Disaster recovery with environment snapshots

Cloud Composer 1 | Cloud Composer 2 | Cloud Composer 3

This page describes how to use environment snapshots for disaster recovery.

Definitions

This guide uses the following definitions:

• Disaster is an event where Cloud Composer or other components essential for your environment's operation are unavailable. This event requires a failover to a different region and Cloud Composer environments. The cause of a disaster could be natural, or human-made, including both downtime of Google Cloud regions and outages on your own infrastructure.
• Disaster Recovery (DR), in the context of Cloud Composer, is a process of restoring the environment's operation after a disaster. The process involves recreating the environment, possibly in another region. For more information about Disaster Recovery, see Disaster recovery planning guide.
• Primary environment is a Cloud Composer environment that you want to enable a DR capability for.
• Failover environment is a Cloud Composer environment designated to take over activities from the primary environment.
• Warm DR scenario is a variant of Disaster Recovery, where you use a standby failover environment, which you create before a disaster occurs.
• Cold DR scenario is a variant of Disaster Recovery, where you create a failover environment after a disaster occurs.
• Cross-region DR is a variant of warm or cold Disaster Recovery where the primary and the failover environment are located in different regions.

About the disaster recovery procedure

The disaster recovery procedure solves the problem when your primary environment has become inoperative (broken or otherwise not accessible) because of a disaster.

This procedure assumes that your primary environment is not going to be fixed in-place to address the disaster. Instead, you create a second (failover) environment side-by-side. This environment operates instead of the primary environment. At a later stage, you might decide to go back to the primary environment or to keep using the failover environment.

Because the procedure uses a failover environment, changes will be introduced when you switch from the primary environment. Changes between the primary and the failover environment include (the list is not comprehensive):

• Web server URL is going to be different. This changes the address of the Airflow UI and the Airflow REST API endpoint.

• Environment's bucket URL is going to be different.

• The network and access permissions configuration might require adjusting.

If you use the warm DR scenario, you know the values for the web server, environment's bucket addresses, and the network configuration in advance.

Before you begin

• Cloud Composer supports scheduled snapshots in 2.0.32 and later versions. Environment snapshots are supported in 2.0.9 and later versions.

• The Airflow database must have less than 20 GB of data to create snapshots.

• The total number of objects in the /dags, /plugins and /data folders in the environment's bucket must be less than 100,000 to create snapshots.

• If you use the XCom mechanism to transfer files, make sure that you use it according to Airflow's guidelines. Transferring big files or a large number of files using XCom impacts Airflow database's performance and can lead to failures when loading snapshots or upgrading your environment. Consider using alternatives such as Cloud Storage to transfer large volumes of data.

Preparation overview

Both DR scenarios include the following preparation steps:

1. Create a failover environment.

   • In the warm DR scenario, you keep this environment available.
   • In the cold DR scenario, you create this environment only to test your disaster recovery procedure. After you complete the preparation, you delete this environment, and create it again after a disaster happens.

2. Create a bucket for snapshots.

   • The bucket must be available in the DR region. For cross-region DR, the snapshots bucket must be either multiregional or located in a different region than the primary environment.

   • Check that DAGs can access regional resources.

3. Set up DB maintenance.

4. Set up scheduled snapshots.

5. Test your disaster recovery procedure.

Disaster recovery overview

After a disaster happens:

1. (Cold DR only) Create a failover environment.
2. If possible, stop the primary environment from executing DAGs.
3. Load a snapshot from the snapshots bucket to the failover environment.
4. If required, adjust the configuration of the failover environment.
5. Decide what to do with the primary environment.

Preparation steps

Do the steps outlined below to set up disaster recovery for your environment.

Create a failover environment

Create an environment that acts as a failover environment.

Use the following guidelines:

• Your primary and failover environment must use the same version of Cloud Composer and Airflow.

• In the warm DR scenario, make sure that you update and upgrade both environments in sync. For example, if you upgrade the primary environment to a later Cloud Composer version, or install PyPI packages, then your failover environment must also have these changes.

• We recommend creating the failover environment in a region different from the primary environment. As a result, a broader range of possible disaster scenarios can be covered, such as a disaster that affects the availability of the whole region.

• We recommend to use Terraform to create primary and failover environments, so that both have consistent configuration. Make sure that Terraform definitions for both primary and failover environments are synchronized.

