Drupal to Google Cloud: Zero Downtime Migration Playbook

Yes, you can migrate Drupal to Google Cloud with zero downtime by using a blue-green deployment strategy, where you build the new environment in parallel and switch traffic over only after it is fully verified. This playbook covers the practical steps to achieve that, from planning and data sync to DNS cutover and rollback, so your site stays live throughout the process.

Formula

The core of a zero-downtime migration isn’t a single formula but a process of continuous synchronization. The key metric is the replication lag – the time difference between the source database and the target database. You want this lag to be as close to zero as possible before you cut over. The formula for replication lag is simply:

Lag = Target_Current_Time - Target_Last_Applied_Transaction_Time

Where Target_Current_Time is the current time on the target database and Target_Last_Applied_Transaction_Time is the timestamp of the last transaction applied from the source. Both are measured in seconds. A lag of 0 means the target is fully up to date.

Worked Example

Suppose you start your migration at 10:00:00 AM. You begin continuous replication from your on-premises MySQL database to Cloud SQL. At 10:15:00 AM, you check the lag. The target database’s current time is 10:15:00, and the last transaction applied was at 10:14:58. The lag is 2 seconds. This is acceptable for a final cutover. You can now schedule the switch for a low-traffic window, knowing that the data loss window is only 2 seconds (which you can then backfill or accept).

Why Zero Downtime Matters for Drupal

Drupal powers many sites that cannot afford even a few minutes of downtime – e-commerce stores, membership portals, government sites, and high-traffic news outlets. A traditional migration that involves taking the site offline for a database dump and restore can result in hours of unavailability, lost revenue, and frustrated users. The blue-green approach avoids this entirely by treating the migration as a deployment, not a maintenance event.

Prerequisites and Planning

Before you start, you need to have a clear picture of your current environment. This includes:

• Drupal version – Are you on Drupal 9, 10, or 11? The migration steps for the codebase and database differ slightly, but the zero-downtime strategy is the same.
• Database type – MySQL or PostgreSQL. Google Cloud SQL supports both, and you’ll need to use the appropriate replication method.
• File system – Drupal stores uploaded files and generated styles. You’ll need to sync these to Google Cloud Storage or a persistent disk.
• Search and cache – If you use Solr or Redis, you’ll need to replicate or rebuild these in the new environment.

Create a detailed runbook that includes every step, the expected time for each, and a rollback plan. Test the entire process in a staging environment first.

Step-by-Step Zero Downtime Migration

Here is the practical sequence of steps to execute a zero-downtime migration from Drupal to Google Cloud:

1. Provision the target environment – Create a Compute Engine VM (or a Kubernetes cluster) with the same OS and PHP version as your current server. Install Drupal and configure the same modules and themes. Set up Cloud SQL (MySQL or PostgreSQL) with a replica of your current database.
2. Set up continuous database replication – Use Database Migration Service (DMS) or native replication to keep the Cloud SQL instance in sync with your on-premises database. This is the heart of the zero-downtime strategy. Monitor the replication lag and ensure it stays low.
3. Sync files to Google Cloud Storage – Use the gsutil rsync command to copy your sites/default/files directory to a Cloud Storage bucket. Run this repeatedly to keep the files in sync. Set up a cron job to run the rsync every few minutes.
4. Run Drupal updates on the target – Once the target is almost in sync, run drush updatedb and drush cache-rebuild on the target environment. This applies any schema changes that are needed for the new Drupal version or modules.
5. Switch traffic to the target – Update your DNS records to point to the new environment’s load balancer IP. Use a low TTL (e.g., 300 seconds) on your DNS records before the cutover. After the switch, monitor the target for errors.
6. Backfill any remaining data – If there was any replication lag at the moment of cutover, you can either accept the small data loss or manually backfill the changes from the source database. In practice, a lag of a few seconds is often acceptable for content sites.
7. Decommission the old environment – After a few days of stable operation, you can shut down the old server and stop the replication.

Common Pitfalls and How to Avoid Them

Several issues can trip up a migration. Here are the most common ones and how to handle them:

• Replication lag spikes – Large database writes can cause lag. Schedule the final cutover during a low-traffic window and monitor the lag closely.
• File sync conflicts – If users upload files during the rsync, you might get conflicts. Use the -d flag in gsutil rsync to delete extraneous files, and run it frequently to minimize the window.
• Drupal cache issues – After the switch, you may see stale cache entries. Clear the cache on the new environment immediately after the cutover.
• Search index mismatch – If you use Solr, you’ll need to re-index the content on the new environment. Do this before the cutover to avoid a period of broken search.
• PHP version differences – Ensure the target environment runs the same PHP version as your current server, or at least a version that is compatible with your Drupal core and modules.

Costs and Performance Considerations

Running a blue-green migration means you’ll have two environments running simultaneously for a period. This doubles your compute and storage costs temporarily. However, you can minimize this by using the smallest viable VM size for the target during the sync phase, then scaling up after the cutover. Cloud SQL replication also incurs additional costs, but they are modest. The performance of your Drupal site on Google Cloud will likely be better than on-premises due to the high-performance SSD persistent disks and low-latency network, especially if you use Cloud CDN for static assets.

Alternatives to the Blue-Green Approach

If your site can tolerate a few minutes of downtime, you can use a simpler approach: take the site offline, do a full database dump and restore, copy all files, and then bring the site back up. This is much easier to implement but requires a maintenance window. Another alternative is to use a module like Migrate to rebuild the site from scratch, but that is only suitable for a complete redesign, not a simple platform migration.

Frequently Asked Questions

Can I use the Database Migration Service for a Drupal site?

Yes, Google Cloud’s Database Migration Service supports MySQL and PostgreSQL, which are the two databases Drupal supports. It handles continuous replication and can be used to keep your Cloud SQL instance in sync with your on-premises database during the migration.

What is the actual downtime during the DNS cutover?

DNS propagation can take anywhere from a few minutes to 24 hours, depending on TTL and ISPs. By using a low TTL (e.g., 300 seconds) before the cutover, you can reduce the effective downtime to a few minutes, but there is no way to make it zero. The key is that your site remains available on either the old or new environment during this period.

How do I handle sessions in Drupal during the migration?

If you use the default file-based sessions, they will not be replicated. To avoid users being logged out, switch to a shared session handler like Redis or a database-backed session store that is replicated. This is a common step before starting the migration.

What if I need to roll back after the cutover?

Keep the old environment running and the replication paused (not stopped) for at least 48 hours after the cutover. If you need to roll back, you can simply switch DNS back to the old environment and resume replication. Any changes made on the new environment will be lost, but you can mitigate this by minimizing the time before rollback.

Do I need to update my Drupal version during the migration?

It is not recommended to upgrade Drupal and migrate platforms at the same time. This adds too many variables. First, migrate as-is to Google Cloud, then plan a separate Drupal core update after the site is stable.

How long does the whole process take?

For a typical Drupal site, the provisioning and sync can take a few hours, but the actual cutover is a matter of minutes. The bulk of the time is spent waiting for the initial database replication to catch up, which depends on the size of your database and network speed.

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