API gateway (application programming interface gateway) 22 API management interview questions and answers


What is a RESTful API?

A RESTful API is an architectural style for an application programming interface that uses HTTP requests to access and use data. That data can be used to GET, PUT, POST and DELETE data types, which refers to reading, updating, creating and deleting operations related to resources.

An API is code that lets two software programs communicate with one another. The API's design spells out the proper way for a developer to write a program, or the client, that uses the API to request services from another application, or the server. APIs have become a vital mechanism for software interoperability.

RESTful APIs are also referred to as RESTful web services and REST APIs. They're based on representational state transfer, an architectural style and approach to communications often used in web services development. This approach can also facilitate communication between other application types.

REST technology is generally preferred over other similar technologies. This is because REST uses less bandwidth, making it more efficient in internet use. RESTful APIs can also be built with common programming languages such as PHP, JavaScript and Python.

REST browsers are thought of as the language of the internet. Cloud consumers use APIs to expose and organize access to web services. REST is a logical choice for building APIs to provide users with ways to flexibly connect to, manage and interact with cloud services in distributed environments. Sites such as Amazon, Google, LinkedIn and Twitter use RESTful APIs.

What are the main elements of RESTful API?

A REST API fundamentally relies on three major elements:

  • Client. The client is the software code or application that requests a resource from a server.
  • Server. The server is the software code or application that controls the resource and responds to client requests for the resource.
  • Resource. The resource is any data or content, such as text, video and images, the server controls and makes available in response to client requests.

To access a resource, the client sends an HTTP request to the server. Client requests include four principal parts:

  • HTTP method. This details what should happen to the specified resource. The four fundamental HTTP methods are known as verbs. They are POST to create a new resource, GET to retrieve an existing resource, PUT to update or change an existing resource, and DELETE to delete a resource. As the table below shows, these HTTP verbs correspond to the Create, Retrieve, Update, and Delete methods or actions, which are referred to as CRUD.
  • Endpoint. The endpoint shows where the resource is located. It typically includes a Uniform Resource Identifier (URI). If the resource is accessed through the internet, the URI can be a URL that provides a web address for the resource.
  • Header. A header has the details needed to execute the call and handle the response. A request header might include authentication data, an encryption key, more details about the server location or access information and details about the desired data format needed for the response.
  • Body. The body contains relevant information to or from the server. For example, a body may contain the new data to be added to the server through a POST or PUT method.
HTTP verb CRUD action
POST Create
GET Read
PUT Update
PATCH Update

The server-side hosting the API processes the call and forms a response. When data is requested, the server sends a machine-readable representation of the requested data that the client then processes. Usually, response details include any information needed to interpret the response, such as whether the data is in Extensible Markup Language (XML), JavaScript Object Notation (JSON) or plain text format. The server provides additional data, such as error codes and time stamps, or other instructions for the client.

In short, calls and responses are self-descriptive. This means calls and responses will include information on how to process and interpret them.

How RESTful APIs work

A RESTful API breaks down a transaction to create a series of small modules. Each module addresses an underlying part of the transaction. This modularity gives developers flexibility, but it can be challenging to design a REST API from scratch. Several companies, including Amazon S3, Cloud Data Management Interface and OpenStack Swift, provide models for developers to use.

A RESTful API uses commands to obtain resources. The state of a resource at any given timestamp is called a resource representation. A RESTful API uses existing HTTP methodologies that the RFC 2616 protocol defined, such as GET, PUT, POST and DELETE.

With REST, networked components are a resource the user requests access to. This is like a black box with unclear implementation details. All calls are stateless; the RESTful service can't retain anything between executions.

Data formats the REST API supports include application/json, application/xml, application/x-web+xml, application/x-www-form-urlencoded, and multipart/form-data.

How is RESTful API used?

Because the calls are stateless, REST is useful in cloud applications. Stateless components can be freely redeployed, re-sent or retried if something fails, and they provide significant scalability to accommodate workload changes.

This approach works because any request can be directed to any instance of a component. Nothing the next transaction must remember is saved. This makes using REST APIs preferable for web applications.

