HBase secondary index with observer coprocessor, .put on the index table results in recursion - indexing

Question

In HBase database I want to create a secondary index by using additional "linking" table. I have followed the example given in this answer: Create secondary index using coprocesor HBase
I am not very well familiar with the entire concept of HBase, and I had read some examples on the issue of creating secondary indexes. I am attaching the coprocessor to single table only, like this:
disable 'Entry2'
alter 'Entry2', METHOD => 'table_att', 'COPROCESSOR' => '/home/user/hbase/rootdir/hcoprocessors.jar|com.acme.hobservers.EntryParentIndex||'
enable 'Entry2'
The source code of it, is as follows:
public class EntryParentIndex extends BaseRegionObserver{
private static final Log LOG = LogFactory.getLog(CoprocessorHost.class);
private HTablePool pool = null;
private final static String INDEX_TABLE = "EntryParentIndex";
private final static String SOURCE_TABLE = "Entry2";
#Override
public void start(CoprocessorEnvironment env) throws IOException {
pool = new HTablePool(env.getConfiguration(), 10);
}
#Override
public void prePut(
final ObserverContext<RegionCoprocessorEnvironment> observerContext,
final Put put,
final WALEdit edit,
final boolean writeToWAL)
throws IOException {
try {
final List<KeyValue> filteredList = put.get(Bytes.toBytes ("data"),Bytes.toBytes("parentId"));
byte [] id = put.getRow(); //Get the Entry ID
KeyValue kv=filteredList.get( 0 ); //get Entry PARENT_ID
byte[] parentId=kv.getValue();
HTableInterface htbl = pool.getTable(Bytes.toBytes(INDEX_TABLE));
//create row key for the index table
byte[] p1=concatTwoByteArrays(parentId, ":".getBytes()); //Insert a semicolon between two UUIDs
byte[] rowkey=concatTwoByteArrays(p1, id);
Put indexput = new Put(rowkey);
//The following call is setting up a strange? recursion, resulting
//...in thesame prePut method invoken again and again. Interestingly
//...the recursion limits itself up to 6 times. The actual row does not
//...get inserted into the INDEX_TABLE
htbl.put(indexput);
htbl.close();
}
catch ( IllegalArgumentException ex) { }
}
#Override
public void stop(CoprocessorEnvironment env) throws IOException {
pool.close();
}
public static final byte[] concatTwoByteArrays(byte[] first, byte[] second) {
byte[] result = Arrays.copyOf(first, first.length + second.length);
System.arraycopy(second, 0, result, first.length, second.length);
return result;
}
}
This executes when I perform put on the SOURCE_TABLE.
There is a comment in the code (please seek it): "The following call is setting up a strange".
I set a debugging print in the log confirming that the prePut method is being executed only on the SOURCE_TABLE, and never on the INDEX_TABLE. Yet I don't understand why this strange recursion is happening despite in the coprocessor I only execute one put on the INDEX_TABLE.
I have also confirmed that the put action on the source table is again only one.

Answer 1

I have fixed my problem. It came out to be that I was adding multiple times the same observer mistakenly thinking that it is getting lost after Hbase restart.
Also the reason why the .put call to the INDEX_TABLE was not working is because I did not set a value to it, but only a rowkey, mistakenly thinking that this is possible. Yet HBase did not throw any excepiton whatsoever, just did not perform the PUT, no info given, which may be confusing for newcommers to this technology.