1. Hibernate
Cache
Alex Verdyan

2. Why cache?
Why it works?
● Temporal locality
● Non uniform distribution of hits

3. Temporal locality

4. Non-uniform distribution

5. 17,000 pageviews
avg load = 250ms
cache 17 pages / 80% of views
cached page load time = 10 ms
new avg load = 58ms
● Trade memory for latency reduction
● Reduce database load

6. Hibernate cache
● Session level cache - L1
● SessionFactory (JVM) cache - L2
● Query Cache

7. Session cache - L1
● Enabled by default
● Used transparently during the session
● All objects that was saved or retrieved
○ save
○ update
○ get
○ list
○ iterate
● flush() - will sync cache to DB
● clear() - will evict all objects

8. L2 cache - configuration
1. Get ehcache.jar and add it to classpath
2.
3. Edit persistence.xml

9. L2 Cache
● Inter-session cache
● Can be clustered
● Configurable with XML or Annotations
● Divided to regions
○ Entity - data
○ Collections - relations
○ Queries - query results
○ Timestamps - last update timestamp for tables

10. L2 cache strategies
● Read-only
○ simplest, safest and fastest
○ supports insert, no update/delete
○
● Nonstrict Read-Write
○ occasional writes, no locking cache
○ low prob. that 2 transactions will write the same
object
○ async - update

11. L2 cache strategies
● Read-Write
○ read committed - transaction isolation
○ uses soft locks
○ async - cache update out of tx
● Transactional
○ only with JTA (i.e. with distributed tx support)
○ serialized transaction isolation
○ sync - cache update inside tx

12. L2 cache - how it works
● Does not cache actual objects
● Caches values of the properties
● Mappings (relations) are not cached by
default
○ it can helps with N+1 queries

13. Example
● cache of the mapping is optional
● although it's the best place to improve
● beware of code altering the associations
and not cascading the change

14. How will the cache look like
*-----------------------------------------------------*
| Person Data Cache |
|-----------------------------------------------------|
| 1 -> [ "John" , "Bon Jovi" , null , [ 2 , 3 ] ] |
| 2 -> [ "Joey" , "Ramone" , 1 , [] ] |
| 3 -> [ "Sara" , "Connor" , 1 , [] ] |
*-----------------------------------------------------*
Hibernate cache stores
"dehydrated" objects

15. Evicting from cache
From L1 cache (Session)
session.evict(entity)
session.evict(Person.class)
From L2 cache (SessionFactory)
sf.getCache().evictEntity(Person.class,2L);
sf.getCache().
evictCollection("com.bla.Person.children",1L)

16. Query cache

17. Query cache
What happens if don't query by Id ?
The query cache will look like:
*---------------------------------------------------------
-*
| Query Cache
|
|---------------------------------------------------------
-|
| ["from Person as p |

18. Query cache
There are 3 cache regions:
● StandardQueryCache
○ cached query results
● UpdateTimestampsCache
○ holding timestamps of the most recent updates
● NamedQueryCaches
○ Query.setCacheRegion(name)

19. Query Cache
QC is useful when you query by "Natural id"
Criteria crit = session.createCriteria(Person.class);
crit.add(
Restrictions.
naturalId().
set("email", "joey@ramones.com")).
setCacheable(true).
uniqueResult();
Since Natural Id immutable, Natural Id -> Primary Key
mapping cannot be invalidated
This allows Hibernate to bypass UpdateTimestampCache

20. Query Cache - Pitfalls
● Any update to the underlying table updates the
timestamp cache - invalidates all entities
● Memory
○ the queries are large pretty strings and the
bind variables can be objects
● Lock contention - QC has a coarse lock on
timestamp cache for insert/update/delete and
lookups

21. Stuff to remember
● Cache cannot know about updates made to
the persistent store by another application
● Query cache - unless you use a natural key
is almost always a bad idea
○ unless you know what you're turning it on
○ you can show an improvement with realistic load
● TTL and TTI
○ tune them

22. Cache providers

23. Bundled cache providers
Cache Type Cluster Safe Query Cache Supported
ConcurrentHashMap memory no yes
EHCache memory, disk,
transactional, clustered
yes yes
Infinispan (JBoss Cache) clustered (ip multicast),
transactional
yes yes (clock sync req.)

24. Bundled cache providers
Cache read-only nonstrict-read-write read-write transactional
ConcurrentHashMap yes yes yes
EHCache yes yes yes yes
Infinispan (JBoss cache) yes yes

25. More cache providers
● Hazelcast
● GemFire
● Oracle Coherence
● Gigaspaces

26. Monitoring

27. Questions

28. References
http://tech.puredanger.com/2009/07/10/hibernate-
query-cache/
http://anirbanchowdhury.wordpress.
com/2012/07/23/hibernate-second-level-cache-
ehcache/#introduction
http://www.javalobby.org/java/forums/t48846.html
http://www.slideshare.net/alexmiller/cold-hard-cache