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NOTE: This is just a proposal. There is no guarantee that any of this will ever end up in the actual codebase, I just felt it was worth experimenting. --JR
Update (09-05-2007): _I've made this work successfully for
Collection
s,
Item
s, and
Bundle
s. The performance improvements aren't fully implemented, but the separation is there, and in theory, that was the hard part. The
[[#org.dspace.core.ArchiveManager|ArchiveManager]]
seems to be working pretty well too._ --JR
Update (11-05-2007): _I've implemented DAOs for the
Community
class as well. The
[[#org.dspace.core.ArchiveManager|ArchiveManager]]
now supports moving
Item
s,
Collection
, and
Communities
between containers._ --JR
Update (23-05-2007): I've totally reimplemented persistent identifiers in DSpace as well (see PersistentIdentifiers). As well as removing the Handle System dependency, they also use DAOs. --JR
Update (20-06-2007): _After a bit of a hiatus while I fixed persistent identifiers I've come back to DAOs, and I've now (mostly) got them in place for
Bitstream
s as well. The two major classes that still need doing are
EPerson
and
Group
; once they're done, there are a few others (eg:
SupervisedItem
,
WorkspaceItem
, etc) but they should be relatively simple._ --JR
Just adding a comment that Handle/Pid management could be greatly improved by such an addition as well. currently with item caching, the DSpaceObject.getHandle method can become stale and using DAO's behind the scene for the HandleManagement might be beneficial – Mark Diggory 13:56, 10 May 2007 (EDT)
Update (14-08-2007): _Everything (apart from the code in
org.dspace.checker
) has been pushed through the DAO+layer. Non-DAO classes no longer
import
the
DatabaseManager
or
throw SQLException
s. There are interfaces for CRUD and link operations in
org.dspace.storage.dao
that I intend to write some tests to for throwing at all the implementing DAOs._ --JR
It has often struck me that DSpace would benefit from the use of Data Access Objects (DAO). If nothing else, it would make porting to alternative database platforms far easier; all we would need to do is provide alternative implementations for the DAO interfaces that worked for a given database. To this end, I have broken up some of the core classes in
org.dspace.content
to use DAOs.
As part of the same effort, I have done some work on making the
Context
less data-layer dependent (by having it hold a
[[#org.dspace.storage.dao.GlobalDAO|org.dspace.storage.dao.GlobalDAO]]
rather than a
java.sql.Connection
, etc). I've also introduced a proxy for the
Item
that is a bit smarter about when it retrieves content from the data layer, and an
[[#org.dspace.core.ArchiveManager|ArchiveManager]]
class that takes care of some core "archive operations" (so that other core classes don't need to).
The process to integration (if at all) would go as follows:
- Incorporate the new DAO classes into the codebase
- Refactor
(etc) to use the DAO implementations of the data access methods internally
org.dspace.content.Item
- Mark relevant methods in
as
org.dspace.content.Item
@Deprecated
- Using the compile-time deprecation warnings as a guide, refactor the rest of the code to use the DAOs explicitly rather than hiding the functionality behind existing methods
Without further ado, here is how I have refactored
org.dspace.content.Item
to use DAOs. A few important things to note:
- "old" code has been used where possible to avoid re-implementing the wheel
- I've never liked
so I've switched to using a "real"
org.dspace.content.ItemIterator
from aIterator
For examples of both of these principles, see the implementation ofList<Item>
below. It is a fairly straightforward wrapper for the currentgetItems()
, except that it returns aItem.findAll()
rather than anList<Item>
.ItemIterator
org.dspace.content
org.dspace.content
The
Item
class will be broken up into the following classes:
- : core class that doesn't go near the database (it doesn't even know about the DAOs); behaves much like the current implementation.
[[#org.dspace.content.Item|org.dspace.content.Item]]
- : interface defining DAO API
[[#org.dspace.content.dao.ItemDAO|org.dspace.content.dao.ItemDAO]]
- : factory for dishing out implementations of the above interface
[[#org.dspace.content.dao.ItemDAOFactory|org.dspace.content.dao.ItemDAOFactory]]
- : default implementation of the above interface for use with PostgreSQL
[[#org.dspace.content.dao.postgres.ItemDAOPostgres|org.dspace.content.dao.postgres.ItemDAOPostgres]]
- : subclass of
[[#org.dspace.content.proxy.ItemProxy|org.dspace.content.proxy.ItemProxy]]
that needs to know about the DAO. It will be used for (eg) only loading metadata on demand, to reduce the memory footprint ofItem
s etc.Item
The following classes have also been introduced:
[[#org.dspace.core.ArchiveManager|org.dspace.core.ArchiveManager]]
[[#org.dspace.storage.dao.GlobalDAO|org.dspace.storage.dao.GlobalDAO]]
[[#org.dspace.storage.dao.GlobalDAOFactory|org.dspace.storage.dao.GlobalDAOFactory]]
[[#org.dspace.storage.dao.postgres.GlobalDAOPostgres|org.dspace.storage.dao.GlobalDAOPostgres]]
Note that it might be preferable to have a more generic implementation of the
ItemDAO
interface that supports both PostgreSQL and Oracle, but given that one motivation for adopting DAOs is to remove db-specificities from the code making it easier to port, I thought it was sensible to start with just PostgreSQL. Eventually, it ought to be possible to drop in
ItemDAOHibernate
(etc) implementations that make db portability far easier.
