An application programming interface, or API, is a crucial element in connecting data flows with client and partner systems. The size of the global API management market is expected to increase from US $4.1 billion in 2021 to US $8.4 bn by 2024.

API is a collection of definitions and protocols used to create and integrate application software. APIs can be private, partner, or public. It's essential to produce a completely functional end result that software engineers will trust and want to use before releasing an API.

At Techstack, we have 8 years of market/domain experience in software development and product development. We will guide you through API, its types, specifications, and protocols. We will also list API testing tools and answer frequently asked questions for CIOs and CTOs who work with APIs.

What is a Web API?

A web API is a software interface for a web server or web browser. A browser API can increase the functionality of a browser, while a server API can extend the functionality of a server. APIs make it simpler for software engineers to build complex functionality. They allow engineers to avoid complex code and make syntax simpler.


A server-side web API is a programmatic interface consisting of one or more accessible endpoints to a specified request-response message system. These endpoints are often written in JSON or XML API format and are accessible via the web, most frequently using an HTTP-based web server.


When processing is client-side, it happens on the user's computer. It requires that browsers execute the scripts locally without utilizing any server-side processing. Client-side APIs are programming tools for changing various settings in the browser and operating system the site uses. They modify data from other websites or services.

Multiple components interacting via their APIs

How APIs Work: A Closer Look

APIs serve as the backbone of software communication, enabling diverse applications to exchange data and functionalities seamlessly. At their core, APIs define the methods and data formats that applications can use to communicate with each other, acting as intermediaries that relay requests and responses between systems.

This deep dive into APIs will focus on their technical workings, particularly how requests and responses are structured, and the role of web protocols like HTTP in their operation.

Understanding API communication

The communication between applications through APIs involves two main parties: the client, who sends a request, and the server, which responds to that request. This exchange is facilitated by a set of predefined rules and standards that both the client and server understand.  These rules and standards are primarily structured around HTTP (Hypertext Transfer Protocol), the foundational protocol of the web.

HTTP requests and responses

HTTP is designed as a request-response protocol in the client-server computing model. An HTTP request from a client to a server includes the following key components:

  • Method: Specifies the action to be performed on the resource identified by the URI. Common methods include GET (retrieve a resource), POST (create a resource), PUT (update a resource), and DELETE (remove a resource).
  • URI (Uniform Resource Identifier): Identifies the specific resource on the server that the client wants to interact with.
  • Headers: Provide additional information about the request or the client itself, such as content type or authentication details.
  • Body: Contains data sent by the client to the server, often used with POST or PUT requests to send the content of the resource being created or updated.

The server processes the request and sends back an HTTP response, which includes:

  • Status code: Indicates the result of the request, such as 200 (OK), 404 (Not Found), or 500 (Internal Server Error).
  • Headers: Similar to request headers, response headers provide additional information about the response or server.
  • Body: Contains the data being sent back to the client, which could be the requested resource, confirmation of a created resource, or error messages.

APIs play a critical role in modern software development, enabling applications to communicate and share data and functionality. Understanding the technical aspects of API communication, including the structure of HTTP requests and responses and the use of web protocols, is essential for developers and anyone involved in software development.

Types of APIs

APIs can be divided into types in terms of release policies and use cases. Regarding release policies, APIs can be public, private, and partner. APIs can be categorized based on their use cases and the systems for which they were created.

APIs by Release Policies

  • Private APIs. The purpose of private APIs is to enhance organizational solutions and services. Software engineers use these APIs to combine a company's technology systems or applications. They also help to create customer-facing apps by utilizing current systems. Even if the apps are made accessible to the general public, access to the interface is restricted to those working directly with the API publisher. A corporation can completely regulate its APIs by using it privately.
  • Partner APIs. Business partners get access to partner APIs, which are openly advertised. Software integration between two companies is the most typical use case for partner APIs. Giving partners access to information or resources results in additional revenue streams for the company. The Amazon API is among the top examples of partner API. Many online businesses can connect directly to Amazon.com through their API to check the inventory and determine shipping choices. However, if you share access to your API, you should keep an eye on how the exposed digital assets are used. It is important to make sure that third-party apps using those APIs offer a good user experience and preserve corporate identity, and most importantly, take security measures to minimize the risks.
  • Public APIs, also known as developer-facing or external, are accessible to all outside software engineers. When effectively implemented, a public API can raise brand awareness and provide a new revenue stream. Public APIs come in two types: open (cost-free) and proprietary. The general public can access open APIs. They are free from limitations and help to develop and test applications. Commercial API users can choose to subscribe for a fee or pay as they go. Offering free trials to users so they can examine APIs before paying subscriptions is a common strategy. Among the examples of public APIs are Google Cloud APIs such as BigQuery API, Cloud Storage, and Cloud Monitoring API.

