SOAP vs. REST: Solving the Testing Challenges of Each

You might already be familiar with SOAP and REST. After all, SOAP and REST are well established with definitions and specifications going back decades. Are you looking to expand your knowledge or get a new perspective? Or maybe you’ve heard about both and are interested in gaining a better understanding?

Please allow me to describe the SOAP and REST differences, compare, and otherwise shed light on these two significant approaches to web service and web API design. I’ll also highlight some of the testing challenges associated with these approaches and how to solve them. First, let’s define what these are and how they relate to the World Wide Web.

Difference Between SOAP vs. REST Web Services

The World Wide Web Consortium (W3C) recommends standards and protocols for the global collection of interconnected resources that we know as the World Wide Web. A “web” resource is accessed at a “web” address and delivered over a “web” protocol.

As someone reading this blog post, you may know that you’re reading an HTML document at the URL shown in your browser’s address bar that was requested and delivered using the HTTP(S) protocol. The W3C defined how these same technologies that are enabling you to read this blog post can facilitate communication between software systems. In particular, the W3C defined “web services,” which led to the creation of many other standards and technologies over the years. Let’s take a high-level look at what they are.

Understanding SOAP

SOAP is an acronym for Simple Object Access Protocol. It’s an object-oriented protocol whereby objects are exchanged between client and server, serialized to and from XML. The SOAP specification builds on top of other web technologies as defined by the W3C including XML and HTTP. Many specifications are based on or extend the SOAP specification, including some that aren’t so simple. A SOAP service exchanges SOAP messages with a SOAP client.

Before I begin to detail the various qualities of SOAP and how they compare to REST, let’s start by explaining how SOAP came to be in the first place. The story of SOAP really begins with the story of XML and the web itself. XML was developed for the web following the success of HTML, the data format used for documents displayed in web browsers.

Like HTML, XML was intended to be both human-readable and machine consumable and served over the web. XML was developed as a way to package and send arbitrary structured data, something HTML was not suited to accomplish. As it turns out, XML only solved some problems, leading up to the introduction of SOAP.

Communication between applications over the web poses various interoperability challenges, especially with applications being developed in different languages having different data types. For instance, a web application may be built with PHP, another with .NET, and another with Java. To address interoperability challenges, one approach is to utilize XML as a data interchange format.

XML provides a standardized way to describe arbitrary, structured data. Other specifications like XML schema define a common type system independent of the programming languages being used. However, using XML poses another challenge: there’s no standard way to describe how the data should be organized within the XML nor how it should be processed. SOAP was developed as a programming language agnostic framework for remote procedure calls based on XML and XML schema.

SOAP defines a set of rules for message structure and communication. A SOAP message is also called an “envelope”. A SOAP envelope has a “Body” element and an optional “Header” element. The “Body” typically wraps or literally “envelopes” another XML document. The header contains optional details such as authentication or metadata, while the body carries the actual content of the message.

One of the key benefits of SOAP is its platform and language independence, meaning that systems developed on different technologies can communicate seamlessly using SOAP as the messaging protocol. Additionally, SOAP is highly extensible and supports many communication styles, including request/response and one-way messaging. It’s often used in enterprise-level systems that require complex operations and strict message contracts.

Understanding REST

REST is an acronym for REpresentational State Transfer. Unlike SOAP, REST is not a specific technology but is an architectural style, defining specific constraints for a software system. A REST-compliant web service or web API is often known as a RESTful API or REST API.

Like SOAP, REST APIs leverage existing HTTP infrastructure and the use of existing web standards to enable interoperability. REST also provides a lightweight and scalable approach to communication between clients and services. RESTful services are built around resources. A resource is any kind of abstract entity which is identified by a URI (Uniform Resource Identifier).

A REST API listens at a base URI with child paths for accessing each of the resources exposed by the API. A resource path may include one or more parameters that can be used to identify a resource, where certain path segments could contain identifiers. Resource parameters may also take the form of query parameters or headers. Clients interact with these resources by sending HTTP requests, using standard CRUD operations: Create, Read, Update, and Delete. REST leverages the HTTP protocol’s verbs (GET, POST, PUT, PATCH, DELETE) and status codes to perform operations on resources and convey information about the response.

