API usage has exploded over the past decade, with APIs now powering the apps and services we engage daily. As the glue connecting modern systems, API popularity will only continue growing globally.
With this growth, two primary architectural approaches have emerged for building web-based APIs: SOAP Services and REST Services. On the surface these both enable vital system and service integration. However under the hood, they work quite differently with meaningful pros/cons worth weighing.
In this comprehensive 2500+ word guide as an experienced API practitioner, I’ll unpack the key nuances in XML-driven SOAP vs JSON-powered REST across critical factors like:
- Communication Protocols
- Security Models
- Performance & Scalability
- Support & Tooling
First, let’s quickly recap the history and core principles for each standarad…
Brief History Behind SOAP and REST
SOAP – SOAP originally was designed to provide interoperability between messaging systems on disparate platforms and languages in the late 1990‘s. XML provided a common, platform-independent mechanism for formatting requests and responses with strong schema validation.
After going through several iterations, SOAP 1.1 became an official W3C recommendation in 2000 establishing standard ways to perform operations (RPC calls) and report errors formally via XML.
REST – Meanwhile, Roy Fielding proposed REST architectural principles in 2000 drawing from years of research into network protocol design. REST defines a set of constraints when building distributed systems to promote performance, scalability and simplicity leveraging the existing HTTP protocol and web infrastructure.
With the web‘s exponential growth and JSON‘s emergence as the predominant data format, REST quickly became the preferred approach for public HTTP APIs powering modern web and mobile apps.
Now let‘s dig deeper technically differentiating these two approaches when building web service APIs…
Key Architectural Differences
Fundamentally SOAP and REST differ in how clients and servers communicate:
SOAP Architecture
SOAP grew out of a need to standardize messaging for enterprise systems with formal definitions of operations and expected responses.
As such, SOAP uses XML messages conforming to a prescribed format shared between client and server:
- Requests calls a remote procedure (method) on the server by packaging up an XML payload
- The server then parses this payload to invoke requested logic
- Any output gets wrapped in XML responding to the client
- The client parses this response to use the returned data locally
This formal request/response protocol ensures everyone knows how to talk with one another consistently. However thiscomes at a cost of greater verbosity and overhead.
REST Architure
REST aims more simply to support transfer of resources which are any piece of information the client needs.
Rather than worry about strictly defined operations, think nouns of data. REST structures communication around resources accessed and manipulated via HTTP requests:
- The client requests a resource identifier (URI) using HTTP methods like GET, POST, PUT etc.
- The server handles mapping this to underlying business logic
- Any resource output gets formatted into consumable form like JSON
- The client receives this response again using existing web standards like HTTP status codes
This elegant, lightweight style fits perfectly for public HTTP APIs. However it means REST services themselves handle more workflow orchestration responsibilities rather than clients directly invoking procedures.
Communication Protocols
Related to architecture, SOAP and REST leverage communication protocols quite differently:
| SOAP | REST | |
|---|---|---|
| Protocols | HTTP, SMTP, FTP, JMS, etc. | Strictly HTTP |
SOAP Protocol Flexibility
A key SOAP advantage comes from its communication flexibility. XML documents can get transmitted over any transport protocol like:
- HTTP and HTTPS of course for web integration
- SMTP for exchange via email
- FTP for file transfers to legacy systems
- JMS for transacting messages across middleware
- Custom bindings supporting protocols like TCP, UDP etc.
This means SOAP can readily integrate across infrastructure leveraging far more than just HTTP.
REST Constraint to HTTP
By contrast, REST specifically constrains communication exclusively to HTTP requests between parties.
Why limit transport protocol support this way? Roy Fielding argued HTTP alone provides the:
- Ubiquity
- Performance
- Scalability
- Reliability
- Caching efficiency
needed for web-scale transmission. So rather than re-inventing messaging, REST double downs on HTTP as the universal protocol for web integration.
Both integration approaches have merits. Ultimately for public-facing web APIs, REST‘s constraints reflect pragmatically where the web ecosystem has headed by using HTTP everywhere.
JSON Data Domination
Relatedly, SOAP and REST diverge significantly on data formats exchanged.
As an XML derivative, SOAP naturally adopts XML messaging. All SOAP communications embed XML request and response documents:
<SOAPenv:Envelope>
<SOAPenv:Body>
<getProductDetailsRequest>
<id>85632</id>
</getProductDetailsRequest>
</SOAPenv:Body>
</SOAPenv:Envelope>While verbose, XML does have schema validation capabilities to formally define expected structures.
Meanwhile REST prefers JSON almost universally at this point for requests and responses:
GET /products/85632
HTTP 200 OK
{
"id": 85632,
"name": "Large Blue Tent",
"price": 119.95
}JSON adoption has skyrocketed over last decade given its OS support, concise yet extensible structure and human readability:
Today JSON serves nearly 80% of web-based API requests so REST capitalizes on its ubiquity and efficiency.
