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The "ThomasBoose/EERD model components to Cassandra Column family's" page has been changed by ThomasBoose. http://wiki.apache.org/cassandra/ThomasBoose/EERD%20model%20components%20to%20Cassandra%20Column%20family%27s -------------------------------------------------- New page: ##master-page:HomepageReadWritePageTemplate ##master-date:Unknown-Date #format wiki #language en = A way to implement EERD components in Cassandra = == Intro == This page describes model tranformations from EERD concepts into Cassandra ColumnFamily concepts. All input is welcome. == DBMS layer == At several spots in this document you wil find suggestions to implement trivial DBMS functionality by hand. At this stage, I would suggest to programmers to implement at least 4 tiers when using cassandra as a backend server. One would be the database layer by cassandra itself, One would be a tier implementing DBMS rules, another for business rules finishing with an application tier. In this DBMS tier functions should be available for keeping data consistend based on datarules and it would throw exceptions when indexes are changed or orders are given to delete key's agains DBMS rules. If this is not yet making sence, read on. == Indexing == In order to add an index to a column, other then the columnfamily's key, we should to create a second columnfamily. Every insert, which can be either an insert or update in cassandra, on the original columnfamily we will update the corresponding index. Think of a columnfamily cf_Person (examples in Python using pycassa) cf_Person.insert('234', {'name':'Karel','City:'Haarlem'}) cfi_Person_City.insert (Haarlem', {'234':''}) This way a hash will be created containing columns for every person's key that lives in a specific City. The ColumnFamily architecture of Cassandra can store a unlimited number of columns for each key. This meens that when deleting a person it reference in the cfi_Person_City index should be removed first. When updating a person, maybe moving to anothor City, we have to remove the element from the cfi_Person_City first and then store it with the corresponding new City. == Relations == === 1 on 1 === Typicly you'll find three kinds of 1 on 1 relations in a relational model. I will address them one at a time. ==== Equal elements ==== Sometimes all the elements are part of both collections on either side of the relationship. The reasons these collections are moddeled seperately are most often based on security issues or functional differences. One solution in a Cassandra database would be the same as you would implement such a relation in an RDBMS. Simply by sharing the same key in both columnfamilies. Inserting a key in one of these columnfamily's would insert the same in the other and vise versa. Updating an existing key in either columnfamily would not result in any change in the other. Deleting a key from one columnfamily will result in deleting the same key in the other family as well, providing this would be allowed. ''I'm not sure to what detaillevel security rules can apply in a Cassandra database. At least I know that one can creat logins per cluster.'' If it gets 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 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 columnfamily. The phone has a phonenumber as key where the employee has a socialsecurity 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. -- CF_Employee ----------------------------- | | name | phone | salary | | 123-12-1234 |John | 0555-123456| 10.000 | -------------------------------------------- | | name | phone | salary | | 321-21-4321 |Jane | 0555-654321| 12.000 | -------------------------------------------- -- CF_Phone --------------------------- | | employee | credit | | 0555-123456 | 123-12-1234 | 10 | --------------------------------------- | | employee | credit | | 0555-654321 | 321-21-4321 | 5 | --------------------------------------- Using a static columnname and requiring input in the foreign key fields, checking the existence of the key in the other columnfamily and processing updates and deletes are all subject to programming in the DBMS layer. Cassandra itself does not, and probably will not, provide foreign key logic. One could imagine an process that makes sure the cross references stay consistend: cf_Employee.insert('321-21-4321', {'name':'Jane', 'phone':'0555-654321'}) if cf_Phone.multiget('0555-654321', columns='employee') == {}: cf_Phone.insert ('0555-654321', {'employee':'321-21-4321'}) else: if cf_Phone.get('0555-654321', columns='employee')["Employee"] <> '321-21-4321': raise error or delete specified employee<<BR>> ==== Subset elements ==== ....
