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From Apache Wiki <>
Subject [Cassandra Wiki] Update of "ThomasBoose/EERD model components to Cassandra Column family's" by ThomasBoose
Date Fri, 10 Dec 2010 15:41:14 GMT
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The "ThomasBoose/EERD model components to Cassandra Column family's" page has been changed
by ThomasBoose.


  If it is necessary to use different keys for both collections, sometimes it is not up to
one designer to select both keys, although the number of element are equal and they are related
one on one, in a relational model the designer gets to select either key to insert into the
other collection with an unique and foreign key constraint.
  In Cassandra modeling you are forced to either croslink both key's, So you'd design both
key's foreign in both ColumnFamily's. Or you create a third ColumnFamily in which you store
both keys preceded by a token to which columfamily you are refering. Lets focus on the first
option. Say we hand out phones to our employees and we agree that every employee will always
have one phone. and phones that are not used are not stored in our database. The phone has
a phonenumber as key where the employee has a social security number. In order to know which
number to dial when looking for employee X and who is calling giving a specific phonenumber
we need to store both keys foreign in both ColumnFamily's.
- ||<tablewidth="400px">'''CF_Employee'''||
+ ||||||||<tablewidth="400px">'''CF_Employee''' ||
- ||<|2>123-12-1234||name||phone||salary||
+ ||<style="text-align: center;" |2>123-12-1234 ||name ||phone ||salary ||
- ||John||0555-123456||10.000||
+ ||John ||0555-123456 ||10.000 ||
- ||<|2>321-21-4321||name||phone||salary||
+ ||<style="text-align: center;" |2>321-21-4321 ||name ||phone ||salary ||
- ||Jane||0555-654321||12.000||
+ ||Jane ||0555-654321 ||12.000 ||
- ||||||<tablewidth="400px" tablealign="left">'''CF_Phone'''||
- ||<|2>0555-123456||employee||credit||
- ||123-12-1234||10||
- ||<|2>0555-654321||employee||credit||
- ||321-21-4321||5||
+ ||||||<tablewidth="400px" tablestyle="text-align: left;"style="text-align: center;">'''CF_Phone'''
+ ||<style="text-align: center;" |2>0555-123456 ||employee ||credit ||
+ ||123-12-1234 ||10 ||
+ ||<style="text-align: center;" |2>0555-654321 ||employee ||credit ||
+ ||321-21-4321 ||5 ||
@@ -71, +66 @@

        raise error or delete specified employee
  ==== Subset elements ====
- ''.... ''
+ One on one relationships with one collection being smaller, in fact being a subset of the
other collections in relational systems are solved by adding the key of the larger collection
as foreign key to the smaler one. Preferably one uses the same key values as decribed above.
This way we prevent null values that are not strictly indicating an unknown value. Null value's
should only meen "We know there is a value but the value is unknown" as we've all learned
in school.
+ As stated we prefer the foreign key to be the same value as the key from the superset ColumnFamily.
In every other case we'll have to introduce logic to keep the relation cosistent. In any case
you have to enforce the existance of all keys in the subset in the superset. Logic must also
be provided when deleting elements from the superset with respect to the related element in
the subset.
+ ==== Overlap ====
+ The easiest one on one relation to implement is the one in which elements in both collections
do not need to be in the other but might. If at all possible create one big super ColumnFamily
that collects all elements from both collections, even if there is no corresponding attribute
(column). If absolutly neccessary you can provide keys from either ColumnFamily if the values
are not the same but one on one related. See above for contraint considerations.
+ === 1 to Many ===
+ In one to many relationships we add the key from the "one" side foreign to the "many" side.
So if we're modelinng students studing at only one school-unit at a time we would add the
unit's key to the student as foreign. Considering that no foreign key logic is provided you
will have to write your own code to enforce consistancy in unit's existing when the unit attribute
of a student is set and defining behaviour when deleting a unit. Cosiddering the fact that
this kind of relation is very common one could best create the logic for this at a seperate
DBMS tier.
+ Every student has only one school-unit so we enforce one static name of a column that will
reference this unit. for instance this column in the cf_Student columnfamily is called "school-unit".
In a cassandra database this is not sufficient to retrieve all student within this unit. One
could find answers to questions like these but it would require quite a lot of processing
power. If a ColumnFamily, the cf_School_unit family in this case, has only one of these relations,
then one could chose to add all student keys to that ColumnFamily it self. I would not count
on this situation persisting in future releases of you system and therefore sugest that you'de
provide seperate ColumnFamily's for each one to many relationship that you model.
+ This would leed to three columnFamily's
+ ||||||||<tablewidth="400px">'''CF_Student''' ||
+ ||<style="text-align: center;" |2>123-12-1234 ||name ||unit ||city ||
+ ||John ||SE ||the hague ||
+ ||<style="text-align: center;" |2>321-21-4321 ||name ||unit ||city ||
+ ||Jane ||SE ||Amsterdam ||
+ ||||||<tablewidth="400px" tablestyle="text-align: left;"style="text-align: center;">'''CF_School_Unit'''
+ ||<style="text-align: center;" |2>SE ||name ||loc ||
+ ||software engineering ||hsl ||
+ ||||||<tablewidth="400px" tablestyle="text-align: left;"style="text-align: center;">'''CFK_School_Unit_Student'''
+ ||<style="text-align: center;" |2>SE ||123-12-1234 ||321-21-4321 ||
+ || || ||
+ No value's are actualy stored in the columns indicating de studentnumbers. These columns
only exist to indicate which students are present in this unit.

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