Cloud computing is an abstraction of compute, storage, and network infrastructure assembled as a platform on which applications and systems can be deployed quickly and scaled on the fly. Crucial to cloud computing is self-service: Users can simply fill in a web form and get up and running.

The vast majority of cloud customers consume public cloud computing services over the internet, which are hosted in large, remote data centers maintained by cloud providers. The most common type of cloud computing, SaaS (software as service), delivers prebuilt applications to the browsers of customers who pay per seat or by usage, exemplified by such popular apps as Salesforce, Google Docs, or Microsoft Teams. Next in line is IaaS (infrastructure as a service), which offers vast, virtualized compute, storage, and network infrastructure upon which customers build their own applications, often with the aid of providers API-accessible services.

When people casually say the cloud, they most often mean the big IaaS providers: AWS (Amazon Web Services), Google Cloud, or Microsoft Azure. All three have become gargantuan ecosystems of services that go way beyond infrastructure: developer tools, serverless computing, machine learning services and APIs, data warehouses, and thousands of other services. With both SaaS and IaaS, a key benefit is agility. Customers gain new capabilities almost instantly without capital investment in hardware or softwareand they can instantly scale the cloud resources they consume up or down as needed.

Way back in 2011, NIST posted a PDF that divided cloud computing into three service modelsSaaS, IaaS, and PaaS (platform as a service)the latter a controlled environment within which customers develop and run applications. These three categories have largely stood the test of time, although most PaaS solutions now make themselves available as services within IaaS ecosystems rather than presenting themselves as their own clouds.

Two evolutionary trends stand out since NISTs threefold definition. One is the long and growing list of subcategories within SaaS, IaaS, and PaaS, some of which blur the lines between categories. The other is the explosion of API-accessible services available in the cloud, particularly within IaaS ecosystems. The cloud has become a crucible of innovation where many emerging technologies appear first as services, a big attraction for business customers who understand the potential competitive advantages of early adoption.

This type of cloud computing delivers applications over the internet, typically with a browser-based user interface. Today, the vast majority of software companies offer their wares via SaaSif not exclusively, then at least as an option.

The most popularSaaS applications for business can be found in Googles G Suite and Microsofts Office 365; most enterprise applications, including giant ERP suites from Oracle and SAP, come in both SaaS and on-prem versions. SaaS applications typically offer extensive configuration options as well as development environments that enable customers to code their own modifications and additions. They also enable data integration with on-prem applications.

At a basic level,IaaS cloud providers offer virtualized compute, storage, and networking over the internet on a pay-per-use basis. Think of it as a data center maintained by someone else, remotely, but with a software layer that virtualizes all those resources and automates customers ability to allocate them with little trouble.

But thats just the basics. The full array of services offered by the major public IaaS providers is staggering:highly scalable databases, virtual private networks,big dataanalytics, developer tools,machine learning, application monitoring, and so on.Amazon Web Serviceswas the first IaaS provider and remains the leader, followed by Microsoft Azure,Google Cloud Platform, Alibaba Cloud, andIBM Cloud.

PaaS provides sets of services and workflows that specifically target developers, who can use shared tools, processes, and APIs to accelerate the development, testing, and deployment of applications. Salesforces Heroku and Salesforce Platform (formerly Force.com) are popular public cloud PaaS offerings; Cloud Foundry and Red Hats OpenShift can be deployed on premises or accessed through the major public clouds. For enterprises, PaaS can ensure that developers have ready access to resources, follow certain processes, and use only a specific array of services, while operators maintain the underlying infrastructure.

FaaS, the cloud version of serverless computing, adds another layer of abstraction to PaaS, so that developers are completely insulated from everything in the stack below their code. Instead of futzing with virtual servers, containers, and application runtimes, developers upload narrowly functional blocks of code, and set them to be triggered by a certain event (such as a form submission or uploaded file). All the major clouds offer FaaS on top of IaaS: AWS Lambda,Azure Functions, Google Cloud Functions, and IBM Cloud Functions. A special benefit of FaaS applications is that they consume no IaaS resources until an event occurs, reducing pay-per-use fees.

A private cloud downsizes the technologies used to run IaaS public clouds into software that can be deployed and operated in a customers data center. As with a public cloud, internal customers can provision their own virtual resources to build, test, and run applications, with metering to charge back departments for resource consumption. For administrators, the private cloud amounts to the ultimate in data center automation, minimizing manual provisioning and management. VMware provides the most popular commercial private cloud software, while OpenStack is the open source leader.

Note, however, that the private cloud does not fully conform to the definition of cloud computing. Cloud computing is a service. A private cloud demands that an organization build and maintain its own underlying cloud infrastructure; only internal users of a private cloud experience it as a cloud computing service.

