origin_node;

     const RBTree& getNSEC3Tree() const; // for ZoneFinder
     RBTree& getNSEC3Tree(); // for ZoneUpdater/loader
 private:
     offset_ptr<TreeType> nsec3_tree_;  // build this on-demand
 };

 This is essentially a private structure for other implementation
 details of in-memory data source client (finder/updater, etc), which
 will use the member variables directly.

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Ticket URL: <http://bind10.isc.org/ticket/2107>
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