Nonstop Exceptions
Typically, application code may access the cache frequently and at various points. Therefore, with a nonstop cache, where your application could encounter NonStopCacheExceptions is difficult to predict. The following sections provide guidance on when to expect NonStopCacheExceptions and how to handle them.
When is NonStopCacheException Thrown?
NonStopCacheException is usually thrown when it is the configured behavior for a nonstop cache in a client that disconnects from the cluster. In the following example, the exception would be thrown 30 seconds after the disconnection (or the "cluster offline" event is received):
<nonstop immediateTimeout="false" timeoutMillis="30000">
<timeoutBehavior type="exception" />
</nonstop>
However, under certain circumstances the NonStopCache exception can be thrown even if a nonstop cache's timeout behavior is not set to throw the exception. This can happen when the cache goes into nonstop mode during an attempt to acquire or release a lock. These lock operations are associated with certain lock APIs and special cache types such as Explicit Locking, BlockingCache, SelfPopulatingCache, and UpdatingSelfPopulatingCache.
A NonStopCacheException can also be thrown if the cache must fault in an element to satisfy a get() operation. If the Terracotta Server Array cannot respond within the configured nonstop timeout, the exception is thrown.
A related exception, InvalidLockAfterRejoinException, can be thrown during or after client rejoin. This exception occurs when an unlock operation takes place on a lock obtained before the rejoin attempt completed. For more information about rejoin, see "Using Rejoin to Automatically Reconnect Terracotta Clients" in the BigMemory Max High-Availability Guide.
Using try-finally Blocks to Release Locks
To ensure that locks are released properly, application code using Ehcache lock APIs should encapsulate lock-unlock operations with try-finally blocks:
myLock.acquireLock();
try {
// Do some work.
} finally {
myLock.unlock();
}
Handling Nonstop Exceptions
Your application can handle nonstop exceptions in the same way it handles other exceptions. For nonstop caches, an unhandled-exceptions handler could, for example, refer to a separate thread any cleanup needed to manage the problem effectively.
Another way to handle nonstop exceptions is by using a dedicated Ehcache decorator that manages the exception outside of the application framework. The following is an example of how the decorator might operate:
try { cache.put(element); }
catch(NonStopCacheException e) {
handler.handleException(cache, element, e);
}