Upgrading Apache Airflow

Prerequisites
General Upgrade Steps
Upgrade-Specific Notes
- Major Version Upgrades
- Minor Version Upgrades
Breaking Changes
Rollback Strategy

Upgrading Apache Airflow is a critical process that ensures you benefit from the latest features, performance improvements, and security patches. This guide outlines the general steps and considerations for a smooth upgrade.

Prerequisites

Before starting any upgrade, ensure you have the following in place:

Backup: A recent, verified backup of your Airflow metadata database. This is the most crucial step.
Version Compatibility: Review the release notes for the target Airflow version to understand any breaking changes or deprecations. Check compatibility with your Python version, operating system, and any custom providers or plugins.
Testing Environment: Ideally, perform the upgrade in a staging or testing environment that mirrors your production setup before applying it to production.
Sufficient Downtime: Plan for a maintenance window. While Airflow aims for minimal downtime, upgrades can sometimes require stopping all components.
Access: Ensure you have the necessary administrative privileges to access your Airflow environment, database, and deployment infrastructure.

General Upgrade Steps

Before You Upgrade

Read Release Notes: Thoroughly read the release notes for the version you are upgrading to. Pay close attention to the "Breaking Changes" and "Deprecations" sections.
Review Configuration: Check your airflow.cfg file for any deprecated or changed configuration options.
Update Dependencies: Ensure your Python environment has the necessary packages and their compatible versions. You might need to update Airflow itself and any related providers.
Run Health Checks: Before upgrading, ensure your current Airflow instance is healthy and all DAGs are running as expected.

During the Upgrade

Stop Airflow Components: Gracefully stop the Airflow Webserver, Scheduler, and any workers. Ensure no tasks are running.
Backup Database: Perform a final backup of your metadata database immediately before proceeding.
Upgrade Airflow Package: Use pip or your package manager to upgrade the Airflow package:
```
pip install apache-airflow --upgrade
```
Or, if you are using specific versions:
```
pip install apache-airflow==X.Y.Z
```
Upgrade Providers: If you use external providers, upgrade them to versions compatible with your new Airflow version.
```
pip install apache-airflow-providers-cncf-kubernetes --upgrade
```
Run Database Migrations: Airflow uses SQLAlchemy for database interactions. You need to run database migrations to update the schema for the new version.
```
airflow db upgrade
```
Reset or Recreate Connections/Variables: In some cases, especially after major upgrades, you might need to reset or re-enter connections and variables.
Start Airflow Components: Start the Scheduler, Webserver, and workers in the new version.

After the Upgrade

Monitor Logs: Closely monitor the logs of the Scheduler, Webserver, and workers for any errors or warnings.
Run Test DAGs: Execute a few test DAGs to ensure they run correctly.
Check UI: Verify that the Airflow UI is functioning as expected and all components are visible.
Gradually Resume Workloads: If everything looks good, gradually resume your normal DAG runs and workloads.

Upgrade-Specific Notes

Major Version Upgrades

Upgrading between major versions (e.g., 2.x to 3.x) typically involves more significant changes. Always consult the specific migration guide for major version upgrades. These often include:

Significant API changes.
Major refactoring of core components.
Deprecation of older features or configurations.
Potential database schema changes that might be more complex.

Minor Version Upgrades

Minor version upgrades (e.g., 2.5.x to 2.6.x) are usually more straightforward. They primarily include new features, bug fixes, and minor enhancements. While still important to read release notes, the risk of breaking changes is generally lower.

Breaking Changes

Breaking changes are the most critical aspect to be aware of during an upgrade. These can affect your DAGs, custom code, or configurations. Always check the official Airflow Release Notes for a comprehensive list of breaking changes for each version.

Common areas for breaking changes include:

Operator parameters or behaviors.
Configuration options.
Internal APIs used by custom plugins or hooks.
Default values for certain settings.

Warning: Never upgrade directly to the latest version from a very old version without intermediate upgrades. It's often recommended to upgrade to intermediate major versions to handle changes incrementally.

Rollback Strategy

Having a rollback plan is essential. If the upgrade fails or causes unexpected issues, you should be able to revert to your previous working state.

Database Restore: The most critical part of rollback is restoring the metadata database from the backup taken before the upgrade.
Code/Configuration Revert: Revert your Airflow installation to the previous version and re-apply the configurations that were present before the upgrade.
Test Rollback: If possible, practice your rollback procedure in your testing environment to ensure it's effective.

By following these guidelines and paying close attention to the official documentation and release notes, you can perform a safe and successful upgrade of your Apache Airflow instance.