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Transactions in Ehcache

Introduction

BigMemory Go suports Global Transactions, with "xa_strict" and "xa" modes, and Local Transactions with "local" mode.

All or nothing

If a cache is enabled for transactions, all operations on it must happen within a transaction context otherwise a TransactionException will be thrown.

Transactional Methods

The following methods require a transactional context to run:

  • put()
  • get()
  • getQuiet()
  • remove()
  • getKeys()
  • getSize()
  • containsKey()
  • removeAll()
  • putWithWriter()
  • removeWithWriter()
  • putIfAbsent()
  • removeElement()
  • replace()

This list is applies to all transactional modes.

All other methods work non-transactionally but can be called on a transactional cache, either within or outside of a transactional context.

Change Visibility

The isolation level offered in BigMemory's Ehcache is READ_COMMITTED. Ehcache can work as an XAResource, in which case, full two-phase commit is supported. Specifically:

  • All mutating changes to the cache are transactional including put, remove, putWithWriter, removeWithWriter and removeAll.
  • Mutating changes are not visible to other transactions in the local JVM or across the cluster until COMMIT has been called.
  • Until COMMIT has been called, reads such as by cache.get(...) by other transactions return the old copy. Reads do not block.

When to use transactional modes

Transactional modes are a powerful extension of Ehcache allowing you to perform atomic operations on your caches and other data stores.

  • "local" — When you want your changes across multiple caches to be performed atomically. Use this mode when you need to update your caches atomically; that is, you can have all your changes be committed or rolled back using a straightforward API. This mode is most useful when a cache contains data calculated from other cached data..
  • "xa" — Use this mode when you cache data from other data stores, such as a DBMS orJMS, and want to do it in an atomic way under the control of the JTA API ("Java Transaction API") but without the overhead of full two-phase commit. In this mode, your cached data can get out of sync with the other resources participating in the transactions in case of a crash. Therefore, only use this mode if you can afford to live with stale data for a brief period of time.
  • "xa_strict" — Similar to "xa" but use it only if you need strict XA disaster recovery guarantees. In this mode, the cached data can never get out of sync with the other resources participating in the transactions, even in case of a crash. However, to get that extra safety the performance decreases significantly.

Requirements

The objects you are going to store in your transactional cache must:

  • implement java.io.Serializable — This is required to store cached objects when the cache is clustered with Terracotta but it is also required by the copy-on-read / copy-on-write mechanism used to implement isolation.
  • override equals and hashcode — Those must be overridden because the transactional stores rely on element value comparison. See ElementValueComparator and the elementValueComparator configuration setting.

Configuration

Transactions are enabled on a cache-by-cache basis with the transactionalMode cache attribute. The allowed values are:

  • "xa_strict"
  • "xa"
  • "local"
  • 'off"

The default value is "off". Enabling a cache for "xa_strict" transactions is shown in the following example:

<cache name="xaCache"
   maxEntriesLocalHeap="500"
   eternal="false"
   timeToIdleSeconds="300"
   timeToLiveSeconds="600"
   diskExpiryThreadIntervalSeconds="1"
   transactionalMode="xa_strict">
 </cache>

Transactional Caches with Spring

Note the following when using Spring:

  • If you access the cache from an @Transactional Spring-annotated method, then begin/commit/rollback statements are not required in application code because they are emitted by Spring.
  • Both Spring and Ehcache need to access the transaction manager internally, and therefore you must inject your chosen transaction manager into Spring's PlatformTransactionManager as well as use an appropriate lookup strategy for Ehcache.
  • The Ehcache default lookup strategy might not be able to detect your chosen transaction manager. For example, it cannot detect the WebSphere transaction manager (see Transactions Managers).
  • Configuring a <tx:method> with read-only=true could be problematic with certain transaction managers such as WebSphere.

Global Transactions

Global Transactions are supported by Ehcache. Ehcache can act as an {XAResouce} to participate in JTA transactions under the control of a Transaction Manager. This is typically provided by your application server, however you can also use a third party transaction manager such as Bitronix. To use Global Transactions, select either "xa_strict" or "xa" mode. The differences are explained in the sections below.

