# API implementation, proxy, interface, client and invocation

*MuleSoft Integration Foundations Study Guide · Recognize and interpret essential integration concepts and terminology · Last updated November 2, 2026 · https://www.certifyforce.com/study-guide/mulesoft-integration-foundations/essential-integration-concepts-and-terminology/api-implementation-proxy-interface-client-and-invocation*

The Anypoint Platform glossary terms for the parts of an API: the interface or specification (the contract), the implementation that realizes it, the instance, the proxy that applies policies in front of a backend, and the client application that invokes the API with its own credentials.

- Certification: Salesforce MuleSoft Integration Foundations
- Exam section: Recognize and interpret essential integration concepts and terminology
- Study guide: https://www.certifyforce.com/study-guide/mulesoft-integration-foundations

**Official exam objective:**

> Define and correctly use the terms API implementation, API proxy, API interface, API client/consumer, and API invocation.

## What the exam expects

- Name each part of an API in a scenario: interface (specification), implementation, instance, proxy, client and invocation.
- Explain what an API proxy does in front of a backend and when MuleSoft recommends one.
- Identify which party holds client credentials when a Client ID Enforcement policy protects an API.
- Avoid calling the first component that receives a call, or the code that defines endpoints, the wrong name.

Key terms:
- **API interface**: the specification, the human- and machine-readable contract for the API.
- **API implementation**: a realization of the API specification that makes the API functional.
- **API proxy**: an intermediary between client applications and the backend that applies policies.
- **API client (consumer)**: the application that calls the API.
- **API invocation**: one call a client makes to the API (Exam convention: the glossary does not define it separately).

## Key facts

| Fact | Detail | Source |
|---|---|---|
| API implementation | A realization of the API specification that makes the API functional | [Glossary](https://docs.mulesoft.com/general/glossary) |
| API specification | A contract that people and software can read | [Glossary](https://docs.mulesoft.com/general/glossary) |
| API instance | Instantiation of the implementation; several per API across environments and gateways | [Glossary](https://docs.mulesoft.com/general/glossary) |
| API proxy former name | API Gateway | [Glossary](https://docs.mulesoft.com/general/glossary) |
| Proxy backend | Any location or language, including non-Mule | [Configure proxies](https://docs.mulesoft.com/api-manager/latest/api-proxy-landing-page) |
| Failed policy at the proxy | Error response; the request does not reach the backend | [Configure proxies](https://docs.mulesoft.com/api-manager/latest/api-proxy-landing-page) |
| Strict validation | Accepts only requests, parameters and headers defined in the specification | [Configure proxies](https://docs.mulesoft.com/api-manager/latest/api-proxy-landing-page) |
| Who holds client ID and secret | The registered client application with an approved contract | [Client ID Enforcement](https://docs.mulesoft.com/mule-gateway/policies-included-client-id-enforcement) |

## Check yourself

- When does MuleSoft suggest a proxy even though you already have a Mule application?
- Why can one API have several API instances?
- What does "API" mean in API Designer, according to the glossary?
- When does a request through the proxy never reach the backend?

## Sources

- [Anypoint Platform Glossary](https://docs.mulesoft.com/general/glossary)
- [Configure Proxies for Your APIs](https://docs.mulesoft.com/api-manager/latest/api-proxy-landing-page)
- [Add API Instances](https://docs.mulesoft.com/api-manager/latest/add-api-instances)
- [What is Full Lifecycle API Management?](https://www.mulesoft.com/api/management/what-is-full-lifecycle-api-management)
- [Client ID Enforcement Policy](https://docs.mulesoft.com/mule-gateway/policies-included-client-id-enforcement)
- [Configuring Mule Gateway API Autodiscovery in a Mule 4 Application](https://docs.mulesoft.com/mule-gateway/mule-gateway-config-autodiscovery-mule4)
- [Difference Between APIs, Connectors and Integration Applications](https://www.mulesoft.com/api/management/difference-between-apis-connectors-integration-applications)

## About the full unit

The "Core concepts", "Exam traps" and "Worked scenario" sections of this unit, its 4 linked practice questions, and unlimited timed mock exams are included with the MuleSoft Integration Foundations pack (USD $19): https://www.certifyforce.com/practice-exams