How payment gateway works?
Nice video presentation https://www.mastercardpaymentgateway.com/mpgpublic/howItWorksIndex.do?
Credit Card Processing is part of every business. We need to choose wisely while going to sign contract with any payment gateway vendor. Here are the few points from my side.
1. They should have 99.99% up time. (Generally this is only on papers. They go down at least once in a month. or They take more time to process if one of their servers goes down.)
2. Never try to tie your fields tightly with payment gateway. This increases the coupling. Reduce coupling by different adapters or mapping.
3. They confuse end user (developer) by providing too much information in return. Be cautious while depending on it.
4. We need to maintain PCI Standards. https://www.pcisecuritystandards.org/
The rest we can read at http://www.sitepoint.com/print/money-where-mouse-is-gateways/
Few Payment gateway vendors
Note: These projects can be done in java technologies ranging from one day to one year. Based on project complexity.