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What is object-relational mapping?. Object-oriented programming technologies are typically used to implement business logicRelational databases are used for persistent data storageImpedance mismatch between the two paradigms: objects vs. relationsEstimated that 30-40% of a JDBC application involv
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1. Glenn Paulley, Director, Engineering
http://iablog.sybase.com/paulley ORMs and Hibernate
2. What is object-relational mapping? Object-oriented programming technologies are typically used to implement business logic
Relational databases are used for persistent data storage
Impedance mismatch between the two paradigms: objects vs. relations
Estimated that 30-40% of a JDBC application involves coercing data from tuples to object instances and back again
ORM toolkits are designed to address this impedance mismatch
61 different ORM toolkits are listed in Wikipedia for Java, C++, Delphi, Ruby, .NET, PHP, Python, Perl 2
3. What is object-relational mapping? To exploit object behaviour fully, data-access from within an object-oriented programming language should offer:
Separation of concerns
Information hiding
Inheritance
Change detection
Uniqueness capability
Database independence
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4. Change detection ORM toolkits require mechanisms to track changes to objects made by the application
When a transaction is complete, write the changes to the database within an atomic transaction
Need appropriate guarantees to prevent lost updates 4
5. Uniqueness Mappings are usually a correspondence between a row in a normalized table and a class instance
Specified using metadata
For example, a row of the Employee table will correspond to an instance of the Employee object within the application
Mappings are often not isomorphic
Sophisticated ORMs such as Hibernate and LINQ permit object models that differ substantially from the underlying relational store
Object-oriented language features offer greater semantic flexibility in application design than 1NF values from a relational database
Need to establish a correspondence between an in-memory object and a database row
Must be independent of how the object was acquired: a database query, or navigating a reference to another object
Predicated on the existence of primary keys in the database 5
6. Database independence Many ORM toolkits attempt to offer database independence, so that applications can be ported from one DBMS to another
Create common APIs and models to interact with a variety of DBMS platforms
Useful with mobilized applications where the consolidated database is one DBMS, and local databases are different 6
7. Why are ORMs useful? Eliminates tedious, repetitive code that instantiates object instances from tuples using a SELECT statement and a CURSOR
Insulates, to some extent, the application developer from vendor-specific SQL extensions
Permits the application developer to exploit object-orientation and model and manipulate the application view differently from the relational model
Data manipulation can be done at the object level, rather than (only) at a SQL statement level 7
8. Challenges of ORMs ORM toolkits introduce an additional level of complexity to the application
Example: Java Hibernate 3.2.6 is
266 packages, 1938 classes, 18,680 functions, over 118K LOC
Can be difficult to debug, perform performance analysis
Most frameworks suffer from a lack of appropriate tools
Performance analysis is problematic because the application’s behaviour is not tied directly to specific interactions with the database
Complex mappings may cause very complex SQL queries to be generated
Can be difficult for the application developer to understand what caused their construction
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9. Complex SQL (LINQ generated) SELECT
[Project9].[ContactID] AS [ContactID],[Project9].[C1] AS [C1],[Project9].[C2] AS [C2],[Project9].[ContactID1] AS [ContactID1],[Project9].[SalesOrderID] AS [SalesOrderID],
[Project9].[TotalDue] AS [TotalDue]
