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From Ian Varley <ivar...@salesforce.com>
Subject Re: When to expand vertically vs. horizontally in Hbase
Date Fri, 05 Jul 2013 23:00:43 GMT
Sure. Maybe it's useful to talk about the functional aspect of relationships in models. In
an RDBMS, explicit relationship play a couple roles:

- foreign key constraints: don't allow a tuple in relation A to point to a row in relation
B that doesn't exist
- join optimization - knowledge of how two relations are logically connected can help perform
joins in a more optimal way

HBase, of course, provides neither of these features out of the box, so there is no difference
between an implied (weakly coupled, to use your term) relationship and something stronger.


Where it gets interesting is in the kind of denormalization you're talking about, where information
that properly belongs to one entity is copied into another one for efficiency's sake, or to
get some kind of atomicity protection. Your scenario below is doing this (duplicating customer
info in the order records). 

To be fair, relational DBs also force this kind of behavior sometimes, again for efficiency
reasons (we've all done it). HBase just starts there. :)

Ian

On Jul 5, 2013, at 4:22 PM, "Michael Segel" <michael_segel@hotmail.com> wrote:

> An entity is an entity. 
> When you couple them you are saying that there's a relationship to them in the model.

> 
> What I am saying is that you can have an HBase model which is not a single table, however
when you look at your use case, you are querying data from a single table at a time. 
> 
> Going back to the order entry system. You may have a customer table which maintains all
of the information about your customer yet you will also duplicate portions of the data in
to the order system.  You still have other entities such as your orders, pick slips, shipping
and invoices. There won't be a hard or strong relationship between the customer table and
the order table. 
> 
> When you go to your ERD tool, you wouldn't show a strong coupling of the data. 
> 
> Does that make sense? 
> 
> On Jul 5, 2013, at 1:56 PM, Ian Varley <ivarley@salesforce.com> wrote:
> 
>> Mike, what do you mean by "you can have entities, except that they are not coupled"?
You mean, they have no relationship to each other? Or the relationship is defined elsewhere
(e.g. application code)? The concept of "coupling" seems a little overloaded and not as concise
here as "relationship". Two tuples in a database can have a wide number of relationships to
each other; the kinds of relationships that are actively supported differs between a traditional
RDBMS and HBase, and proper HBase design requires understand these limitations precisely.
>> 
>> I'm not trying to be an ER<http://en.wikipedia.org/wiki/Entity%E2%80%93relationship_model>
apologist, there are a lot of ways in which it sucks. :) But if we want to evolve, we can't
just pretend there's no history here to build on.
>> 
>> Ian
>> 
>> On Jul 5, 2013, at 1:41 PM, Michael Segel wrote:
>> 
>> LOL...
>> 
>> Ian wrote:
>> "But, something just occurred to me: just because your physical implementation (HBase)
doesn't support normalized entities and relationships doesn't mean your *problem* doesn't
have entities and relationships. :) An Author is one entity, a Title is another, and a Genre
is a third. Understanding how they interact is a prerequisite for translating into a physical
model that works well in HBase. (ERD modeling is not categorically the only way to understand
that, but I've yet to hear a credible alternative that doesn't boil down to either ERD or
"do it in your head").
>> "
>> 
>> You can have entities, except that they are not coupled.
>> 
>> If you have a common key, then you may have a use for column families, it just depends
on your data and how you access your data.
>> 
>> Its not rocket science, but its a non-trivial matter. Not doing it right may mean
that you are not going to get the most out of your system.
>> 
>> 
>> On Jul 5, 2013, at 1:26 PM, Ian Varley <ivarley@salesforce.com<mailto:ivarley@salesforce.com>>
wrote:
>> 
>> But, something just occurred to me: just because your physical implementation (HBase)
doesn't support normalized entities and relationships doesn't mean your *problem* doesn't
have entities and relationships. :) An Author is one entity, a Title is another, and a Genre
is a third. Understanding how they interact is a prerequisite for translating into a physical
model that works well in HBase. (ERD modeling is not categorically the only way to understand
that, but I've yet to hear a credible alternative that doesn't boil down to either ERD or
"do it in your head").
>> 
>> 
> 

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