The preceding discussion has gone into a lot of detail about what Hibernate wants to accomplish as it caches data, and what JBoss Cache configuration options are available. What should be clear is that the configurations that are best for caching one type of data are not the best (and are sometimes completely incorrect) for other types. Entities likely work best with synchronous invalidation; timestamps require replication; query caching might do best in local mode.
Prior to Hibernate 3.3 and JBoss Cache 2.1, the conflicting requirements between the different cache types led to a real dilemna, particularly if query caching was enabled. This conflict arose because all four cache types needed to share a single underlying cache, with a single configuration. If query caching was enabled, the requirements of the timestamps cache basically forced use of synchronous replication, which is the worst performing choice for the more critical entity cache and is often inappropriate for the query cache.
With Hibernate 3.3 and JBoss Cache 2.1 it has become possible, even easy, to use separate underlying JBoss Cache instances for the different cache types. As a result, the entity cache can be optimally configured for entities while the necessary configuration for the timestamps cache is maintained.
There were three key changes that make this improvement possible:
As mentioned previously, Hibernate 3.3 introduced the
RegionFactory API as its mechanism for managing
the Second Level Cache. This API makes it possible for implementations
to know at all times whether they are working with entities,
collections, queries or timestamps. That knowledge allows the
Hibernate/JBoss Cache integration layer to make the best use of the
various options JBoss Cache provides.
A Hibernate user doesn't need to understand the RegionFactory
API in any detail at all; the main point is internally it makes
possible independent management of the different cache types.
The CacheManager API is a new feature of JBoss
Cache 2.1. It provides an API for managing multiple distinct
JBoss Cache instances in the same VM. Basically a
CacheManager is instantiated and provided a set
of named cache configurations. An application
like the Hibernate/JBoss Cache integration layer accesses the
CacheManager and asks for a cache configured with
a particular named configuration.
Again,a Hibernate user doesn't need to understand the
CacheManager; it's an internal detail. The thing
to understand is that the task of a Hibernate Second Level Cache user
is to:
Provide a set of named JBoss Cache configurations in an XML
file (or just use the default set included in the
jbc2-configs.xml file found in the
org.hibernate.cache.jbc2.builder package
in hibernate-jbosscache2.jar).
Tell Hibernate which cache configurations to use for entity, collection, query and timestamp caching. In practice, this can be quite simple, as there is a reasonable set of defaults.
See Chapter 3, Configuration for more on how to do this.
JGroups is the group communication library JBoss Cache uses to send
messages around a cluster. Each cache has a JGroups
Channel; different channels
around the cluster that have the same name and compatible
configurations detect each other and form a group for message
transmission.
A Channel is a fairly heavy object, typically
using a good number of threads, several sockets and some good sized
network I/O buffers. Creating multiple different channels in the
same VM was therefore costly, and was an administrative burden as
well, since each channel would need separate configuration to use
different network addresses or ports. Architecturally, this
mitigated against having multiple JBoss Cache instances in an
application, since each would need its own Channel.
Added in JGroups 2.5 and much improved in the JGroups 2.6
series is the concept of sharable JGroups resources. Basically,
the heavyweight JGroups elements can be shared. An application
(e.g. the Hibernate/JBoss Cache integration layer) uses a JGroups
ChannelFactory. The ChannelFactory
is provided with a set of named channel
configurations. When a Channel is needed (e.g.
by a JBoss Cache instance), the application asks the
ChannelFactory for the channel by name. If
different callers ask for a channel with the same name, the
ChannelFactory ensures that they get
channels that share resources.
The effect of all this is that if a user wants to use four separate JBoss Cache instances, one for entity caching, one for collection caching, one for query caching and one for timestamp caching, those four caches can all share the same underlying JGroups resources.
The task of a Hibernate Second Level Cache user is to:
Provide a set of named JGroups configurations in an XML file
(or just use the default set included in the
jgroups-stacks.xml file found in the
org.hibernate.cache.jbc2.builder package
in the hibernate-jbosscache2.jar).
Tell Hibernate where to find that set of configurations
on the classpath. See Section 3.1, “Configuring the Hibernate Session Factory” for
details on how to do this. This is not necessary if the
default set included in hibernate-jbosscache2.jar
is used.
In the JBoss Cache configurations you are using specify
the name of the channel you want to use. This should be
one of the named configurations in the JGroups XML file.
The default set of JBoss Cache configurations found in the
hibernate-jbosscache2.jar already have
appropriate default choices. See Section 3.2.3.4, “JGroups Channel Configuration”
for details on how to set this if you don't wish to use the
defaults.
See Section 3.3, “JGroups Configuration” for more on JGroups.
So, we've seen that Hibernate caches up to four different types of data (entities, collections, queries and timestamps) and that Hibernate 3.3 + JBoss Cache 2 gives you the flexibility to use a separate underlying JBoss Cache, with different behavior, for each type. You can actually deploy four separate caches, one for each type.
In practice, four separate caches are unnecessary. For example,
entities and collection caching have similar enough semantics that
there is no reason not to share a JBoss Cache instance between them.
The queries can usually use the same cache as well. Similarly,
queries and timestamps can share a JBoss Cache instance configured
for replication, with the
hibernate.cache.region.jbc2.query.localonly=true
configuration letting you turn off replication for the queries if
you want to.
Here's a decision tree you can follow:
Decide if you want to enable query caching.
Decide if you want to use invalidation or replication for your entities and collections. Invalidation is generally recommended for entities and collections.
If you want invalidation, and you want query caching, you will need two JBoss Cache instances, one with synchronous invalidation for the entities and collections, and one with synchronous replication for the timestamps. The queries will go in the timestamp cache if you want them to replicate; they can go with the entities and collections otherwise.
If you want invalidation but don't want query caching, you can use a single JBoss Cache instance, configured for synchronous invalidation.
If you want replication, whether or not you want query caching, you can use a single JBoss Cache instance, configured for synchronous replication.
If you are using query caching, from the above decision tree you've either got your timestamps sharing a cache with other data types, or they are by themselves. Either way, the cache being used for timestamps must have initial state transfer enabled. Now, if the timestamps are sharing a cache with entities, collections or queries, decide whether you want initial state transfer for that other data. See Section 2.2.5, “Initial State Transfer” for the implications of this. If you don't want initial state transfer for the other data, you'll need to have a separate cache for the timestamps.
Finally, if your queries are sharing a cache configured
for replication, decide if you want the cached query results
to replicate. (The timestamps cache must
replicate.) If not, you'll want to set the
hibernate.cache.region.jbc2.query.localonly=true
option when you configure your SessionFactory
Once you've made these decisions, you know whether you need just one underlying JBoss Cache instance, or more than one. Next we'll see how to actually configure the setup you've selected.