REST primarily employs JavaScript Object Notation (JSON) for data representation. JSON is a lightweight and human-readable format, making it easier to work with and understand. It also aligns well with modern web development technologies and JavaScript-based frameworks.

One notable feature of REST is its statelessness. Each request from a client to a RESTful service contains all the necessary information to process that request. The server does not maintain any session or client state, making it highly scalable and allowing for load balancing across multiple servers.

Now, let’s dig into some details about the differences between SOAP and REST, understanding how these approaches compare to one another

Operations

A SOAP service defines a set of operations. The operations can be arbitrary in that there are no restrictions on the scope or purpose of the operations being defined. Operations have a signature, typically representative of the fully qualified name of the element inside the envelope’s Body element. For example, the element’s name might be “calculateSomething” or “doSomethingFun.”

A REST API has a collection of operations for each resource. The operations that are available are limited to CRUD (create, read, update, and delete). The operations are mapped to HTTP methods like GET, POST, PUT, PATCH, and DELETE.

Positive Comparison

SOAP operations offer a higher degree of flexibility since they are not limited to CRUD and do not have to be structured around specific resource types like REST. However, operations can also be used for CRUD, where the XML in the SOAP Body can include an XML representation of an object along with its identifier.

REST operations offer a higher degree of simplicity. A URI is used to identify the resource, decoupled from the representation of the resource being exchanged. Additionally, operations must be stateless, meaning that the operations behave in a consistent way and not based on the state of the conversation between the client and the service endpoint.

Critical Comparison

SOAP services can have much higher complexity by supporting arbitrary operations and by potentially tracking state. An example could be a bookstore service having an “addToCart” operation. Every time a client calls “addToCart”, the service tracks the item in the client’s shopping cart. A different client can call “addToCart” and not impact a different client’s shopping cart. The service tracks the state of each client separately.

REST APIs are more restricted than SOAP by having operations limited to CRUD. Additionally, clients have the burden of tracking their own state. In the bookstore example, the client needs to know the ID of its cart so that it can build the correct URI for their shopping cart, like “cart/{id}”. The client could perform a GET at that URI to fetch a structured representation of their cart. The client could also do a PUT at that same URI to update their cart with an updated representation.

Data Representation

SOAP messaging involves the exchange of XML documents called SOAP envelopes. A SOAP envelope contains a “Body” element and an optional “Header” element. The XML within the “Body” element can be arbitrary but typically represents one or more entities or objects. The content type is either “text/xml” or “application/soap+xml” depending whether SOAP version 1.1 or SOAP version 1.2 is being followed.

XML elements in the SOAP can also be used to wrap other types of data, textual or binary. The W3C defined optimizations called “XOP” and “MTOM” which describe how to efficiently package binary data in XML and SOAP messages as a MIME “Multipart/Related”, avoiding the need to base64 encode the binary data directly within XML tags.

REST API messaging involves the exchange of representations of a resource. A “representation” could be any data format. It could be a structured data exchange/interchange format such as XML or JSON, or something completely different like PDF or JPEG. There’s no content-type restriction. A REST API could support multiple data formats or different data formats for different resources.

Positive Comparison

SOAP. XML is standardized and well understood, defined by various W3C recommendations. XML has a concept of namespaces, which help disambiguate elements, avoiding naming conflicts. SOAP envelopes provide a layer of encapsulation, enabling the inclusion of additional metadata under the “Header” element.

REST. REST APIs aren’t restricted in terms of what data formats they can use. For structured data, the use of JSON is common and much faster to consume and produce than XML. However, there are other formats for serializing structured data that can be even more compact and efficient than JSON, like BSON (Binary JSON) or Google’s Protocol Buffers (Protobuf), or Apache Avro. Any content type is a possibility.

Critical Comparison

SOAP. XML can also be verbose or bloated compared to other data formats with a relatively high serialization cost to both produce and consume. However, message size can be reduced using compression like the “Content-Encoding: gzip” HTTP compression scheme. Serialization cost can be reduced by using alternative or binary XML encodings, like Microsoft’s .NET Binary Format for XML.

