Frequently Asked Questions About JBossCache
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1. Frequently Asked Questions About JBossCache Ben Wang lt ben wang jboss com gt Bela Ban lt bela jboss com gt June 2005 This is frequently asked questions regarding to JBossCache 1 General Information Q What is JBossCache A JBossCache is a replicated and transactional cache It is replicated since multiple JBossCache instances can be distributed within the same JVM or across several JVMs whether they reside on the same machine or on different machines on a network and data is replicated across the whole group It is transactional because a user can configure a JTA transaction manager and make the cache operation transactional Note that the cache can also be run without any replication this is the local mode Currently it consists of two components TreeCache and TreeCacheAop TreeCache is a tree structured cache that provides replication and transaction context while TreeCacheAop extends the functionality of TreeCache but behaves as a true object cache providing transparent and finer grained object mapping into in ternal cache Q Who are the JBossCache developers A JBossCache has been developed by Bela Ban Ben Wang Harald Gliebe Manik Surtani and Brian Stans berry Currently Bela is the overall lead while Ben leads JBossCacheAop Q sie What is the license for JBossCache A JBossCache is licensed under LGPL Q Where can I download the package A You can either obtain it from JBoss cvs repository2. UserTransaction ER new TreeCache config new PropertyConfigurator config configure tree META INF replSync service xml tx begin tree put fqn key value tx commit Oe How do I control the cache locking level JBossCache let you control the cache locking level through the transaction isolation level This is configured through the attribute IsolationLevel Currently JBossCache employs pessimistic locking internally And the transaction isolation level from the pessimist locking corresponds to JDBC isolation levels namely NONE READ_UNCOMMITTED READ_COMMITTED REPEATABLE_READ and SERIALIZABLE For details please refer to the user manual How does the write lock applies to a fqn node say org jboss test First of all JBossCache has a notion of root that serves as a starting point for every navigational operation The default is since the default separator is for the fqn The locking then is applied to the node under root for example org no locking Furthermore let s say when JBossCache needs to apply a write lock on node org jboss test it will first try to obtain read lock from the parent nodes recursively in this example org and org jboss Only when it succeeds then it will try to obtain a write lock on org jboss test Can I use the cache locking level even without a transaction context Yes JBossCache controls the indiv
3. although it is also on our to do list Our internal implementation does use a similar 2PC procedure to co ordinate transaction among different instances What kind of TransactionManager is supported by JBossCache JBossCache supports any TransactionManager that is JTA compliant such as JBossTM A user can configure the transaction manager through the configuration xml setting JBossCache also has a built in dummy trans action manager org jboss cache tm DummyTransactionManager for testing purpose only But note that DummyTransactionManager is not thread safe though I e it does not support concurrent transaction Instead only one transaction is allowed at one time How do I setup the cache to be transactional You either use the default JBoss TransactionManager to run JBossCache inside JBoss or you have to im plement the TransactionManagerLookup interface and return an instance of your javax transaction TransactionManager The configuration property TransactionManagerLookupClass defines the class to be used by the cache to fetch a reference to a TransactionManager It is trivial to imple ment this class to support other TransactionManagers Once this attribute is specified the cache will look up the transaction context from this transaction manager Frequently Asked Questions About JBossCache For the client code here is a snippet to start and commit a transaction tx UserTransaction new InitialContext prop lookup
4. different names you will need to use the class proxy to insert the second time to ensure both are managed by the cache Here is the code snippet Ar Ia cache putObject list list Put the list under the aop cache ArrayList myList List cache getObject list we are getting a dynamic proxy instead List add second it works now my cache putObject list_alias myList Note you will need to use the proxy here List remove second my rayList list new ArrayList j Sh ada first OK so I know I am supposed to use proxy when manipulating the Collection classes once they are managed by the cache But what happens to Pojos that share the Collection objects e g a List instance that is shared by 2 Pojos Pojos that share Collection instance references will be handled by the cache automatically That is when you ask the Cache to manage it the Cache will dynamically swap out the regular Collection referecens with the dynamic proxy ones As a result it is transparent to the users What happens when my aspectized POJO has field members that are of Collection class When a user puts a POJO into the cache through the call putobject it will recursively map the field mem bers into the cache store as well When the field member is of a Collection class e g List Set or Map TreeCacheAop will first map the collection into cache Then it will swap out dynamically the field refere
