The developers'

API Guide

Everything you need to know about building, purchasing and integrating with APIs.

What is an API?

An application programming interface (API) is a set of rules and protocols for building and interacting with software applications. APIs enable different software systems to communicate with each other, allowing for data exchange and functional integration without requiring the end-user to understand the underlying code.

APIs have become the backbone of modern software development, enabling developers to leverage existing functionality and services without reinventing the wheel. They facilitate modularity, scalability, and innovation, allowing developers to build complex applications more efficiently by integrating with third-party services.

API Example

The following API examples show how to return the five most recent email messages from an account’s inbox by making a request to the Nylas Email API.







app.get("/nylas/recent-emails", async (req, res) => {
 try {
   const identifier = process.env.USER_GRANT_ID;
   const messages = await nylas.messages.list({
     queryParams: {
       limit: 5,

 } catch (error) {
   console.error("Error fetching emails:", error);
require 'nylas'

nylas = 'API_KEY')
query_params = { limit: 5 }
messages, _ = nylas.messages.list(identifier: '<GRANT_ID>', query_params: query_params)

messages.each {|message|
 puts "[#{[:date]).strftime("%d/%m/%Y at %H:%M:%S")}] \
from dotenv import load_dotenv

import os
import sys
from nylas import Client

nylas = Client(

grant_id = os.environ.get("NYLAS_GRANT_ID")

messages = nylas.messages.list(
   "limit": 5

import com.nylas.NylasClient;
import com.nylas.models.*;
import java.text.SimpleDateFormat;

public class ReadInbox {
 public static void main(String[] args) throws NylasSdkTimeoutError, NylasApiError {
   NylasClient nylas = new NylasClient.Builder("<NYLAS_API_KEY>").build();
   ListMessagesQueryParams queryParams = new ListMessagesQueryParams.Builder().limit(5).build();
   ListResponse<Message> message = nylas.messages().list("<GRANT_ID>", queryParams);

   for(Message email : message.getData()) {
     String date = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss").
         format(new java.util.Date((email.getDate() * 1000L)));

     System.out.println("[" + date + "] | " + email.getSubject());
curl --request GET \
  --url "" \
  --header 'Accept: application/json' \
  --header 'Authorization: Bearer <NYLAS_API_KEY_OR_ACCESS_TOKEN>' \
  --header 'Content-Type: application/json'   
  "request_id": "d0c951b9-61db-4daa-ab19-cd44afeeabac",
  "data": [
          "starred": false,
          "unread": true,
          "folders": [
          "grant_id": "1",
          "date": 1706811644,
          "attachments": [
                  "id": "1",
                  "grant_id": "1",
                  "filename": "invite.ics",
                  "size": 2504,
                  "content_type": "text/calendar; charset=\"UTF-8\"; method=REQUEST"
                  "id": "2",
                  "grant_id": "1",
                  "filename": "invite.ics",
                  "size": 2504,
                  "content_type": "application/ics; name=\"invite.ics\"",
                  "is_inline": false,
                  "content_disposition": "attachment; filename=\"invite.ics\""
          "from": [
                  "name": "Nylas DevRel",
                  "email": ""
          "id": "1",
          "object": "message",
          "snippet": "Send Email with Nylas APIs",
          "subject": "Learn how to Send Email with Nylas APIs",
          "thread_id": "1",
          "to": [
                  "name": "Nyla",
                  "email": ""
          "created_at": 1706811644,
          "body": "Learn how to send emails using the Nylas APIs!"
  "next_cursor": "123"

History of APIs

APIs serve as a crucial bridge between different software programs, allowing them to communicate and exchange data efficiently. They have become integral to modern computing, powering everything from web applications to mobile and cloud services.

API beginnings

Before the Internet became ubiquitous, APIs were used in software libraries and operating systems to enable different software components to communicate. For example, the C Standard Library provided a set of APIs for tasks like input/output processing, which developers could use to build their applications without dealing directly with the hardware’s complexities.

APIs also played a crucial role in inter-process communication (IPC), which allowed different processes running on the same machine to share information. Techniques like shared memory, message queues, and semaphores were used, and they formed the basis for modern IPC.

API explosion

The advent of the World Wide Web in the early 1990s revolutionized how APIs were used and developed. The web provided a unified, network-based interface for accessing various services, and Hypertext Transfer Protocol (HTTP) became the standard for web-based APIs. This led to a significant increase in the development and consumption of APIs as web services became more popular.

HTTP is a stateless protocol that uses a request-response model for communication between clients and servers. This simplicity and statelessness made HTTP an excellent choice for web API development. It allowed for the creation of accessible APIs over the web, using standard HTTP methods like GET, POST, PUT, and DELETE to interact with resources represented by URLs.