REST. There’s no standard data interchange format for REST APIs. So, you may need a different set of libraries to consume and produce structured data depending on the REST API you are communicating with. However, JSON is very popular and often available.

Extensibility

SOAP and REST are extensible but in very different ways. Let’s dive into the comparison.

Positive Comparison

SOAP. The protocol binding does not have to be HTTP but could be anything. SOAP Messages could be sent over some other TCP-based protocols like SMTP or could be sent over UDP socket as is done in WS-Discovery and UPnP. Microsoft’s .NET WCF SOAP framework has TCP and named-pipe transports. Event-driven or message-queuing interfaces like JMS (Java Message Service) or AMQP (Advanced Message Queuing Protocol) are also used for SOAP.

SOAP also allows for different message exchange patterns. It does not have to be request-reply. It could be asynchronous, one-way, or fire-and-forget. SOAP is used in service-oriented architecture (SOA) where services are loosely coupled, pushing, or reacting to messages routed by an enterprise service bus (ESB).

REST. REST APIs are extensible in that they can potentially represent resources with different message formats. Unlike SOAP, you aren’t restricted to XML-based representations. New resources can be added without affecting existing resources. REST APIs can be versioned by including some version number or version identifier as part of the URI.

Critical Comparison

SOAP. With higher extensibility comes greater complexity. There’s no single way to implement SOAP messaging, given the various protocols, messaging patterns, and WS-* specifications that may be applied.

REST. REST APIs are generally restricted to HTTP, where method and resource URI are sent in the HTTP request header. However, RESTful approaches have been accomplished with other technologies like Constrained Application Protocol (CoAP), which is an implementation of REST for constrained environments for IoT (Internet of Things) applications. RESTful principles can also be followed in messaging brokers like RabbitMQ and MQTT, where resource identifier and CRUD operation could be potentially mapped to message destinations or topics.

Interoperability

SOAP was designed with interoperability in mind, following open standards, and not bound to any specific implementation, platform, or programming language. However, some things in the specification were left open to interpretation. Some parts may be confusing or have errors or typos or bad examples. The issues stem from simple things like whether:

• A particular value is supposed to be wrapped in quotes or not.
• Certain XML constructs are OK or should be avoided.
• Specific types of things should be allowed or restricted in the SOAP Body.

Another standards body, the Web Services Interoperability Organization (WS-I), came about to provide guidelines for web service interoperability. The WS-I provides various interoperability profiles. Each profile has a list of requirements and a list of assertions, which define how to check the requirements. In short, the WS-I profiles say things like “You should do this” and “You shouldn’t do that.”

Fun fact! Parasoft was a contributor to the WS-I Basic Profile 1.1 Test Assertions Document (TAD).

REST APIs are interoperable in that they are simple to invoke. There are many tools and APIs that can make HTTP requests. Popular tools include cURL and Postman. Even a simple form on a web page can be used to make HTTP requests. There are also various open standards that are typically used for REST APIs aside from HTTP, including open message formats like JSON. REST APIs can also implement various open standards for security and authorization. More on that later.

Positive Comparison

SOAP was designed with interoperability in mind. The W3C SOAP specifications are primarily authored by Microsoft, which has its own SOAP stack that’s implemented as part of Microsoft’s .NET Framework, originally .NET Web Service Enhancements (WSE) and later .NET Windows Communication Foundation (WCF). However, SOAP stacks are available from many other vendors including open source projects like the Apache project. Aside from .NET, SOAP stacks are also available for different platforms and programming languages like Java, Python, and TypeScript. SOAP clients and SOAP services implemented with different SOAP stacks are capable of communication if they follow the same set of open SOAP standards.

REST APIs follow the KISS (“keep it simple, stupid”) principle, following the general design principles of REST software architecture. Being simple to invoke, REST APIs do not necessarily require a complex software stack like you typically need for communicating with SOAP endpoints.

