Request Builder
Request Builder is a component of the Magento payment gateway responsible for building a request from several parts. It allows implementing complex, yet atomic and testable, building strategies. Each builder can have simple logic or contain builder composites.
Basic interface
The basic interface for a request builder is \Magento\Payment\Gateway\Request\BuilderInterface.
Builder composite
\Magento\Payment\Gateway\Request\BuilderComposite is a container for a list of \Magento\Payment\Gateway\Request\BuilderInterface implementations. It gets a list of classes, or types, or virtual type names, and performs a lazy instantiation on an actual BuilderComposite::build([]) call. So that you can have as many objects, as required, but only those, which are needed for a request are instantiated.
BuilderComposite implements the composite design pattern.
The concatenation strategy is defined in the BuilderComposite::merge() method. So if you need to alter the strategy, you need to add your custom implementation of BuilderComposite.
Adding a builder composite
Builder composites are added using dependency injection in di.xml. A builder composite might comprise simple builders as well as other builder composites.
Example of adding composite builders for the Braintree payment provider (app/code/Magento/Braintree/etc/di.xml):
...
<!-- is a builder composite comprising a number of builders -->
<virtualType name="BraintreeAuthorizeRequest" type="Magento\Payment\Gateway\Request\BuilderComposite">
<arguments>
<argument name="builders" xsi:type="array">
<item name="customer" xsi:type="string">Magento\Braintree\Gateway\Request\CustomerDataBuilder</item>
<item name="payment" xsi:type="string">Magento\Braintree\Gateway\Request\PaymentDataBuilder</item>
<item name="channel" xsi:type="string">Magento\Braintree\Gateway\Request\ChannelDataBuilder</item>
<item name="address" xsi:type="string">Magento\Braintree\Gateway\Request\AddressDataBuilder</item>
<item name="3dsecure" xsi:type="string">Magento\Braintree\Gateway\Request\ThreeDSecureDataBuilder</item>
<item name="device_data" xsi:type="string">Magento\Braintree\Gateway\Request\KountPaymentDataBuilder</item>
</argument>
</arguments>
</virtualType>
...
<!-- The same BraintreeAuthorizeRequest builder composite is a part of the BraintreeSaleRequest builder composite -->
<virtualType name="BraintreeSaleRequest" type="Magento\Payment\Gateway\Request\BuilderComposite">
<arguments>
<argument name="builders" xsi:type="array">
<item name="authorize" xsi:type="string">BraintreeAuthorizeRequest</item>
<item name="settlement" xsi:type="string">Magento\Braintree\Gateway\Request\SettlementDataBuilder</item>
</argument>
</arguments>
</virtualType>(The code sample is from Magento Open Source v2.1. Although the payment provider gateway was added in v2.0, the particular default implementation using the gateway were added in v2.1)