Cassandra provides row-level isolation where a transaction is atomic for a single query executed on one node. A transaction in Cassandra updates either all columns in a row or none of them. Complex multi-row or multi-query transactions are not supported. The implementation uses a copy-on-write approach utilizing a concurrent data structure called SnapTree to clone columns being updated, ensuring atomic and isolated updates at the row level for a single query.

1. What is Row Level Isolation on Cassandra
INTHEFOERST Co., Ltd. tomitakazutaka

2. What is meaning of Cassandra’s transaction?
One query is one transaction !
In Cassandra, a transaction is provided atomicity guarantees in executed
one query. Complex transactions isn’t supported.
And it is only by one node.

3. Ex)
CQL)
UPDATE CBench SET Tag1 = ‘data1’, Tag2=‘data2’ where KEY = ‘john smith’;
Atomic
Hector API)
cluster = HFactory.getOrCreateCluster("Intheforest Cluster",chc);
keyspace = HFactory.createKeyspace("CassandraBenchmark", cluster);
mutator = HFactory.createMutator(keyspace, StringSerializer.get());
mutator.addInsertion("venezia-20120613004300-000", "CBench",HFactory.createStringColumn("Tag1", "data1"))
.addInsertion("venezia-20120613004300-000", "CBench", HFactory.createStringColumn("Tag2", "data2")) Atomic
.addInsertion("venezia-20120613004300-000", "CBench",HFactory.createStringColumn("Tag3", "data3"));
mutator.execute();

4. Implementation
Read is not changed.
As long as I investigated ....
Perhaps….

5. Implementation
Write to Commitlog
Commitlog is not changed for Isolation.
As long as I investigated ....
Perhaps….

6. Implementation
Writing process to MemTable
org.apache.cassandra.db.RowMutation.apply()
↓
org.apache.cassandra.db.Table.apply()
↓
org.apache.cassandra.db.ColumnFamilyStore.apply()
↓
org.apache.cassandra.db. Memtable.put()
↓
org.apache.cassandra.db. Memtable. resolve()

7. Implementation
1.0.10
org.apache.cassandra.db. Memtable. resolve()
private void resolve(DecoratedKey key, ColumnFamily cf)
{
~~~~ Omission ~~~~
clonedCf.delete(cf);
for (IColumn column : cf.getSortedColumns())
clonedCf.addColumn(column.localCopy(cfs, allocator), allocator);
}
increment simply for every column.

8. Implementation
1.1.1
org.apache.cassandra.db.Memtable.resolve()
private void resolve(DecoratedKey key, ColumnFamily cf)
{
~~~~ Omission ~~~~
previous.addAll(cf, allocator, localCopyFunction);
}

9. Implementation
org.apache.cassandra.db.AbstractColumnContainer.addAll()
org.apache.cassandra.db.AtomicSortedColumns.addAll()
public void addAll(ISortedColumns cm, Allocator allocator, Function<IColumn, IColumn> transformation)
{
Holder current, modified;
main_loop:
do
{
current = ref.get();
DeletionInfo newDelInfo = current.deletionInfo;
if (newDelInfo.markedForDeleteAt < cm.getDeletionInfo().markedForDeleteAt)
newDelInfo = cm.getDeletionInfo();
modified = new Holder(current.map.clone(), newDelInfo);
SnapTree
for (IColumn column : cm.getSortedColumns())
{ (copy-on-write clone facilities)
modified.addColumn(transformation.apply(column), allocator);
// bail early if we know we've been beaten
if (ref.get() != current)
continue main_loop; CAS
}
}
(See:http://en.wikipedia.org/wiki/Compare-and-swap)
while (!ref.compareAndSet(current, modified));
}

10. Implementation
What’s SnapTree ?
SnapTree is a concurrent AVL tree with fast cloning.
See) http://ppl.stanford.edu/papers/ppopp207-bronson.pdf

11. Conclusion
 Notice the Cassandra’s transaction about the range.
 Cassandra's row level isolation is atomic by one
query and by one node

12. Thank you !