The RESTful model is helpful in cloud services because binding to a service through an API is a matter of controlling how the URL is decoded. Cloud computing and microservices are almost certain to make RESTful API design the rule in the future.

RESTful APIs are frequently used in mobile and web-based applications to access and change data on remote systems across the Internet. There are countless examples of use cases, but the following four are some of the most popular:

  • Mobility. Mobile apps such as Lyft and Uber use REST APIs to access maps and schedule rides.
  • Banking. These apps use REST APIs to access account data and support transactions.
  • Streaming services. Spotify, Netflix and other streaming services use REST APIs to access media from remote servers.
  • Social media. Services such as X and Facebook use REST APIs to make and manage posts as well as integrate with other applications.

RESTful API design and architecture constraints

Dr. Roy Fielding, senior principal scientist at Adobe, defined RESTful API design in his 2000 doctoral dissertation as a web service that adheres to the following six REST architectural constraints:

  • A uniform interface. Resources should be uniquely identifiable through a single URL. Manipulating a resource should occur only via the network protocol methods DELETE, PUT and GET with HTTP.
  • Client-server based. Delineation between the client and server should be clear. UI and request-gathering concerns are the client's domain. Data access, workload management and security are the server's domain. This loose coupling of the client and server lets each to developed and enhanced independent of the other.
  • Stateless operations. All client-server operations must be stateless. Any state management required should occur on the client, not the server.
  • RESTful resource caching. All resources should allow caching unless that isn't possible.
  • Layered system. REST enables an architecture composed of multiple layers of servers.
  • Code on demand. Usually, a server will send back static representations of resources in the form of XML or JSON. However, when necessary, servers can send executable code to the client.

What are the benefits of RESTful API?

REST APIs have gained enormous popularity due to the numerous benefits available to developers and organizations, including the following:

  • Simplicity. REST APIs use common HTTP methods, including GET, PUT, POST and DELETE requests, making them easy to design, implement and use.
  • Independence. Developers enjoy platform independence because they can use almost any programming language to create REST APIs. They work with various client devices, such as traditional web browsers, mobile devices and internet-of-things devices.
  • Flexible. REST APIs support many different data formats, including JSON, XML and plain text. Developers can choose the data format that best suits client needs and available server-side data.
  • Scalable. The stateless nature of REST APIs supports horizontal scaling, where many API calls run in parallel to handle significant API call loads.
  • Cacheable. REST APIs support caching, allowing data to be stored in local memory. This approach can speed server-side response time, potentially improving API performance. It might even eliminate the need for an API call if required data is already on the client from a prior call.
  • Secure. REST APIs can secure calls and data exchanges with Open Authorization (OAuth) authentication and Secure Sockets Layer/Transport Layer Security encryption.
  • Compatible. Proper use of versioning lets developers treat APIs as any other evolving software, adding new features over time with backward compatibility and support of legacy features for existing clients.

Common REST API challenges

REST APIs aren't a panacea. Some concepts that may be challenging include the following:

  • Endpoint consistency. To be consistent, endpoint paths should follow common web standards, which might be difficult to manage.
  • API versioning. Endpoint URLs shouldn't be invalidated when used internally or with other applications.
  • Long response times and too much data. The amount of returned resources can grow over time, increasing load and response times.
  • Navigation paths and user input locations. Because REST uses URL paths for input parameters, determining URL spaces can be challenging.
  • Security. There are a lot of security issues to manage, such as the use of HTTPS, blocking access from unknown IP addresses and domains, validating URLs, blocking unexpectedly large payloads, monitoring and logging requests, and investigating failures.
  • Authentication. This requires awareness and use of common authentication methods such as HTTP basic authentication, which allows for a base64-encoded username and password string; API keys; JSON Web Tokens; and other access tokens, such as OAuth 2.0, which is good for access control.
  • Requests and data. Requests might have more data and metadata than needed, or more requests might be needed to obtain all the data. APIs can be adjusted for this.
  • Error codes and messages. A common practice is to use standard HTTP error codes. Error handling might not be able to distinguish whether a response is successful besides parsing the body or checking for an error.
  • API testing. This might be a long process to set up and run. Each part of the process can be challenging. Testing can also occur in the command line with the client URL (cURL) utility. Testing-related challenges include the initial setup, schema updates, test parameter combinations, sequencing of API calls, validation of test parameters and system integration.
    curl -k -u "cisco:cisco"
    Code showing use of the cURL utility to access a router remotely.
    The cURL utility supports multiple protocols and can be used to access network devices remotely, such as this router with IP address