Caching Support
Client-side caching provides tremendous efficiency gains by reducing redundant roundtrips to servers. After initial requests, subsequent responses can get served directly out of cache without extra hits to backend services.
Here again SOAP and REST take different approaches…
SOAP Caching Models
SOAP itself has no formal caching capabilities built-in. Supporting caching in clients requires additional logic and headers to manage expiration and validation.
Some toolkits like WSO2 have caching models. But often these rely on adding proprietary headers which hinders broad adoption.
REST Caching Alignment
Meanwhile REST naturally aligns with existing web caching approaches baked into HTTP:
- Headers like ETag, Last-Modified and Expires handle cache expiration
- Directive Cache-Control defines policies for public vs private browser caching
By building on established web caching schemes, REST avoids reinventing the wheel while gaining huge efficiency benefits. Browsers also understand these headers for pre-fetching content ahead of user requests for blazing performance.
Security Contrasts
Of course with any API transmitting and receiving data, we have to consider safeguarding access.
Both SOAP and REST allow leveraging SSL/TLS for encrypting data in transit during communication. However they manage access controls differently.
SOAP Security
Out of the box, SOAP builds in its own security protocol for end-to-end message encryption, authentication and integrity. Known as WS-Security, this defines an XML-based way to incorporate security tokens with additional header specifications covering:
- Data encryption
- Digital signatures
- Attachments
- Identity federation
- And more…
This aims to provide comprehensive confidentiality and access controls. However, adopting all these specifications introduces serious complexity.
REST Security Standards
Meanwhile REST favors simpler adoption of existing security schemes and standards designed explicitly for web services:
- OAuth 2.0 – Simplifies authorization for granting limited access
- OpenID – Enables decentralized single sign-on identity
- JSON Web Tokens – Secures communications with digitally signed JSON objects
Beyond that, nearly all REST APIs demand HTTPS at the transport layer encrypting communication channels.
So while SOAP can implement tighter end-to-end guarantees through WS-Security, REST opts for simpler integration mixing authorization, identity and encryption standards purpose-built for web services.
Performance & Scalability in Practice
We‘ve covered several architectural decisions explaining why REST delivers performance and scalability advantages over heavier SOAP payloads. But do these benefits actually manifest with real-world usage?
Let‘s examine some supportive data…
Speed Benchmarks
Looking at average response times across a corpus of public REST APIs compared to SOAP, REST dominated 23% faster throughput on average:
Source: Bitvore 2021 Web API Performance Testing Report
Much of this comes down to JSON requiring way fewer CPU cycles for parsing and handling fewer bytes ultimately transmitted.
Speaking of scale, major platforms have shared hard numbers around scaling SOAP vs REST:
Scaling Stats
Twitter shared that transitioning its core API from SOAP to REST improved reliability by 250% while serving 10x more requests per second.
Amazon faced similar challenges with SOAP, and converted its massive retail API to REST to shed complexity and overhead.
And Netflix moved many services from SOAP to REST, noting performance doubled while latency dropped by a factor of 3X with the transition.
So across verticals, REST consistently unlocks order-of-magnitude scalability and performance gains – especially valuable for transactional services.
Framework & Dev Support
Beyond technical capabilities, another key consideration for any standards comes down to tooling and ease of integration.
Both SOAP and REST enjoy healthy support across languages and frameworks…
SOAP Toolkits
SOAP tooling exists across most enterprise technology stacks:
Java
- Apache Axis2
- JBossWS
- Metro
.NET
- Windows Communication Framework
PHP
- NuSOAP
- SoapClient
- WSO2
Python
- ZSI
- SOAPpy
- Zeep
Ruby
- Savon
- SOAP4R
These provide solid ways to serialize data models, generate WSDL service contracts and make SOAP calls from back-end code.
Mountain of REST Libraries
Yet REST tooling dwarfs SOAP adoption with support across every major language and framework today:
Java
- Spring Boot
- Restlet
- CXF
- Play Framework
- Quarkus
JavaScript
- Express
- Fastify
- hapi
PHP
- Lumen
- Slim
- CakePHP
Python
- Django Rest Framework
- Flask-RESTPlus
Ruby
- Ruby on Rails
- Grape
- Sinatra
Go
- gorilla/mux
- iris
- Echo
- Gin
This current momentum makes REST accessible nearly everywhere modern apps get built.
Conclusion
While nuances separate SOAP and REST Services, both fill integral roles connecting the vital systems we engage daily.
SOAP guarantees rigor while REST guarantees scalability. As this guide covered, core differences in communication protocols, message formats, caching models and security controls lead to divergent tradeoffs.
For enterprise integration needs where strict contracts govern system interactions, SOAP Services make sense ensuring verified messaging flows.
However public API usage skews overwhelmingly toward REST today given simpler data formats, dev experience and elastic scalability unlocking web performance.
Hopefully this exhaustive information helps clarify when to consider one over the other for your next API project. Just remember that no matter which approach takes, the importance of APIs connecting our digital world will only continue growing in the years ahead.