FROM ( SELECT [Distinct1].[ContactID] AS [ContactID], 1 AS [C1], [Project8].[ContactID] AS [ContactID1], [Project8].[SalesOrderID] AS [SalesOrderID],
[Project8].[TotalDue] AS [TotalDue], [Project8].[C1] AS [C2]
FROM
(SELECT DISTINCT [Extent1].[ContactID] AS [ContactID]
FROM [DBA].[Contact] AS [Extent1]
INNER JOIN [DBA].[SalesOrderHeader] AS [Extent2]
ON EXISTS (SELECT cast(1 as bit) AS [C1]
FROM ( SELECT cast(1 as bit) AS X ) AS [SingleRowTable1]
LEFT OUTER JOIN (SELECT [Extent3].[ContactID] AS [ContactID]
FROM [DBA].[Contact] AS [Extent3] WHERE [Extent2].[ContactID] = [Extent3].[ContactID] )AS [Project1] ON cast(1 as bit) = cast(1 as bit)
LEFT OUTER JOIN (SELECT [Extent4].[ContactID] AS [ContactID]
FROM [DBA].[Contact] AS [Extent4] WHERE [Extent2].[ContactID] = [Extent4].[ContactID] ) AS [Project2] ON cast(1 as bit) = cast(1 as bit)
WHERE ([Extent1].[ContactID] = [Project1].[ContactID]) OR (([Extent1].[ContactID] IS NULL) AND ([Project2].[ContactID] IS NULL)) )
) AS [Distinct1]
LEFT OUTER JOIN
(SELECT [Extent5].[ContactID] AS [ContactID], [Extent6].[SalesOrderID] AS [SalesOrderID], [Extent6].[TotalDue] AS [TotalDue], 1 AS [C1]
FROM [DBA].[Contact] AS [Extent5]
INNER JOIN [DBA].[SalesOrderHeader] AS [Extent6]
ON EXISTS (SELECT cast(1 as bit) AS [C1]
FROM ( SELECT cast(1 as bit) AS X ) AS [SingleRowTable2]
LEFT OUTER JOIN (SELECT [Extent7].[ContactID] AS [ContactID]
FROM [DBA].[Contact] AS [Extent7] WHERE [Extent6].[ContactID] = [Extent7].[ContactID] )AS [Project5] ON cast(1 as bit) = cast(1 as bit)
LEFT OUTER JOIN (SELECT [Extent8].[ContactID] AS [ContactID]
FROM [DBA].[Contact] AS [Extent8] WHERE [Extent6].[ContactID] = [Extent8].[ContactID] )AS [Project6] ON cast(1 as bit) = cast(1 as bit)
WHERE ([Extent5].[ContactID] = [Project5].[ContactID]) OR (([Extent5].[ContactID] IS NULL) AND ([Project6].[ContactID] IS NULL))
)
) AS [Project8]
ON ([Project8].[ContactID] = [Distinct1].[ContactID]) OR (([Project8].[ContactID] IS NULL) AND ([Distinct1].[ContactID] IS NULL))
) AS [Project9]
ORDER BY [Project9].[ContactID] ASC, [Project9].[C2] ASC 9
10. Equivalent SQL query select Extent6.ContactID,
1 as C1,
1 as C2,
Extent6.ContactID as ContactID1,
Extent6.SalesOrderID as SalesOrderID,
Extent6.TotalDue as TotalDue
from DBA.SalesOrderHeader as Extent6
order by Extent6.ContactID asc
Can your query optimizer get there? 10
11. Challenges of ORMs Object-instance-at-a-time navigation through the object model can result in multiple, separate interactions to the database server to retrieve the rows to create the objects
Termed the N+1 Selects problem
Application developer must tradeoff prefetching applicability in various scenarios
Objects are not partially instantiated; all object attributes are required for the constructor
Transactional semantics are complex once caching is introduced
Transactional semantics often differ across DBMSs, even with the identical isolation level
Developers must (still) be aware of the potential for lost updates
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12. Challenges of ORMs The SQL dialect supported by ORM toolkits is typically a very restricted subset of ANSI SQL
ORM toolkits often support only limited, straightforward DDL for schema creation/modification
Not recommended for production applications 12
13. Important aspects of ORM toolkits Mapping specification
Query language
Persistence
Class inheritance
Fetch strategies
Caching
We look and see how these are implemented in Hibernate, a popular open-source ORM toolkit for Java applications
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14. Hibernate as an example of an ORM
15. Brief introduction to Hibernate Open-source, LGPL Java ORM toolkit
Originally developed by Christian Bauer, Gavin King, and a worldwide team of developers
Now maintained by a team at JBoss (Redhat) led by Steve Ebersole
Ported to the .NET environment (C#), called Nhibernate
http://hibernate.org
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16. Hibernate mapping specification CREATE TABLE "GROUPO"."SalesOrders" (
"ID" integer NOT NULL DEFAULT autoincrement
,"CustomerID" integer NOT NULL
,"OrderDate" date NOT NULL
,"FinancialCode" char(2) NULL
,"Region" char(7) NULL
,"SalesRepresentative" integer NOT NULL
