A Closer and Detailed Look at SOAP API

An API, or Application Programming Interface, is software that allows two different applications to send messages to each other. 

For example, say you need to log into an app using your Google account because you want a check-in email to arrive in your main inbox. Here, a Web API is used. It’s an agreement between these web services about how they are going to exchange data.

Most APIs use the REST (Representational State Transfer) pattern. Occasionally, you may stumble across the SOAP protocol approach.
What are SOAP’s unique advantages? Let’s take an in-depth look.

What Is SOAP?

SOAP is short for Simple Objects Access Protocol. It is a method for communication between applications. It’s mostly used for transmitting data over HTTP/HTTPS.

SOAP only works with XML and imposes its own encoding rules and conventions. SOAP can be used regardless of whether the applications are built on different platforms or use different languages.

Though SOAP has been in use since 1998, it’s unlikely to disappear anytime soon. It’s secure, highly standardized, and allows for automation testing.

SOAP and XML

The most common formats for structuring web-transmitted data are JSON and XML. XML (Extensible Markup Language) is a markup language like HTML, designed to store data. It’s self-descriptive and simple to understand. However, it’s also fairly text-heavy and uses a lot of bandwidth.
Requests for data sent through a SOAP API over HTTP (or another application layer protocol) are processed before the data is returned in an XML document.

SOAP Message Structure

SOAP messages contain four elements, each with specific functions:

Envelope:

The envelope is the essential element that identifies the document as a SOAP message, as opposed to any other type of XML document. Messages begin and end with the envelope’s tags. They are literally ‘enveloped’.

Header: 

The header is an optional element that can make it possible to add new features and functionalities using SOAP Modules. An envelope can contain more than one header.

Body:

The body contains the actual message: the request or response. This element is mandatory. 

Fault: 

Used for error messages and status information if there is an issue while processing. Problems may be caused by bad SOAP formatting, mismatched data types, or more. Though a Fault tag is technically optional, it is advised to encompass any potential problems.

How Does SOAP Differ From REST?

SOAP is a protocol. On the other hand, REST is a set of test execution principles for building applications in many different ways. 

An application is RESTful if it complies with these constraints:

Uniform Interface

A uniform interface simplifies the architecture. Individual resources must be identified in requests. A representation of a resource must contain enough information to modify the resource. Messages must be self-descriptive and state their cacheability. Hypermedia is used as the engine of application state – ie. except for the URL of the application, all interactions should be driven by hyperlinks.

Client-Server

Separate the interface concerns from the data storage concerns to ensure that the application is portable across platforms. Simplify the server components to make them more easily scalable.

Stateless

Every request must contain the information necessary to understand the request. Information from the past cannot be stored. Each request is separate.

Cacheable

Caching allows web browsers to save and reuse data. This improves speed and performance and allows for scalability. Therefore, caching should be used for resources whenever possible. Responses must be labelled as cacheable or non-cacheable.

Layered System

The layers of the system are hierarchical. All components are connected but do not depend on each other, and cannot see beyond the layer they interact with.

Code on Demand

Instead of returning a response in JSON or XML, the API can provide code such as Javascript or applets, thus simplifying it. This constraint is optional.

Four basic types of commands are used in REST clients: GET (to retrieve resource representation), POST (to create a resource), PUT (to edit a resource), and DELETE (to delete a resource).

To summarize, SOAP and REST are designed differently. While SOAP is a strict protocol, REST is a style with a set of guidelines. SOAP is heavyweight, while REST is lightweight and requires fewer resources and input, which could free up time and boost your team’s overall productivity.

Let’s examine their features in closer detail.

Bandwidth Usage

Bandwidth is one of the key differences between the two APIs. SOAP messages contain a lot of data. SOAP messages, with their hefty XML, tend to be longer than REST messages. 

If you are a fast-growing business with a large team, you may have the resources to handle SOAP. For smaller projects, REST could be implemented.

