Standby Server

hot_standby

Parameter description: Specifies whether to allow connections and queries on a standby node during its recovery.

This parameter is a POSTMASTER parameter. Set it based on instructions provided in Table 1.

• If this parameter is set to on, wal_level must be set to hot_standby or higher. Otherwise, the database startup fails.
• In an HA system, hot_standby cannot be set to off, because this setting can affect other features of the HA system.
• If the hot_standby parameter was disabled and the wal_level parameter was set to a value lower than hot_standby, perform the following operations to ensure that the logs to be replayed on the standby node can be queried on the standby node before enabling the hot_standby parameter again:
  1. Change the wal_level value of the primary and standby nodes to hot_standby or higher, and restart the instances for the change to take effect.
  2. Perform the checkpoint operation on the primary node and query the pg_stat_get_wal_senders() function to ensure that the receiver_replay_location value of each standby node is the same as the sender_flush_location value of the primary node. Ensure that the value adjustment of wal_level is synchronized to standby nodes and takes effect, and standby nodes do not need to replay low-level logs.
  3. Set the hot_standby parameter of the primary and standby nodes to on, and restart the instances for the setting to take effect.

Value range: Boolean

• on indicates that connections and queries are allowed on the standby node during the recovery.
• off indicates that connections and queries are not allowed on the standby node during the recovery.

Default value: on

max_standby_archive_delay

Parameter description: Specifies the wait period before queries on a standby node are canceled when the queries conflict with WAL processing and archiving in hot standby mode.

This parameter is a SIGHUP parameter. Set it based on instructions provided in Table 1.

–1 indicates that the standby node waits until the conflicting queries are complete.

Value range: an integer ranging from –1 to INT_MAX. The unit is ms.

Default value: 3s (3000 ms)

max_standby_streaming_delay

Parameter description: Specifies the wait period before queries on a standby node are canceled when the queries conflict with WAL data receiving through streaming replication in hot standby mode.

This parameter is a SIGHUP parameter. Set it based on instructions provided in Table 1. If this parameter is set to a large value or the service load is heavy, an error may be reported for waiting for transaction replay and flushing to disks.

–1 indicates that the standby node waits until the conflicting queries are complete.

Value range: an integer ranging from –1 to INT_MAX. The unit is ms.

Default value: 3s (3000 ms)

wal_receiver_status_interval

Parameter description: Specifies the maximum interval for notifying the primary node of the WAL Receiver status.

This parameter is a SIGHUP parameter. Set it based on instructions provided in Table 1.

Value range: an integer ranging from 0 to INT_MAX. The unit is ms.

Default value: 5s (5000 ms)

If this parameter is set to 0, the standby node does not send information, such as the log receiving location, to the primary node. As a result, the transaction commit on the primary node may be blocked, and the switchover may fail. In normal service scenarios, you are not advised to set this parameter to 0.

hot_standby_feedback

Parameter description: Specifies whether a standby node is allowed to send the result of a query performed on it to the primary node, preventing a query conflict.

This parameter is a SIGHUP parameter. Set it based on instructions provided in Table 1.

Value range: Boolean

• on indicates that the ID of the oldest transaction active on standby nodes will be sent to the primary node.
• off indicates that the ID of the oldest transaction active on standby nodes will not be sent to the primary node.

Default value: off

If this parameter is set to on, VACUUM on the primary node will not clean up tuples modified in transactions later than the oldest transaction active on standby nodes.

Therefore, the performance of the primary node will be affected. If playback conflicts with query on the standby node and a query error is reported, you are advised to increase the value of max_standby_streaming_delay.

wal_receiver_timeout

Parameter description: Specifies the maximum wait period for a standby node to receive data from the primary node.

This parameter is a SIGHUP parameter. Set it based on instructions provided in Table 1.

Value range: an integer ranging from 0 to INT_MAX. The unit is ms.

Default value: 6s (6000 ms)

wal_receiver_connect_timeout

Parameter description: Specifies the timeout period for a standby node to connect to the primary node.

This parameter is a SIGHUP parameter. Set it based on instructions provided in Table 1.

Value range: an integer ranging from 0 to INT_MAX/1000. The unit is s.

Default value: 2s

wal_receiver_connect_retries

Parameter description: Specifies the maximum attempts that a standby node connects to the primary node

This parameter is a SIGHUP parameter. Set it based on instructions provided in Table 1.

Value range: an integer ranging from 1 to INT_MAX

Default value: 1

wal_receiver_buffer_size

Parameter description: Specifies the size of the memory buffer that stores the Xlogs received by the standby and secondary nodes. Currently, primary/standby/secondary deployment is not supported by default.

This parameter is a POSTMASTER parameter. Set it based on instructions provided in Table 1.

Value range: an integer ranging from 4096 to 1047552. The unit is KB.

Default value: 64 MB (65536 KB)

primary_slotname

Parameter description: Specifies the slot name of the primary node corresponding to a standby node. This parameter is used for the mechanisms to verify the primary-standby relationship and delete WALs.

This parameter is a SIGHUP parameter. Set it based on instructions provided in Table 1.

Value range: a string

Default value: empty

max_logical_replication_workers

Parameter description: Specifies the maximum number of apply worker threads on the subscriber side.

This parameter is a POSTMASTER parameter. Set it based on instructions provided in Table 1.

Value range: an integer ranging from 0 to 262143

Default value: 4