APIs by use cases

APIs can be categorized by the systems they are designed for:

  • Database APIs. An application and a database management system can communicate via database APIs. When working with databases, developers write queries to retrieve data, modify tables, etc.
  • Operating systems APIs. This category of APIs describes applications' utilization of the resources and services provided by operating systems. Each OS has a unique collection of APIs, such as the Windows API or the Linux API.
  • Remote APIs. The standards of interaction for programs operating on several machines are set via remote APIs. In other words, as the term suggests, one software product accesses resources that are not on the device making the request.
  • Web APIs. Client-server web-based systems can exchange machine-readable data and functional information thanks to web APIs. These APIs transfer HTTP requests from web programs and server responses (HTTP).
  • API Libraries. These are directory lists of different APIs. From many of their API profiles, you can find your way to related SDKs, Tutorials, and sample source code for consuming those APIs. Some of the libraries are open so that everyone can make contributions.

API Specifications/Protocols

A set of API specifications helps to standardize data flow across web services. ‘Standardization’ means the ability of multiple systems, made using various technologies, to interact without error.

RPC (Remote Procedure Call)

Based on a Remote Procedure Call, web APIs may follow resource exchange principles. This protocol defines how client-server-based apps will communicate with one another. One program (the client) asks another program (the server), running on a different machine on the network, for data or functionality. The server provides the necessary response.

SOAP (Service Object Access Protocol)

SOAP is a protocol for transferring structured information in a decentralized, distributed context. This API specification contains the syntax rules for web applications sending request and response messages. SOAP-compliant APIs enable XML messaging between systems through HTTP or Simple Mail Transfer Protocol (SMTP) for transferring mail.

REST (Representational State Transfer)

By utilizing the REST architecture style, computer systems can communicate efficiently. This stateless structure reduces complexity, allowing data transactions to occur quickly between two points of access, offering convenience for users and maximizing reliability on both sides of interactions.

As opposed to SOAP, a protocol, REST is a software architectural style with six limitations for creating HTTP-based applications, commonly web services. The most widespread use of this architecture type is on the World Wide Web. SOAP is challenging to utilize because it necessitates writing a lot of code, while REST is considered an easier option.


gRPC is an open-source universal API framework, also known as RPC. It is much ‘younger’ than SOAP. Google made it available to the general public in 2015. A client program using gRPC can call a server application running on a remote machine as though it was a local object. As a result, developing distributed services and applications becomes simpler. gRPC is mostly utilized for communication between microservices.


GraphQL is a query language for APIs. The developer can use a single API call to request all the required data. It enables the client to specify the required data and makes data aggregation from many sources easier. The fact that GraphQL uses a type system to represent data is another unique characteristic of the language. GraphQL-based apps can choose which information to retrieve from a server, which enables them to operate quickly.

API Testing Tools

To make sure that your API components respond appropriately to a wide range of expected and unexpected queries, it is crucial to test them. Along with its functionality, this technique aims to confirm the API's dependability, performance, and security. The most popular API functionality testing tools are listed below.


Postman helps test API services. It is a potent HTTP client for web service verification. Postman is a decent option for testing APIs for manual or exploratory testing. Teams can congregate in collaborative Postman workspaces to work on problems.

Key features

  • Help to extract almost any current web API data.
  • Build a group of REST calls and save each for later execution.
  • Dependable for sending and receiving REST data.

Ping API

Ping-API is an API testing tool that can develop test scripts in JavaScript and CoffeeScript to test APIs. It will allow the full inspection of the HTTP API call’s request and response data.

Key features

  • Scheduling tests using the Ping-API for a particular period
  • Support for setting request body, headers, and URL parameters with a script. It allows users to create scripts to check the content and headers of responses.
  • Validate Create Read Update and Delete (CRUD) flow and log in to Ping API


vREST API tool is an online solution for the automated testing, modeling, automatic recording, and design of REST/HTTP APIs/RESTful APIs.

Key features

  • A thorough tool to quickly validate REST APIs.
  • Aid in API testing to deliver web applications with zero defects more easily.
  • Validation of web applications.
  • API specs documentation production without any specialized resources.

Application of API

In today's economy, APIs enable us to exchange crucial data and offer proper functionality easily. And even though we might not notice them, APIs are present almost everywhere. Here are some of their applications:

  • Batch distribution and traffic generation.
  • Increasing footprints.
  • Feature development
  • Fresh business model investigation.
  • Content Acquisition Advance Business Model.
  • Internal Content Distribution and Innovation.

How to Work With APIs

APIs have become a crucial component of tech modernization at many companies, including most banks and insurance companies. APIs are essential for improving the responsiveness and adaptability of software systems, since they link systems and data.

Companies ultimately need to make significant progress in several aspects to realize the full potential of APIs. Technology leaders need to deal with six major issues to avoid these mistakes and maximize the benefits of their API initiatives. Here are the answers to the questions frequently asked by technology leaders when working with APIs.

Where Do I Start?

Since APIs are adaptable, they may be applied to practically every area of tech. To choose which APIs to create, you should consider both their potential for enabling advanced customer-facing solutions and their potential to provide a solid technical foundation.

Tech leaders can decide how to prioritize API development based on their business plan, the impact of modernization, and the ability to execute. Consider the customer journeys while choosing which APIs to create. Consider three additional benefits APIs might offer to optimize the impact:

  • Data access and integration
  • Cloud migration
  • Core transformation

What Do I Track?

