Hibernate is an ORM framework which allows developers to handle database from application easily. It also allows multi level caching. Which is a great feature.
I know how does it maintain first level caching.
https://howtodoinjava.com/hibernate/understanding-hibernate-first-level-cache-with-example
My concern is how it maintain first level caching while I have multiple instances of same application related/transact with same database?
My concern is how it maintain first level caching while I have
multiple instances of same application related/transact with same
database?
Each application runs in its JVM, so will have its own Hibernate Session (used for the first level cache) and its own Hibernate second level cache.
Note that in cases of running multiple instances of a same application, you generally wonder how to share the second level cache, not the first level that is specific most of time to the current transaction.
And to get a distributed cache, you have to favor a distributed cache solution as EHCache or HazelCast that can set in front of Hibernate.
Related
I use technologies like spring boot, jpa and java 8. I have a question, how can I check if the cache is empty and I should send a query to the database to reload it (how to check that I need to reload the cache)?
As your question is not clear about regarding what type of cache you are using ??
JPA uses the first level of caching is the persistence context.
The Entity Manager guarantees that within a single Persistence Context, for any particular database row, there will be only one object instance. However the same entity could be managed in another User's transaction, so you should use either optimistic or pessimistic locking.
If you mean 2nd level cache ,This level of cache came due to performance reasons.this 2nd level cache sits between Entity Manager and the database. Persistence context shares the cache, making the second level cache available throughout the application. Database traffic is reduced considerably because entities are loaded in to the shared cache and made available from there. So actually laying you dont need to worry about reloading of data from database if cache miss happens.
Now if you are implementing your own logic to implement the cache , then you need to do more research on how actually caching works and different algorithms for caching like LRU,MRU etc. (which I would personally not recommend as you can use existing available providers like ehcache, redis,hazelcast just few names for 2nd level caching )
We have a new Java project which we are planning to deploy in a cluster environment.
I just want to clarify if Hibernate is suitable for us as I am new in the technology. As far as i know, Hibernate is basically a set of Java APIs which will be working in a JVM, so the caching of objects, first/second level whatever it is, will be bind with that particular JVM. Is that right?
If yes, then in the cluster environment there will be many cluster nodes, each with their own JVM. So it will lead to a logical mistake, right?
If second-level cache is not enabled, there will be no problems, because first level cache is bound to the session (persistence context).
If second-level cache is enabled, then all nodes in the cluster must be aware of each other, so that cache entries are properly invalidated across the cluster when changed. For example, see the documentation about how to do this with Infinispan as cache provider.
We'd like to use another L2 cache for our big JPA application. We are trying to achieve a shared cache between multiple servers.
We use Eclipselink as JPA implementation, and some legacy codes uses internal Eclipselink API's, so switching is not an option.
Coherence/Toplink Grid seems too expensive (4000$/cpu?).
Is there a way we could plug another cache implementation? Is something specified in JPA 2 (I can't find anything in the specs, but maybe I just misread it)? Proprietary (=Eclipselink specific) solutions are ok, as long as they are somewhat documented or simple enough (we don't want that to break).
Is there a way we could plug another cache implementation?
Did you investigate the use of the EclipseLink shared object cache that comes with EclipseLink? Going by the description, the shared object cache is not confined to a single EntityManager alone, and is available across the lifecycles of several Entity managers, i.e. across several transactions. It is of course, constrained to the lifecycle of an EntityManagerFactory, which may be as live as long as the application is running in the container.
The EclipseLink shared object cache is different from Oracle Coherence, and I believe it is not licensed and packaged separately, thereby making it available on all containers.
JPA does not specify a pluggable cache interface. I don't know if it ever will, but if it does, my bet is that it won't be until after the resurrected JSR-107 finishes defining a standard API to object caches, which JPA would then be able to use. It might also have to wait for JSR 347, which is defining another cache interface, whose relationship to JCache is somewhat unclear (there is open factional warfare between and within the groups, with some members of the 107 expert group trying to declare 347 an independent republic, and invade Mexico).
So, until then, you're at the mercy of your provider's cache interface. I am not an EclipseLink expert, but last time i looked, i couldn't see a pluggable second-level cache interface. In fact, i think only Hibernate and, of course, DataNucleus, have them.
Most cache implementations are not distributed (other than Coherence), just local.
EclipseLink already supports a share cache and cache coordination for caching in a cluster.
What cache do you intent to use, and what benefit do you intend to get from it?
EclipseLink does support integration with 3rd party caches, this API was created for the Coherence integration, although Coherence is the only cache that currently provides an integration.
I am currently working on a project that uses JPA (Toplink, currently) for its persistence. Currently, we are running a single application server, but, for redundancy, we would like to add a load balancer and another application sever (and possibly more as it grows).
First, I'm running into the issue of JPA caching. Since two processes will be updating the same database, the JPA cache returns the cached value rather than going to the database. I see how to turn that off, and the database itself implements a level of caching. Is turning off the cache completely the way to go here? I see the ways to tell JPA to always get from the database at a query level, but in a multi-server environment, it seems that you'll always want that to happen.
