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    PolarDB:Logical replication slot failover

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    PolarDB:Logical replication slot failover

    Last Updated:Apr 24, 2025

    This topic describes the logical replication slot failover feature of PolarDB for PostgreSQL (Compatible with Oracle). In case the primary node fails, this feature ensures a logical replication slot is synced from the primary to the standby node.

    Prerequisites

    The PolarDB for PostgreSQL (Compatible with Oracle) cluster runs PolarDB for PostgreSQL (Compatible with Oracle) 1.0 (revision version 1.1.27 or later).

    Note

    Execute the SHOW polar_version statement to view the revision version of your cluster. Upgrade the revision version if necessary.

    Background information

    According to PostgreSQL's streaming replication protocol, the logical replication slot on the primary node is not synced to the read-only node that serves as the standby node. In the event of a failover, the logical replication slot is lost, and consequently the logical subscriptions are disrupted. To resolve this issue, PolarDB for PostgreSQL (Compatible with Oracle) enhances the system by supporting the failover of the logical replication slot.

    Note

    • The failover of the physical replication slot is not supported.

    • For more information, see the Replication slots section of PostgreSQL's Logical decoding concepts.

    Enable or disable the logical replication slot failover feature

    You can enable or disable the logical replication slot failover feature by configuring the polar_failover_slot_mode parameter. Valid values are as follows:

    • sync: enables logical replication slot failover and sets it to the synchronous mode.

      Note

      • The synchronous mode ensures that no data is lost in logical replication during the failover of a cluster.

      • This is made possible by ensuring that the logical replication client does not receive changes earlier than the standby node. However, in the event of long-time disconnection and reconnection or the rebuild of the standby node, the time window becomes large enough that the primary-standby sync latency is relatively high and the logical replication client may receive changes earlier than the standby node. In case a failover occurs during this time window, data may be lost in logical replication. In addition, the data that is generated on the new primary node may also be lost.

        To avoid such data loss, contact us to set the polar_priority_replication_force_wait parameter to on, so that when the standby node is disconnected, the primary node does not send changes to the logical replication client until the standby node is reconnected or rebuilt. However, this reduces the availability of logical replication. Proceed with caution.

    • async (default): enables logical replication slot failover and sets it to the asynchronous mode.

      Note

      • In the asynchronous mode, no data is lost in logical replication during the failover of an HA cluster. However, duplicate data may be sent to the logical replication client. If the client has newer data than the new primary after failover is complete, the client may have retained lost data, if any, during failover.

      • If the inconsistent data affects your business, we recommend that you use the synchronous mode.

    • off: disables logical replication slot failover.