The Google App Engine datastore facilitates a scalable storage for web applications with main focus on the efficiency of read and query performance. An App engine application creates objects called entities with have values called properties. There are two options for App engine datastore: the master-slave datastore and the high replication datastore.
The two type of datastores differ on the availability and consistency of data. The Master/Slave datastore uses a master-slave replication system wherein data are replicated as you write it in a physical data center, providing a strong consistency of data but at the cost of the system's temporary downtime. High replication system on the other hand provides high availability on read and write but results into high-latency of writes. However, queries become consistent eventually. In terms of Storage Quota and CPU cost, the high Replication datastore uses three times greater than the Master/Slave datastore.
to be continued...
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