    REST API best practices

    REST APIs are dedicated software applications designed to support network communication and execution of specific tasks. API development and management are often approached using the same principles and guidelines applied to any other software project. However, several common REST API best practices can improve API designs and implementations:

    • Use nouns in endpoint paths. Where a REST API already employs a verb, such as GET or PUT, in the formation of a request, the resource being accessed should use a noun designation, like GET/datasource or POST/articles. This makes request formation more intuitive for designers.
    • Use a well-established data format. JSON is the most common and popular data format for REST API payloads. It's also the default format for popular programming languages such as JavaScript.
    • Employ graceful error handling. Be sure to include complete error handling in REST API responses and use standard HTTP response codes to indicate error type. This helps ensure that errors don't crash the client app or server, and they can be understood and corrected quickly.
    • Restrict data sets. Use filtering, sorting and other data access techniques to restrict or reduce the volume of data that can be returned to a client. Trying to transfer huge data sets can impair or crash the API. Design the API to request and send only the minimum data requested.
    • Maintain security. Design the REST API to use both authentication and encryption as standard practices. Comprehensive security should be applied even to public APIs using freely available data.
    • Use caching wherever possible. REST API supports caching on the server and client sides. Using cache on the server side speeds up the server's response time when a frequent request arrives. Using cache on the client side means the data is already on the client and need not make the API call at all. In both cases, API performance improves.

    REST vs. SOAP

    REST and Simple Object Access Protocol offer different methods to invoke a web service. REST is an architectural style, while SOAP defines a standard communication protocol specification for XML-based message exchange. REST applications can use SOAP.

    RESTful web services are stateless. A REST-based implementation is simple compared to SOAP. However, users must understand the context and content being passed along. There's no standard set of rules to describe the REST web services interface. REST services are useful for restricted profile devices, such as mobile devices, and are easy to integrate with existing websites.

    SOAP requires less plumbing code -- the low-level, infrastructural code that connects main code modules together -- than REST services design. The Web Services Description Language describes a common set of rules to define the messages, bindings, operations and locations of the service. SOAP web services are useful for asynchronous processing and invocation. The structured formality found in SOAP is often better-suited to complex enterprise-level software integrations and workflows that might overwhelm the design of a similar REST API.

    REST and SOAP are both useful and effective methods for building APIs. Choosing between them depends on the API's intended purpose and characteristics.

    History of RESTful APIs

    Prior to REST, developers used SOAP to integrate APIs. To make a call, developers handwrote an XML document with a Remote Procedure Call in the body. They then specified the endpoint and would POST their SOAP envelope to the endpoint.

    In 2000, Roy Fielding and a group of developers decided to create a standard so that any server could talk to any other server. He defined REST. REST's universal rules make it easier for developers to integrate software.

    Salesforce was the first company to sell a RESTful API as part of its internet-as-a-service package in 2000. However, few developers were able to use the complicated XML API. Then eBay built a REST API, which expanded its market to any site that could access its API. This caught the attention of another e-commerce giant, and Amazon announced its API in 2002.

    Flickr launched its RESTful API in August 2004, letting bloggers easily embed images on their sites and social media feeds. Facebook and Twitter both released their APIs in 2006, buckling under the pressure of developers who scraped the sites and created "Frankenstein APIs." When Amazon Web Services helped launch the cloud in 2006, developers could access data space in minutes using AWS's REST API. The request for public APIs quickly escalated.

    Since then, developers have embraced RESTful APIs, using them to add functionality to their websites and applications. Today, REST APIs are considered the backbone of the internet.

    Understanding of RESTful APIs is important for developers. Test your knowledge with this quiz.

    This was last updated in May 2024

    Continue Reading About RESTful API

    Dig Deeper on API design and management

    Software Quality
    Cloud Computing