5. guide for more de tails To solve the second kind of issue the only solution that we know of is to cache serialized byte code and only de serialize it during every object get and this will be expensive That is during a put operation the object instance will be serialized and therefore can be deserialized safely by a foreign classloader However the performance penalty of this approach is quite severe so in general another local in vm version will need to be used as a near line cache Note also that each time the serialized bytes are deserialized a new instance of the object is created To help with this kind of handling JBoss has a utility class called Marshalledvalue that wraps around the serialized object Here is a code snippet that illustrates how you can create a wrapper around JBossCache to handle the classloader issue import org jboss invocation MarshalledValue Frequently Asked Questions About JBossCache public class CacheService private TreeCache cache_ public object get Fqn fqn String key return getUnMarshalledValue cache_ get fqn key public object set Fqn fqn String key Object value cache_ put fqn key getMarshalledValue value return value only if successful private Object getUnMarshalledValue object value assuming we use the calling thread context classloader return MarshalledValue value get private Object getMarshalledValue
6. the Collection classes upon the putobject call is invoked The proxy will delegate the operations to a cache interceptor that implements the actual Collection classes APIs That is the system classes won t be invoked when used in TreeCacheAop Internally the cache interceptor implements the APIs by direct interaction with respect to the underlying cache store Note that this can have implication in performance for certain APIs For example both arrayL ist and LinkedList will have the same implementation Plan is currently underway to optimize these APIs How do I use List Set and Map dynamic proxy TreeCacheAop supports classes extending from List Set and Map without users to declare them aspect ized It is done via a dynamic proxy Here is a code snippet to use an ArrayList proxy class ArrayList list new ArrayList Mist 360 rir st iy cache putObject list test list Put the list under the aop cache list add second Won t work since AOP intercepts the dynamic proxy not the original POJO ArrayList myList List cache getObject list test we are getting a dynamic proxy instead 11 Frequently Asked Questions About JBossCache my my my List add second it works now ae SA ela st yelt 6 List remove third A What is the propery way of assigning two different keys with Collection class object Let s say you want to assign a List object under two
7. Object value return new MarshalledValue value Does JBossCache currently support pre event and post event notification Yes Starting with release 1 2 4 JBossCache has introduced ExtendedTreeCacheListener which takes in con sideration pre and post event notification See documentation for more details Note that TreeCacheListener and ExtendedTreeCacheListener will be merged into TreeCacheListener in release 1 3 How do I implement a custom listener to listen to TreeCache events You create a class myListener that extends AbstractTreeCacheListener and provide concrete implementa tion for the node events that you are interested in Then you add this listener to the TreeCache instance on startup to listen to the events as they occur by calling TreeCache addTreeCacheListener my Listener public class MyListener extends AbstractTreeCacheListener public void nodeModify Fqn fqn boolean pre boolean isLocal if log isTraceEnabled if pre log trace Event DataNode about to be modified fqn else log trace Event DataNode modified fqn Frequently Asked Questions About JBossCache 3 TreeCacheAop Q What is TreeCacheAop TreeCacheAop currently implemented as a sub class of TreeCache is a fine grained field level replicated and transactional object oriented cache By object oriented we mean that the cache 1 automatically manages object mapping and relation
8. ache on 3 2 releases A Yes The JBossCache source code is also up to date on the jboss 3 2 cvs branch However only TreeCache is supported there since JBossAop lives only under jboss head Q Can I run multiple JBossCache instances on the same VM A Yes There are couple scenarios that you want to run multiple instances of JBossCache For example you want to run multiple local cache instances with each instance has their own configuration e g different cache policy In this case you will need to multiple xml configuration files Q Can TreeCache run as a second level cache inside Hibernate A Yes Since Hibernate 2 1 release you can configure it to use JBossCache namely TreeCache as a second level cache For details see Hibernate documentation Note that since Hibernate3 0 2 and JBossCache1 2 2 we have fixed a critical bug that depends on the usage pattern can cause deadlock during query caching Q How can I configure JBossCache Frequently Asked Questions About JBossCache You can configure the JBossCache through a configuration xml file Or you can set it programatically through its get set methods Check with the documentation for both examples In the configuration xml file there are tags such as class MBean etc What are these These are tags for deploying JBossCache as a JBoss MBean service For reason of compatibility we have kept them in the standalone package as well specifically the MBean tag If y
