The High Replication Datastore provides the highest level of availability for your reads and writes, at the cost of increased latency for writes and changes in consistency guarantees in the API," writes Kevin Gibbs in the announcement. "API. The High Replication Datastore increases the number of data centers that maintain replicas of your data by using the Paxos algorithm to synchronize that data across datacenters in real time." A detailed comparison of the two datastore options is available in App Engine documentation.
The price for the new datastore is starting out at three times the cost of the Master/Slave option, but the pricing will likely change in the future.
For the time being, the traditional Master/Slave datastore will remain the default configuration option. The datastore cannot be changed after an application is created, so existing applications can't be switched to the High Replication Datastore. However, Google is providing some migration tools.
There's a new option in the admin console that allow users to put their applications in read-only mode so that data can be reliable copied between applications. Google is also providing a migration tool with the Python SDK that allows code to be copied from one application to another. The documentation for the migration tools can be found here.
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