,CONSTRAINT "SalesOrdersKey" PRIMARY KEY ("ID")
)
ALTER TABLE "GROUPO"."SalesOrders“
ADD FOREIGN KEY "FK_SalesRepresentative_EmployeeID" ("SalesRepresentative“) REFERENCES "GROUPO"."Employees" ("EmployeeID")
ALTER TABLE "GROUPO"."SalesOrders“
ADD FOREIGN KEY "FK_FinancialCode_Code" ("FinancialCode")
REFERENCES "GROUPO"."FinancialCodes" ("Code“)
ON DELETE SET NULL
ALTER TABLE "GROUPO"."SalesOrders“
ADD FOREIGN KEY "FK_CustomerID_ID" ("CustomerID")
REFERENCES "GROUPO"."Customers" ("ID“) 16
17. Hibernate mapping specification Hibernate mapping file SalesOrders.hbm.xml:
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<!-- Generated Mar 3, 2009 11:59:55 AM by Hibernate Tools 3.2.2.GA -->
<hibernate-mapping>
<class name="SalesOrders" table="SalesOrders">
<comment>sales orders that customers have submitted to the sporting goods company</comment>
<id name="id" type="int">
<column name="ID" />
<generator class="assigned" />
</id>
<many-to-one name="employees" class="Employees" fetch="select">
<column name="SalesRepresentative" not-null="true" />
</many-to-one>
<many-to-one name="financialCodes" class="FinancialCodes" fetch="select">
<column name="FinancialCode" length="2" />
</many-to-one>
<many-to-one name="customers" class="Customers" fetch="select">
<column name="CustomerID" not-null="true" />
</many-to-one>
<property name="orderDate" type="date">
<column name="OrderDate" length="10" not-null="true" />
</property> .............[snip]
</class>
</hibernate-mapping>
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18. Java class implementation // default package
// Generated Mar 3, 2009 1:39:06 PM by Hibernate Tools 3.2.2.GA
import java.util.Date;
import java.util.HashSet;
import java.util.Set;
/**
* SalesOrders generated by hbm2java
*/
public class SalesOrders implements java.io.Serializable {
private int id;
private Employees employees;
private FinancialCodes financialCodes;
private Customers customers;
private Date orderDate;
private String region;
private Set salesOrderItemses = new HashSet(0);
public SalesOrders() {
}
public SalesOrders(int id, Employees employees, Customers customers,
Date orderDate) {
this.id = id;
this.employees = employees;
this.customers = customers;
this.orderDate = orderDate;
}
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19. HQL: Hibernate Query Language Subset of ANSI query specification with support for
DISTINCT
GROUP BY, simple aggregation
INNER, LEFT- and RIGHT-OUTER JOIN
Quantified subqueries
Supports dot-notation for many-to-one, one-to-one associations, for example:
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20. HQL: Hibernate Query Language Hibernate’s HQL supports SELECT, inner and outer JOIN, WHERE, HAVING, simple GROUP BY, UNION, ORDER BY, self-joins with different correlation names
HQL does not support recursion, common table expressions, window functions, derived tables, other set operators, table functions, array or structured types, APPLY/OUTER APPLY, CROSS JOIN, GROUP BY CUBE/ROLLUP/GROUPING SETS, FULL OUTER JOIN 20
21. HQL: Hibernate Query Language HQL is augmented by “dialects” that implement specific methods to modify the SQL generated by Hibernate before execution on the server
HQL has optional support for (implemented by a specific dialect):
UNION ALL (for entity-type hierarchies)
LIMIT (SELECT TOP N), OFFSET
IDENTITY, GUID data types
Syntax to declare an updateable cursor and locking mode
Case-insensitive string comparisons
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22. Object persistence in Hibernate Saving objects
Once an object is created or modified, it must be saved explicitly and then the transaction must be committed:
session.save(<object name>);
tx.commit();
Loading objects
The Hibernate session interface offers several load() methods for loading objects into memory from database tuples:
public Object load(Class theClass, Serializable id)
public Object load(String entityname, Serializable id)
public Object load(Object object, Serializable id)
Other load() methods permit specification of a lock mode, ie Select for Update
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23. Object persistence in Hibernate A refresh() method is implemented to reload objects from the database
