I realize that the other night in my presentation on Google App Engine and Django I may not have clearly stated how reads and transactions are related. If you perform a read outside of a transcation the value on disk will be returned. If you perform a read within a transaction, any uncommitted transactions are rolled forward before returning the result.

Remember that transactions are entered in the journal of BigTable (in memory) and committed (in memory) sequentially. After the commit they are applied to disk

In most cases, since operations take place so quickly this distinction will not be significant. However, if you have logic where your reads are based on the state of a previous write transaction, put your read in the transaction and ask yourself if you relly need to make your read request stateful .

You can find the slides at http://docs.google.co....