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- 2. Caching Scopes
Professional Open Source™
Transaction scope cache—Attached to the current unit of work,
which may be a database transaction or even a conversation. It’s
valid and used only as long as the unit of work runs. Every unit of
work has its own cache. Data in this cache isn’t accessed
concurrently.
Process scope cache—Shared between many (possibly
concurrent) units of work or transactions. This means that data in the
process scope cache is accessed by concurrently running threads,
obviously with implications on transaction isolation.
Cluster scope cache—Shared between multiple processes on the
same machine or between multiple machines in a cluster. Here,
network communication is an important point worth consideration.
© JBoss, Inc. 2003, 2004. 2
- 3. Professional Open Source™
Hibernate has a two-level cache architecture:
The first-level cache is the persistence context cache. This is at the
unit-of-work level.
It corresponds to one session in Hibernate for a single request and is
by default enabled for the Hibernate session.
The second-level cache is either at the process scope or the cluster
scope. This is the cache of the state of the persistence entities.
A cache-concurrency strategy defines the transaction isolation details
for a particular item of data, and the cache provider represents the
physical cache implementation.
© JBoss, Inc. 2003, 2004. 3
- 4. Using the Second-Level Cache in Hibernate
Professional Open Source™
The cache policy involves setting the following:
– Whether the second-level cache is enabled
– The Hibernate concurrency strategy
– Cache expiration policies (such as timeout, least recently used,
and memory sensitive)
– The physical format of the cache (memory, indexed files, or
cluster-replicated)
© JBoss, Inc. 2003, 2004. 4
- 5. Concurrency Strategies
Professional Open Source™
– Transactional: This strategy should be used when there is more
data to read. It prevents stale data in concurrency strategies. It’s
equivalent to isolation level: repeatable read.
– Read-write: This maintains a read-committed isolation level.
– Non-strict read-write: This doesn’t guarantee that you won’t
read stale data. It doesn’t guarantee consistency between cache
and database.
– Read-only: This is appropriate for data that never changes.
© JBoss, Inc. 2003, 2004. 5
- 7. Professional Open Source™
Not every cache provider is compatible with every concurrency
strategies . The compatibility matrix is given in below Table :
© JBoss, Inc. 2003, 2004. 7
- 8. Cache Regions
Professional Open Source™
Cache regions are handles by which you can reference classes and
collections in the cache provider configuration and set the expiration
policies applicable to that region.
Regions are buckets of data of two types:
one type contains disassembled data of entity instances,
the other contains only identifiers of entities that are linked through a
collection.
The name of the region is the class name in the case of a class cache
or the class name together with the property name in the case of a
collection cache.
© JBoss, Inc. 2003, 2004. 8
- 9. Caching Query Results
Professional Open Source™
A query’s result set can be configured to be cached. By default,
caching is disabled, and every HQL, JPA QL, and Criteria query hits
the database.
You enable the query cache as follows:
hibernate.cache.use_query_cache = true
In addition to setting this configuration property, you should use the
org.hibernate.Query interface:
Query bookQuery = session.createQuery("from Book");
bookQuery.setCacheable(true);
The setCacheable() method enables the result to be cached.
© JBoss, Inc. 2003, 2004. 9
- 10. First Level Cache
Professional Open Source™
If you inspect the SQL statements executed by Hibernate, you find
that only one database query is made.
That means Hibernate is caching your objects in the same session.
This kind of caching is called first-level caching, and its caching
scope is a session.
© JBoss, Inc. 2003, 2004. 10
- 11. Professional Open Source™
Two database queries are made. That means Hibernate isn’t caching
the persistent objects across different sessions by default.
You need to turn on this second-level caching, whose caching
scope is a session factory.
© JBoss, Inc. 2003, 2004. 11
- 13. Configuring cache provider
Professional Open Source™
To turn on the second-level cache, the first step is to choose a cache
provider in hibernate.cfg.xml.
© JBoss, Inc. 2003, 2004. 13
- 14. Professional Open Source™
You can configure EHCache through the configuration file
ehcache.xml, located in the source root folder.
