title: How to execute tasks in (FIFO) order using Google App Engine Task Queues?
body: <p>I believe task queues (push, pull, deferred) in Google App Engine do not guarantee that tasks will be executed in FIFO order.  For example, suppose I have a task queue with tasks A, B, and C, and each task has timestamps t_A, t_B, and t_C, such that t_A &lt; t_B &lt; t_C.  How can I ensure that tasks A, B, and C are executed in order of timestamp?  If task B fails, I would like to delay the execution of task C until task B executes successfully.  I've seen an ETA field to set the earliest time that a task can be sent, but this seems like more of a heuristic, not a guarantee.</p>