org.dspace.content.Item
org.dspace.content.Item
Basic implementation of the
Item
object. This class has been stripped down to remove all contact with the database, including (but not limited to) contstructors, factory methods,
update()
,
delete()
,
find()
, etc. I haven't decided exactly how the
Item
API will look, but it will probably be much the same as before, only with any of the aforementioned methods. Another key difference is that it will have actual Java objects as member variables instead of pulling everything out of a
TableRow
.
org.dspace.content.proxy.ItemProxy
org.dspace.content.proxy.ItemProxy
This will be a fairly simple proxy implementation. Specifically, it will be closest to being a virtual proxy, in that it will appear to be a regular
Item
object, but will have a slightly smarter implementation (not loading metadata until requested, keeping track of what has changed to make updates more efficient etc).
public class ItemProxy extends Item
{
// Overrides relevant methods of Item.
}
org.dspace.content.dao.ItemDAO
org.dspace.content.dao.ItemDAO
This isn't final, but it's a good start.
public interface ItemDAO extends ContentDAO
implements CRUD<Item>, Link<Item, Bundle>
{
public Item create(); throws AuthorizeException
public Item retrieve(int id);
public Item retrieve(UUID uuid);
public void update(Item item); throws AuthorizeException
public void delete(int id); throws AuthorizeException
public List<Item> getItems();
public List<Item> getItemsBySubmitter(EPerson eperson);
public List<Item> getItemsByCollection(Collection collection);
public List<Item> getParentItems(Bundle bundle);
}
org.dspace.content.dao.ItemDAOFactory
org.dspace.content.dao.ItemDAOFactory
public class ItemDAOFactory
{
public static ItemDAO getInstance(Context context)
{
// Eventually, the implementation that is returned will be
// defined in the configuration.
return new ItemDAOPostgres(context);
}
}
org.dspace.content.dao.postgres.ItemDAOPostgres
org.dspace.content.dao.postgres.ItemDAOPostgres
This is a fairly straightforward implementation of the above interface. As much as possible, code from the original
Item
class will be used. For instance, this is how
getItems()
is implemented:
public List<Item> getItems()
{
try
{
TableRowIterator tri = DatabaseManager.queryTable(context, "item",
"SELECT item_id FROM item WHERE in_archive = '1'");
List<Item> items = new ArrayList<Item>();
for (TableRow row : tri.toList())
{
int id = row.getIntColumn("item_id");
items.add(retrieve(id));
}
return items;
}
catch (SQLException sqle)
{
// Need to think more carefully about how we deal with SQLExceptions
throw new RuntimeException(sqle);
}
}
Some changes have been made to eliminate
ItemIterator
s, and to generally make things a little more consistent with the rest of the code (this looks almost identical to, eg,
CollectionDAO.getCollections()
).
org.dspace.core
org.dspace.core
org.dspace.core.ArchiveManager
org.dspace.core.ArchiveManager
The idea behind this class came from the realisation that
Item.withdraw()
and
Item.reinstate()
don't really make sense. What I'd much rather do is call (eg)
ArchiveManager.withdrawItem(Item item)
.
I've been thinking that the
ArchiveManager
could be used for certain maintenance operations as well, such as moving
Item
s between
Collection
s, and maybe acting as a wrapper for the
CommunityFiliator
.
public class ArchiveManager
{
public static void withdrawItem(Context context, Item item)
{
// ...
}
public static void reinstateItem(Context context, Item item)
{
// ...
}
public static void moveItem(Context context,
Item item, Collection source, Collection dest)
{
// ...
}
}
org.dspace.storage
org.dspace.storage
org.dspace.storage.dao.GlobalDAO
org.dspace.storage.dao.GlobalDAO
As suggested by Richard Jones, there probably ought to be a top-level general-purpose DAO interface that has implementations for the various storage mechanisms (
GlobalDAOPostgres
etc). The idea is to have this top-level object capture any implementation-specific details in a single top-level object, rather than in every Postgres DAO implementation. For example, with the current database "abstraction layer", the top-level implementation of
GlobalDAO
understands the
Context
object, whereas a Hibernate implementation would know what a
SessionFactory
is.
public interface GlobalDAO
{
// The following methods actually currently throw SQLExceptions to
// keep things simple, but in future SQLExceptions should be
// eliminated from any code that doesn't directly touch a database.
public void startTransaction() throws GlobalDAOException;
public void endTransaction() throws GlobalDAOException;
public void saveTransaction() throws GlobalDAOException;
public void abortTransaction();
public boolean transactionOpen();
@Deprecated Connection getConnection();
}
org.dspace.storage.dao.GlobalDAOFactory
org.dspace.storage.dao.GlobalDAOFactory
Super-simple
GlobalDAO
factory.
org.dspace.storage.dao.GlobalDAOPostgres
org.dspace.storage.dao.GlobalDAOPostgres
Implementation of the
GlobalDAO
interface for PostgreSQL.
public class GlobalDAOPostgres implements GlobalDAO
{
private Connection connection;
// ...
public void startTransaction()
{
connection = DatabaseManager.getConnection();
connection.setAutoCommit(false);
}
// ...
}
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