Implementation

Global transactions support is implemented at the Store level, through XATransactionStore and JtaLocalTransactionStore. The former decorates the underlying MemoryStore implementation, augmenting it with transaction isolation and two-phase commit support through an <XAResouce> implementation. The latter decorates a LocalTransactionStore-decorated cache to make it controllable by the standard JTA API instead of the proprietary TransactionController API. During its initialization, the Cache does a lookup the TransactionManager using the provided TransactionManagerLookup implementation. Then, using the TransactionManagerLookup.register(XAResouce), the newly created XAResource is registered. The store is automatically configured to copy every Element read from the cache or written to it. Cache is copy-on-read and copy-on-write.

Failure Recovery

In support of the JTA specification, only prepared transaction data is recoverable. Prepared data is persisted onto the cluster and locks on the memory are held. This means that non-clustered caches cannot persist transactions data, therefore recovery errors after a crash may be reported by the transaction manager.

Recovery

At any time after something went wrong, an XAResource might be asked to recover. Data that has been prepared might either be committed or rolled back during recovery. In accordance with XA, data that has not yet been prepared is discarded. The recovery guarantee differs depending on the XA mode.

xa Mode

With "xa", the cache doesn't get registered as an {XAResource} with the transaction manager but merely can follow the flow of a JTA transaction by registering a JTA {Synchronization}. The cache can end up inconsistent with the other resources if there is a JVM crash in the mutating node. In this mode, some inconsistency might occur between a cache and other XA resources (such as databases) after a crash. However, the cache data remains consistent because the transaction is still fully atomic on the cache itself.

xa_strict Mode

With "xa_strict", the cache always responds to the TransactionManager's recover calls with the list of prepared XIDs of failed transactions. Those transaction branches can then be committed or rolled back by the transaction manager. This mode supports the basic XA mechanism of the JTA standard.

Sample Apps

We have three sample applications showing how to use XA with a variety of technologies.

XA Sample App

This sample application uses JBoss application server. It shows an example using User managed transactions. Although most people will use JTA from within Spring or EJB container rather than managing it themselves, this sample application is useful as a demonstration.

The following snippet from our SimpleTX servlet shows a complete transaction.

Ehcache cache = cacheManager.getEhcache("xaCache");
UserTransaction ut = getUserTransaction();
printLine(servletResponse, "Hello...");
try {
   ut.begin();
   int index = serviceWithinTx(servletResponse, cache);
   printLine(servletResponse, "Bye #" + index);
   ut.commit();
} catch(Exception e) {
   printLine(servletResponse,
       "Caught a " + e.getClass() + "! Rolling Tx back");
   if(!printStackTrace) {
       PrintWriter s = servletResponse.getWriter();
       e.printStackTrace(s);
       s.flush();
   }
   rollbackTransaction(ut);
}

The source code for the demo can be checked out from the Terracotta Forge. A README.txt explains how to get the sample app going.

XA Banking Application

This application is to show a real world scenario. A Web app reads <account transfer> messages from a queue and tries to execute these account transfers. With JTA turned on, failures are rolled back so that the cached account balance is always the same as the true balance summed from the database. This app is a Spring-based Java web app running in a Jetty container. It has (embedded) the following components:

  • A message broker (ActiveMQ)
  • 2 databases (embedded Derby XA instances)
  • 2 caches (transactional Ehcache)

All XA Resources are managed by Atomikos TransactionManager. Transaction demarcation is done using Spring AOP's @Transactional annotation. You can run it with: mvn clean jetty:run. Then point your browser at: http://localhost:9080. To see what happens without XA transactions: mvn clean jetty:run -Dxa=no

The source code for the demo can be checked out from the Terracotta Forge. A README.txt explains how to get the sample app going.

