I had this discussion in my workplace and wanted to share and get opinions from the folks here. (I suspect StackOverflow might not appreciate such open ended questions).
Context: We have a microservice involved in pricing signalling to our users. We have an endpoint which have the following:
- Input: an array of item ID’s
- Output: the expected final price of the given items.
The item prices are quite volatile (and no, it is not crypto related), and is dependent on things like instantaneous supply-demand, promotions, etc.
Since the prices change quite frequently, it became a requirement that we commit to the price that was shown to the user initially, up to a certain time period (eg 5 min after the price was calculated). This improves the UX since the user will be charged as according to what they expected at the start.
Currently, in our system, we achieve this via a JWT, which contains all the details in the request, the obligatory signature, and the expiry set to 5 min from the time it was generated.
After generating this receipt, the FE can then call the endpoint with the JWT which does the actual payment processing using the params encoded in the token. This way, we know that the params + the total cost that is quoted in the JWT originates from our service since we verify that we signed it.
And the system evolves once more. We see that in the system, there is this mechanism, that if the token is expired, we do not reject the request at the charging step. Instead, we call the price endpoint internally using the params provided, and check if the price is the same as in the expired JWT. If it is the same, we process it as normal despite the JWT being expired.
This is where the contention lies. I believe that we should force the user to procure another non-expired JWT and removing this complex logic while others believe in the value of this improved UX where the user doesn’t need to restart the whole flow again.
What do y’all think? Which way would y’all architect the endpoint? Or is there something fundamentally wrong with our design (maybe JWT is not the best suited for this use case)?
What are the alternatives to a JWT. I know it is a bit bloated and we could just use the HS256 signature itself, but that doesn’t really change the core problem of expiry vs auto-refetch
My comment about JWT wasn’t that it complicates the current problem, but that it complicates the design in general.
The alternative is to do it the traditional way, with DB records associated with the user, rather than pushing everything to the client. It wont solve the problem you’re describing, but it might make working on a solution easier.
The feature you’re describing is just a slight tweak of the standard shopping cart, so the standard tried-and-true shopping cart design would serve you well, barring extenuating circumstances like some kind of significant DB limitation. DB tables for the items, and for the shopping cart itself. When the user goes to check out the cart, you make an offer on the cart, which is basically just a clone of the shopping cart into a new table with all the item prices denormalized (and therefore locked in). Add a field for the offer expiration date and you’ve got a working design that is very similar to the standard well-worn design, and without any complicated JWT stuff. You still need the client to retry with a new offer if the current offer has expired, but adding retry logic to clients is a pretty normal thing for clients to have.
It’s a pretty significant departure from your current design, so it’s probably not actually a useful answer, but this is what I meant about the JWT making things more complicated in general.