SOAP vs REST APIs: 5 Critical Differences

SOAP (Simple Object Access Protocol) is known as a protocol that was designed to ensure that programs designed using different programming languages and on different platforms could exchange data in an easy manner. SOAP leverages service interfaces to expose its functionality to client applications.

REST API is defined as a Software Architectural Style built to guide the development and design of the World Wide Web’s architecture. REST APIs define a set of constraints on how the architecture of a distributed system should behave.

This blog talks about the SOAP vs REST differences in great detail. It also gives a brief introduction to SOAP and REST APIs before diving into the difference between SOAP and REST.

What is REST API?

REST APIs provide a flexible, lightweight way of integrating applications. REST APIs allow you to search for some items, and then the results are returned from the service you requested from. A REST API has the following components:

• The Headers: Headers refer to the Authentication Mode and the Content Types.
• The Method: There are various methods that you can use to call a REST API, the most common ones being POST and GET. The GET method is used to gather information while the POST method can be used to send information to the resources on the web.
• The Endpoint: The Endpoint is defined as the URL of the resource to which you are going to place the requests. For instance, https://api.powerbi.com/v1.0/myorg/.
• The Data or Body: This refers to the textual data, typically in the form of JSON, which needs to be sent to the resource.

Here are a few REST Design Principles that are integral for the smooth functioning of a REST API:

• Client-Server Decoupling: In REST API design, Server and Client applications must be completely independent of each other. The only information available to the Client-side should be the URI of the requested resource. Similarly, a Server application shouldn’t change the Client application other than passing the requested data through HTTP.
• Statelessness: REST APIs are Stateless. This means that each request should contain all the information required to process it. This can also mean that REST APIs don’t require any server-side sessions. Server applications aren’t permitted to store any data related to the Client request.
• Cacheability: Whenever possible, resources should be Cacheable on the Server or Client-side. Server responses should also contain information on whether caching is allowed for the delivered resource. You should aim for improving performance on the Client-side while increasing scalability on the Server-side.
• Uniform Interface: All API requests for the same resource must look the same, irrespective of where the request comes from. The REST API should ensure that the same piece of data, like the email address or name of a user, belongs to only one Uniform Resource Identifier (URI). Resources shouldn’t be too huge but should contain every piece of information that the client may need.
• Layered System Architecture: In REST APIs the responses and calls go through different layers. You shouldn’t assume that the Client and Server applications connect directly to each other. There may be various intermediaries involved in the communication loop. Therefore, REST APIs need to be designed such that neither the Server nor the Client can tell if it communicates with an intermediary or an end application. 

What is SOAP?

SOAP is slightly more complex as it defines more standards than REST APIs- things like how messages are sent and security. These built-in standards carry a bit more overhead. Despite this, these standards can be the deciding factor for organizations that need more comprehensive features in the way of Transactions, Security, and ACID (Atomicity, Consistency, Isolation, Durability) compliance. Here are a few key features of SOAP that give it an edge over other protocols:

• Successful/Retry Logic for Reliable Messaging Functionality: SOAP has a built-in Retry/Successful Logic that provides end-to-end reliability even across SOAP intermediaries.
• Built-in ACID Compliance: ACID Compliance decreases the occurrence of anomalies and protects the integrity of your database by prescribing how transactions can interact with your database. The ACID model is considered more conservative than other Data Consistency models. This is why it is favored when handling sensitive information such as financial transactions.
• Robust Security: On top of SSL support, WS-Security is a built-in standard that allows SOAP to offer some more enterprise-level security features should you need them.

Examples of REST & SOAP

To better understand the practical differences between REST and SOAP, here is an example that demonstrates how the same operation can be performed using the two technologies. In this example, you will be making a request for User Details.

