The Gatekeeper language is a special-purpose declarative language for specifying payment instructions in the Flexible Payments Service (FPS). The payment instructions capture a set of conditions that must be met for the service to perform a transaction. Participants in a transaction each specify the conditions they will impose on the payment, and the transaction will only go ahead if it complies with all of these conditions.
The Gatekeeper language allows FPS developers to express powerful transaction and business logic rules if required for sophisticated applications. Most developers will not need to use all the features of this language, because the FPS API interface is designed to capture the fundamental conditions that will be needed by the majority of FPS applications. Unless your application or payment model is unusually sophisticated, you will be able to use FPS effectively without doing any more than applying boilerplate Gatekeeper documents that express the most common payment instruction patterns.
Payment instructions must be installed as tokens in an Amazon Payments account before they can be applied to a transaction. There are two ways to install payment instructions: directly via the API, or indirectly via the Co-branded UI (CBUI) pipeline. The two mechanisms are designed for different situations.
You install payment instructions in your Amazon Payments developer account with the InstallPaymentInstruction ...