Question

I want to make a simple GAE app in Go that will let users vote and store their answers in two ways. First way will be raw data (Database store of “voted for X”), the second will be a running count of those votes (“12 votes for X, 10 votes for Y”). What is an effective way to store both of those values with the app being accessed by multiple people at the same time? If I retrieve the data from the Datastore, change it, and save it back for one instance, another might be wanting to do the same in parallel, and I`m not sure if the final result will be correct.

Answer 1

It seems like a good way to do that is to simply store all vote events as separate entities (the “voted for X” way) and use the Task Queue for the recalculation (the “12 votes for X, 10 votes for Y” way), so the recalculation is done offline and sequentially (without any races and other concurrency issues). Then you’d have to put the recalc task every once in a while to the queue so the results are updated.

The Task Queue doesn’t allow adding another task with the same name as an existing one, but doesn’t allow checking whether a specific task is already enqueued, so maybe simply trying adding a task with a same name to the queue will be enough to be sure that multiple recalc tasks are not there.

Another way would be to use a goroutine waiting for a poke from an input channel in order to recalculate the results. I haven’t run such goroutines on App Engine so I’m not sure of the general behavior of this approach.