# Workflow Engine Concepts

Workflow Engine provides seventeen concepts organised into two groups. The seven core concepts cover everything you need to configure, run,
and persist a workflow. The ten additional concepts control what happens when a transition fires, handle advanced scenarios, or extend the
workflow with logging, localization, and scheduling. Use this page to find the concept you need and jump to its full description.

## Core Concepts

These seven concepts are required for every Workflow Engine integration.

- [Activity](https://docs.workflowengine.io/get-started/concepts/activity/): A stable state where the process waits for input. Think of it as a step in the process, like Draft, Review, or Approved.
- [Transition](https://docs.workflowengine.io/get-started/concepts/transition/): A directed connection between two activities. A transition fires when its trigger activates and its conditions pass.
- [Command](https://docs.workflowengine.io/get-started/concepts/command/): A named action that advances the process from one activity to the next. A command can be triggered by a user or by the system.
- [Scheme](https://docs.workflowengine.io/get-started/concepts/scheme/): The blueprint that defines a process: its activities, transitions, and commands. One scheme can spawn many process instances.
- [Process Instance](https://docs.workflowengine.io/get-started/concepts/process-instance/): A single running instance of a scheme. Each instance tracks its own current activity and history.
- [Workflow Runtime](https://docs.workflowengine.io/get-started/concepts/workflow-runtime/): The engine that executes schemes, manages process instances, and fires transitions.
- [Persistence](https://docs.workflowengine.io/get-started/concepts/persistence/): The database adapter that stores schemes, instances, history, timers, and parameters.

## Additional Concepts

How transitions get triggered, gated, enriched with code, and extended with metadata, logging, localization, and scheduling.

- [Action](https://docs.workflowengine.io/get-started/concepts/action/): C# code that runs when a transition fires. Use actions to send emails, call APIs, update databases, or any side-effect.
- [Parameter](https://docs.workflowengine.io/get-started/concepts/parameter/): Named data attached to a process instance. Parameters carry values through the lifecycle.
- [Condition](https://docs.workflowengine.io/get-started/concepts/condition/): A guard on a transition that decides whether it actually fires. A transition with a failing condition stays dormant.
- [Rule](https://docs.workflowengine.io/get-started/concepts/rule/): A permission check that determines which users or roles can execute a command from the current activity.
- [Timer](https://docs.workflowengine.io/get-started/concepts/timer/): An automatic trigger that fires a transition after a delay or on a schedule, without any user action.
- [Subprocess](https://docs.workflowengine.io/get-started/concepts/subprocess/): A child process instance spawned by a parent. The parent can wait for the subprocess to complete before continuing.
- [Annotation](https://docs.workflowengine.io/get-started/concepts/annotation/): Key-value metadata attached to activities and transitions in a scheme.
- [Process Logs](https://docs.workflowengine.io/get-started/concepts/process-logs/): A record of events and errors scoped to a single process instance.
- [Localization](https://docs.workflowengine.io/get-started/concepts/localization/): Translations for activity names, command names, and parameter labels displayed to users.
- [Work Calendar](https://docs.workflowengine.io/get-started/concepts/work-calendar/): A schedule that defines working days and hours for calendar-aware timers.

## Frequently asked questions

**Do I need to understand all concepts before building my first workflow?**

No. The seven core concepts are enough to build and run a workflow. Behaviour concepts can be added as your workflows need conditions,
custom code, or automatic triggers.

**Which concepts are required for a minimal workflow?**

Activity, Transition, Command, Scheme, Process Instance, Workflow Runtime, and Persistence. Create a scheme with at least two activities and
one transition, configure the runtime with a persistence provider, then create an instance and execute a command.

**Can I use Workflow Engine without a database?**

No. The runtime requires a persistence provider to store schemes, instances, and history. SQLite is the lightest option for local
development.

**What is the difference between a Condition and a Rule?**

A condition guards whether a transition fires - it evaluates data or state. A rule guards who can fire a transition - it evaluates the
user's identity. Both run during transition evaluation but serve different purposes.

## See also

- [Web API Quick Start](https://docs.workflowengine.io/get-started/install-workflow-engine/): Start coding with the built-in REST API.
- [Your First Workflow](https://docs.workflowengine.io/get-started/create-your-first-workflow/): Define a minimal scheme and create a process instance.