SOAP vs. REST APIs: Understand the Key Differences

These days, it’s more true than ever that “no company is an island.” From social logins to selling their data, many businesses rely on each other by exchanging information over the Internet–and much of that exchange is done via an API. When delving deeper into the question of developing APIs, you’ll undoubtedly encounter the question: SOAP or REST? Although REST APIs have become the most popular choice for today’s businesses, the decision isn’t always an easy one. In this article, we’ll go over everything you need to know about SOAP and REST APIs, so that you can come to the conclusion that’s ultimately right for your situation.

What is an API?

There are a lot of definitions of the term “API” out there, and many can leave you feeling more confused than before you read them. At its core, an API (application program interface) is a way for you to get the information that you need from a website in a consistent format. You can think of an API as like an interaction between a business and a customer, such as placing an order at a restaurant or getting cash from an ATM.

• Customers first read the menu or the ATM screen. Then, they decide what food they would like to order, or what transaction they would like to select.
• The waiter or ATM serves as the “middleman” between the customer and the business. They take the request from the customer and present it to the business in the way that’s most comprehensible and efficient.
• The business reviews the request and sends back a response to the customer, such as a plate of food or the customer’s account balance.

It’s important to note that in both of these examples, the interaction is entirely predictable. When customers go to a restaurant, they can assume that they’ll be presented with a menu, use that menu to place an order, and receive the food that they ordered. Meanwhile, most ATMs have a similar user interface that customers can easily navigate in order to withdraw money and check their balance. In the same way, APIs offer consistency and regularity to users who want to query a website for its data. By establishing a common set of rules for exchanging information, APIs make it easier for two parties to communicate. Suppose that you want to download 100 different articles from Wikipedia. You’d also like to know the date that each page was created, and which other Wikipedia pages link to that page. The good news is that you don’t have to visit each page individually and compile this information yourself. Wikipedia offers an API through which it can deliver this data (and more) to the user. You include the name of the article in your API request, and then you can parse the content of the API response to get what you’re looking for: the article text, the creation date, and the list of other pages. Some organizations offer their APIs as a product that other businesses can purchase, such as commercial weather service Weather Underground. The company sells access to its complete weather data and forecasts in the form of an API. Its customers can use that data for their own business purposes and in their own products.

What is a SOAP API?

SOAP (which stands for Simple Object Access Protocol) is an API protocol that uses the XML Information Set specification in order to exchange information. A standard SOAP message consists of the following XML elements:

• An Envelope element that identifies the document as a valid SOAP message.
• An optional Header element that specifies additional requirements for the message, such as authentication.
• A Body element that contains the details of the request or response.
• An optional Fault element that contains information about any errors encountered during the API request and response.

An example SOAP request for the weather.gov API might look like this:

<SOAP-ENV:Envelope xmlns:SOAP-ENV="http://schemas.xmlsoap.org/soap/envelope/" 
   xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
   xmlns:SOAP-ENC="http://schemas.xmlsoap.org/soap/encoding/" SOAP-ENV:encodingStyle="http://schemas.xmlsoap.org/soap/encoding/">

<SOAP-ENV:Body>

<ns8023:NDFDgenLatLonList xmlns:ns8023="uri:DWMLgen">

<listLatLon xsi:type="xsd:string">
   39.965506,-77.997048 39.916268,-77.947228
</listLatLon>

<product xsi:type="xsd:string">time-series</product>

<startTime xsi:type="xsd:string">2004-01-01T00:00:00</startTime>

<endTime xsi:type="xsd:string">2012-02-12T00:00:00</endTime>

<Unit xsi:type="xsd:string">e</Unit>

<weatherParameters>
   <maxt xsi:type="xsd:boolean">1</maxt>
   <mint xsi:type="xsd:boolean">1</mint>
</weatherParameters>

</ns8023:NDFDgenLatLonList>

</SOAP-ENV:Body>

</SOAP-ENV:Envelope>

This simple SOAP API request asks for the minimum and maximum temperatures for two locations in Pennsylvania between 2004 and 2012. As you can see, it contains a base Envelope element, which itself contains a Body element with the details of the request:

• The “ns8023:NDFDgenLatLonList” element, which contains the latitudes and longitudes of the two locations.
• The “startTime” and “endTime” elements, which denote the time boundaries of the request.
• The “weatherParameters” element, which denote the information that we are interested in seeing (here, the maximum and minimum temperatures).

What is a REST API?