• The configuration of the failover environment (such as environment size, number of schedulers, and IAM permissions) is recommended to conform to the configuration of the primary environment. IAM permissions for both environments must give appropriate access to users and snapshots.

Check the resource availability

DAGs can operate on external resources, and access to those resources might be dependent on the environment's configuration (such as permissions granted to the environment's service account, network configuration, or project). Make sure that those resources are available to the failover environment.

An environment might interact with some external resources through connections stored in Airflow. Check if these resources should be adjusted in the failover environment compared to the primary environment.

Create a storage bucket for snapshots

Create a new storage bucket for environment snapshots. Do not use environment buckets for disaster recovery, since configuration for the retention policy and lifecycle is applied on the bucket level.

Make sure that this storage bucket has IAM permissions, a retention policy, and a lifecycle configuration set in a way that prevents accidental deletion or unauthorized access. For more information about configuring a bucket for snapshots, see Configuring scheduled snapshots.

You can:

• Create a bucket in a different region.
• Create a multi-regional bucket.

Set up DB maintenance

Keep the Airflow database small and within the size limit by setting up database cleanup. Doing so makes the process of saving and loading snapshots quicker. The Airflow database must have less than 20 GB of data to create snapshots.

Set up scheduled snapshots

Set up scheduled snapshots for the primary environment.

Snapshots can be created only in a healthy environment, so snapshots must be saved before the disaster occurs.

For more information about how snapshots work, see Save and load environment snapshots. See the Save an environment snapshot section of the documentation for information about where to find the saved snapshots.

(Optional) Set up monitoring for scheduled snapshot operations

For scheduled snapshots with a frequency of at least once every 12 hours, you can use Cloud Monitoring to alert you when a snapshot is not automatically created.

For lower frequency schedules, you use Google Cloud CLI to verify snapshot operations results. See Verify save snapshot operations.

1. In the Google Cloud console, go to the Monitoring page.

   Go to Monitoring

2. In the Monitoring navigation pane, select notificationsAlerting.
3. If you haven't created your notification channels and if you want to be notified, then click Edit Notification Channels and add your notification channels. Return to the Alerting page after you add your channels.
4. From the Alerting page, select Create policy.
5. To select the metric, expand the Select a metric menu and then do the following:
   1. To limit the menu to relevant entries, enter Composer Snapshot into the filter bar. If there are no results after you filter the menu, then disable the Show only active resources & metrics toggle.
   2. For the Resource type, select Cloud Composer Environment.
   3. For the Metric category, select Environment.
   4. For the Metric, select Snapshot creation count.
   5. Select Apply.
6. Click Add filter, and use the dropdown menus to add the following filters:

   Filter	Comparator	Value
   Resource label > environment_name	=	The environment name where you want to monitor scheduled snapshots.
   Monitor label > result	=	SUCCEEDED

7. In the Transform data section, set the following attributes:
   • For Rolling window, select the monitoring window for this alert. This value impacts the threshold configuration in the next step.

     Recommended value for scheduled snapshot monitoring: 1 day.

   • For Rolling window function, select delta.
8. Click Next.
9. The settings in the Configure alert trigger page determine when the alert is triggered. Complete this page with the settings in the following table.
   

   Field	Value
   Condition type	Threshold
   Alert trigger	Any time series violates
   Threshold position	Below threshold
   Threshold value	The number of scheduled snapshots you expect to be saved within the amount of time configured as the Rolling window for the alert. Calculate this value using the following formula: (rolling window in hours / schedule frequency in hours) - 1 Note: Deducting 1 hour in the formula is to account for varying snapshot completion times. This helps prevent raising false positives if the latest snapshot is still running during a monitoring check. Example: If you use the recommended rolling window of 1 day, and your schedule frequency is once every 2 hours, set this value to 11 (as per calculation: 24 / 2 - 1 = 11). If your schedule runs correctly, within any 24 hour window you should have at least 11 snapshots. If you do not, it means a snapshot operation did not complete successfully and Cloud Monitoring triggers this alert.
   Condition name	Your custom name for the condition.

10. Click Next.
11. Optional: To add notifications to your alerting policy, click Notification channels. In the dialog, select one or more notification channels from the menu, and then click OK.
12. Optional: Update the Incident autoclose duration. This field determines when Monitoring closes incidents in the absence of metric data.
13. Optional: Click Documentation, and then add any information that you want included in a notification message.
14. Click Alert name and enter a name for the alerting policy.
15. Click Create Policy.

