SaaS vs PaaS vs IaaS: The Three Cloud Service Models Explained Clearly
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The Three Cloud Service Models: What Each One Means
SaaS, PaaS, and IaaS are the three fundamental cloud service models. Each one represents a different division of responsibility between you and the cloud provider. Understanding where that division sits in each model is the foundation for making sensible decisions about which cloud services to buy, build, and manage.
SaaS (Software as a Service): A complete application delivered over the internet. You use the software through a browser or app. The provider manages everything: the application, the runtime, the operating system, the servers, and the data centre. You manage only your data and user configuration. Examples: Slack, Salesforce, Google Workspace, Notion, HubSpot, Zoom. SaaS is the right choice when you need a specific business function and do not need to customise the underlying application.
PaaS (Platform as a Service): A managed environment for building and deploying your own applications. The provider manages the operating system, runtime, middleware, and infrastructure. You manage the application code and data. Examples: Heroku, Render, Railway, Vercel, Google App Engine, AWS Elastic Beanstalk. PaaS is the right choice when you need to run custom application code without managing servers.
IaaS (Infrastructure as a Service): Virtualised computing resources delivered over the internet: virtual machines, storage, networking. You manage the operating system and everything above it. The provider manages the physical hardware and virtualisation layer. Examples: AWS EC2, Microsoft Azure VMs, Google Compute Engine, DigitalOcean Droplets. IaaS is the right choice when you need maximum control over your infrastructure or when your workloads do not fit PaaS constraints.
What You Manage in Each Model
The clearest way to understand the three models is to look at the infrastructure stack and see which layers each model requires you to manage. Going from the bottom to the top of the stack:
On-premise (for comparison): You manage everything: physical hardware, networking, data centre, virtualisation, operating system, runtime, middleware, data, and application. Maximum control, maximum cost, maximum operational overhead.
IaaS: Provider manages: physical hardware, data centre, virtualisation. You manage: operating system, runtime, middleware, data, application. You still need to patch operating systems, configure runtimes, manage storage, and handle network security groups.
PaaS: Provider manages: physical hardware, data centre, virtualisation, operating system, runtime, middleware. You manage: data and application. No server patching, no runtime configuration, no middleware updates. You focus entirely on the application.
SaaS: Provider manages everything in the stack including the application. You manage: data (within the application’s data model) and user configuration. No infrastructure, no code, no deployment.
The trade-off across all three models is control versus convenience. IaaS provides the most control and the lowest unit cost at scale but requires the most operational expertise. SaaS provides the least control and the highest per-unit cost but requires no infrastructure expertise. PaaS sits in the middle and is the right starting point for most development teams.
- SaaS examples: Slack, HubSpot, Notion, Salesforce, Zoom, Google Workspace
- PaaS examples: Render, Railway, Vercel, Heroku, Google App Engine
- IaaS examples: AWS EC2, Azure VMs, Google Compute Engine, DigitalOcean
- SaaS: you manage data and config only, zero infrastructure responsibility
- PaaS: you manage application code and data, no OS or runtime management
- IaaS: you manage OS and above, full control over server configuration
Which Cloud Model Does Your Organisation Actually Need?
Start with SaaS for business functions: For CRM, email, communication, project management, HR software, accounting, and marketing tools: use SaaS. There is no advantage to building or self-hosting business software that is available as a well-supported SaaS product. The cost of maintaining custom infrastructure for a business function is almost always higher than the SaaS subscription price, even before accounting for security and compliance obligations.
Use PaaS for custom application deployments: If your team writes application code that needs to run somewhere, PaaS is the right starting point for most organisations under 100 engineers. Render, Railway, and Vercel for web applications and APIs. AWS Elastic Beanstalk, Google App Engine, or Azure App Service when you are already invested in that cloud provider. PaaS removes the operational overhead of infrastructure management while keeping full flexibility in the application layer.
