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....