Evolution of API technologies

The 2000s marked the rise of web APIs to a diverse array of types, each tailored to specific needs in software development. Simple Object Access Protocol (SOAP) and Web Services established a new standard for exchanging structured information in a platform-independent way.

The emergence of RESTful APIs further revolutionized the field, emphasizing simplicity, scalability, and statelessness, quickly becoming a preferred choice for web-based interactions. The proliferation of public APIs in the Web 2.0 era unlocked new business models and integration opportunities, introducing a new era of openness and interconnectivity. 

Modern API practices have since evolved, incorporating sophisticated authentication mechanisms like OAuth and descriptive frameworks like Swagger (OpenAPI) for better API management and API documentation. The mobile era’s unique demands gave rise to APIs catering specifically to mobile applications. At the same time, the advent of microservices architecture further emphasized the role of APIs in modular and scalable system designs. 

Additionally, the introduction of GraphQL API demonstrated a continuous pursuit of efficiency and flexibility in API design, catering to more complex data relationships and query possibilities. Each API, from SOAP to GraphQL, represents a significant development in how software systems communicate, showcasing API technology’s dynamic and ever-evolving nature.

Importance of APIs in app development

In the world of app development, APIs are incredibly important. They’re not just technical tools but critical parts that help shape how apps are built, work, and connect with other digital services. As we previously mentioned, APIs act like bridges, allowing different parts of the digital world to talk to each other easily and work together smoothly.

Enabling interoperability and integration

APIs are crucial in app development for seamless integration between software systems and external services, facilitating access to data sources. This integration capacity extends an app’s functionality without the need to develop complex systems from scratch. 

For instance, consider a healthcare application designed to transmit patient data to laboratories for analysis. By integrating a secure data transmission API, the application can efficiently and securely send sensitive information, utilizing the API’s advanced encryption and authentication protocols. Such integration is essential for safeguarding patient confidentiality and upholding legal and ethical standards in healthcare, thereby maintaining patient trust. 

Facilitating modular development

The use of APIs promotes a modular approach to app development. This methodology breaks down the app development process into smaller, manageable pieces, where each module can be developed, tested, and deployed independently. APIs serve as the connecting links between these modules. 

Using an API for modular development speeds up the development process and enhances the maintenance and scalability of the application. For example, a marketplace app can have separate modules for connecting vendors and users to streamline pick-up and delivery schedules, connect directly with users’ email providers, and build other features for order processing, each interconnected through specific APIs.

Enhancing user experience

By tapping into the power of APIs, developers significantly enhance the user experience in digital applications. Developers can create more feature-rich and interactive apps by connecting to APIs that provide various functionalities. These functionalities range from accessing real-time information and first-party email and calendar data using an email API to tracking user trends and behaviors to delivering better vendor-user experiences. The ability to fetch and display real-time data and interact with other services makes apps more engaging and valuable for the end-user.

Supporting platform agnosticism

Users today engage with applications across multiple devices and platforms, and APIs play a pivotal role in fostering platform uniformity. They empower developers to build applications that provide a consistent user experience and functionality, regardless of the platform. 

For example, consider a cloud storage API that enables users to access their data from web and mobile apps and allows real-time synchronization of files across devices. This means users can start editing a document on their computer and continue seamlessly on their smartphone without any manual transfer or compatibility issues. Such seamless integration across platforms significantly enhances user convenience and application versatility.

Streamlining updates and maintenance

APIs greatly simplify the process of updating and maintaining apps. By categorizing different functionalities into distinct services accessed via APIs, developers gain the ability to update or repair specific components of an application without the risk of affecting the entire system. This modular approach significantly reduces downtime during maintenance periods, enhancing the app’s overall stability and reliability. It also enables quicker deployment of new features and more efficient bug fixes, leading to a better user experience. 

For instance, consider an e-commerce app: updating the payment processing logic through a dedicated API means that enhancements can be made without interrupting other functionalities like product browsing or order tracking. Additionally, this separation allows for more effortless scalability and adaptability of the app to changing needs or technologies.

Enabling scalability and flexibility

Scalability and flexibility are essential attributes of modern applications, and APIs are vital enablers. They allow apps to scale up their operations by integrating with more robust or additional services as needed. For example, building a calendar integration for one service provider from scratch can take as long as 12 months for a team of four engineers. With a Calendar API, you can go from an idea to execution in less than two days.

APIs also provide the flexibility to replace or upgrade backend services without major overhauls in the front-end application. This ability is crucial for businesses adapting quickly to changing market demands or technological advancements.

Who uses APIs?

Diverse users, from individual developers to large corporations, utilize APIs. Whether it’s for enhancing the capabilities of an app, integrating different systems, or accessing data, APIs are the invisible yet essential tools that enable these interactions seamlessly. 

Let’s explore the variety of users of APIs and how they are instrumental in different sectors and for various purposes, driving innovation and efficiency in the digital world.