Move to IaaS when PaaS becomes a constraint: Organisations move from PaaS to IaaS when they hit PaaS cost ceilings (where IaaS provides lower unit cost at scale), when they need infrastructure configurations that PaaS abstractions cannot accommodate, when they need granular control over network topology and security groups, or when they have dedicated DevOps teams that can justify the operational overhead in return for the control and cost benefits.
Use multiple models simultaneously: Most organisations use all three models at once. The internal communication tool is SaaS (Slack). The product is deployed on PaaS (Render or AWS Elastic Beanstalk). The data pipeline runs on IaaS (AWS EC2 or Kubernetes on GKE). The question is not which model to use but which model fits which workload. Understanding the trade-offs in each model lets you allocate workloads to the layer that best fits their requirements.
SaaS is software delivered as a subscription - you use the app, the vendor manages everything below it. Examples: Slack, HubSpot, Zoom. PaaS is a managed platform where you deploy code without managing servers - the vendor handles OS and runtime. Examples: Railway, Render, Vercel. IaaS is raw compute, storage, and networking you configure yourself. Examples: DigitalOcean, Hetzner Cloud, AWS EC2. The key difference is how much infrastructure you manage yourself.
Most startups should start with SaaS tools for business functions (CRM, team communication, project management) and a PaaS for their application infrastructure. Move to IaaS evaluation when infrastructure costs are a meaningful fraction of revenue and your team has DevOps capability to manage the operational overhead.
SaaS: Slack, HubSpot, Zoom, Notion, Salesforce. PaaS: Railway, Render, Vercel, Heroku (legacy), Google App Engine. IaaS: AWS EC2, DigitalOcean Droplets, Hetzner Cloud, Vultr, Azure Virtual Machines.
It defines which security obligations belong to the vendor and which belong to you. With IaaS: you manage everything from OS upward. With PaaS: the vendor additionally manages OS and runtime; you manage your application and data. With SaaS: the vendor manages the full stack; you manage configuration, access control, and the data you put in. Understanding this matters for GDPR, SOC 2, and customer security questionnaires.
Use PaaS when your team wants to deploy code rather than manage servers, when you're at startup or scale-up stage without dedicated DevOps resource, and when the speed-to-deployment advantage outweighs the flexibility benefit of direct infrastructure control. Evaluate IaaS when your infrastructure costs at current scale justify the operational investment to optimise them.
AWS is primarily an IaaS and PaaS provider, not SaaS. EC2 is IaaS (raw virtual machines). Elastic Beanstalk is PaaS (managed deployment). Lambda is FaaS (serverless functions). AWS does offer some SaaS products - Amazon Connect, AWS WorkDocs - but its core business is cloud infrastructure, not application software.
No installation or local server management, automatic updates, accessible from any device with internet access, vendor-managed security patching, lower upfront cost with subscription billing, built-in collaboration for multi-user products, and usage-based scaling. The main trade-offs are data residency with the vendor, internet dependency, and ongoing subscription costs across a growing tool stack.
SaaS: per user per month, usually with a free tier and paid tiers unlocking more features. PaaS: per resource per month - usage-based (pay for what you consume) or plan-based (fixed monthly per service), often with egress charges at scale. IaaS: pay-per-second for compute, per-GB for storage, per-GB for data transferred out. IaaS is cheapest at scale when optimised; SaaS is cheapest to get started without technical resources.
FaaS (Function as a Service), also called serverless, runs individual functions on demand rather than keeping an always-on server running. It scales automatically including to zero when idle, which makes it cost-efficient for event-driven or infrequently-used workloads. The trade-offs versus PaaS are cold starts on infrequently-called functions, execution time limits, and stateless design that requires external state management.
Vendor lock-in means becoming dependent on one provider's proprietary services, making migration expensive. The practical mitigation strategies: use standard databases (PostgreSQL) over proprietary options, choose S3-compatible object storage that works across providers, define infrastructure as code with Terraform or OpenTofu, and containerise applications with Docker so they're portable between platforms.