9. ame and the class name E g instead of lt mbean code org jboss cache TreeCache name jboss cache service TreeCache gt lt mbean code org jboss cache aop TreeCacheAop name jboss cache service TreeCacheAop gt Q What is the difference between TreeCache and TreeCacheAop Think of TreeCacheAop as a TreeCache on steroid Seriously both are cache store However while TreeCache only provides pure object reference storage e g put FON fqn Objecy key Object value TreeCacheAop goes beyond that and performs fine grained field level replication object mapping and rela tionship management for a user behind the scene As a result if you have a complex object systems that you Frequently Asked Questions About JBossCache would like to cache you can have TreeCacheAop manages for you You simply treat your object systems as they are residing in memory e g use your regular POJO methods without worrying about cache manage ment Furthermore this is true in replication mode as well Can I pre compile the aop classes such that I don t need to use the system classloader and jboss aop configur ation xml Yes The latest versions of JBossCache has a pre compiler option called aopc You can use this option to pre compile your aspectized POJO Once the classes have been byte code generated it can be treated as regular class files i e you will not need to include any jboss aop xm1 that specifie
10. ess to frequently accessed data This is all configured through XML and the programmer doesn t have to take care of loading and eviction JBossCache currently ships with 3 CacheLoader implementations e FileCacheLoader this implementation uses the file system to store and retrieve data JBossCache nodes are mapped to directories subnodes to subdirectories etc Attributes of a node are mapped to a file data inside the directory e BdbjeCacheLoader this implementation is based on the Sleepycat Java Edition database a fast and effi cient transactional database It uses a single file for the entire store Note that if you use Sleepycat s Cach eLoader with JBossCache and ship your product you have to acquire a commercial license from Sleepycat see http www sleepycat com jeforjbosscache for details e JDBCCacheLoader this implementation uses the relational database as the persistent storage 14 Frequently Asked Questions About JBossCache Can writing to CacheLoaders be asynchronous As of JBossCache 1 2 4 yes Set the CacheLoaderAsynchronous property to true See the JBossCache docu mentation for a more detailed discussion By default though all cache loader writes are synchronous and will block Can I write my own CacheLoader Yes A CacheLoader is a class implementing org jboss cache loader CacheLoader It is configured via the XML file see JBossCache and Tutorial documentation Does a CacheLoader have to use a pe
11. idual node locking behavior through the isolation level semantics This means even if you don t use a transaction you can specify the lock level via isolation level You can think of the node locking behavior outside of a transaction as if it is under transaction with auto_commit on During replication mode e g REPL_SYNC and REPL_ASYNC how often does the cache do the replica tion If the updates are under transaction then the replication happens only when the transaction is about to com mit actually during the prepare stage internally That is it will be a batch update However if the opera tions are not under transaction context then each update will trigger replication Note that this has perform ance implication if network transport is heavy it usually is How can do a mass removal Frequently Asked Questions About JBossCache If you do a cache remove root it will recursively remove all the entries under root Can I monitor and manage the JBossCache If you are running JBossCache as a MBean service inside JBoss you can monitor and manage through the jmx console We plan to add more attributes such as cache hit miss numbers in the future release Does JBossCache support partitioning Not right now JBossCache does not support partitioning that a user can configure to have different set of data residing on different cache instances while still participating as a replication group Does JBossCache handle
12. imit timeToLiveInSeconds Age in seconds for the node to be evicted in the queue 0 denotes no limit I have turned on the eviction policy why do I still get out of memory OOM exception OOM can happen when the speed of cache access exceeds the speed of eviction policy handling timer Evic tion policy handler will wake up every wakeUpIntervalInSeconds seconds to process the eviction event queue And the queue size is fixed at 20000 now So when the queue size is full it will create a backlog and cause OOM to happen unless the eviction timer catches up To address this problem in addition to increase the VM heap size you can also reduce the wakeUpIntervaleInSeconds so the timer thread processes the queue more frequently We will also externalize the queue size so it will be configurable in the next release 5 CacheLoader What is a CacheLoader A CacheLoader is the connection of JBossCache to a persistent data store The CacheLoader is called by JBossCache to fetch data from a store when that data is not in the cache and when modifications are made to data in the cache the CacheLoader is called to store those modifications back to the store In conjunction with eviction policies JBossCache with a CacheLoader allows a user to maintain a bounded cache for a large backend datastore Frequently used data is fetched from the datastore into the cache and the least used data is evicted in order to provide fast acc
