question

So the idea is that you're creating an "upgrade" system for items in the player inventory, where if the player chooses to upgrade, it consumes both items and (potentially) virtual currencies? Granted, we have purchase systems now, and we're planning on providing an alchemy/crafting system later on (items and/or VCs go in, different items and/or VCs come out, according to recipes you set up in the game configuration), but taking items/VCs in order to set Custom Data on an item would clearly not fit into either of those models cleanly.

Since each Cloud Script call would take some time, given the ping time from the user to the service backend (as well as the potential for dropped packets if their connection is bad), I would go with doing the upgrades locally and then syncing it with the backend. You'll want to sync before they use the items, of course, but I'm thinking you'd want to when they exit the interface, when the number of items being upgraded "caps" the Cloud Script call (more on that in a sec), or when they next come back into the game. For "capping" the Cloud Script call, I'm referring to keeping track of how many API calls you'll need to make in Cloud Script to account for the exchange. So, one call to get the catalog, another to decrement a count from an item, still another to subtract the VC, and then one more to grant the items. Your savings on API calls would then be on all but the decrement of the count from the item instances, though you'll definitely want to check all the balances before doing the decrement. Also, you'll want to have try/catch statements throughout, so that you can reverse the process if this fails anywhere.

Alternately, if you have animations for the upgrades, that might introduce enough time between calls that you could just do each upgrade when they ask for it.  :)

In terms of protecting yourself against hacked clients, it is possible to be so extreme that it slows the gameplay overall (as well as blocks it when you don't have a connection). The key thing to decide is how much you need to protect the experience, and whether requiring a connection is valid for your user base. In many cases, having the core game require that connection while you have an alternate mode of play for when the player is offline is an option.