Useful for when attributes are modified by database triggers upon INSERT or UPDATE
Highly error-prone
Updating objects
Hibernate manages changes to persistent objects transparently and automatically
If an attribute is altered, the appropriate Hibernate session will queue the change for writing to the database using SQL
One can force changes to be written at a certain point using the flush() method, controlled by isDirty() and setFlushMode()
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24. Class inheritance Hibernate offers a variety of built-in techniques to handle different normalizations of entity-type hierarchies:
Single table with discriminator value
Multiple tables fully normalized into BCNF
A hybrid model consisting of a mixture of the two
If represented as different objects in a mapping, an ETH requires careful construction and different equals() and hashcode() implementations 24
25. FETCH strategies A major pain point is the N+1 SELECTs problem
Navigation through the object model iteratively causes additional rows to be retrieved from the database, using independent SQL requests
“client-side” join; performance tends to be extremely poor due to the additional latency
Adaptive, client-side prefetching and SQL rewriting may be of benefit in these scenarios: see Ivan Bowman’s PhD thesis on Scalpel
Alternatively, in the mapping one may specify the selection method: either “lazy” or “eager”
One must tradeoff this method will global application behaviour, or override it on a case-by-case basis using HQL-specific syntax 25
26. Concurrency control Hibernate relies on the database’s locking scheme for concurrency control
ANSI isolation levels 0-3 are supported directly; 1 or 2 is recommended (READ COMMITTED and REPEATABLE READ)
DBMS that support snapshot isolation require tweaks to their Hibernate dialect implementation
Lock mode for individual HQL statements or instantiations of objects can be specified directly, ie
Customer c = (Customer) session.get(Customer.class, 101, LockMode.Upgrade);
Hibernate has builtin support for optimistic concurrency control
Can use either version numbers (added to the schema of the table), or base change control on all of the values in the row
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27. Caching and concurrency control Like many runtime environments, Hibernate supports built-in caching controls to speed-up database interaction
Two levels of caching are supported:
Level 1: Persistence context cache
Lifespan is a transaction or a conversation without sharing.
Guarantees scope of the object and object identity. Mandatory.
Level 2: Pluggable, scope is process or cluster (shared)
Configurable on a class-by-class basis
Selectable concurrency control strategies: Transactional, Read-write, non-strict read-write, read-only
EHCache, OpenSymphony, SwarmCache, JBoss Cache
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28. Research opportunities Robust, industrial-strength debugging frameworks
Identify how a particular database interaction was caused by what specific application program behaviour
Analysis of concurrency control behaviour to determine application correctness, for example with respect to lost updates
Identifying common classes of SQL queries for specific sets of optimizations
Place in the dialect layer, in the DBMS itself, or in a separate tool that can support the optimization of multiple inter-related statements
Mechanisms to support additional types of mappings and more complex schemas
Support for recursion
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29. Questions? Watch my blog for continuing articles on supporting ORM toolkits with SQL Anywhere
http://iablog.sybase.com/paulley
References:
Christian Bauer and Gavin King (November 2007). Java Persistence with Hibernate (revised edition of Hibernate in Action). Manning Publications, New York, New York. ISBN 1-932394-88-5. Seventh printing.
Dave Minter and Jeff Linwood (2005) Pro Hibernate 3. Apress Books, Berkeley, CA.
ACM Queue 6(3), May/June 2008, “Bridging the Object-Relational Divide”.
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