Software Knowledge → 04 · Backend → External API

PART IV · BACKEND

External API

An API provided by another system that the backend calls to use external data or capabilities.

01 · What is it?

What is it?

An external API is an interface owned or operated by another system.

A backend can call it to retrieve data or trigger actions outside the current application.

Simple DefinitionAn external API lets your backend use another system’s service.

02 · Think of it like...

Think of it like...

Think of outsourcing a task.

Your backend needs something another system provides. It sends an API request and waits for a response.

BackendExternal APIExternal SystemResponse
03 · What does it do?

What does it do?

External APIs let a backend use capabilities that are not built into the current application.

Market PricesRead current or historical pricing data.
PaymentsStart or confirm a payment.
ShippingCreate labels or track deliveries.
Identity ServicesVerify an identity or account.
EmailSend transactional messages.
AI ServicesUse an external model or capability.
04 · Where is it used?

Where is it used?

External APIs are common wherever a system depends on a specialized outside capability.

Trading Systems
Payment Systems
Cloud Services
Maps
Messaging
AI Applications
05 · Example

Example

Backend needs a market price before it calculates an order.

01 · Order Service

Needs a market price

The order service starts the workflow.

02 · API Request

Pricing API Request

The backend sends a request to the external API.

03 · External System

External Pricing System

The other system looks up the price.

04 · Response

Price Response

The backend uses the returned price for calculation.

Order Service→Pricing API Request→External Pricing System→Price Response→Order Calculation
06 · How it connects

How it connects

Service→HTTP Client→External API→Response→Business Logic

The HTTP client is the backend component that sends the external request and receives the response.

07 · Do Not Confuse

Do Not Confuse

External API

Provided by another system or organization. The current backend calls it as a dependency.

Internal API

Provided within the same organization or platform. It connects parts of the same system boundary.

External API = another system’s interface.
Internal API = an interface inside the same organization or platform.

08 · Topic Glossary

Topic Glossary

ENGLISH TERM中文SIMPLE ENGLISH中文小白理解
External API外部 APIAn API provided by another system.当前系统需要调用的其他系统或公司的接口。
Internal API内部 APIAn API used inside the same organization or platform.同一平台或公司内部系统之间调用的接口。
HTTP ClientHTTP 客户端Code used to send HTTP requests to another API.后端用来调用其他 API 的程序组件。
API KeyAPI 密钥A value used to identify or authorize an API caller.调用外部 API 时常用来识别调用方身份的凭证。
Timeout超时A limit on how long the backend waits for a response.后端等待外部 API 返回结果的最长时间。
Retry重试Trying the external request again after a temporary failure.外部 API 调用失败后再次尝试请求。
09 · What to Remember

What to Remember

External APIs provide capabilities from other systems.

Backend services often call them through HTTP clients.

Timeouts, retries, authentication, and error handling are important when using external APIs.

Backend→External API→External Capability
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