Transaction Managers

Automatically Detected Transaction Managers

Ehcache automatically detects and uses the following transaction managers in the following order:

  • GenericJNDI (e.g. Glassfish, JBoss, JTOM and any others that register themselves in JNDI at the standard location of java:/TransactionManager
  • Weblogic (since 2.4.0)
  • Bitronix
  • Atomikos

No configuration is required; they work out of the box. The first found is used.

Configuring a Transaction Manager

If your Transaction Manager is not in the above list or you wish to change the priority, provide your own lookup class based on an implementation of net.sf.ehcache.transaction.manager.TransactionManagerLookup and specify it in place of the DefaultTransactionManagerLookup in ehcache.xml:

<transactionManagerLookup
  class= "com.mycompany.transaction.manager.MyTransactionManagerLookupClass"
  properties="" propertySeparator=":"/>

Another option is to provide a different location for the JNDI lookup by passing the jndiName property to the DefaultTransactionManagerLookup. The example below provides the proper location for the TransactionManager in GlassFish v3:

<transactionManagerLookup
  class="net.sf.ehcache.transaction.manager.DefaultTransactionManagerLookup"
  properties="jndiName=java:appserver/TransactionManager" propertySeparator=";"/>

Local Transactions

Local Transactions allow single phase commit across multiple cache operations, across one or more caches, and in the same CacheManager. This lets you apply multiple changes to a CacheManager all in your own transaction. If you also want to apply changes to other resources, such as a database, then you need to open a transaction to them and manually handle commit and rollback to ensure consistency. Local transactions are not controlled by a Transaction Manager. Instead there is an explicit API where a reference is obtained to a TransactionController for the CacheManager using cacheManager.getTransactionController() and the steps in the transaction are called explicitly. The steps in a local transaction are:

  • transactionController.begin() - This marks the beginning of the local transaction on the current thread. The changes are not visible to other threads or to other transactions.
  • transactionController.commit() - Commits work done in the current transaction on the calling thread.
  • transactionController.rollback() - Rolls back work done in the current transaction on the calling thread. The changes done since begin are not applied to the cache. These steps should be placed in a try-catch block which catches TransactionException. If any exceptions are thrown, rollback() should be called. Local Transactions has its own exceptions that can be thrown, which are all subclasses of CacheException. They are:
  • TransactionException - a general exception
  • TransactionInterruptedException - if Thread.interrupt() was called while the cache was processing a transaction.
  • TransactionTimeoutException - if a cache operation or commit is called after the transaction timeout has elapsed.

Introduction Video

Ludovic Orban, the primary author of Local Transactions, presents an introductory video on Local Transactions.

Configuration

Local transactions are configured as follows:

<cache name="sampleCache"
   ...
   transactionalMode="local"
 </cache>

Isolation Level

As with the other transaction modes, the isolation level is READ_COMMITTED.

Transaction Timeouts

If a transaction cannot complete within the timeout period, then a TransactionTimeoutException will be thrown. To return the cache to a consistent state you need to call transactionController.rollback(). Because TransactionController is at the level of the CacheManager, a default timeout can be set which applies to all transactions across all caches in a CacheManager. If not set, the default is 15 seconds. To change the defaultTimeout:

transactionController.setDefaultTransactionTimeout(int defaultTransactionTimeoutSeconds)

The countdown starts when begin() is called. You might have another local transaction on a JDBC connection and you might be making multiple changes. If you think it could take longer than 15 seconds for an individual transaction, you can override the default when you begin the transaction with:

transactionController.begin(int transactionTimeoutSeconds) { ...

Sample Code

This example shows a transaction which performs multiple operations across two caches.

CacheManager cacheManager = CacheManager.getInstance();
try {
   cacheManager.getTransactionController().begin();
    cache1.put(new Element(1, "one"));
   cache2.put(new Element(2, "two"));
   cache1.remove(4);
    cacheManager.getTransactionController().commit();
} catch (CacheException e) {
   cacheManager.getTransactionController().rollback()
}

Performance

Managing Contention

If two transactions, either standalone or across the cluster, attempt to perform a cache operation on the same element then the following rules apply:

  • The first transaction gets access
  • The following transactions block on the cache operation until either the first transaction completes or the transaction timeout occurs.

