What is REST vs SOAP?

Application programming interfaces (APIs) allow different software systems to communicate with each other. There are two main architectural styles used for web APIs: REST (Representational State Transfer) and SOAP (Simple Object Access Protocol). Choosing between REST and SOAP depends on the specific needs and goals of your API project.

Table of Contents

What is REST?

REST is an architectural style for building distributed systems based on hypermedia. REST aims to leverage existing HTTP protocols and infrastructure to enable communication between systems on the web. The key principles of REST include:

Client-server separation – REST systems have a client-server architecture. Clients initiate requests to servers, which process requests and return appropriate responses.
Statelessness – REST APIs do not maintain client session state between requests. All request interactions are stateless.
Cacheability – REST responses should indicate if they are cacheable to improve performance.

Layered system – REST allows for an architectural model with intermediary servers for security, load balancing, caching, etc.
Uniform interface – REST uses standard HTTP methods and response codes for cacheable, idempotent interactions.
Code on demand (optional) – REST allows client functionality to be extended by downloading code like Javascript.

REST APIs expose endpoints (URIs) that correspond to resources. Clients interact with services by exchanging representations of resources in agreed-upon formats like JSON and XML. REST uses HTTP as its underlying protocol and supports operations like GET, POST, PUT, DELETE for CRUD operations.

Key characteristics of REST APIs:

Resources are identified by URIs
Resources are represented and addressed separately from their representation

Messages are self-descriptive, with enough information for processing
Hypermedia is used to define state transitions through resources

What is SOAP?

SOAP is a protocol for exchanging structured information between systems using XML. SOAP provides a standard messaging framework that allows communication between applications running on different operating systems, programming languages, and architectures. Key features of SOAP include:

XML-based messaging format – SOAP messages contain information encoded in XML.
Platform and language independent – SOAP is not tied to any specific platform or language.
Standardized – SOAP is an open standard defined by the W3C consortium.

Supports RPC style communication – SOAP permits remote procedure calls between systems.
Uses HTTP and SMTP for message transmission – SOAP relies on existing internet protocols.
Supports intermediate processing – SOAP messages can be processed by intermediary servers.

SOAP defines an envelope structure for messages that contains header and body sections. The contents of SOAP messages are formatted according to XML schema. SOAP uses HTTP and SMTP as transport protocols and supports different messaging patterns like request-response and asynchronous messaging.

Key elements of a SOAP message:

Envelope – Root XML element that defines the message.
Header – Optional header containing metadata.

Body – Contains call and response information.
Fault – Provides information about errors.

Key Differences Between REST and SOAP

Criteria	REST	SOAP
Architectural Style	REST is an architectural style	SOAP is a protocol
Message Format	No official format. Most REST services use JSON or XML.	Messages must be XML
Transport Protocol	Typically HTTP	HTTP, SMTP, TCP, MQ, etc
Caching	REST APIs are cacheable	SOAP has no caching support
State Management	Stateless	Can be stateful
Bandwidth Needs	Smaller and more lightweight	Has significant overhead from XML
Client Usage	More suited for web clients	APIs can be consumed by diverse platforms

Summary of Key Differences:

REST uses HTTP methods like GET, POST, PUT, DELETE to interact with resources while SOAP only uses POST with different message bodies.

REST is designed for distributed information systems while SOAP is better suited for message-based RPC communication.
REST has better performance and scalability since it requires less bandwidth.
SOAP defines standards and is more strictly structured than REST.

SOAP has built-in error handling while REST uses HTTP status codes.
SOAP has wider compatibility across platforms compared to REST.

When to Use REST vs SOAP

Here are some key considerations for when to use REST vs SOAP APIs:

Use REST when:

You need to expose resources for consumption by diverse web-based clients.
Statelessness is important. Clients manage state.
Caching is important for performance.

You want to keep bandwidth usage low.
You want to leverage existing HTTP infrastructure.

Use SOAP when:

You need formal contracts between systems.

You need guaranteed delivery and transactions.
Your systems expose complex operations and data structures.
You need to support intermediate processing by middleware.

Security standards like WS-Security are important.

Here is a summary of when to prefer REST or SOAP:

	Prefer REST	Prefer SOAP
Client Type	Web clients	Diverse platforms
Message Size	Small	Large
Bandwidth	Low	High
Caching	Important	Not needed
State	Stateless	Stateful
Complexity	Simpler	More complex

Conclusion

REST and SOAP represent two leading architectural approaches for building web service APIs. While they achieve similar ends, the fundamental philosophies and underlying protocols differ significantly.

REST leverages existing HTTP and web infrastructure to expose resources in a simple, scalable way. It is best suited for web applications and lightweight mobile apps. SOAP provides more rigid structure and messaging standards between systems. It has broader compatibility and supports more complex interactions.

The choice between REST vs SOAP depends on the specific requirements of an API. Factors like client types, performance needs, message sizes, security concerns, and complexity determine what approach makes the most sense for a particular use case. By understanding the core principles and trade-offs of REST and SOAP, API developers can make informed decisions about the appropriate architecture.