Test your disaster recovery procedure

Make sure to test your disaster recovery procedure after you set it up and then periodically afterwards. This lets you address potential problems that might impact the actual disaster recovery process.

In the cold DR scenario, you can delete the failover environment after you finish testing the disaster recovery procedure.

Verify save snapshot operations

You can use Google Cloud CLI to retrieve the list of save snapshot operations and verify if your snapshots are ready for disaster recovery scenarios.

This method is useful if you save snapshots less often than at least once every 12 hours. For verifying snapshots saved more frequently, it's best to configure Cloud Monitoring alerting. See Set up monitoring for scheduled snapshot operations.

gcloud composer operations list \
    --locations LOCATION \
    --filter="metadata.operationType=SAVE_SNAPSHOT AND 
    metadata.resource=projects/PROJECT_ID/locations/LOCATION/environments/ENVIRONMENT_ID"
    --format yaml

gcloud composer operations list \
    --locations us-central1 \
    --filter="metadata.operationType=SAVE_SNAPSHOT AND 
    metadata.resource=projects/my-project/locations/us-central1/environments/my-environment"
    --format yaml

After a disaster happens

Do the steps outlined below after a disaster happens to recover your primary environment.

(Cold DR only) Create a failover environment

Follow the instructions in the Create a failover environment section.

Stop the primary environment from executing DAGs

If possible, stop the primary environment from executing DAGs:

• If the primary environment is still accessible, pause all DAGs.
• If the primary environment's bucket is accessible, move all DAGs from the environment's bucket, or to a folder ouside /dags in the primary environment's bucket.

Load a snapshot to the failover environment

Load a snapshot from the primary environment into the failover environment.

Once the snapshot is loaded to the failover environment, it schedules and executes tasks as if nothing was executed by the primary environment after creating a snapshot. However, some of those tasks might have already been executed by the primary environment. The failover environment does not have any means to recognize which tasks have been executed after creating the snapshot and before a disaster. As a result, some tasks might be executed twice (in both the primary and the failover environment). We recommend that all tasks are idempotent and that the scheduled snapshots are created every two hours.

(If required) Adjust the configuration of the failover environment

In some cases, you might want to change the configuration of the failover environment after you load the snapshot of the primary environment into it.

For example, in a cold DR scenario, you might need to use a different set of Airflow environment variables in the failover environment. As another example, in a warm DR scenario, you might need to grant permissions to users in the Airflow UI, so that they can access the failover environment.

You can either perform these changes manually, or prepare a shell script with commands that change the configuration of the failover environment by running gcloud composer environment update commands.

Decide what to do with the primary environment

Some disasters might happen because the primary environment is not reachable but still operative or does not operate properly. For example, you cannot access the primary environment through the network because of an infrastructure failure. As another example, the environment operates with some errors or with reduced capacity, but some DAGs are still executed.

If the original environment is still running, then it might generate costs directly related to Cloud Composer or other services accessed through the DAGs, even though a new environment was created as a replacement. This environment can still execute some DAGs; as a result, some operations might be executed twice: in the primary environment that is still running and in the failover environment after loading the snapshot.

If the primary environment exists, but does not operate properly

Primary environment can be deleted, if all relevant data was recovered. For example, you might want to recover data that is not included in the environment snapshots, such as networking configuration, or contents of the environment's bucket outside the /dags and /plugins folders.

If the primary environment becomes accessible and healthy again

If the primary environment was inaccessible only temporarily, and becomes accessible and healthy again, then you can choose an approach:

• Keep using the failover environment.
• Return to the primary environment.

To keep using the failover environment:

1. If the primary environment still executes DAGs, pause them as soon as possible.
2. Make sure that all relevant data is recovered, then delete the primary environment.
3. Repeat the DR preparation steps for the failover environment, such as setting up scheduled snapshots.

To return to the primary environment:

1. Pause all DAGs in the failover environment.
2. Wait for all DAG runs to complete in the failover environment, or stop them.
3. Save a snapshot of the failover environment.
4. Load this snapshot to the primary environment.
5. Unpause the DAGs in the primary environment.
6. If required, delete the failover environment.

What's next

• Configure scheduled snapshots
• Create environments