6. Service Models

Cloud service models define how much of the stack the cloud provider manages versus how much the customer manages. The more the provider manages, the less control the customer has — and the faster they can move.

The three foundational models defined by NIST¹ are IaaS, PaaS, and SaaS. Beyond them, several specialised models have emerged — FaaS, CaaS, DBaaS, and others — each carving out a narrower slice of the stack.

The shared responsibility stack

Every cloud service model can be mapped to the same six-layer stack. The dividing line between what the provider manages (green) and what the customer manages (orange) shifts as you move up the model hierarchy:

graph TD
  subgraph On-Premises
    direction TB
    A1[Hardware] --> A2[Virtualisation]
    A2 --> A3[OS]
    A3 --> A4[Middleware]
    A4 --> A5[Application]
    A5 --> A6[Data]
    style A1 fill:#ff9999,stroke:#333
    style A2 fill:#ff9999,stroke:#333
    style A3 fill:#ff9999,stroke:#333
    style A4 fill:#ff9999,stroke:#333
    style A5 fill:#ff9999,stroke:#333
    style A6 fill:#ff9999,stroke:#333
  end
  subgraph IaaS
    direction TB
    B1[Hardware] --> B2[Virtualisation]
    B2 --> B3[OS]
    B3 --> B4[Middleware]
    B4 --> B5[Application]
    B5 --> B6[Data]
    style B1 fill:#a8e6cf,stroke:#333
    style B2 fill:#a8e6cf,stroke:#333
    style B3 fill:#ffd3b6,stroke:#333
    style B4 fill:#ffd3b6,stroke:#333
    style B5 fill:#ffd3b6,stroke:#333
    style B6 fill:#ffd3b6,stroke:#333
  end
  subgraph PaaS
    direction TB
    C1[Hardware] --> C2[Virtualisation]
    C2 --> C3[OS]
    C3 --> C4[Middleware]
    C4 --> C5[Application]
    C5 --> C6[Data]
    style C1 fill:#a8e6cf,stroke:#333
    style C2 fill:#a8e6cf,stroke:#333
    style C3 fill:#a8e6cf,stroke:#333
    style C4 fill:#a8e6cf,stroke:#333
    style C5 fill:#ffd3b6,stroke:#333
    style C6 fill:#ffd3b6,stroke:#333
  end
  subgraph SaaS
    direction TB
    D1[Hardware] --> D2[Virtualisation]
    D2 --> D3[OS]
    D3 --> D4[Middleware]
    D4 --> D5[Application]
    D5 --> D6[Data]
    style D1 fill:#a8e6cf,stroke:#333
    style D2 fill:#a8e6cf,stroke:#333
    style D3 fill:#a8e6cf,stroke:#333
    style D4 fill:#a8e6cf,stroke:#333
    style D5 fill:#a8e6cf,stroke:#333
    style D6 fill:#a8e6cf,stroke:#333
  end

Provider manages    Customer manages    Customer manages (on-premises: everything)

IaaS — Infrastructure as a Service

You rent raw infrastructure. The provider supplies virtualised compute, storage, and networking. You install the OS, runtime, middleware, and everything above.

Examples: AWS EC2, Google Compute Engine, Azure Virtual Machines, DigitalOcean Droplets.

When to choose IaaS: - You need full OS-level control (custom kernel, specific OS version) - Lift-and-shift of existing on-premises workloads - High-performance computing with specialised hardware (GPU instances) - Strict compliance that requires controlling the entire stack

PaaS — Platform as a Service

You deploy code; the provider runs it. The provider manages the OS, runtime, middleware, scaling, and networking. You write the application and configure it via environment variables.

Examples: Heroku, AWS Elastic Beanstalk, Google App Engine, Azure App Service, Railway, Render.

Architectural implication: PaaS favours the 12-factor app pattern — stateless processes, config via environment variables, logs to stdout. Persistent state must live in external managed services (database, object storage).

When to choose PaaS: - Building a new application without infrastructure expertise on the team - Rapid prototyping and MVPs - API backends and microservices that don't need OS control - Teams that want to ship features, not manage servers

SaaS — Software as a Service

You use the software; the provider runs everything. The customer manages only users, permissions, and the data they put in. There is no infrastructure or application to operate.

Examples: Google Workspace, Salesforce, Slack, Notion, GitHub, Datadog, PagerDuty.

Multi-tenancy: A single application instance serves all customers, with data isolated by tenant ID. This enables economies of scale — and is why SaaS has the lowest time-to-value.

When to choose SaaS: - Off-the-shelf software already meets the need (CRM, HR, email, monitoring) - The team should not be building or operating the tool itself - Speed of adoption matters more than customisation depth

Beyond the Big Three

The foundational models have spawned specialised variants targeting a narrower part of the stack:

FaaS — Function as a Service (Serverless)

The extreme end of PaaS. You deploy individual functions rather than applications. The provider manages everything — including scaling to zero when idle — billing only for the milliseconds the function runs.

Best for: event-driven workloads, webhooks, scheduled jobs, sporadic or bursty tasks. Not suited for long-running or consistently low-latency applications.

CaaS — Container as a Service

Between IaaS and PaaS. You provide Docker images; the provider manages the container runtime, orchestration, networking, and scaling. You retain full control of the runtime inside the container.

Best for: microservices teams already using Docker who want managed orchestration without operating Kubernetes themselves.

DBaaS — Database as a Service

A managed database where the provider handles provisioning, backups, replication, patching, and failover. You connect with a standard client and run queries.

Other specialised models

Full comparison

Decision framework

flowchart TD
  start([What do you need?]) --> q1{Need full OS control\nor lift-and-shift?}
  q1 -->|Yes| iaas[IaaS\nEC2, GCE, Azure VM]
  q1 -->|No| q2{Deploying containers?}
  q2 -->|Yes| q3{Want to manage\nKubernetes yourself?}
  q3 -->|Yes| k8s[CaaS on Kubernetes\nEKS, GKE, AKS]
  q3 -->|No| caas[CaaS managed\nCloud Run, ECS Fargate]
  q2 -->|No| q4{Event-driven /\nsporadic workload?}
  q4 -->|Yes| faas[FaaS\nLambda, Cloud Functions]
  q4 -->|No| q5{Building a\ncustom application?}
  q5 -->|Yes| paas[PaaS\nHeroku, App Engine]
  q5 -->|No| saas[SaaS\nUse an existing product]

IaaS vs PaaS vs SaaS — practical overview of the three foundational models.