- Webserver:
- Role: Provides the Airflow web-based user interface.
- Functionality:
- Allows users to visualize DAGs (Directed Acyclic Graphs), task dependencies, and task logs.
- Enables users to trigger and monitor DAG runs, view task logs, and manage configurations.
- Provides an interface for managing connections, variables, and other configurations.
- Interaction: Users primarily interact with the webserver when working with Airflow.
- Scheduler:
- Role: Orchestrates the execution of jobs on a trigger or schedule.
- Functionality:
- Continuously checks the
dags folder for new DAGs and updates.
- Determines which tasks need to run, when, and in what order.
- Checks for any missed or scheduled runs and triggers them.
- Assigns tasks to available worker processes (or nodes) for execution.
- Interaction: The scheduler operates in the background, ensuring that tasks run at their scheduled times or when triggered. Users don't directly interact with the scheduler, but it's crucial for executing workflows.
Why Both Are Needed:
- Separation of Concerns: By separating the webserver and scheduler, Airflow ensures that a potential issue in one component doesn't affect the other. For instance, if the web interface experiences high traffic or an issue, it won't impact the scheduler's ability to execute tasks.
- Scalability: In larger deployments, you might want to scale the number of schedulers or webservers independently based on the load. Having them as separate services facilitates this.
- Reliability: If the webserver goes down, the scheduler can still continue to schedule and execute tasks. Conversely, if the scheduler has an issue, you can still access the web interface to diagnose problems.
- Resource Allocation: In some cases, you might want to allocate different resources (CPU, memory) to the webserver and scheduler based on their workloads.