why banks are trying consensus algorithms like ripple I am wondering that why do we need consensus algorithm like ripple if all the nodes in a system is honest. There won't exist an use case of double spending. I am asking this question because recently i am reading the news that banks are using ripple algorithm to transfer money between their international branches.
Double spending doesn't just include cases where a participant tries to spend his own funds twice, but it also includes cases where two honest participants each want the same liquidity.

For example, say I have 100,000 Euros that I'm willing to exchange for 113,000 USD and there are two people who each want to take me up on that offer. Somehow, we have to agree on which of those gets the offer, and that's precisely the same issue as a double spend.

You either need a consensus algorithm or you need a central authority.