1) REST Example

REST APIs can be easily called with all HTTP verbs. To obtain a resource (here, a user), a GET request can be used. While the SOAP request holds the user’s name in the body, a REST API accepts GET parameters from the URI. Here is what that looks like:

GET https://restexample.com/users?name=John

REST APIs generally use the JSON data format. So, this is what the user details will look like when represented in JSON:

{ "name": "John", "age": 5, "address": "Greenville" }

2) SOAP Example

When you use SOAP, the request made to the API is an HTTP POST request with an XML request body. The request body contains an envelope which is a kind of SOAP Wrapper that identifies the requested API along with a SOAP body that contains the request parameters. For this example, you will be fetching the user with the name “John”. Here is the SOAP snippet you can use:

<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:sch="http://www.soapexample.com/xml/users">
<soapenv:Header/> 
<soapenv:Body>
<sch:UserDetailsRequest>
<sch:name>John</sch:name> 
</sch:UserDetailsRequest> 
</soapenv:Body> 
</soapenv:Envelope>

The response similar to the request is made up of a SOAP envelope and a SOAP body. Here, the SOAP body represents the requested user data. Here is the code snippet for the response:

<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/"> <soapenv:Header/> <soapenv:Body> <ns2:UserDetailsResponse xmlns:ns2="http://www.soapexample.com/xml/users"> <ns2:User> <ns2:name>John</ns2:name> <ns2:age>5</ns2:age> <ns2:address>Greenville</ns2:address> </ns2:User </ns2:UserDetailsResponse> </soapenv:Body> </soapenv:Envelope>

Differences between SOAP and REST

Here are the critical differences between the topic SOAP vs REST:

• SOAP vs REST API: Architecture
• SOAP vs REST API: Security 
• SOAP vs REST API: Caching
• SOAP vs REST API: Handling Payloads
• SOAP vs REST API: Bandwidth and Resources

SOAP vs REST API: Architecture

An API is designed to primarily display certain aspects of an application’s business logic on a server. SOAP uses a Service Interface to do this, while REST leverages URIs for the same. While SOAP APIs are designed after the functions that the API depicts, REST APIs are designed after the data. REST is an architecture that is more Data-driven as opposed to SOAP which is more Function-driven.

SOAP vs REST API: Security

SOAP supports WS-security, which is desirable at the transport level and slightly more comprehensive than SSL. It is also ideal for integration with enterprise-level security tools. Both SOAP and REST support SSL for end-to-end security and REST can use the secure version of the HTTP protocol, known as HTTPS.

SOAP vs REST API: Caching

You can mark data as cacheable, which allows it to be reused by browsers without initiating another request back to the server. This helps save valuable time and effort. Since SOAP requests are sent using a POST request, which is considered as non-idempotent by the HTTP standard, responses won’t be cached at the HTTP level. REST APIs don’t have this implementation but you still need to implement the caching mechanisms yourself if you wish to use caching.

SOAP vs REST API: Handling Payloads

A payload is defined as data sent over the Internet. When a payload is deemed ‘heavy’, it requires more resources. REST tends to use JSON and HTTP, which help lighten the payload, as opposed to SOAP, which uses XML. SOAP APIs have a very strict communication contract that usually requires the Client to use a specific Client library with generated code to access them. This means that SOAP is more tightly coupled with the server and provides a lower abstraction layer as compared to REST.

SOAP vs REST API: Bandwidth and Resources

SOAP needs more bandwidth since there is a slightly greater overhead with SOAP because of the envelope style of payload transport. Since REST is primarily used for web services, its lightweight nature is an advantage in these scenarios.

When to use REST API?

Here are a few use cases where leveraging REST APIs is a good option:

• Error Reporting and Monitoring: REST allows you to build a monitoring system based on API responses. Other alternatives like GraphQL don’t provide the facility for this feature since it always returns a 200 OK status response.
• Non-existent HTTP Caching Mechanism: Nowadays, all browsers offer an implementation of the HTTP cache to easily avoid refetching resources. It is also used to identify if two resources are the same.
• Resource Attacks: Alternatives like GraphQL allow you to query exactly what you want whenever you want it. But, this can lead to some serious security complications. For example, if a malicious party attempts to submit an expensive nested query to overload your database or server and your server doesn’t have the right protections, you might fall victim to DoS (Denial-of-Service) Attacks. This is where REST APIs can come in handy to help protect you against DoS attacks.

