[Taler] transaction history UX and fulfillment URL semantics

[Florian Dold]

Hi,

I'm wondering what's the best way to go back to completed purchases that
show up in the transaction history.

One way would be to *always* do replay, and never rely on the cached
fulfillment URL.  The user is free to remember or bookmark the
fulfillment URL, but the wallet will never remember it.

I guess this is currently the "correct" way to do it, since we can never
know whether the user agent still has the necessary state to view the
fulfillment URL, and we can't really detect if going to the fulfillment
URL in the browser fails and offer re-playing the purchase.

The alternative would be to require the merchant-internal contract
identifier to be also added to the *fulfillment* URL.

This way the merchant's fulfillment page can detect that the session
state is missing, and redirect the user agent to the execution page,
where the payment will be replayed.  If the contract can't be replayed
(because it's missing in the wallet), the user agent will be redirected
to a checkout page.

The second approach requires changes to the merchant, but I like it
better for these reasons:

* The user can bookmark fulfillment URLs, and they will always work as
long as the contract is in the user agent's wallet.
* The wallet does not have to implement any special logic and user
interface for contract replay.  It's just a link to the fulfillment page
(that contains the UUID).

Again, this would restrict the merchant a bit, but for the user there's
much less magic going on, and common things (like bookmarking or sending
URLs to a friend) just works.

Alternatively, strange things will happen:  I bookmark the fulfillment
page for a donation I made (maybe because I want to print it later), but
then I make another donation and suddenly my bookmarked page is gone,
but it *does* work when I click the "replay" button from the wallet ...
this doesn't sound right from the user's perspective.

Thoughts?

- Florian