Availability needs are determined by how a system recovers when a database crashes or becomes unavailable. Availability is a balance between how much data may be lost after a failure and how long it takes for the database to get back up and running. The former is called the Recovery Point Objective (RTO), and the latter is called the Recovery Time Objective (RTO). High availability usually means keeping the system operational during a failure while minimizing any data loss.
Timescale Cloud provides very low RTO and RPO for all instances. Timescale Cloud also offers features such as replicas to satisfy high availability requirements. Replicas ensure near-zero downtime and near-zero data loss if the database fails. This section covers some strategies used to increase service availability on Timescale Cloud.
For all services, Timescale Cloud decouples the database's compute and storage. This allows databases to self-heal gracefully in many situations, rather than crashing and recovering from backup in all cases. Without self-healing, failure recovery usually requires a full restore from backup. This can result in hours of downtime. Timescale Cloud is able to replace only the failed part of the database, dramatically reducing potential downtime in the most common failure scenarios.
Compute failing is by far the most common cause of a database failure. It can be caused by unoptimized queries or increased load that maxes out the CPU usage, causing a failure. In compute failure scenarios, only the instance (compute and memory) needs replacing since the data on disk is unaffected. Once a compute failure is detected, Timescale Cloud immediately provisions a new database instance and mounts the database's existing storage (disk) to the new instance. Any available WAL then replays. This process is similar to recovering from a filesystem-level backup. The impact is that the database is unavailable while the storage mounts to the new server instance, and database connections reset. This process typically only takes thirty seconds, though it may take up to twenty minutes in some circumstances. Even in the worst-case scenario, this recovery is an order of magnitude faster than a standard recovery from backup procedure. In addition, the entire process for detecting and recovering from a compute failure is fully automated, with no action required by the user.
Compute failures are by far the most common types of failures. That said, sometimes the disk itself can fail, although this is far less common. In the event of a storage failure, Timescale Cloud automatically performs a full recovery from backup. Similar to compute failures, any unarchived WAL (up to 16 MB or 5 minutes) is also lost. You can learn more about backups and recovery here.
Timescale Cloud's rapid recovery strategy can dramatically reduce the RTO in the most common failure scenarios, bringing the time to recover to mere seconds or minutes instead of hours. The potential data loss (RPO) is minimal due to the WAL streaming strategy. The only potential data loss is of WAL segments that had not yet been written to disk at the time of failure. Fortunately, these segments are written to disk every 16 MB or 5 minutes, whichever comes first. For systems that require very low RTO and near-zero RPO in nearly every disaster scenario, Timescale Cloud recommends using replicas.
Timescale Cloud offers different tools to help improve the availability of services, but also needs help from the user. We recommend that Cloud users follow best practices to avoid situations like consistently maxing out CPU usage. These practices can result in worst-case scenarios like WAL archiving getting queued behind other processes, causing a failure to result in larger data loss. To mitigate this, Cloud actively monitors for such scenarios to help catch them before a failure occurs.
Timescale Cloud offers replicas for systems requiring higher availability than rapid recovery. Replicas on Timescale Cloud are "hot standbys," meaning that they take over operations if the primary fails and can also be used for read queries during normal operations. Timescale Cloud replicas are also deployed in a different Availability Zone (AZ) than the primary to protect against scenarios where an entire AZ becomes unavailable.
Adding a replica to your service helps significantly reduce the likelihood of downtime in the event of a failure. If there is a failure, the only noticeable impact on your service is connections resetting while the replica is promoted to the primary. For replicas, WAL is streamed to the replica directly, not in chunks like in services without replicas. In certain high-traffic scenarios, there can be a small lag (at most a few seconds) between the primary and replica, causing transactions to be committed but not replicated before a failure. In this specific case, there may be a small amount of data loss, though this is rare. In general, replicas are considered a best practice for services with high availability requirements.
For more information about database replicas, including how they work, see the Service Operations - Replicas section.
Some operations on your database cannot avoid downtime, such as upgrading a major version of PostgreSQL. If Timescale Cloud has to apply a critical update, like a security patch, it is only applied during the set Maintenance Window. However, these situations are rare or triggered manually by the user. See the Maintenance section in the docs to learn more about how maintenance is handled.
Adding replicas to your service can help reduce the downtime during a maintenance event, as maintenance is applied to each node individually. For example, your replica may have maintenance performed on it while the primary remains operational; once it is completed, the replica is promoted to the primary while the (former) primary node undergoes maintenance.
On Timescale Cloud, full backups are taken weekly and incremental backups are performed daily. Additionally, WAL is archived as soon as it is written to disk. In the event of a catastrophic failure, if the service is unable to self-heal, Timescale Cloud automatically recovers your database from backup and replays any WAL to close the gap between the backup and time of failure. This strategy ensures that data can be retrieved almost right up to the point of failure, with the exception of data in memory (up to 16 MB or the last 5 minutes). For more information about backups on Timescale Cloud, see the backup and restore section.
Found an issue on this page?Report an issue!