13. in SourceForge net or the standalone package also in SourceForge net Q How do I build the JBossCache myself from cvs Frequently Asked Questions About JBossCache JBossCache now is a separate standalone project from jboss head Check ht tp www jboss org wiki Wiki jsp page C VSRepository for details The module name is JBossCache To build do sh build sh jar This will produce jboss cache jar in the dist 1lib directory Or if you want to build the standalone package do sh build sh dist this will produce dist jboss cache dist zip What JVM is supported by JBossCache JBossCache has been tested and supported on JDK1 4 and JDK5 0 On jboss 3 2 cvs tree it also compiles on JDK1 3 but there is no official support for it though How do I know the version of JBossCache that I am using Since release 1 2 you can check the jar version by running java jar jboss cache jar org jboss cache Version Can I run JBossCache outside of JBoss Application Server Of course JBossCache comes with two flavors e Integrated with JBoss server as a MBean service e Asa standalone that can run outside of JBoss server such as under Weblogic or Websphere Of course it can also run under a standalone Java process i e outside J2EE context Where can I report bugs or problems Please report any bugs or problems to JBossCache forum http www jboss org index html module bb amp op viewforum amp f 157 2 JBossCache in general and T
14. l be in sync However you still can run eviction policy with cache mode set to either REPL_SYNC or REPL_ASYNC Depends on your use case you can set multiple cache instances to have their own eviction policy of which is operated locally or just have selected instance with eviction policy activated Also note that with cache loader option an locally evicted node can also be persisted to the backend store and a user can retrieve it from the store later on Does JBossCache support Region Yes JBossCache has the notion of region where a user can configure the eviction policy parameters e g maxNodes Or timeToIdelSeconds A region in JBossCache denotes a portion of tree hierarchy e g a fully qualified name Fon For example a user can define org jboss and org foocom as two separate regions But note that you can configure the re gion programatically now i e everything has to be configured through the xml file What are the EvictionPolicyConfig tag parameters for org jboss cache eviction LRUPolicy They are Table 1 Parameters wakeUpIntervalInSeconds Interval where the clean up thread wakes to process the sitting queue and sweep away the old data region A area where each eviction policy parameters are specified Note that it needs a minimum of 13 Frequently Asked Questions About JBossCache _default region maxNodes Max number of nodes allowed in the eviction queue 0 means no l
15. n string have to be given JBossCache will then instantiate a CacheLoader See JBossCache documentation for details 15 Frequently Asked Questions About JBossCache Do I have to pay to use Sleepycat s CacheLoader Not if you use it only for personal use As soon as you distribute your product with BdbjeCacheLoader you have to purchase a commercial license from Sleepycat See details at ht tp www sleepycat com jeforjbosscache 16
16. nce with an corresponding proxy reference This is necessary so that an internal update on the field member will be intercepted by the cache 4 Eviction Policy Q Does JBossCache support eviction policy Yes JBossCache currently implements a LRU eviction policy for both TreeCache org jboss cache eviction LRUPolicy and TreeCacheAop org jboss cache eviction AopLRUPolicy Users can also plug in their own eviction policy algorithms See user manual for details Currently there is user contributed policy called FIFOPolicy that evicts the node 12 Frequently Asked Questions About JBossCache based on FIFO principle only Why can t I use org jboss cache eviction LRUPolicy for TreeCacheAop as well For TreeCacheAop you will need to use org jboss cache eviction AopLRUPolicy because AOP has its eviction algorithm although is LRU but has totally different notion of an object for example Does JBossCache s implemented LRU eviction policy operates in replication mode Yes and no The LRU policy only operates in local mode That is nodes that are only evicted locally This may cause the cache contents not to be synchronized temporarily But when a user tries to obtain a cache content of an evicted node and finds out that is null e g get returns null it should get it from other data source and re populate the data in the cache During this moment the node content will be propagated and the cache con tent wil
