Thursday, February 6, 2014

Determining if a conditional update was applied w/ CQL Java-Driver


Sorry, I should have included this in my previous post on conditional updates.   One of the critical aspects to using conditional updates is determining whether the update was applied. Here is how you do it.

Given our previous update (all fluentized):
        
Update updateStatement = update(KEYSPACE_NAME, TABLE_NAME);
updateStatement.with(set(VALUE_NAME, 15)).where(eq(KEY_NAME, "DE")).onlyIf(eq(VALUE_NAME, 10));
this.executeAndAssert(updateStatement, "DE", 15);
When we execute the updateStatement with our session and examine the ResultSet returned:
LOG.debug("EXECUTING [{}]", statement.toString());
ResultSet results = clientFactory.getSession().execute(statement);
for (ColumnDefinitions.Definition definition : results.getColumnDefinitions().asList()) {
   for (Row row : results.all()) {
      LOG.debug("[{}]=[{}]", definition.getName(), row.getBool(definition.getName()));
   }
}
You'll notice that you get a ResultSet back that contains one row, with a column named "[applied]". The above code results in:
DEBUG EXECUTING [UPDATE mykeyspace.incrementaltable SET v=15 WHERE k='DE' IF v=10;]
DEBUG [[applied]]=[true]
Thus, to check to see if a conditional update is applied or not, you can use the concise code:
Row row = results.one();
if (row != null)
   LOG.debug("APPLIED?[{}]", row.getBool("[applied]"));

1 comment:

Brian O'Neill said...

UPDATE: In Cassandra, there is a constant that you can use so you don't need to hardcode the "[applied]" column name. Instead use:
ModificationStatement.CAS_RESULT_COLUMN.text