防止无状态系统中的重复表单提交

Question

When working in PHP, to avoid duplicate form submissions, I used to generate a unique id of some sort, store it into a session variable, and have the id in the form, so on submission I can compare the values, regenerating the session value at that point. I never considered it a great solution, but I was never able to think of/find a better solution.

Now, I'm doing an Angular front end with a PHP backend (Lumen), and I'm struggling to think of a solution that doesn't involve me writing into a database. Unless I'm misunderstanding something, I can't use sessions between Angular and PHP, right? So this solution won't work. The only other thing I can think of is to have a key/pair value in a DB, but I never quite understood how that prevents duplicates on something like an accidental double click, wherein the session/database may not update it's key before the second submission starts processing. And as I'm learning more about stateless systems, it feels like a session isn't the best place to put this sort of thing?

Overall, I'm having trouble with creating a secure, backend system to avoid duplicate forms. With angular, I can always prevent duplicate submissions through preventing the button from being clicked, the API call from firing, etc, but I'd like to add backend protection too, and I'd love to hear how the experts do it.

Answer 1

I'm sure there are ways to hack it, but I think the short answer is that you don't want to hack it. When you transition to a split back/front end, one thing you are doing is specifically making your API calls stateless. This is a good thing! The statelessness, lack of sessions, etc, can dramatically simplify your back-end application. In short, statelessness is half the reason why you do something like this.

Preventing double submits as you are used to doing is decidedly something that you need a stateful application to do. As a result, it is now the job of the front-end application exclusively.

Your best bet is to think about your application in a whole new way. Your PHP backend handles stateless REST requests, and as such it is not PHP's problem if it gets duplicate submissions. In practice angular should have no problem making sure duplicates don't get submitted (it is really easy to prevent it on the front-end). Your PHP backend does need to make sure it always returns appropriate responses. So for instance on a registration page, back-to-back duplicate registration requests would result a successful registration followed by a failed registration with a message of "That email already exists" (or something like that). Otherwise, your PHP backend doesn't care. I can still do its job. It's the client's job to make sure the double submit doesn't happen, or make sense of the conflicting answers if it does.

Statelessness is a desirable quality in API calls: you'll make your life much more difficult if you muck that up.