Along with this specific question, I'm interested in anyone out there who has implemented a JPA solution with multiple application servers and what problems arose during the implementation (and any suggestions you have).
Thanks much.
As you have found, you can disable the shared cache, see http://wiki.eclipse.org/EclipseLink/Examples/JPA/Caching or http://wiki.eclipse.org/EclipseLink/FAQ/How_to_disable_the_shared_cache%3F
There are also other options available in EclipseLink depending on your data and requirements.
A list of option include:
Disable shared cache
Enable cache coordination (see, http://www.eclipse.org/eclipselink/api/2.1/org/eclipse/persistence/config/PersistenceUnitProperties.html#COORDINATION_PROTOCOL)
Set a cache invalidation timeout (see, http://www.eclipse.org/eclipselink/api/2.1/org/eclipse/persistence/annotations/Cache.html#expiry%28%29)
Enable optimistic locking, this will ensure that any stale object cannot be updated, when an update on stale data occurs it will fail, and EclipseLink will automatically invalidate the object in the cache.
Investigate the Oracle TopLink integration of EclipseLink and Oracle Coherence to provide a distributed cache.
See also, http://en.wikibooks.org/wiki/Java_Persistence/Caching#Caching_in_a_Cluster
There is no perfect solution, the solution used normally depend on the data/class, normally an application has a set of read-only classes, read-mostly classes and write mostly classes. Personally I would enable the cache for the read-only with a 1 day timeout, enable the cache with cache coordination for the read-mostly, and disable the cache for the write mostly.
Our design has one jvm that is a jboss/webapp (read/write) that is used to maintain the data via hibernate (using jpa) to the db. The model has 10-15 persistent classes with 3-5 levels of depth in the relationships.
We then have a separate jvm that is the server using this data. As it is running continuously we just have one long db session (read only).
There is currently no intra-jvm cache involved - so we manually signal one jvm from the other.
Now when the webapp changes some data, it signals the server to reload the changed data. What we have found is that we need to tell hibernate to purge the data and then reload it. Just doing a fetch/merge with the db does not do the job - mainly in respect of the objects several layers down the hierarchy.
Any thoughts on whether there is anything fundamentally wrong with this design or if anyone is doing this and has had better luck with working with hibernate on the reloads.
Thanks,
Chris
A Hibernate session loads all data it reads from the DB into what they call the first-level cache. Once a row is loaded from the DB, any subsequent fetches for a row with the same PK will return the data from this cache. Furthermore, Hibernate gaurentees reference equality for objects with the same PK in a single Session.
From what I understand, your read-only server application never closes its Hibernate session. So when the DB gets updated by the read-write application, the Session on read-only server is unaware of the change. Effectively, your read-only application is loading an in-memory copy of the database and using that copy, which gets stale in due course.
The simplest and best course of action I can suggest is to close and open Sessions as needed. This sidesteps the whole problem. Hibernate Sessions are intended to be a window for a short-lived interaction with the DB. I agree that there is a performance gain by not reloading the object-graph again and again; but you need to measure it and convince yourself that it is worth the pains.
Another option is to close and reopen the Session periodically. This ensures that the read-only application works with data not older than a given time interval. But there definitely is a window where the read-only application works with stale data (although the design guarantees that it gets the up-to-date data eventually). This might be permissible in many applications - you need to evaluate your situation.
The third option is to use a second level cache implementation, and use short-lived Sessions. There are various caching packages that work with Hibernate with relative merits and demerits.
Chris, I'm a little confused about your circumstances. If I understand correctly, you have a both a web app (read/write) a standalone application (read-only?) using Hibernate to access a shared database. The changes you make with the web app aren't visible to the standalone app. Is that right?
If so, have you considered using a different second-level cache implementation? I'm wondering if you might be able to use a clustered cache that is shared by both the web application and the standalone application. I believe that SwarmCache, which is integrated with Hibernate, will allow this, but I haven't tried it myself.
In general, though, you should know that the contents of a given cache will never be aware of activity by another application (that's why I suggest having both apps share a cache). Good luck!
From my point of view, you should change your underline Hibernate cache to that one, which supports clustered mode. It could be a JBoss Cache or a Swarm Cache. The first one has a better support of data synchronization (replication and invalidation) and also supports JTA.
Then you will able to configure cache synchronization between webapp and server. Also look at isolation level if you will use JBoss Cache. I believe you should use READ_COMMITTED mode if you want to get new data on a server from the same session.
The most used practice is to have a Container-Managed Entity Manager so that two or more applications in the same container (ie Glassfish, Tomcat, Websphere) can share the same caches.
But if you don't use an Application container, because you use Play! for instance, then I would build some webservices in the primary Application to read/write consistently in the cache.
I think using stale data is an open door for disaster. Just like Singletons become Multitons, read-only applications are often a write sometimes.
Belt and braces :)