Service operations - Replicas

Replicas are up-to-date, running databases that contain the same data as your primary database. If your primary database fails, the replica can take over within a few seconds. Having replicas of your database is also known as instance redundancy.

You can enable a replica for your single-node services. If your primary database fails, the replica automatically assumes the role of primary, and a new standby replica is created. Replicas are resilient to availability zone failure, because they are created in different availability zones. If the availability zone of your primary goes down, your replica can take over.

To learn more about how replicas work in Timescale Cloud, see the section on how replicas work. To learn how to create a replica, see the section on creating replicas.

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How replicas work

Replicas in Timescale Cloud are asynchronous hot standbys. They use streaming replication to minimize the chance of data loss during failover. To learn more, see the blog post on Timescale Cloud replicas.

Asynchronous commits

Timescale Cloud replicas are asynchronous. That means the primary database reports success as soon as a transaction is completedly locally. It doesn’t wait to see if the replica successfully commits the transaction as well. The improves ingest rates and allows you to keep writing to your database even when the replica fails. But it might cause a small amount of data loss if the primary fails. To learn more, see the failover section.

Timescale Cloud doesn’t currently offer synchronous replicas.

Hot standbys

Timescale Cloud replicas are hot standbys. That means they are ready to take over as soon as the primary fails. It also means you can read from your replica, even when the primary is running. You can reduce the load on your primary by distributing your read queries.

Streaming replication

To keep data in sync between the primary and the replicas, the primary streams its write-ahead log (WAL). WAL records are streamed as soon as they’re written, rather than waiting to be batched and shipped. This reduces the chance of data loss.

Failover

In a normal operating state, your application is connected to the primary. It can optionally be connected to the replica for read queries. Timescale Cloud manages these connections automatically through a load balancer.

If the primary fails, the platform promotes the replica to the primary and redirects traffic to it. Any missing data writes are retrieved from backup and replayed on the new primary, to account for any lag at the time of failure. Because replicas are asynchronous, it is possible that not all data is captured, so there might be a small amount of data loss. Closing and reopening connections to the databases usually takes a few seconds, meaning there is minimal downtime.

To maintain a stable number of replicas, either the failed node recovers or a new one is created. This node becomes the new replica, while the promoted node remains the primary.

Multi-AZ

By default, Timescale Cloud replicas are created in a different availability zone (AZ) than the primary. This provides additional availability for Timescale Cloud services with replicas, as it protects against entire AZ outages. If a primary is in an AZ that experiences an outage, the service can easily fail over to the replica.

Create a database replica

warning

If your service was created before June 2022, adding a replica may cause your service to restart. Restarts typically take about one minute to complete.

Creating a database replica

1. Log in to your Timescale Cloud account and click the service you want to replicate.
2. Navigate to the Operations tab, and select Replication.
3. Check the pricing of the replica, and click Add a replica. Confirm the action by clicking Add replica.
4. You can see the replicas for each service by clicking on the service name, navigating to the Operations tab, and selecting Replication. Replicas are not shown in the main Services section, as they are not independent.
5. You can see connection information for the replica by navigating to the Overview tab. In the Connection info section, select the replica from the Role drop down menu to populate the section with the replica’s connection details.