Data Formats

While SOAP APIs only use XML for their messages, REST APIs typically use JSON. JSON messages are shorter and less dense than those in XML. REST also has the capability to support other formats, including XML, HTML, YAML, and plain text. 

Security 

SOAP is a highly secure API. It is compatible with WS security and SSL. Additionally, SOAP is ACID-compliant. ACID is an enterprise standard for security. ACID compliance means that database transactions have the following standards:

Atomicity

If one component of a transaction doesn’t work, the whole transaction won’t work. Therefore transactions must work as a single unit or not at all. 

Consistency

Only data that follows data validation rules can be written into the database. Transactions that don’t follow the rules will be ‘rolled back' to a previous state.

Isolation

The ability to process multiple transactions or requests at the same time without any of them affecting the other. This is crucial for e-commerce platforms, inventory management software, and other retail-related applications.

Durability

Data is saved after transactions are completed, even in cases of system failure. This ensures that even if an app breaks down in the middle of a customer purchase, the transaction isn’t lost.

Thus, SOAP is an excellent choice for applications that involve the exchange of sensitive information or otherwise require a high level of data protection.

REST uses HTTP. It can support WS security as well as TLS (Transport Layer Security). TLS keeps internet connections private and checks that any data sent between systems is encrypted and has not been modified. This protects data from malicious hackers.

Overall, SOAP APIs have more comprehensive security measures which adhere to enterprise standards. APIs which handle financial transactions, such as eCommerce or affiliate programs, should opt for SOAP.

REST APIs should not handle sensitive data without additional security measures.

Caching

SOAP does not use caching mechanisms, though it is possible to configure a cache module. However, REST enables caching via HTTP. In general, if your application requires the use of caching, REST is the better API. 

Statelessness

As previously mentioned, one of REST’s key constraints is statelessness: information cannot be retained from previous requests. Every request is treated individually, without context. The user preferences or data from previous queries, for example, may be remembered and assisted by the stack flow community that supplements your API with user data storage capabilities.

But in some cases, an “information flow” is required, so statefulness must be implemented. Although statefulness is not one of SOAP’s protocol principles, usability testing can be easily adapted for this purpose.

REST is always stateless, while SOAP can be stateless or stateful depending on the design.

Time and Effort

A final factor that should not be overlooked is labour intensiveness.

SOAP is powerful, adaptable, and highly secure. It can be used across platforms with different languages and is compatible with automation. However, SOAP has an intimidating learning curve. It’s highly complex and can take a long time to set up.

On the other hand, REST is intuitive, straightforward to understand, and easy to code. It’s also widely used, so it may be easier to find support when you do have queries. Though REST is not suitable for handling confidential data, it’s quick, lightweight, and convenient. The simplicity is a major plus.

While e-commerce platforms may prefer to use SOAP for their APIs, REST could be used for quick integration, such as syncing email to CRMs or other alternatives to apps like Vonage.

When To Use Soap

Though your choice of API will vary based on the specifications of your project, SOAP is usually used in the following instances: 

Asynchronous Operations:

Asynchronous programming is parallel programming in which a unit of work can run separately from the main application thread. In asynchronous operations, timing is important as one event needs to be able to trigger another. SOAP can also provide more robust security.

Stateful Operations:

Repetitive tasks which require statefulness cannot be performed in REST. SOAP is more statefulness-friendly. 

Formal Contracts:

In certain instances, an application needs to create a contract between its user and its service provider. This is particularly important for applications that are used on a large scale. Examples could include billing or navigation (ie. maps combining information from different sources to forge the best route).

SOAP uses WSDL to describe the terms of formal contracts. It is clear with little room for misinterpretation.

Summing Up

We use APIs for everything in technology, from syncing our data to a new app, to pairing a new Bluetooth wireless headset. Choosing an API solution is a crucial stage in the development process. SOAP and REST are two different approaches to the problem of data transmission.

SOAP is an important protocol that helped introduce the use of APIs, and continues to run many internal systems worldwide today. Though it imposes rules, it is extensible and customizable. In terms of security, SOAP sets a peerless standard.