High availability in Azure Database for PostgreSQL – Hyperscale (Citus)
APPLIES TO: Azure Database for PostgreSQL - Hyperscale (Citus)
High availability (HA) avoids database downtime by maintaining standby replicas of every node in a server group. If a node goes down, Hyperscale (Citus) switches incoming connections from the failed node to its standby. Failover happens within a few minutes, and promoted nodes always have fresh data through PostgreSQL synchronous streaming replication.
All primary nodes in a server group are provisioned into one availability zone for better latency between the nodes. The standby nodes are provisioned into another zone. The Azure portal displays the availability zone of each node in a server group.
Even without HA enabled, each Hyperscale (Citus) node has its own locally redundant storage (LRS) with three synchronous replicas maintained by Azure Storage service. If there's a single replica failure, it’s detected by Azure Storage service and is transparently re-created. For LRS storage durability, see metrics on this page.
When HA is enabled, Hyperscale (Citus) runs one standby node for each primary node in the server group. The primary and its standby use synchronous PostgreSQL replication. This replication allows customers to have predictable downtime if a primary node fails. In a nutshell, our service detects a failure on primary nodes, and fails over to standby nodes with zero data loss.
To take advantage of HA on the coordinator node, database applications need to detect and retry dropped connections and failed transactions. The newly promoted coordinator will be accessible with the same connection string.
High availability states
Recovery can be broken into three stages: detection, failover, and full recovery. Hyperscale (Citus) runs periodic health checks on every node, and after four failed checks it determines that a node is down. Hyperscale (Citus) then promotes a standby to primary node status (failover), and creates a new standby-to-be. Streaming replication begins, bringing the new node up to date. When all data has been replicated, the node has reached full recovery.
Hyperscale (Citus) displays its failover progress state on the Overview page for server groups in the Azure portal.
- Healthy: HA is enabled and the node is fully replicated to its standby.
- Failover in progress: A failure was detected on the primary node and a failover to standby was initiated. This state will transition into Creating standby once failover to the standby node is completed, and the standby becomes the new primary.
- Creating standby: The previous standby was promoted to primary, and a new standby is being created for it. When the new secondary is ready, this state will transition into Replication in progress.
- Replication in progress: The new standby node is provisioned and data synchronization is in progress. Once all data is replicated to the new standby, synchronous replication will be enabled between the primary and standby nodes, and the nodes' state will transition back to Healthy.
- No: HA isn't enabled on this node.
- Learn how to enable high availability in a Hyperscale (Citus) server group.
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