Critical Comparison

SOAP has a myriad of extensions often referred to as WS-*. There is WS-Addressing, WS-Policy, WS-Discovery, WS-MetadataExchange, WS-SecureConversation, WS-SecurityPolicy, WS-Trust, and WS-Federation. There is also WS-Security with various related specifications including those related to XML and SOAP signature, XML and SOAP encryption, and SAML (Security Assertion Markup Language). The list goes on, and on, and on. Chances are, a SOAP service is going to be following several WS-* specifications, adding complexity to what was originally defined as being a “simple” protocol. Your client has to follow the same WS-* standards as the service, or they won’t be able to communicate correctly.

REST APIs do not necessarily have to follow open standards. Although JSON is very popular, there’s no standard data interchange format. Any content type is a possibility, including possibly proprietary data formats. Additionally, certain security or authorization frameworks can introduce extra complexity, requiring a compatible implementation on the client side.

Security

Security is an important consideration for both SOAP and REST. Transport layer security is needed to encrypt messages as they’re sent over the wire to prevent eavesdropping. Message layer security is necessary for full end-to-end security, so the message is protected from any intermediaries that may have access to it before reaching its intended destination. Authentication or authorization mechanisms are needed to establish the identity between client and server.

Positive Comparison

SOAP has a large collection of security specifications known as WS-Security, published by the standards body OASIS. Aside from transport layer security mechanisms like HTTPS, WS-Security specifications describe how to embed security directly within the SOAP messages themselves (including signatures, encrypted data), and how to package security tokens for establishing identity like SAML (Security Assertion Markup Language).

REST can leverage existing mechanisms in HTTP for security including SSL and HTTP-based authentication schemes. There are also open standards for authorization including OpenID Connect (OIDC), which builds upon OAuth, and some other open specifications like JSON Web Token (JWT).

Critical Comparison

SOAP. OASIS WS-Security specifications are complex. Services implementing multiple WS-Security and other WS-* specifications pose challenges for building clients.

• What key stores do I need?
• Do I need to sign the message or encrypt it too?
• What XML canonicalization algorithm am I supposed to use?
• Do I need to first get a SAML token and include it in a SOAP Header?
• What parts of the message do I need to sign and encrypt?

REST. Message layer security is not standard, some being proprietary. For example, Amazon AWS provides server-side and client-side mechanisms for encrypting messages sent to or from their APIs.

Service Definitions

There are various types of machine consumable documentation formats for SOAP services and REST. Service definition documents enable automated processing, like automated code generation for clients or service stubs. Service definition documents can also be translated into a human-friendly documentation format like a web page.

Positive Comparison

SOAP. SOAP services are described by WSDL, another open specification by the W3C. A WSDL is an XML document. It defines the service endpoints, operations, and XML schemas for request, reply, and fault messages.

REST. There are various service definition formats for REST APIs. These include OpenAPI, RAML, API Blueprint, and WADL. They all provide different ways to describe things common to REST APIs like resource paths, parameters, operations, and type and format or schema of representations.

OpenAPI is based on JSON Schema specifications and can be represented as either JSON or YAML. RAML documents are based on YAML and support both JSON Schema and XML Schema (XSD). API Blueprint is based on Markdown Syntax for Object Notation (MSON) with support for JSON Schema. WADL is a W3C submission, based on XML with support for describing XML representations using XML Schema, a bit analogous to WSDL for SOAP.

Critical Comparison

SOAP. The WSDL specification has some issues with additional clarification and interoperability recommendations provided by the Web Services Interoperability Organization (WS-I). There are also multiple versions of WSDL. WSDL 1.1 is most commonly implemented. WSDL 2.0, formerly known as WSDL 1.2, was not adopted by all SOAP stacks including Microsoft’s .NET WCF. Interestingly, WSDL 2.0 introduced support for describing REST APIs.

REST. There is no standard service definition format for REST APIs. However, OpenAPI is a close consideration for being the standard. OpenAPI was originally developed by SmartBear as “Swagger”, later donated to the Linux Foundation as OpenAPI. The specification is overseen by the OpenAPI Initiative, which includes members from a diverse collection of large corporations including Google, IBM, and Microsoft.

Each service definition format has its own collection of tooling for code and doc generation. This means you need to use a different set of tooling depending on the service implementation. However, converters exist so you could convert an OpenAPI document to a RAML or vice versa.

