Hibernate Persistence

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Hibernate Persistence
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What is Persistence

• Persist data to database or other storage.
• In OO world, persistence means persist object to
  external storage.
• Relational database
• SQL
• Using SQL in Java
• Persistence object-oriented applications ORM

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What is Hibernate?

• Popular Open Source (LGPL) Object/Relational
  Mapping (ORM) tool
• Transparent persistence for POJOs (Plain Old
  Java Objects)
• Core of JBoss CMP 2.0 impl.

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Why Hibernate?

• Minimizes Code
• Does not require a container
• Model is not tied to persistence implementation,
  Retains natural object model (transparent)

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Object/Relational Mapping
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Course Object Model
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Persistence Tasks

• Class mapping
• Class to Table mapping
• Object ID to Primary Key
• Attribute Mapping
• Primary Attribute to Column
• User defined data type to embedded type
• Inheritance Mapping
• Relationship Mapping
• One to Many
• Many to One

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Persistence /Concurrency Pattern Introduced

• Martin Fowler????????

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Persistence

• What is Persistence
• Why Persistence is Importance
• History of persistence
• Plan text
• RDB(OODB?)
• XML

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Mapping to a Relational Database

• Metadata Mapping
• Identity Field
• Lazy Load
• Query Object

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Metadata Mapping

• Hold details of object-relational mapping in
  metadata.
• Hibernate as sample

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Identity Field

• Save a database id field in an object to maintain
  identity between an in-memory object and a
  database row.

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Identity Field

• Choosing your Key
• meaningful key
• meaningless key
• simple key
• compound key
• table-unique key
• database-unique key

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How to get a new Key

• Database counter
• it's non-standard and not available in all
  databases.
• Key table
• Separate transaction is needed
• GUID
• Large keys may also lead to performance problems,
  particularly with indexes

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Identity Map

• Ensure each object only gets loaded once by
  keeping every loaded object in a map. Lookup
  objects using the map when referring to them
• A Identity Map keeps a record of all the objects
  that have been read from the database in a single
  business transaction. Whenever you want an
  object, you check the Identity Map first to see
  if you already have it.

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Lazy Load

• An object that doesn't contain all of the data
  you need, but knows how to get it.

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Query Object

• An object that represents a database query
• SQL can be an involved language, and many
  developers are not particularly familiar with
  it..

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Query Object( cont. )
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Concurrency

• When Concurrency problem raised?
• Why Concurrency is difficult to deal with
• It is difficult to enumerate the possible
  scenarios that can get you into trouble
• it is hard to test for

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Execution Contexts

• Connection
• Session
• A session is a long running interaction between a
  client and server
• Transaction
• Process
• Thread

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Transactions

• The primary tool for handling concurrency in
  enterprise applications is the transaction.
• ATM machine example

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Transaction Resources

• Most enterprise applications run into
  transactions in terms of databases. But there are
  plenty of other things than can be controlled
  using transactions, such as
• message queues,
• printers,
• As a result technical discussions of transactions
  use the term 'transactional resource' to mean
  anything that is transactional that is uses
  transactions to control concurrency.

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Transaction types

• long transaction.
• making a transaction span multiple requests is
  generally known as a long transaction .
• Not recommended
• request transaction
• start a transaction at the beginning of a request
  and complete it at the end.
• late transaction
• open a transaction as late as possible ,

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Business and System Transactions

• System transactions
• transactions supported by RDBMS systems and
  transaction monitors
• Business transaction
• Transaction that logically defined by a business
  requirement

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Concurrency Patterns

• Unit of work
• Optimistic Offline Lock
• Pessimistic Offline Lock

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Unit of Work

• Maintains a list of objects that are affected by
  a business transaction and coordinates the
  writing out of changes and resolution of
  concurrency problems.

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How it Works

• Unit of Work is an object that keeps track of
  these changed things, such as inserted, updated
  or deleted.

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Unit of Work
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Optimistic Offline Lock

• Prevent conflicts between concurrent business
  transactions, by detecting a conflict and rolling
  back the transaction.

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Optimistic Offline Lock
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How it Works
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Pessimistic Offline Lock

• Prevent conflicts between concurrent business
  transactions by allowing only one business
  transaction to access data at once

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Pessimistic Offline Lock
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How it Works

• have an exclusive write lock. That is, require
  only that a business transaction acquire a lock
  in order to edit session data. This avoids
  conflict by not allowing two business
  transactions to simultaneously make changes to
  the same record.