Developers and programmers

The most direct users of APIs are developers and programmers. They use APIs to enhance the functionality of their applications without having to create complex code from scratch. For example, a web developer might use an email API to seamlessly integrate communications features into a website. APIs provide developers with a toolkit to build rich, interactive applications efficiently.

Mobile and web app developers extensively use APIs to enhance their applications’ capabilities. They integrate APIs for various functionalities like payment processing, social media integration, and data analytics. This integration is crucial for creating feature-rich and interactive mobile apps.

Businesses and organizations

Businesses and organizations across diverse sectors increasingly turn to APIs to enhance operational efficiency and customer service quality. For example, in the financial sector, institutions utilize APIs for many reasons. Secure transaction APIs ensure safe and swift financial exchanges, while data analysis APIs help understand customer behavior, manage risk, and personalize services. These tools are vital in maintaining the security of sensitive information and in providing insights that shape financial products and customer engagement strategies.

APIs help support supply chain management and customer relations in the retail industry. Inventory management APIs allow real-time tracking of stock levels, ensuring optimal inventory control and reducing the risk of overstocking or stockouts. This directly translates to cost savings and improved customer satisfaction. Another example is customer relationship management (CRM) APIs, which facilitate better customer communication and service by integrating customer data across different touchpoints. Having all the data in one place gives users a unified view that helps personalize marketing and improve customer service. 

Third-party service providers

Many third-party service providers, such as those specializing in payment processing or shipping logistics, also offer APIs to enable other businesses to easily integrate their specialized services into their applications. These API offerings are more than just a convenience; they help companies provide customers with a holistic and streamlined experience. 

For instance, by integrating a third-party payment processing API, an online retailer can offer secure and diverse payment options, enhancing customer trust and satisfaction. Similarly, incorporating a shipping API allows for real-time tracking and efficient logistics management, which is crucial for customer transparency and operational efficiency. These integrations expand the capabilities of the businesses and contribute to a more cohesive and user-friendly customer journey. 

End users

While end users might not interact with APIs directly, they benefit significantly from the functionalities APIs provide. Let’s take the travel industry, for example. When a user books a hotel through a travel app, APIs work behind the scenes to pull information from various sources, providing a streamlined user experience.

APIs span various disciplines and industries, underlining their significance in the modern digital ecosystem. They enable the development of innovative applications and the seamless operation and integration of various digital services, catering to the needs of a wide range of users.

What makes an API great?

An outstanding API isn’t just about technical proficiency; it’s about delivering users an easy, intuitive, and reliable experience. A great API goes beyond basic functionality, embodying qualities like ease of use, comprehensive documentation, robust security, and scalability. It is designed with both the developer and the end user in mind, ensuring that it meets current needs and is adaptable to future demands. 

Let’s explore the key attributes that distinguish a great API, focusing on how these qualities contribute to its effectiveness, usability, and long-term success across different applications.

Ease of use and intuitive design

A great API should be easy to understand and use. This means having a clear, logical structure, consistent naming conventions, and a design that follows industry best practices. An intuitive API reduces developers’ learning curve and helps quicken project integration.

Comprehensive and clear documentation

Well-documented APIs are essential for effective usage. Documentation should include detailed instructions, examples, and best practices, helping developers understand how to use the API effectively and troubleshoot any issues that arise.

Robust security measures

Security is paramount for any API, particularly those handling sensitive data. A great API incorporates strong authentication, encryption, and access control measures to protect against unauthorized access and data breaches.

High performance and reliability

Performance is a critical aspect of a great API. It should have fast response times and be able to handle a high number of requests without significant latency. Reliability also plays a key role, as users depend on the API to be available and functional consistently.


An API should be scalable and handle growing users and data without compromising performance. This adaptability is crucial for supporting the growth of the applications and services that depend on it.

Regular updates and maintenance

Continual updates and maintenance reflect an API’s commitment to improvement and adaptation to evolving technological trends. Regular updates ensure that the API stays relevant, secure, and efficient.

Developer support and community

A strong developer support system, including active community forums, tutorials, and customer support, can significantly enhance the API’s usability. A vibrant community also fosters innovation and provides valuable feedback for further improvements.

Compatibility and interoperability

A great API should be compatible with various platforms and programming languages, allowing for broader integration. Interoperability with other APIs and systems is also a key factor in its effectiveness in diverse digital ecosystems.

Demonstrable Value

Lastly, a great API should offer clear value to its users. It should solve a real problem, improve existing processes, or significantly enhance the applications it integrates with.

A great API embodies technical excellence, user-centric design, and ongoing support and development. It stands out not just for its functionality but for its ability to provide a secure, efficient, and enjoyable experience for developers and end users, thereby playing a pivotal role in the success of the applications it powers.