Use Cases & Examples

SOAP and REST can satisfy similar use cases but vary in the way they handle requests and responses. Below are some examples.

SOAP Use Cases

Here are some use cases of SOAP-based web services.

1. Currency Conversion Service

A web service that allows clients to convert between different currencies. The client sends a SOAP request containing the source currency, target currency, and the amount to be converted. The server processes the request and sends back a SOAP response containing the converted amount.

2. Flight Reservation System

A web service that enables clients to book flights. The client sends a SOAP request with the necessary flight details like departure city, destination, date, and passenger information. The server processes the request and sends back a SOAP response confirming the reservation.

3. Weather Information Service

A web service that provides weather information for a given location. The client sends a SOAP request with the desired location, and the server responds with a SOAP message containing details such as temperature, humidity, and forecast.

4. Customer Management System

A web service that handles customer information for an e-commerce platform. The client can send SOAP requests to create, update, or retrieve customer data. The server processes the requests and sends SOAP responses with the appropriate status or requested customer information.

In each use case, the SOAP requests and responses are structured XML messages exchanged between the client and the server. The example below explains it better.

SOAP Example

Here is a code snippet example of a banking app that uses SOAP to interact with a server for account management.

In this example, the SOAP request is for a fund transfer operation. The request includes the source account, destination account, and the transfer amount. The SOAP response includes the transaction ID and the status of the transfer.

REST Use Cases

Here are some use cases of REST APIs that leverage HTTP and JSON.

1. Social Media Platform

The REST API for a social media platform allows users to create, retrieve, update, and delete posts, comments, and user profiles. Clients can use HTTP methods like POST, GET, PUT, and DELETE to interact with the API endpoints. For example, a client can send a POST request to create a new post, a GET request to retrieve a list of posts, or a PUT request to update a user’s profile information.

2. Ecommerce Store

Using a RESTful API for an e-commerce store enables clients to browse products, add items to a shopping cart, place orders, and retrieve order history. Clients can use HTTP methods to perform actions like GET for retrieving product information, POST for adding items to the cart, and PUT or DELETE for updating or removing items from the cart.

3. Financial Service

A RESTful API for a financial service allows clients to perform various operations such as account balance inquiries, fund transfers, and transaction history retrieval. Clients can use HTTP methods like GET to retrieve account information, POST to initiate a fund transfer, and GET to fetch transaction history.

4. Mapping and Geolocation Service

When used for mapping and geolocation services, a REST API can provide functionality for address lookup, geocoding, and routing. Clients can send requests to the API endpoints with parameters like addresses, coordinates, or specific routes to obtain the desired information. The API responds with the corresponding data in a format such as JSON or XML.

REST Example

Now, let’s consider a movie database API that uses REST for retrieving movie information.

In the above example, the REST request is to retrieve information about a specific movie with the ID “123456”. The REST response has a status code of 200 (OK) and is in JSON format, providing details such as the movie title, year, director, genre, and rating.

The above examples highlight the differences in how requests and responses are exchanged but they can also satisfy the same use cases. In other words, the above REST APIs could be implemented as SOAP services and vice versa.

For a given REST API, a SOAP operation can be defined for each CRUD operation and any resource parameters can translate to XML elements within the SOAP Body. The above SOAP examples can also be implemented as REST APIs. One approach is to map different SOAP operations to different HTTP verbs. The customer management system example works well for this, having CRUD-based SOAP operations that would cleanly map to corresponding HTTP verbs.

The other approach is to map different SOAP operations to different resource paths. The currency conversion example could be made RESTful by having the type of currency represented in the resource URI. Then you can literally “GET” the representation of the desired currency where the conversion parameters are submitted as query parameters like “/conversion/dollar?fromAmount=5&fromType=pesos” which could return the result as a JSON number.

Comparing SOAP & REST for API Testing

REST and SOAP provide their own unique tradeoffs and challenges, especially with respect to testing. To test an API, you need to be able to build clients, send input data, and then be able to view and validate the output that was returned. To draw a comprehensive comparison between the two, let us first look at SOAP and REST for API testing along the following lines.