17. ou run in standalone mode JBossCache will ignore these elements What is the difference between the different cache modes JBossCache has three different cache modes i e LOCAL REPL_SYNC and REPL_async If you want to run JBossCache as a single instance then you should set the cache mode to Locat so that it won t invoke replica tion layer If you want to have synchronous replication among different JBossCache instances you set it to REPL_SYNC Finally for asynchronous replication use AYSNC_REPL Note that asyNc_REPL is non blocking This can be useful when you want to have another JBossCache serving as a mirror or backup How does JBossCache s replication mechanism work JBossCache leverage JGroups as a replication layer A user can configure the cluster of JBossCache in stances by sharing the same cluster name cluster name There is also an option of whether to populate the cache data upon starting a new instance in the clusterConfig attribute Note that once all instances join the same replication group every replication change is propagated to all par ticipating members There is no mechanism for sub partitioning where some replication can be done within only a subset of members This is in our to do list If I have the need for different treecache properties e g CacheMode and IsolationLevel do I simply need to create multiple treecache instances with the appropriate configuration Yes All the above mentioned proper
18. reeCache Q How do I deploy JBossCache as a MBean service To deploy JBossCache as a MBean inside JBoss you can copy the configuration xml file over to the deploy directory from all configuration whereby the necessary jars are present Under the standalone package etc META INF directory there are example configuration files for different cache modes that can be used to deploy JBossCache as well How do I know if my JBossCache MBean has been deployed To verify that your JBossCache MBean is deployed correctly you can first check the log output under the Frequently Asked Questions About JBossCache command console Next you can verify it from JBoss JMX console Look for jboss cache domain Q How do I access the JBossCache MBean A Accessing the JBossCache MBean is just like accessing any JBoss MBean Here is a code snippet import org jboss mx util MBeanServerLocator import javax management MBeanServer import javax management ObjectName public init throws Exception try cacheService new ObjectName jboss cache service TreeCache server MBeanServerLocator locate catch Exception ex throw new CreateException ex toString public Object get String fqn String key throws Exception return server invoke cacheService get new Object fqn key new String String class getName Object class getName Q Can I run JBossC
19. rsistent store No a CacheLoader could for example fetch and possibly store its data from a webdav capable webserver Another example is a caching proxy server which fetches contents from the web Note that an implementa tion of CacheLoader may not implement the store functionality in this case but just the load functionality What can I use a CacheLoader for Some applications e HTTP sessions can be persisted besides being replicated by JBossCache The CacheLoader can be con figured to be shared or unshared meaning that every node in a cluster has its own local store It is also possible to attach a CacheLoader to just one of the nodes e Simple persistence for POJOs Use of JBossCache aop and a local CacheLoader persist POJOs transpar ently into the store provided by the CacheLoader e Highly available replicated and persisted data store The service is up as long as at least 1 node is run ning but even if all nodes are taken offline when the first node is started again the data previously saved will still be available e g a shopping cart e A caching web proxy a la Squid all data are contents of URLs users access the proxy and if the URL is not in the cache the CacheLoader fetches it from the web This could actually be a replicated and transactional version of Squid How do I configure JBossCache with a CacheLoader Through XML both the fully qualified classname of the CacheLoader and its configuratio
20. s the advisable POJO and to spe cify the JBossAop system class loader For an example of how to use aopc please see the build xml in the standalone package look for the aopc Ant target How do use aopc on multiple module directories In aopc you specify the src path for a specific directory To pre compile multiple ones you will need to in voke aopc multiple times What s in the jboss aop xm1 configuration jboss aop xml is needed for POJO instrumentation In jboss aop xm1 you can declare your POJO e g Person to be prepared a JBOssAop term to denote that the object will be aspectized by the system After this declaration JBossAop will invoke any interceptor that associates with this POJO TreeCacheAop will dynamically add an org jboss cache aop CacheInterceptor to this POJO to perform object mapping and relationship management Note that to add your POJO you should declare all the fields to be prepared as in the example Can I use annotation instead of the xml declaration Yes starting with JBossCache1 2 3 you can use annotation to instrument your POJO Check the documenta tion for details However keep in mind JBossCache only supports JDK1 4 so far So JDK5 0 style annota tion is not fully supported yet As a result you will need a JDK1 4 annotation pre compiler provided by JBossAop Is it possible to store the same object multiple time but with different FQN paths Like foo byName and foo bylId Yes