When to use SOAP?

Here are a few instances where leveraging SOAP can come in handy:

• Stateful Operations: If your application has a requirement that the state needs to be maintained from one request to another, then the SOAP 1.2 standard gives you the WS* structure to complement these requirements.
• Asynchronous Processing and Subsequent Invocation: If the application poses a requirement that the Client needs a certain level of assured security and reliability then the new SOAP standard of SOAP 1.2 offers a lot of extra features that can especially help establish a robust security framework. 
• Formal Means of Communication: If both the Client and Server have an agreement on the exchange format, then SOAP 1.2 offers the rigid specifications for this kind of interaction. A good instance would be an online purchasing site in which users add items to a cart before the payment is made. Suppose, you have a web service that does the final payment. You can have a firm agreement that the web service will only accept the cart quantity, unit price, and item name. In the light of such a scenario, it is always better to use the SOAP protocol.

SOAP vs REST: Summary Table

Here is the summary table for the SOAP vs REST discussion:

SOAP	REST
SOAP is an abbreviation for Simple Object Access Protocol.	REST is an abbreviation for Representational State Transfer.
SOAP is a protocol that is designed with a specification. This consists of a WSDL file that has the required information on what the web service is capable of. It also provides the location of the web service.	REST is known as an architectural style where a web service can only be treated as a RESTful service if it adheres to the constraints like being Cacheable, having a Layered System, a Client-Server, a Uniform Interface, and being Stateless.
SOAP needs more bandwidth for its usage. Since SOAP messages contain a lot of information, the amount of data transfer leveraging SOAP is a lot.	REST, on the other hand, doesn’t need as much bandwidth when requests are sent to the server. REST messages usually just contain JSON messages. Here is an example of a JSON message that has been forwarded to a web server. You can always notice that the size of the message is comparatively smaller than SOAP:
SOAP cannot utilize REST since SOAP is a protocol as opposed to REST (architectural style).	REST, on the other hand, can leverage SOAP as the underlying protocol for web services. This gives REST a slight edge in the SOAP vs REST discussion.
SOAP is only compatible with the XML format. As seen in the SOAP snippets before, all data is passed in XML format.	REST is compatible with various different formats as opposed to SOAP such as Plain Text, JSON, HTML, and XML. The most favored format for transferring data through a REST API is JSON. So in terms of compatibility with languages, REST is the clear winner in the SOAP vs REST discussion.
SOAP leverages service interfaces to display its functionality to client applications.	REST leverages Uniform Service Locators to access the components on the hardware device. For instance, if there is an object that represents the data of an employee hosted on a URL.

Which API is the Best for your Project?

To summarize the discussion of SOAP vs REST for this article, when it comes to APIs for web services, developers tend to be partial towards a RESTful architecture unless the SOAP is clearly the better choice. This can be the case for an enterprise app that’s backed by more resources, has more requirements, and needs robust security.

Here are a few extra advantages of leveraging REST APIs for your use case:

• Enables the use of effective caching.
• Lightweight communication with the help of HTTP and lighter payloads, for instance, in the JSON format.
• Fewer requirements for external libraries on the Client-side.

Here are a few instances where you might be better off choosing SOAP:

• Need to integrate with legacy systems already leveraging SOAP.
• Enterprise-level requirements on security.
• Requirements on ACID transactions or the use of the built-in retry mechanisms provided by SOAP.

Irrespective of the technology you use from SOAP vs REST, the most crucial part of building a good API is designing it by leveraging the best practices to make it easy to understand and use for the clients. A well-designed API can vastly increase your delivery speed and future-proof your technology stack.     

