Write behind strategy may be very useful in certain cases. Using Coherence you can use this pattern in very smart way - synchronously replicate your data in memory over several nodes, then asynchronously write to slow external storage. This way you will not lose any data in case of single server outage. One unpleasant thing is that Coherence does not retain sequence of updates in certain cases, but wait, this is a distributed system after all :)
But there is a catch. Let's assume you are receiving message via JMS, put data to Coherence grid, start processing etc. But you do not want to acknowledge message until it is written to persistent storage.
While Coherence let your data survive single node failure, there could be network failure, logical bug in your system or outage of persistent storage your are writing to. You want to be consistent, you can process message, but you do not want to confirm its delivery (delete it from one persistent storage) until it is not written into another persistent storage.
So, is it possible to find which entries in cache are not persisted yet?
Yes, it is. Internally Coherence marks not-yet-stored entries with special flag. This flag is usually invisible for application, but we can access it using some low level Coherence API.
Below is StoreFlagExtractor returning FALSE for vulnerable entries.
public class StoreFlagExtractor extends AbstractExtractor implements PortableObject {
private static final long serialVersionUID = 20010915L;
public StoreFlagExtractor() {
}
@Override
public int compare(Object object1, Object object2) {
// make no sense
throw new UnsupportedOperationException();
}
@Override
public int compareEntries(com.tangosol.util.QueryMap.Entry entry1, com.tangosol.util.QueryMap.Entry entry2) {
// make no sense
throw new UnsupportedOperationException();
}
@Override
public Object extract(Object object) {
// decorator can be extracted only from binary entry
throw new UnsupportedOperationException();
}
@Override
@SuppressWarnings("rawtypes")
public Object extractFromEntry(java.util.Map.Entry entry) {
BinaryEntry binEntry = (BinaryEntry) entry;
Binary binValue = binEntry.getBinaryValue();
return extractInternal(binValue, binEntry);
}
@Override
public Object extractOriginalFromEntry(com.tangosol.util.MapTrigger.Entry entry) {
BinaryEntry binEntry = (BinaryEntry) entry;
Binary binValue = binEntry.getOriginalBinaryValue();
return extractInternal(binValue, binEntry);
}
private Object extractInternal(Binary binValue, BinaryEntry entry) {
if (ExternalizableHelper.isDecorated(binValue)) {
Binary store = ExternalizableHelper.getDecoration(binValue, ExternalizableHelper.DECO_STORE);
if (store != null) {
Object st = ExternalizableHelper.fromBinary(store, entry.getSerializer());
return st;
}
}
return Boolean.TRUE;
}
@Override
public void readExternal(PofReader paramPofReader) throws IOException {
// do nothing
}
@Override
public void writeExternal(PofWriter paramPofWriter) throws IOException {
// do nothing
}
}
UPDATE: Changes of STORE decoration are not triggering cache events (only backing map events) and index updates. Nor CQ nor indexes nor listener would not work.
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