A hybrid cloud is the integration of a private cloud with a public cloud. At its most developed, the hybrid cloud involves creating parallel environments in which applications can move easily between private and public clouds. In other instances, databases may stay in the customer data center and integrate with public cloud applicationsor virtualized data center workloads may be replicated to the cloud during times of peak demand. The types of integrations between private and public cloud vary widely, but they must be extensive to earn a hybrid cloud designation.

Just as SaaS delivers applications to users over the internet, public APIs offer developers application functionality that can be accessed programmatically. For example, in building web applications, developers often tap into the Google Maps API to provide driving directions; to integrate with social media, developers may call upon APIs maintained by Twitter, Facebook, or LinkedIn. Twilio has built a successful business delivering telephony and messaging services via public APIs. Ultimately, any business can provision its own public APIs to enable customers to consume data or access application functionality.

Data integration is a key issue for any sizeable company, but particularly for those that adopt SaaS at scale. iPaaS providers typically offer prebuilt connectors for sharing data among popular SaaS applications and on-premises enterprise applications, though providers may focus more or less on business-to-business and e-commerce integrations, cloud integrations, or traditional SOA-style integrations. iPaaS offerings in the cloud from such providers as Dell Boomi, Informatica, MuleSoft, and SnapLogic also let users implement data mapping, transformations, and workflows as part of the integration-building process.

The most difficult security issue related to cloud computing is the management of user identity and its associated rights and permissions across private data centers and pubic cloud sites. IDaaS providers maintain cloud-based user profiles that authenticate users and enable access to resources or applications based on security policies, user groups, and individual privileges. The ability to integrate with various directory services (Active Directory, LDAP, etc.) and provide single sign-on across business-oriented SaaS applications is essential. Okta is the clear leader in cloud-based IDaaS; CA, Centrify, IBM, Microsoft, Oracle, and Ping provide both on-premises and cloud solutions.

Collaboration solutions such as Slack and Microsoft Teams have become vital messaging platforms that enable groups to communicate and work together effectively. Basically, these solutions are relatively simple SaaS applications that support chat-style messaging along with file sharing and audio or video communication. Most offer APIs to facilitate integrations with other systems and enable third-party developers to create and share add-ins that augment functionality.

Key providers in such industries as financial services, health care, retail, life sciences, and manufacturing provide PaaS clouds to enable customers to build vertical applications that tap into industry-specific, API-accessible services. Vertical clouds can dramatically reduce the time to market for vertical applications and accelerate domain-specific B-to-B integrations. Most vertical clouds are built with the intent of nurturing partner ecosystems.

The most widely accepted definition of cloud computing means that you run your workloads on someone elses servers, but this is not the same as outsourcing. Virtual cloud resources and even SaaS applications must be configured and maintained by the customer. Consider these factors when planning a cloud initiative.

Objections to the public cloud generally begin with cloud security, although the major public clouds have proven themselves much less susceptible to attack than the average enterprise data center.

Of greater concern is the integration of security policy and identity management between customers and public cloud providers. In addition, government regulation may forbid customers from allowing sensitive data off premises. Other concerns include the risk of outages and the long-term operational costs of public cloud services.

The bar to qualify as a multicloud adopter is low: A customer just needs to use more than one public cloud service. However, depending on the number and variety of cloud services involved, managing multiple clouds can become quite complex from both a cost optimization and technology perspective.

In some cases, customers subscribe to multiple cloud services simply to avoid dependence on a single provider. A more sophisticated approach is to select public clouds based on the unique services they offer and, in some cases, integrate them. For example, developers might want to use Googles TensorFlow machine learning service on Google Cloud Platform to build AI-driven applications, but prefer Jenkins hosted on the CloudBees platform for continuous integration.

To control costs and reduce management overhead, some customers opt for cloud management platforms (CMPs) and/or cloud service brokers (CSBs), which let you manage multiple clouds as if they were one cloud. The problem is that these solutions tend to limit customers to such common-denominator services as storage and compute, ignoring the panoply of services that make each cloud unique.

You often see edge computing described as an alternative to cloud computing. But it is not. Edge computing is about moving compute to local devices in a highly distributed system, typically as a layer around a cloud computing core. There is typically a cloud involved to orchestrate all the devices and take in their data, then analyze it or otherwise act on it.

The clouds main appeal is to reduce the time to market of applications that need to scale dynamically. Increasingly, however, developers are drawn to the cloud by the abundance of advanced new services that can be incorporated into applications, from machine learning to internet of things (IoT) connectivity.

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What is cloud computing? Everything you need to know now