Webapp Blog Post 3

The Google App Engine eliminates the need to worry about web servers interacting with data that is spread around two many different locations around the world. The data repository is called a High Replication Datastore and uses the Paxos algorithm to replicate data across datacenters. Entities are data written to the datastore. Each entity has a key that uniquely identifies it in the database. There is also the option with each entity to designate another entity as a parent entity, making it the child of the parent entity. In this manner the datastore entities form a hierarchical structure. The hierarchical structure consists of ancestors, descendants and root entities. Ancestors are anything above the entity (its parents or its parents parents, etc.), descendants are anything below the entity, and a root entity is an entity without a parent. An entity descending from a common ancestor with another entity is said to belong to the same entity group as that other entity. These groups have a parent key, which is the common ancestor's key. This key identifies the entire group and ancestor queries which query of an entity group refer to the parent key instead of the specific entity key. A primary strength of the datastore is that it is very resilient in the face of catastrophic failure.