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Prevayler Quietly Reaches 2.0 Alpha, Bye RDBMS?
"We've used relational databases for years despite incompatibilities in SQL implementation. Accessing them from an OOP paradigm has been so tedious, that Object-Relational mapping technologies have sprouted all over the Open Source landscape. Some competing examples and models are Hibernate, OJB, TJDO, XORM, and Castor; which in turn have supporting frameworks such as Spring and SQLExecutor. Because SQL is the dominant form of interfacing with the data in an RDBMS, there's now a specification to offer it a friendlier OO face.
Most of the above, including the SQL-variants, arguably appear to add yet another layer of complexity (even if only at the integration level) where they should be taking complexity away. These solutions are put together by some very smart people, but it's inescapable to get that feeling someone is missing the forest (simple answer) because all the trees (incompatible models) are in the way. If there are so many after-the-fact solutions attempting to simplify relational database access and manipulation from OO, isn't it reasonable to think that there is something generally wrong with trying to cobble-together two disparate concepts with what are essentially high-caliber hacks? Is Prevayler a better way?"
I tried Prevayler and even loaded in 50k records from an old product table and object-ized them.
Yah, it was fast. Searches were great. I even figured out how to do complex object joins.
But I started having trouble when I tried to figure out how all the transactions worked. This was complicated by the wiki they're using, which was quite useless to me. It lead to many dead ends.
However, the real reason I found that I could not (possibly ever) use Prevayler is becuase it seemed the approach was for one machine and one machine only. There were no distributed mechanisms at all. Or at least, not how I'm used to working.
All the systems I've worked on in the past five years have all been with clusters of app servers. If all the objects on one machine were all in memory, then I couldn't think of an easy way to get them into the memory of the other machines. There was some talk about using Java Spaces, but that's kinda where I dropped off.
And the other issue was getting to the data from non app server machines. Like stuff to do back end reporting and things like that. I bascially figured out that for n-machine access, I needed something that, well, acted like a database.
I thought the idea was very interesting and maybe these things have been addressed. But when I really sat down with it for a few weeks, it just didn't pan out for me.
I'm not sure what the ACs' problems are. My post has plenty of grammatical errors as well.
But the point is, in business, not only do we have a SQL database, there are thousands of vendors with products that expect SQL/ODBC databases.
Sure, if I'm writing something that requires a database, but will never ever have to interoperate with anything else that I haven't predicted, then Prevaylent is in the running. Otherwise, not. Their website is gleefully ignorant of this fact.
They imply that anyone not using their product is idiotic. They do it many times, and here the poster acts like their relative unpopularity is an enigma. They sound like assholes. I'll stand by that.
There are no trails. There are no trees out here.
The object-oriented ideology as instantiated in C++ and Java is founded upon breaking data into objects, bearers of identity, which belong to classes, bearers of structure and behavior. (C++ and Java make little account of metaclasses, which are used in more dynamic object systems such as Python's class system and Common Lisp's CLOS. Templates are not metaclasses.) Objects have identity, so they can be equated; they are the unique bearers of attributes about themselves; and each object's structure is dictated by the class to which it belongs.
When object-oriented partisans look at a database, they see its relvars (or table headers) as bearers of structure and think of classes, and its tuples (or rows) as bearers of identity and think of objects. They see a database as a place to store objects persistently.
But this is not what an RDBMS does. An RDBMS isn't an object store; its relvars are not classes and its tuples are not objects. So what is an RDBMS? What is "relational" anyhow? Relational databases are founded upon relational mathematics, which is what you get when you cross set theory with predicate calculus.
Set theory is the branch of math that deals with collections of elements which behave according to formal axioms. Set theory lets you say, for instance, that if you have a non-null set R and a non-null set S, that you can construct a set R*S of all the possible pairs of elements from R and S.
Predicate calculus is the branch of logic that deals with quantified statements about entities. It lets you formalize logical arguments such as the syllogism: All men are mortal; Socrates is a man; therefore, Socrates is mortal. Predicate calculus deals with generalizations and instantiations of those generalizations.
What do we get by combining set theory and predicate calculus? We get a system that allows us to operate upon sets of tuples of values satisfying predicates. A relation holds tuples of values which make some predicate true. For instance, consider the predicate "Person x owes me y dollars." Tuples which satisfy this predicate will be pairs (x,y) for which the sentence is true. For instance, if Fred owes me 40 dollars, (Fred, 40) satisfies the predicate. It could thus be a tuple in the relation described by the predicate -- the one relating people's names to how much they owe me.
With the relational algebra (or an RDBMS) we can do operations upon this relation and others. We could, for instance, select a result set of all those people who owe me more than 50 dollars -- or join this result set with those people's addresses. Whatever result set we ask for will be calculated from the facts in the database. We might get back this result set:
(Barney, 75, 40 Elm Road)
(Megan, 60, 9 High Street)
Now, are the elements of this result set objects in the object-oriented sense? They are not. They do not have identity. The tuple about Barney is not Barney himself, or even a machine representation of him. It doesn't uniquely store attributes of Barney -- after all, we created it by joining tables which also contain such attributes. It is not even, truly, a fact about Barney exclusively -- for it is also a fact about the number 75, and about the address 40 Elm Road. It isn't an object; it's a tuple value, and values do not have identity as objects do.
Moreover, note that by joining, we can construct new relations from old ones. Thus, not only are tuples not objects, but relvars are not classes. After all, in OO we do not create new classes by joining, but by inheritance or encapsulation of old ones -- and creating a new class does not cause it to be instantiated into known-correct objects.
So what does this matter to OO people faced with RDBMS as a