[TYPO3-mvc] The different layers of persistence

[Jochen Rau]

Hi.

I just want to give you some hints what the different classes and 
(sub)layers of the persistence layers are responsible for. This should 
enable you to dig more easily into the system (and report bugs ;-)).

Let's begin from the top level

1. Repository
A Repository is the first objects a developer comes in contact to. It 
handles only domain objects and provide task oriented interface to 
retrieve them by saying findByFoo('Bar'). Most of the time you are done.

2. Query
The next level is the Query object. It provides methods to create an 
abstract query (abstract in sense of abstracted from where the objects 
come from). It has no clue about the underlying structure of the 
storage. It even does not now if it is based on a relational model or a 
tree model (like the content repository of FLOW3). It also invokes the 
DataMapper (see below). An extension developer uses the Query object to 
create his own findByFoo() methods.

3. QueryObjectModel
Deeper and deeper ... In comparison to the Query object Query Object 
Model knows about the underlying concepts of relations and thus Joins 
and Orderings. An extension developer may use this class to specify more 
complex and "relational model" aware queries. That makes the creation of 
queries (esp. Joins) al little bit laborious. But it helps to keep 
abstraction.

4. StorageBackend (aka Typo3DbBackend)
The Storage Backend translates the Query Object Model in syntactical 
correct SQL (hopefully ;-)). It also invokes the call to the database 
via a handler. An extension developer normally does not have to mak his 
hands dirty with SQL (which is in most cases the best way to improve 
performance and security ;-) ).

5. DatabaseHandler
The Database Handler prepares queries, fetches the result set. It can be 
simply the t3lib_db or PDO or ...

6. DataMapper
And finally the Data Mapper. It maps the resulting rows on the Domain 
Object (it creates an empty object and fills it with properties). It 
also resolves relations according to the loading strategy (eg. Lazy 
Loading).

Puh! Now we are done. Hope you enjoyed the journey.

FAQ
Q: Are you nuts! Six different layers, dozens of objects, and bunch of 
new things to learn! I only wanted to say "SELECT * FROM Tx_MyExt_Foo 
WHERE ..."!
A: You are right: I'am nuts (at least a litte bit ;-)). But now we are 
fully compatible to the (Query) API of FLOW3 and reached the best level 
to port Extensions forward. And I really like the Idea of having totally 
freedom of choice.
Q: And what about performance?
A: That’s indeed a very important point. We are trying our best to meet 
the goals (see also http://forge.typo3.org/issues/show/3876)

And please keep in mind: 90% of the work is based on the concepts and 
code of FLOW3. Credits goes to the FLOW3 team.

Regards
Jochen

-- 
Every nit picked is a bug fixed