Time-Windows Exercise

In the previous chapter, you learned about enforcing a time-window to ensure that a transaction is notarized within a range of acceptable times.

Now, how do you use this enforcement of a time-window to make the car sales flow more robust and enforce the expiration date on the sales proposal? What you want to achieve is:

• The proposal has an expiration date.
• The buyer can only accept before this date.
• And the seller can only reject the acceptance after this date.

Try to implement that on your own before looking below.

The way to go is:

• The SalesProposal defines an expiration date as a point in time in the future.
• The offer flow adds to the transaction a time-window with an “until” end.
• On Offer, the contract checks that the expiration date is after this “until” end. If it is not, the buyer is not able to buy.
• The accept flow adds to the transaction a time-window with an “until” end too.
• On Accept, the contract checks that the expiration date is after this “until” end.
• The reject flow adds to the transaction a time-window with a “from” end too.
• On Reject, the contract checks that the expiration date is before this “from” end.

As you can see:

• The contract verifies that the time-window is on the right side of the expiration date.
• The notary verifies that it is signed within the time-window.

The interaction of these 2 attestations in effect enforces the expiration date.

Go ahead and add an expiration date to your project before moving on to the next chapter for an example solution.