When we consider API (application programming interface) architecture, a common comparison is SOAP vs. REST, two of the most common API paradigms. Despite being compared all the time, they are inherently different technologies and have differences on a granular level.

This is because SOAP is a protocol, and REST is an architectural style. A REST API can actually utilize the SOAP protocol, just like it can use HTTP. Hence, they are going to be packaged differently, function differently, and be used in different scenarios.

What is SOAP?

SOAP stands for Simple Object Access Protocol. It is a messaging protocol to interchange data in a decentralized and distributed environment. SOAP can work with any application layer protocol, for instance HTTP, SMTP, TCP, or UDP. It returns data to the receiver in an XML format. Security, authorization, and error-handling are built into the protocol and unlike REST, it does not assume direct point-to-point communication.

SOAP follows a formal and standardized approach that specifies how to encode XML files returned by the API.  A SOAP message is, in fact, an ordinary XML file that consists of the following parts:

  1. Envelope (required) – This is the starting and ending tags of the message.
  2. Header (optional) – Contains the optional attributes of the message. It allows you to extend a SOAP message in a modular and decentralized way.
  3. Body (required) – Contains the XML data that the server transmits to the receiver.
  4. Fault (optional) – Carries information about errors occurring during processing the message.
Here is how an ordinary SOAP message looks like. 
The example is from the W3C SOAP docs and it contains a SOAP envelope, a header block, and a body:

Reasons you may want to build an application with a SOAP API include higher levels of security (e.g., a mobile application interfacing with a bank), messaging apps that need reliable communication, or ACID compliance.

  • SOAP has tighter security: WS-Security, in addition to SSL support, is a built-in standard that gives SOAP some more enterprise-level security features, if you have a requirement for them.
  • Successful/retry logic for reliable messaging functionality: REST does not have a standard messaging system and can only address communication failures by retrying. SOAP has successful/retry logic built in and provides end-to-end reliability even through SOAP intermediaries.
  • SOAP has built-in ACID compliance: ACID compliance reduces anomalies and protects the integrity of a database by prescribing exactly how transactions can interact with the database. ACID is more conservative than other data consistency models, which is why it’s typically favored when handling financial or otherwise sensitive transactions.

What is a REST API?

REST (Representational State Transfer) is truly a “web services” API. REST APIs are based on URIs (Uniform Resource Identifier, of which a URL is a specific type) and the HTTP protocol, and use JSON for a data format, which is super browser-compatible. (It could also theoretically use the SOAP protocol, as we mentioned above.) REST APIs can be simple to build and scale, but they can also be massive and complicated—it’s all in how they’re built, added on to, and what they’re designed to do.

Why build an API to be RESTful?  – Resource limitations, fewer security requirements, browser client compatibility, discoverability, data health, and scalability—things that completely apply to web services.

Some quick REST information:

  1. Uniform interface – Requests from different clients should look the same, for example, the same resource shouldn’t have more than one URI.
  2. Client-server separation – The client and the server should act independently. They should interact with each other only through requests and responses.
  3. Statelessness – There shouldn’t be any server-side sessions. Each request should contain all the information the server needs to know.
  4. Cacheable resources – Server responses should contain information about whether the data they send is cacheable or not. Cacheable resources should arrive with a version number so that the client can avoid requesting the same data more than once.
  5. Layered system – There might be several layers of servers between the client and the server that returns the response. This shouldn’t affect either the request or the response.

REST and JSON

The REST architecture allows API providers to deliver data in multiple formats such as plain text, HTML, XML, YAML, and JSON, which is one of its most loved features. Thanks to the increasing popularity of REST, the lightweight and human-readable JSON FORMAT has also quickly gained traction, since it is very suitable for quick and painless data exchange.

JSON stands for JavaScript Object Notation. It is an easy-to-parse and lightweight data-interchange format. In spite of its name, JSON is completely language-agnostic, so it can be used with any programming language, not just JavaScript. JSON files consist of collections of name/value pairs and ordered lists of values that are universal data structures used by most programming languages. Therefore, JSON can be easily integrated with any language.

To see the difference between XML and JSON, here is an example code:This is how the above XML code looks like in JSON:

SOAP vs. REST: The key differences

REST APIs vs Microservices: The Differences

The terms RESTful API and Microservices go hand-in-hand when building a microservices-based application. However, they refer to very different things. The easiest way to understand the difference between RESTful APIs and microservices is as follows:

  • Microservices: The individual services and functions – or building blocks – that form a larger microservices-based application.
  • RESTful APIs: The rules, routines, commands, and protocols – or the glue – that integrates the individual microservices, so they function as a single application.

How They Work Together

Now that you have a better understanding of RESTful APIs and microservices, you can see how these two concepts work together to build a microservices-based application architecture: Microservices function as the “building-blocks” of the application by performing various services, while “RESTful APIs” function as the “glue” that integrates the microservices into an application.

When developers use RESTful APIs and microservices to create a modular, service-oriented architecture like this, enterprises can achieve the following benefits:

  • Scalability: Applications are easier and more cost-effective to scale because you only need to scale the services that need it and not the complete architecture.
  • Cost savings: Cost savings on development as upgrades are faster and easier
  • Resiliency: Greater ability to prevent and contain failure cascades
  • Easy upgrades: A modular architecture that facilitates adding, upgrading, or removing services and functions faster and with less chance of coding conflicts
  • Rapid development: Faster time to market when it comes to developing new functions and services
  • Security and compliance: Improved data security and compliance due to the containment of microservices – which work independently and don’t have knowledge of each other. RESTful API connections between microservices let you define strict rules for data access and sharing
  • Language agnostic: The ability to connect modular services that were programmed in different languages regardless of what kind of platform they’re running on
  • Smaller teams: Smaller, more agile development teams for each microservice
  • Agility: Greater agility to respond faster to changing business needs
  • Cloud-based architecture: Microservices usually run on cloud-based services like Amazon AWS

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