Programming Google App Engine with Java

Chapter 6. Datastore Entities

Most scalable web applications use separate systems for handling web requests and for storing data. The request handling system routes each request to one of many servers, and the server handles the request without knowledge of other requests going to other servers. Each request handler behaves as if it is stateless, acting solely on the content of the request to produce the response. But most web applications need to maintain state, whether it’s remembering that a customer ordered a product, or just remembering that the user who made the current request is the same user who made an earlier request handled by another server. For this, request handlers must interact with a central database to fetch and update the latest information about the state of the application.

Just as the request handling system distributes web requests across many machines for scaling and robustness, so does the database. But unlike the request handlers, databases are by definition stateful, and this poses a variety of questions. Which server remembers which piece of data? How does the system route a data query to the server or servers that can answer the query? When a client updates data, how long does it take for all servers that know that data to get the latest version, and what does the system return for queries about that data in the meantime? What happens when two clients try to update the same data at the same time? What happens when a server goes down?

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