REST (which stands for Representational State Transfer) is an architectural style for APIs that relies on the HTTP protocol and JSON data format to send and receive messages. Let’s use the example of the API for the copywriting marketplace Scripted.com. If you want to get all of the writing jobs that a particular business has ordered, for example, then you would make the following REST API request:

GET https://api.scripted.com/abcd1234/v1/jobs

where “abcd1234” is replaced with a key that is unique to the organization. With REST APIs, the details of the request–such as the type of request (jobs) and the organization (abcd1234)–are explicitly embedded in the URL itself, rather than being wrapped in an XML document like we saw with SOAP. REST APIs typically send back data in JSON format rather than XML. The corresponding JSON response would look something like:

HTTP/1.1 200 OK{

"id": "5654ec06a6e02a37e7000318",

"topic": "Where to Buy an Orangutan",

"state": "copyediting",

"quantity": 1,

"delivery": "standard",

"deadline_at": "2015-12-04T01:30:00Z",

"created_at": "2015-11-24T23:00:22Z",

"content_format": {

"id": "5654ec02a6e02a37e70000d5",

"name": "Standard Blog Post",

"pitchable": true,

"length_metric": "350-450 words",

},

"pricing": {

"total": 9900

},

"writer": {

"id": "5654ec01a6e02a37e700003b",

"nickname": "Bob L.",

},

"document": {

"id": "5654ec06a6e02a37e700031a",

"type": "Document"

}

}

Here, the HTTP response object contains details about the API request: for example, the title of the article, the length of the article, and the writer assigned to the article.

The Pros and Cons of SOAP and REST

When comparing REST and SOAP, people often use the analogy of a postcard and an envelope. REST is like a postcard in that it’s lightweight and consumes less bandwidth (paper). Meanwhile, SOAP is like an envelope: there’s an extra overhead required on both ends to package and unpackage it. Note that the analogy isn’t perfect: unlike a postcard, the content of REST requests and responses isn’t (necessarily) insecure. Instead, REST uses the security of the underlying transport mechanism, which is usually HTTPS. On the other hand, SOAP implements its own security measure, which is known as WS-Security. Some people believe that REST is largely a “replacement” for SOAP, due to its lower overhead and improved ease of use. According to Cloud Elements’ 2017 State of API Integration report, 83 percent of APIs now use REST, while only 15 percent continue to use SOAP. Some of these businesses primarily use REST, but continue to integrate SOAP APIs into their projects using tools such as DreamFactory’s SOAP connector. However, this conception of SOAP as outmoded isn’t quite accurate. Even as REST becomes the API style of choice for most businesses, SOAP remains a tool that is better-suited for certain use cases, mainly in large enterprises who need  additional extensibility and logic features native to the protocol. The advantages of REST include:

• Flexibility: Although REST is most commonly implemented with HTTP and JSON, developers are by no means obligated to use them. Websites can send back responses using data formats including JSON, XML, HTML, or even plaintext–whatever best suits their needs.
• Speed: Because it tends to use much less overhead, REST APIs are typically significantly faster than SOAP. While the differences might be imperceptible for a single request, the disparity grows larger and larger as you place more and more requests.
• Popularity: REST has reached critical mass on the Internet. Major websites such as Google, Twitter, and YouTube all use REST APIs for users to send and receive messages. Due to this familiarity, it’s typically easier for developers to get up and running with REST.
• Scalability: Thanks to their speed and simplicity, REST APIs usually perform very well at scale.

Despite the major benefits of using REST, SOAP remains the preferred protocol in certain use cases. Some organizations find that SOAP offers the transactional reliability that they’re looking for, while others simply continue to use SOAP because they need legacy system support. The advantages of SOAP include:

• Formality: SOAP can use WSDL (Web Services Description Language) to enforce the use of formal contracts between the user and the website. SOAP is also inherently compliant with ACID database standards, which ensures that the transactions it performs will be valid even in the event of errors or hardware issues.
• Logic: If a REST API request is unsuccessful, it can only be addressed by retrying until the request successfully goes through. On the other hand, SOAP includes built-in successful/retry logic so that the requesting system knows how to behave.
• Security: SOAP comes with its own security mechanism, WS-Security, built into the protocol. If you want to ensure that your messages are secure, rather than relying on the underlying transport mechanism as does REST, then SOAP may be the right choice.
• Extensibility: In addition to WS-Security, SOAP includes support for other protocols such as WS-Addressing and WS-ReliableMessaging that can define other standards of communication and information exchange.

Final Thoughts

For most cases, REST should be considered the “default” option as adoption continues to grow across the web. Most public-facing APIs now use REST, because it consumes less bandwidth and its compatibility with HTTP makes it easier for web browsers to use. However, you may find that the additional features and security offered by SOAP are enough to sway your decision. In the end, the “right” choice between SOAP and REST will be highly dependent on your own situation. Even better, the choice of SOAP and REST doesn’t have to be between one and the other. If you want to communicate with REST but still need access to legacy SOAP services, DreamFactory offers the ability to add a REST API onto any database or SOAP API. Reach out to us today to get a free demo from our team of API experts.