21. ship for a client under both local and replicated cache mode 2 provides support for inheritance relationship between aspectized POJOs By leveraging the dynamic AOP in JBossAop it is able to map a complex object into the cache store preserve and manage the object relation ship behind the scene During replication mode it performs fine granularity i e on a per field basis update and thus has the potential to boost cache performance and minimize network traffic From a user perspective once the object is managed by the cache all cache operations are transparent Therefore all the usual in VM POJO method semantics are still preserved providing ease of use For ex ample if a POJO has been put in TreeCacheAop by calling putObject for example then any get set meth od will be intercepted by TreeCacheAop to provide the data from the cache Does TreeCacheAop have all the functional capabilities of TreeCache Yes TreeCacheAop extends TreeCache so it has all the same features TreeCache such as cache mode trans action isolation level and eviction policy Can I run TreeCacheAop in JBoss3 2 x application server Yes and no Yes since JBossAop can also be back ported to 3 2 x see JBossAop wiki for details However it will take some effort Therefore the recommended JBoss version is 4 x to run TreeCacheAop Can TreeCacheAop run as a MBean as well Yes It is almost the same as TreeCache MBean The only difference is the object n
22. the concept of application classloading inside say a J2EE container Application specific classloading is used widely inside a J2EE container For example a web application may require a new classloader to scope a specific version of the user library However by default JBoss Cache is agnostic to the classloader In general this leads to two kinds of problems e Object instance is stored in cachel and replicated to cache2 As a result the instance in cache2 is created by the system classloader The replication may fail if the system classloader on cache2 does not have ac cess to the required class Even if replication doesn t fail a user thread in cache2 may not be able to ac cess the object if the user thread is expecting a type defined by the application classloader e Object instance is created by thread 1 and will be accessed by thread 2 with two different classloaders JBossCache has no notion of the different classloaders involved As a result you will have a classCas tException This is a standard problem in passing an object from one application space to another JBossCache just adds a level of indirection in passing the object To solve the first kind of issue in 1 2 4 JBossCache introduced the concept of a TreeCacheMarshaller Ba sically this allows application code to register a classloader with a portion of the cache tree for use in hand ling objects replicated to that portion See the TreeCacheMarshaller section of the user
23. ties are per cache instance Therefore you will need a separate JBoss Cache instance Does the TreeCache config clusterName have any relation to the JBoss cluster Part itionName Yes They are both JGroups group name Besides the notion of channel is JGroups it also can partition the channel into different group names When using multiple JGroups based components cluster service xml treecache multiple instances what is the correct valid way to configure those components to make sure my multicast addresses don t conflict Frequently Asked Questions About JBossCache There are two parameters to consider multicast address plus port and the group name At minimum you will have to run components on different group name But whether to run them on the same channel depends upon whether the communication performance is critical for you or not If it is then it d be the best to run them on different channels Does JBossCache currently supports cache persistence storage Yes Starting with release 1 1 JBossCache has a CacheLoader interface that supports the cache persistency See below Does JBossCache currently supports cache passivation overflow to a data store Yes Starting with release 1 2 4 JBossCache uses the CacheLoader to support cache passivation overflow See documentation on how to configigure and use this feature Is JBossCache thread safe Yes It is Does JBossCache support XA 2PC transaction now No
24. you can use TreeCacheAop to do that It supports the notion of object reference TreeCacheAop man ages the unique object through association of the dynamic cache interceptor 10 Frequently Asked Questions About JBossCache Do I need to declare all my objects prepared in jboss aop xml Not necessarily If there is some objects that you don t need the cache to manage it for you you can leave it out of the declaration The cache will treat this object as a primitive type However the object will need to implement Serializable interface for replication though Can the cache aop intercept update via reflection No The update via reflection will not be intercepted in JBossAop and therefore TreeCacheAop will not be able to perform the necessary synchronization When I declared my POJO to be aspectized what happens to the fields with transient static and final modifiers TreeCacheAop currently will ignore the fields with these modifiers That is it won t put these fields into the cache and thus no replication either What are those keys such as JBoss internal class and AOPInstance They are for internal use only User should ignore these key and values in the node hashmap What about Collection classes Do I need to declare them prepared No Since the Collection classes such as ArrayList are java util classes aop by default won t instrument these classes Instead TreeCacheAop will generate a dynamic class proxy for
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