Companies frequently evaluate API performance using unsuitable measures. You should access teams' activities using a set of standard agile metrics. These metrics include direct business value for customer-facing APIs and reuse/reduction of technological debt for back-end APIs.

Sample metrics for API teams

Other possible measures include developer adoption, contribution to architecture simplification or infrastructure cost reduction, and data-specific KPIs.

How Do I Make Sure My APIs Genuinely Add Value to My Business?

In the past, APIs were categorized as middleware, integrating and exchanging data among several systems. The taxonomy used to classify them is frequently technical and illogical. It prevented business stakeholders from participating in the design and prioritization.

Today's top companies define their API taxonomies in a way that establishes a shared language for executives and technical experts. The idea is to distinguish between APIs that are technological enablers and those that directly support the business, where business input is vital.

A solid taxonomy enables business people and technology people to discuss which APIs directly influence customer experiences and which infrastructure components are required.

What Technology Tools Do We Need?

Working with product stakeholders will help you identify the proper use cases for the APIs. They should be based on technical viability (for example, back-end readiness), business rules, and error conditions. The software engineering team must then decide on data formats for the APIs and create a modular, adaptable, and extensible API reference architecture.

This way, you will avoid unnecessary technical debt and layers of technology. To encourage developers to follow best practices and API reuse, engineering leaders should also design API guiding principles, inform product owners about the advantages of good versus bad APIs, and provide toolkits and API catalogs.

Who Owns APIs?

In some companies, business units pay for customer-facing APIs while the technology department creates them. However, other companies are still developing their digital strategies and need to prepare to offer large-scale customer-experience APIs.

Joint ownership of APIs is the best approach, especially when cross-functional digital teams create APIs to improve customer journeys. In situations like this, the technology group owns the necessary process and system APIs. At the same time, business divisions control and pay for APIs.

Where Can I Get Experts to Build APIs?

Companies starting API development often need to hire seasoned professionals. Such professionals require experience in streamlining current infrastructure into API-enabled systems. It is necessary to recruit specialists who have access to monolithic cores with microservices utilizing open-source tools.

These professionals need to be well-versed in API DevOps procedures. They should be capable of iterating continually on the API strategy, development, and life cycle maintenance tasks. They are the foundation of an organization's API strategy and must collaborate closely with technology and business leaders.

From our experience, any API development team should have mastery over skills like API design, cross-platform development, front-end development, understanding integrations, API orchestration skills, etc. The team should interact with the stakeholders directly and be aware of product value creation.

The Importance of APIs in Modern Technology

APIs enable separate software systems to communicate with each other, making the exchange of data and functionalities effortless. This interoperability is the cornerstone of innovation in various technology sectors, from software development and cloud computing to the Internet of Things (IoT).

Fueling innovation in software development

APIs revolutionize software development by allowing for modular programming, where developers can build upon existing services instead of creating systems from scratch. This accelerates the development process, reduces costs, and enhances the reliability of software applications. For example, APIs facilitate the integration of third-party services, such as payment gateways, social media feeds, or mapping services, into applications to enrich user experiences without the need for extensive in-house development.

Transforming cloud computing

In cloud computing, APIs are essential for the orchestration and automation of services. They enable developers to interact with cloud resources, manage storage, compute instances, and more, all through programmable web interfaces. This capability allows for scalable, flexible, and efficient cloud architectures that can dynamically adjust to changing demands. Major cloud providers like AWS, Google Cloud, and Azure offer comprehensive APIs that empower organizations to leverage the cloud's full potential.

Enabling the Internet of Things (IoT)

The IoT realm is where APIs truly showcase their potential to connect the physical and digital worlds. APIs facilitate communication between IoT devices and the cloud, enabling data collection, device management, and real-time analytics. This interconnectedness brings about smart homes, wearable technology, and industrial IoT solutions, transforming everyday objects into intelligent systems that improve efficiency, safety, and quality of life.

Examples of API use cases:

  • Payment processing in e-commerce platforms.
  • Social media integration for sharing and authentication.
  • Cloud storage services for backup and data synchronization.
  • Smart home devices controlled via mobile apps.
  • Wearable health monitors syncing data with health applications.
  • Industrial sensors monitoring and managing factory operations.

The importance of APIs in modern technology cannot be overstated. They are the glue that binds together the components of the digital ecosystem to enable seamless interactions and pave the way for continuous innovation across all sectors of technology.

Harness the Power of APIs with Techstack

Knowing the answer to the question "What is an API?" might bring significant additional earnings or financial savings. APIs come in many shapes and sizes. Your business situation determines the type of API you should use. Whichever option you choose, you will need experienced professionals to create and implement such a solution.

If you are looking for experts who can help you develop APIs for your app, Techstack has got you covered. A full-stack software development company with years of experience, Techstack offers custom software development services and provides technology solutions for startups, mid-sized businesses, and large corporations. We pay special attention to technology culture inside the company and leverage our community of technology experts who have years of experience adjusting the development process and developing software solutions. Techstack offers both autonomous, cross-functional teams and individual tech specialists who may easily integrate into your development process.

Contact us today to learn more about web APIs and how they can help your business.