Google App Engine：排队任务与Cron作业

Question

I'm very confused about GAE's concepts of Tasks, Task Queues (both push and pull), Cron Jobs and how each of these relate to Frontend vs. Backend instances.

I'm trying to achieve a situation where some HTTP requests can be serviced immediately, whereas some get queued. Queued requests might ultimately end up triggering my own code to execute (once they are consumed) or they might hit one of the GAE service APIs ( LogQuery , etc.).

I can't seem to wrap my head around how to design these two scenarios and let alone do the code up. To make things worse I've read literature that suggests there's certain task/queue-related coding you want to do differently depending on whether the code is executing on a Frontend or Backend instance. Thanks in advance for any help here! Bonus points for some concrete examples!

Answer 1

You write the code, Tasks and Cron execute it.

Task is a wraper for a set of properties, the main one is Url that should execute. You code (handler, servlet) should reside on that url. Task sit in the TaskQueues, which have certain default properties on how fast, how many in parallel, etc.. they execute the Tasks. So basically a To-Do list, that sequentially executes tasks with no guarantee when a task will start.

Cron is a service that periodically calls Url that you provided. In a sense its a scheduler.

Your Url (= your handler/servlet) can reside on frontend instance (default) or backend instance (must set special property on Task or in Cron settings). The main difference being that front requests must complete in 10min, while backend requests can take indefinitelly.