Understanding the relevance of checkNonce method

[aum.s...@gmail.com]

Hi,

I was playing around with the API and its working perfectly fine and i
am able to login to Yahoo/Gmail.
i was refreing the code of OpenIdServlet under "sample".

In this sample implimentation i saw a method checkNonce() where
"openid.response_nonce" is being passed as parameter.

then this parameter is being passed through some checks and in last 2
lines

if (isNonceExist(nonce))
throw new OpenIdException("Verify nonce failed.");
storeNonce(nonce, nonceTime + TWO_HOUR);

this has been checked in the database.I am unable to understand what
the purpose storing this in the database and checking it.
Please help me to understand this

Thanks in advance