Testability

SOAP APIs are more complex and require specialized tools that understand the SOAP protocol. Testing SOAP-based services involves XML parsing, interpreting WSDL and WS-* specifications, and handling SOAP envelope structures. Fortunately, a sophisticated SOAP testing tool like Parasoft SOAtest provides features such as automated test generation, simplifying these complexities and making SOAP API testing faster and more efficient.

On the other hand, REST APIs follow a more straightforward design, leveraging standard HTTP methods and data formats such as JSON. This simplicity makes REST APIs more approachable for testing. Parasoft SOAtest also offers excellent support for REST API testing, enabling testers to easily construct HTTP requests, validate responses, and perform functional and integration testing.

Test Execution

SOAP API testing involves executing test cases that interact with the API through SOAP envelopes. This requires handling XML parsing, SOAP headers, and adherence to the SOAP protocol and various WS-* standards. SOAP services may also be implemented over transport protocols other than HTTP. While this may sound way too complex, the extensive features provided by SOAtest, with full support for many transport protocols and complex WS-* SOAP specifications, make it possible to automate these processes.

REST API testing, with its simplified nature, offers quicker test execution, making it easier to verify the API’s behavior and response. REST APIs also lend themselves well to automation, as popular testing frameworks often provide robust libraries for making HTTP requests and assertions. This enables the creation of automated test suites that can be easily integrated into continuous integration (CI/CD) pipelines.

Test Case Generation

SOAP APIs rely on Web Service Description Language (WSDL) specifications to define their operations, data types, and message structures. This formal specification facilitates the generation of test cases based on the defined contract. While some tools can help with test case generation, Parasoft SOAtest builds sophisticated clients that correctly adhere to the XML Schema types and WS-* requirements described by the WSDL, reducing the manual effort required to create test scenarios.

REST APIs are often described by a formal document like an OpenAPI definition. Service definition documents can be large or complex, depending on the number of resources, parameters, and JSON schema types. Parasoft SOAtest builds sophisticated clients that correctly adhere to the parameter and JSON schema types expected by each resource and HTTP method. However, REST APIs sometimes lack a formal specification like OpenAPI, which can make test case generation more challenging.

Testers often rely on pre-recorded traffic messages to define their test scenarios. While this normally introduces a higher degree of manual effort, Parasoft SOAtest‘s Smart API Test Generator automates test creation by monitoring API calls and leveraging artificial intelligence (AI) to automatically construct and configure test scenarios.

That said, it’s crucial to note that the choice between SOAP and REST for API testing depends on the project’s specific requirements. SOAP’s strict contract and validation features suit enterprise-level applications that demand standardization and interoperability. On the other hand, REST excels in simplicity, flexibility, and ease of testing, making it a preferred choice for many modern web services. Nonetheless, whichever option is most suitable for your project will benefit from API test automation offered by Parasoft SOAtest.

Positive Comparison

SOAP. A WSDL document can provide a full description of a SOAP service, including its security requirements. There are various commercial and open source tools available that can consume WSDL documents to automatically produce clients for testing SOAP endpoints. One simple example is Microsoft’s WCF Test Client application.

REST. REST APIs can similarly provide a service definition document, which can be consumed to produce test clients. For OpenAPI, Swagger UI provides a simple web interface to “try out” every single operation exposed by the API.

Critical Comparison

SOAP services may be implemented using protocols other than HTTP, where communication may require a specific messaging interface like JMS. Various WS-* protocols are complex. Free and open source tools have limitations and lack of support for all transport and WS-* protocols. However, Parasoft SOAtest helps solve this, having comprehensive support for SOAP and related protocols.

REST services don’t necessarily have service definitions. Manually building clients can be difficult and tedious, determining the right sequence of API calls to string together to create the desired scenarios. However, Parasoft SOAtest helps solve this. In addition to being able to create test clients from various service definition formats, SOAtest‘s Smart API Test Generator automates API test creation by monitoring API calls and using AI to automatically construct and configure test scenarios.

Is your head spinning yet? Let Parasoft help. Reduce the cost, time, and complexity of testing service interfaces with the full end-to-end API testing solution, Parasoft SOAtest.