XML: Structuring Complex Data

Modern software rarely works in isolation. A weather application may need data from a meteorological service, while a payment processor may need to verify a transaction with a bank. In each case, one application needs a controlled way to use a capability provided by another application.

An application programming interface, or API, provides that controlled connection. It is an application-to-application contract: a published set of rules describing how one program can access services provided by another program. The contract lets applications exchange requests and responses without requiring the consumer to understand the provider's internal implementation.

Every API involves three essential elements. The service provider is the application offering a service. The API specification is the published set of rules. The service consumer is the application using that service. The consumer reads the specification and creates requests that follow its rules.

This cycle takes place over HTTP, which acts as the transport protocol. The data carried in the cycle is represented using XML or JSON, formats that both applications can understand and parse. HTTP answers how the exchange travels; XML or JSON answers how the exchanged data is represented.

XML is one of the data representation formats an API can use when applications exchange information. Its role is to represent the data in a form that both the consumer and provider can understand and parse. The API contract specifies the expected format, so the consumer can interpret the provider's response without learning the provider's internal storage system.

The hierarchy illustrates the organizing idea: a response can contain a larger piece of information, which can contain related pieces of information. For example, a weather service may expose separate services for the current temperature, a forecast, and historical data. The exact representation rules come from the API specification; the consumer should use those rules rather than assume how the provider organizes its internal data.

These technologies have different jobs. HTTP carries the communication between applications. XML and JSON represent the data being exchanged. The API specification establishes which services are available, what parameters are accepted, and what request and response formats the consumer should use.

This abstraction protects the provider's internal systems because the consumer can access only the functionality explicitly exposed through the API. It also allows the provider to change its internal implementation without breaking consumers, provided the API contract remains the same. At the same time, the consumer's code is simpler because it only needs to learn the published API.