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Programming Google App Engine, 2nd Edition by Dan Sanderson

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Chapter 16. Task Queues and Scheduled Tasks

The App Engine architecture is well suited for handling web requests, small amounts of work that run in a stateless environment with the intent of returning a response to the user as fast as possible. But many web applications have other kinds of work that need to get done, work that doesn’t fit in the fast response model. Instead of doing the work while the user waits, it’s often acceptable to record what work needs to get done, respond to the user right away, then do the work later, within seconds or minutes. The ability to make this trade-off is especially useful with scalable web applications that use a read-optimized datastore, since updating an element of data may require several related but time-consuming updates, and it may not be essential for those updates to happen right away.

What we need is a way to do work outside of a user-facing request handler. By “outside,” we mean code that is run separately from the code that evaluates a request from a user and returns a response to the user. This work can run in parallel to the user-facing request handler, or after the request handler has returned a response, or completely independently of user requests. We also need a way to request that this work be done.

App Engine has two major mechanisms for initiating this kind of work: task queues and scheduled tasks. A task is simply a request that a unit of work be performed separately from the code requesting the task. Any application code ...

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