A read replica<\/b><\/strong>\u00a0is a copy of your primary Cloud SQL instance that stays in near real time sync with it and can serve read only queries. Pointing reporting or analytics traffic at a replica instead of your primary instance keeps that load from competing with the writes and application traffic your primary is handling. A replica can also be created in a different region from your primary, which additionally gives you an option for disaster recovery, covered further down.<\/p>\r\n
Prerequisites<\/h2>\r\n
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A Google Cloud Platform account<\/li>\r\n
An existing Cloud SQL instance to replicate from<\/li>\r\n<\/ul>\r\n
Step by Step Process of Creating Replicas<\/h2>\r\n
Step One: Open Your Instance<\/h3>\r\n
In the console, open the menu, then SQL.<\/p>\r\n\r\n\r\n\r\n
Menu > SQL<\/p>\r\n\r\n\r\n\r\n<\/figure>\r\n\r\n\r\n\r\n
Select your Cloud SQL instance.<\/p>\r\n\r\n\r\n\r\n<\/figure>\r\n\r\n\r\n\r\n
Give the replica an instance ID and choose its region. A replica can be created in the same region as your primary instance or in a different one, a cross region replica, which is what makes disaster recovery possible later. A multi zone, highly available configuration is not offered for a replica itself in this step, that setting applies to your primary instance’s own availability, not to the replica you are creating from it.<\/p>\r\n\r\n\r\n\r\n<\/figure>\r\n\r\n\r\n\r\n
Step Four: Show Configuration Options<\/h3>\r\n
Click Show Configuration Options.<\/p>\r\n\r\n\r\n\r\n<\/figure>\r\n\r\n\r\n\r\n
Choose the machine specification for the replica.<\/p>\r\n\r\n\r\n\r\n<\/figure>\r\n\r\n\r\n\r\n
Step Five: Choose a Connection Type<\/h3>\r\n
Choose Public IP for the simplest setup. If you want the replica reachable only over a private IP instead, you will need to enable the relevant API and configure the right permissions first, covered in Google’s own private IP setup documentation. Assign a network either way.<\/p>\r\n\r\n\r\n\r\n<\/figure>\r\n\r\n\r\n\r\n
Step Six: Create the Replica<\/h3>\r\n
Click Create Replica. This takes around 5 minutes.<\/p>\r\n\r\n\r\n\r\n<\/figure>\r\n\r\n\r\n\r\n
The replica is created and appears alongside your primary instance.<\/p>\r\n\r\n\r\n\r\n<\/figure>\r\n\r\n\r\n\r\n
Read Replicas Compared to High Availability<\/h2>\r\n
These solve different problems, and it is worth being clear on the difference. High availability, HA, uses a standby instance in a different zone within the same region, and it fails over automatically the moment Cloud SQL detects a problem with your primary, with no action needed from you. A read replica is a separate thing entirely, you control it directly, it does not fail over on its own, and promoting one to replace a failed primary is always a manual, deliberate action you trigger yourself. A replica also does not protect against a regional outage on its own unless it is a cross region replica, while HA today only protects against a zonal failure within the same region.<\/p>\r\n
Promoting a Replica for Disaster Recovery<\/h2>\r\n
If your primary instance’s region becomes unavailable for an extended period, or you are performing a planned migration to a different region, you can promote a cross region replica into a standalone, writable instance of its own. Open the replica and choose the promote option from its menu.<\/p>\r\n
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Promotion is manual and intentional. Nothing gets promoted automatically, you decide when and if it happens.<\/li>\r\n
Promotion is irreversible. Once a replica becomes a standalone instance, that action cannot be undone.<\/li>\r\n
After promoting, check that the new instance’s machine type is actually sized for full production load, since a replica is sometimes deliberately provisioned smaller than the primary it was replicating.<\/li>\r\n<\/ul>\r\n
Step Seven: Disable Replication<\/h3>\r\n
Open the replica and click Disable Replication.<\/p>\r\n\r\n\r\n\r\n<\/figure>\r\n<\/figure>\r\n\r\n\r\n\r\n
Type the replica ID to confirm, then press delete. The replica will be removed.<\/p>\r\n\r\n\r\n\r\n<\/figure>\r\n
Common Mistakes to Avoid<\/h2>\r\n
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Assuming a read replica gives you automatic failover the way high availability does. It does not, promoting a replica is always a manual step you have to trigger yourself.<\/li>\r\n
Creating a replica in the same region only, then expecting it to protect against a regional outage. Only a cross region replica gives you that option.<\/li>\r\n
Promoting a replica without first checking its machine type against your actual production load. A replica sized for light read traffic may need resizing before it can handle everything the old primary was handling.<\/li>\r\n
Treating promotion as reversible. It is not, so be certain before you commit to it, particularly outside a genuine emergency.<\/li>\r\n<\/ul>\r\n
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