The application domain is the layer where software and services live. It covers the programs users interact with and the back-end logic that powers them. Examples include web apps, mobile apps, desktop apps and background services. It also includes APIs that other systems call. This domain focuses on what the software does, how it is built and how it talks to other parts of the environment. Hosting can be on-premises, in private cloud or in public cloud. The application domain sits above the operating system and network layers, and it depends on them for compute, storage and connectivity. Typical components are front ends, back ends and data access code. A front end might be a browser app or a mobile client. A back end might be a web server, an application server, a container or a serverless function. Many modern systems use microservices, message queues and caches. Applications call databases or object stores to read and write data. They also rely on identity providers for login, often using SAML, OAuth or OpenID Connect. Configuration is part of the domain as well. Think environment variables, feature flags and secrets such as keys or tokens. Data flows here are easy to recognise. A user sends a request over HTTPS to a front end. The front end calls a back end. The back end queries a database, publishes to a queue or fetches a file. Services call other services through internal APIs. Logs, metrics and traces flow out to monitoring tools. Webhooks carry events to partners. Batch jobs and schedulers run regular tasks. Each of these flows has a clear source, a destination and a purpose. The lifecycle is a key part of the application domain. Software is designed, developed, tested and released. It is then operated and supported. Teams use version control, build systems and CI/CD pipelines to move changes from dev to test and on to production. Environments are often separated to reduce mix-ups. Dependencies are tracked with package managers. Licences and third-party terms apply to those dependencies. This domain meets many others. It meets the user and device domains through the user interface. It meets the data domain at the databases and file stores. It meets the LAN, WAN and cloud domains through the networks it uses. Clear boundaries help teams reason about behaviour and risk. In the application domain the main concerns are code quality, dependency health, authentication and authorisation, input handling, business logic and how failures are handled. Understanding this scope makes it easier to explain how a feature works, how data moves and why an incident happened.