© JBoss, Inc. 2003, 2004. 14
- 15. Professional Open Source™
To monitor Hibernate’s caching activities at runtime, add the following
line to the log4j configuration file log4j.properties:
© JBoss, Inc. 2003, 2004. 15
- 16. Professional Open Source™
When you enable a class with the second-level cache, Hibernate
doesn’t store the actual instances in cache.
Instead it caches the individual properties of that object.
For example, the instance of Book isn’t cached—rather, the
properties in the book object like name and price are cached.
The cached objects are stored in a region that has the same name as
the persistent class, such as com.training.Book.
© JBoss, Inc. 2003, 2004. 16
- 18. Professional Open Source™
When the Book class is cached, only one SQL query is made,
even though the book object is loaded
more than once in two different sessions. Because the second call to
the database is avoided, the query’s performance improves:
© JBoss, Inc. 2003, 2004. 18
- 19. Professional Open Source™
If you modify the book object in one session and flush the
changes, an exception will be thrown for updating a read-only
object.
© JBoss, Inc. 2003, 2004. 19
- 20. Professional Open Source™
So, for an updatable object, you should choose another cache usage:
read-write.
Hibernate invalidates the object from cache before it’s updated:
© JBoss, Inc. 2003, 2004. 20
- 21. Professional Open Source™
You can invalidate either one instance or all instances of a persistent
class:
factory.evict(Book.class);
factory.evict(Book.class, id);
factory.evictEntity("com.training.Book");
factory.evictEntity("com.training.Book", id);
© JBoss, Inc. 2003, 2004. 21
- 22. Professional Open Source™
CacheMode options are provided to control the interaction of
Hibernate with the second level cache:
© JBoss, Inc. 2003, 2004. 22
- 23. CacheModes
Professional Open Source™
CacheMode.IGNORE: Hibernate doesn’t interact with the second-
level cache. When entities cached in the second-level cache are
updated, Hibernate invalidates them.
• CacheMode.GET: Hibernate only reads and doesn’t add items
to the second-level cache. When entities cached in the second-level
cache are updated, Hibernate invalidates them.
CacheMode.PUT: Hibernate only adds and doesn’t add read from
the second-level cache. When entities cached in the second-level
cache are updated, Hibernate invalidates them.
• CacheMode.REFRESH: Hibernate only adds and doesn’t read
from the second-level cache. In this mode, the setting of
hibernate.cache.use_minimal_puts is bypassed,
as the refresh is forced.
© JBoss, Inc. 2003, 2004. 23
- 25. Professional Open Source™
Is a book’s associated publisher object cached when its parent
book object is cached?
© JBoss, Inc. 2003, 2004. 25
- 26. Professional Open Source™
If you initialize the association in two different sessions, it’s
loaded as many times as the initialization is made.
This is because Hibernate caches only the identifier of
publisher in Book’s region
© JBoss, Inc. 2003, 2004. 26
- 27. Professional Open Source™
To cache the publisher objects in their own region, you need to
enable caching for the Publisher class. You can do it the same way
as for the Book class.
The cached publisher objects are stored in a region with the name
com.training.bookshop.Publisher:
<hibernate-mapping package="com.training.bookshop">
<class name="Publisher" table="PUBLISHER">
<cache usage="read-write" />
...
</class>
</hibernate-mapping>
© JBoss, Inc. 2003, 2004. 27
- 28. Caching Collections
Professional Open Source™
factory.evictCollection("com.training.Book.chapters");
factory.evictCollection("com.training.Book.chapters",
id);
© JBoss, Inc. 2003, 2004. 28
- 29. Caching Queries
Professional Open Source™
By default, the HQL queries aren’t cached. You must first enable the
query cache in the Hibernate configuration file:
© JBoss, Inc. 2003, 2004. 29
- 30. Professional Open Source™
The setting cache.use_query_cache creates two cache regions:
– one holding the cached query result sets
– the other holding timestamps of the more recent updates to
queryable tables.
By default, the queries aren’t cached. In addition to using the previous
setting, to enable caching,
you need to call Query.setCacheable(true).
This allows the query to look for existing cache results or add
the results of the query to the cache.
The query result is cached in a
region named org.hibernate.cache.QueryCache by default.
© JBoss, Inc. 2003, 2004. 30