Challenges With SOAP API

WSDL file

One of the most complex aspects of the SOAP API is the WSDL document itself. The WSDL document informs the customer of all the actions that the web service is capable of doing. The WSDL document will contain all information, such as the data formats used in SOAP messages and the functions provided through the web service.

The code snippet below is only a portion of an example WSDL file.

<?xml version="1.0"?>
<definitions name="Tutorial"             
	targetNamespace=http://demo.guru99.com/Tutorial.wsdl             
	xmlns:tns=http://demo.guru99.com/Tutorial.wsdl             
	xmlns:xsd1=http://demo.guru99.com/Tutorial.xsd            
	xmlns:soap=http://schemas.xmlsoap.org/wsdl/soap/
	xmlns="http://schemas.xmlsoap.org/wsdl/"> 

	<types>  
		<schema targetNamespace=http://Demo.guru99.com/Tutorial.xsd
		xmlns="http://www.w3.org/2000/10/XMLSchema">

      	<element name="TutorialNameRequest">    
			<complexType>          
				<all>           
					<element name="TutorialName" type="string"/>         
				</all>       
			</complexType>    
		</element>     
	<element name="TutorialIDRequest">        
		<complexType>          
			<all>           
				<element name="TutorialID" type="number"/>         
			</all>       
		</complexType>      
	</element>   
	</schema>  
</types>	

According to the WSDL file above, we have an element named “TutorialName” of the type String that is part of the element TutorialNameRequest.

For example, the WSDL file needs to be changed due to business requirements, and the TutorialName has to be changed to TutorialDescription. This would imply that all clients presently connecting to this web service would need to make the necessary adjustment in their code to accommodate the WSDL file update.

This demonstrates the WSDL file’s most problematic issue, which is the impenetrable contract between the client and the server, and how a single modification may have a tremendous impact on all client applications.

Document Size

The size of the SOAP messages that are transmitted from the client to the server is another significant difficulty. Because of the high message sizes, employing SOAP in regions where bandwidth is limited — a significant challenge, indeed!

Challenges in REST API

Lack of Security

Unlike SOAP, REST does not impose any type of security. This is why REST is ideal for publicly accessible URLs, but when it comes to passing sensitive data between the client and the server, REST is the worst technology to utilize for web services.

Lack of state

Most online applications need the use of a stateful method. For example, if you had a purchasing site with a shopping cart mechanism, it is necessary to know the number of products in the shopping cart before the actual transaction is done. Unfortunately, the client bears the burden of preserving this state, which makes the client application heavier and more difficult to maintain.

REST & SOAP Alternatives

Here are a few key alternatives to REST and SOAP:

• GraphQL
• JSON
• gRPC

1) GraphQL

GraphQL is a Query Language that can be used to work with APIs. It enables the client to make HTTP requests and get responses.

As the name suggests, any set of information in GraphQL is seen in the context of a graph. Nodes defined under the GraphQL Schema System are used to depict objects. Edges between these Nodes are used to represent the connection between the nodes of a graph. This ensures clear relationships between queries and improves the connectivity between objects. GraphQL can also allow the users to request data from various resources using a single request. As opposed to making multiple requests to fetch data, you can use GraphQL to make ad-hoc queries to a single endpoint and access all the necessary data.

2) JSON

JSON is an Open-standard file format that is used to transmit data objects across applications. It is a lightweight format to transfer and store data. It is usually used when sending data from a server to a webpage. The quick and simple transmission of SOAP makes it a suitable alternative in various situations.    

3) gRPC

gRPC is an Open-source System that leverages HTTP/2. It was developed by Google to commonly connect services in a microservices architecture and to connect mobile devices to backend services. The benefits of gRPC include more lightweight messages than JSON, built-in code generation, high performance, and support for more connection options like streaming data. 

Conclusion

This blog talks about the different aspects of SOAP and REST APIs before diving into the differences between the two. It also gives a glimpse of the viable alternatives of the two that you can avail yourself of in the market.

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