Apache Cassandra: Iterate over all columns in a row

Recently I have been using Cassandra for one of my projects, and one of the needs is to iterate over all columns of a row. Each column represents an individual data, of type identified by row id, and keeps changing. So I can’t simply use a set of known column names. Using the setRange call on a SliceQuery and setting a large count is also not an option, since Cassandra will try to load the entire set of columns into memory. Instead I’ve written this iterator which takes a query on which row key and column family has been set, and will load columns as they are requested. By default it loads a 100 columns at a time. You could make it take the count as a parameter and all, but this works for me for now.

	// Made by Nikhil Marathe <nsm.nikhil@gmail.com>
	// This code is in the public domain.
	import java.util.Iterator;
	import java.util.List;
	import me.prettyprint.hector.api.beans.ColumnSlice;
	import me.prettyprint.hector.api.beans.HColumn;
	import me.prettyprint.hector.api.exceptions.HectorException;
	import me.prettyprint.hector.api.query.SliceQuery;
	public class AllColumnsIterator<N, V> implements Iterator<HColumn<N, V>> {
	private N start;
	private int count;
	Iterator<HColumn<N, V>> columnsIterator;
	SliceQuery<?, N, V> query;
	private boolean isLastIteration;
	public AllColumnsIterator(SliceQuery<?, N, V> query) {
	start = null;
	count = 100;
	columnsIterator = null;
	this.query = query;
	isLastIteration = false;
	}
	public Iterator<HColumn<N, V>> iterator() {
	return this;
	}
	public boolean hasNext() {
	if (columnsIterator == null || !columnsIterator.hasNext()) {
	if (isLastIteration)
	return false;
	if (!fetchMore())
	return false;
	}
	return true;
	}
	public HColumn<N, V> next() {
	return columnsIterator.next();
	}
	private boolean fetchMore() {
	try {
	query.setRange(start, null, false, count);
	ColumnSlice<N, V> slice = query.execute().get();
	List<HColumn<N, V>> columns = slice.getColumns();
	int origSize = columns.size();
	if (origSize == 0) {
	return false;
	}
	if (origSize >= count)
	start = columns.remove(columns.size()-1).getName();
	columnsIterator = columns.iterator();
	if (origSize < count)
	isLastIteration = true;
	return true;
	} catch (HectorException e) {
	return false;
	}
	}
	public void remove() {
	throw new UnsupportedOperationException();
	}
	}

view raw AllColumnsIterator.java hosted with ❤ by GitHub

The one ‘problem’ with this is the removal of the last column to ensure that there are no duplicates, but still having a start point for the next query. This is because each column is independent, so you cannot ask a column who it’s next neighbour is and start the next query from there. If anybody has a tip to make it more elegant, I’d love to hear it.