Note that when an element is involved in a transaction, it is replaced with a new element with a marker that is locked, along with the transaction ID. The normal cluster semantics are used. Because transactions only work with consistency=strong caches, the first transaction is the thread that manages to atomically place a soft lock on the Element. (This is done with the CAS based putIfAbsent and replace methods.)

What granularity of locking is used?

Ehcache uses soft locks stored in the Element itself and is on a key basis.

Performance Comparisons

Any transactional cache adds an overhead which is significant for writes and nearly negligible for reads. Compared to transactionalMode="off", the time it takes to perform writes will be noticeably slower with either "xa" or "local" specified, and "xa_strict" will be the slowest.

Accordingly, "xa_strict" should only be used where full guarantees are required, otherwise one of the other modes should be used.

FAQ

Why do some threads regularly time out and throw an exception?

In transactional caches, write locks are in force whenever an element is updated, deleted, or added. With concurrent access, these locks cause some threads to block and appear to deadlock. Eventually the deadlocked threads time out (and throw an exception) to avoid being stuck in a deadlock condition.

Is IBM Websphere Transaction Manager supported?

Mostly. "xa_strict" is not supported due to each version of Websphere being a custom implementation; that is, there is no stable interface to implement against. However, "xa", which uses TransactionManager callbacks and "local" are supported.

When using Spring, make sure your configuration is set up correctly with respect to the PlatformTransactionManager and the Websphere TM.

To confirm that Ehcache will succeed, try to manually register a com.ibm.websphere.jtaextensions.SynchronizationCallback in the com.ibm.websphere.jtaextensions.ExtendedJTATransaction. Simply get java:comp/websphere/ExtendedJTATransaction from JNDI, cast that to com.ibm.websphere.jtaextensions.ExtendedJTATransaction and call the registerSynchronizationCallbackForCurrentTran method. If you succeed, then Ehcache should too.

How do transactions interact with Write-behind and Write-through caches?

If your transactional enabled cache is being used with a writer, write operations are queued until transaction commit time. Solely a Write-through approach would have its potential XAResource participate in the same transaction. Write-behind is supported, however it should probably not be used with an XA transactional Cache, as the operations would never be part of the same transaction. Your writer would also be responsible for obtaining a new transaction. Using Write-through with a non XA resource would also work, but there is no guarantee the transaction will succeed after the write operations have been executed. On the other hand, any thrown exception during these write operations would cause the transaction to be rolled back by having UserTransaction.commit() throw a RollbackException.

Are Hibernate Transactions supported?

Ehcache is a "transactional" cache for Hibernate purposes. The net.sf.ehcache.hibernate.EhCacheRegionFactory supports Hibernate entities configured with <cache usage="transactional"/>.

How do I make WebLogic 10 work with transactional Ehcache?

WebLogic uses an optimization that is not supported by our implementation. By default WebLogic 10 spawns threads to start the Transaction on each XAResource in parallel. Because we need transaction work to be performed on the same Thread, you must turn off this optimization by setting parallel-xa-enabled option to false in your domain configuration:

<jta>
  ...
  <checkpoint-interval-seconds>300</checkpoint-interval-seconds>
  <parallel-xa-enabled>false</parallel-xa-enabled>
  <unregister-resource-grace-period>30</unregister-resource-grace-period>
...
</jta>

How do I make Atomikos work with the Ehcache "xa" mode?

Atomikos has a bug which makes the "xa" mode's normal transaction termination mechanism unreliable, There is an alternative termination mechanism built in that transaction mode that is automatically enabled when net.sf.ehcache.transaction.xa.alternativeTerminationMode is set to true or when Atomikos is detected as the controlling transaction manager. This alternative termination mode has strict requirement on the way threads are used by the transaction manager and Atomikos's default settings won't work. You have to configure the following property to make it work:

com.atomikos.icatch.threaded_2pc=false