A replication function synchronizes logs and data between instances. It is a statistics or operation method provided by the system to implement HA.

Replication functions except statistics queries are internal functions. You are not advised to use them directly.

pg_create_logical_replication_slot('slot_name', 'plugin_name')

Description: Creates a logical replication slot.

Parameter:

slot_name
Indicates the name of the streaming replication slot.

Value range: a string, supporting only letters, digits, and the following special characters: _?-.
plugin_name
Indicates the name of the plugin.

Value range: a string, supporting only mppdb_decoding

Return type: name, text

Note: The first return value is the slot name, and the second is the start LSN position for decoding in the logical replication slot.

pg_create_physical_replication_slot ('slot_name', isDummyStandby)

Description: Creates a physical replication slot.

Parameter:

slot_name
Indicates the name of the streaming replication slot.

Value range: a string, supporting only letters, digits, and the following special characters: _?.-
isDummyStandby
Indicates whether the replication slot is the secondary one.

Value range: a boolean value, true or false

Return type: name, text

Note: The first return value is the slot name, and the second is the start LSN position for decoding in the physical replication slot.

pg_get_replication_slots()

Description: Displays information about all replication slots on the current DN.

Return type: record

The following information is returned:

**Table 1** pg_get_replication_slots() fields
Field	Type	Description
slot_name	text	Replication slot name
plugin	name	Name of the output plug-in of the logical replication slot
slot_type	text	Replication slot type
datoid	oid	Replication slot's database OID
active	boolean	Whether the replication slot is active
xmin	xid	Transaction ID of the replication slot
catalog_xmin	text	ID of the earliest-decoded transaction corresponding to the logical replication slot.
restart_lsn	text	Xlog file information on the replication slot.
dummy_standby	boolean	Indicates whether the replication slot is the secondary one.

Example:

SELECT * FROM pg_get_replication_slots();
    slot_name    | plugin | slot_type | datoid | active | xmin | catalog_xmin |    restart_lsn    | dummy_standby
-----------------+--------+-----------+--------+--------+------+--------------+-------------------+---------------
 gs_roach_common |        | physical  |      0 | f      |      |    602861775 | FFFFFFFF/FFFFFFFF | f
(1 row)

pg_drop_replication_slot('slot_name')

Description: Deletes a streaming replication slot.

Parameter:

slot_name
Indicates the name of the streaming replication slot.

Value range: a string, supporting only letters, digits, and the following special characters: _?-.

Return type: void

pg_logical_slot_peek_changes('slot_name', 'LSN', upto_nchanges, 'options_name', 'options_value')

Description: Performs decoding but does not go to the next streaming replication slot. (The decoding result will be returned again on future calls.)

Parameter:

slot_name
Indicates the name of the streaming replication slot.

Value range: a string, supporting only letters, digits, and the following special characters: _?-.
LSN
Indicates a target LSN. Decoding is performed only when an LSN is less than or equal to this value.

Value range: a string, in the format of xlogid/xrecoff, for example, '1/2AAFC60' (If this parameter is set to NULL, the target LSN indicating the end position of decoding is not specified.)
upto_nchanges
Indicates the number of decoded records (including the begin and commit timestamps). Assume that there are three transactions, which involve 3, 5, and 7 records, respectively. If upto_nchanges is 4, 8 records of the first two transactions will be decoded. Specifically, decoding is stopped when the number of decoded records exceeds 4 after decoding in the first two transactions is finished.

Value range: a non-negative integer

If any of the LSN and upto_nchanges values are reached, decoding ends.
options (optional)
- include-xids
  Indicates whether the decoded data column contains XID information.
  
  Valid value: 0 and 1. The default value is 1.
  - 0: The decoded data column does not contain XID information.
  - 1: The decoded data column contains XID information.
- skip-empty-xacts
  Indicates whether to ignore empty transaction information during decoding.
  
  Valid value: 0 and 1. The default value is 0.
  - 0: The empty transaction information is not ignored during decoding.
  - 1: The empty transaction information is ignored during decoding.
- include-timestamp
  Indicates whether decoding information contains the commit timestamp.
  
  Valid value: 0 and 1. The default value is 0.
  - 0: The decoding information does not contain the commit timestamp.
  - 1: The decoding information contains the commit timestamp.

Return type: text, uint, text

Note: The function returns the decoding result. Each decoding result contains three columns, corresponding to the above return types and indicating the LSN position, XID, and decoded content, respectively.

pg_logical_slot_get_changes('slot_name', 'LSN', upto_nchanges, 'options_name', 'options_value')

Description: Performs decoding and goes to the next streaming replication slot.

Parameter: This function has the same parameters as pg_logical_slot_peek_changes. For details, see pg_logical_slot_peek_changes('slot_name', 'LSN', upto_nchanges, 'options_name', 'options_value').

pg_replication_slot_advance ('slot_name', 'LSN')

Description: Directly goes to the streaming replication slot for a specified LSN, without outputting any decoding result.

Parameter:

slot_name
Indicates the name of the streaming replication slot.

Value range: a string, supporting only letters, digits, and the following special characters: _?-.
LSN
Indicates a target LSN. Next decoding will be performed only in transactions whose commission position is greater than this value. If an input LSN is smaller than the position recorded in the current streaming replication slot, the function directly returns. If the input LSN is greater than the LSN of the current physical log, the latter LSN will be directly used for decoding.

Value range: a string, in the format of xlogid/xrecoff

Return type: name, text

Note: A return result contains the slot name and LSN that is actually used for decoding.

pg_stat_get_data_senders()

Description: Displays statistics about replication sending threads on all data page on the current DN.

Return type: record

The following information is returned:

**Table 2** pg_stat_get_data_senders() fields
Field	Type	Description
pid	bigint	Thread PID
sender_pid	integer	Current sender PID
local_role	text	Local role
peer_role	text	Peer role
state	text	Current sender's replication status
catchup_start	timestamp with time zone	Startup time of a catchup task
catchup_end	timestamp with time zone	End time of a catchup task
queue_size	text	Data queue size
queue_lower_tail	text	Position of data queue tail 1
queue_header	text	Position of data queue header
queue_upper_tail	text	Position of data queue tail 2
send_position	text	Sending position of the sender
receive_position	text	Receiving position of the receiver
catchup_type	text	Catchup task type, full or incremental
catchup_bcm_filename	text	BCM file executed by the current catchup task
catchup_bcm_finished	integer	Number of BCM files completed by a catchup task
catchup_bcm_total	integer	Total number of BCM files to be operated by a catchup task
catchup_percent	text	Completion percentage of a catchup task
catchup_remaining_time	text	Estimated remaining time of a catchup task

pg_stat_get_wal_senders()

Description: Displays statistics about replication sending threads on all WALs on the current DN.

Return type: record

The following information is returned:

**Table 3** pg_stat_get_wal_senders() fields
Field	Type	Description
pid	bigint	Thread PID
sender_pid	integer	Current sender PID
local_role	text	Local role
peer_role	text	Peer role
peer_state	text	Peer status
state	text	Current sender's replication status
catchup_start	timestamp with time zone	Startup time of a catchup task
catchup_end	timestamp with time zone	End time of a catchup task
sender_sent_location	text	Location where the sender sends LSNs
sender_write_location	text	Location where the sender writes LSNs
sender_flush_location	text	Location where the sender flushes LSNs
sender_replay_location	text	Location where the sender replays LSNs
receiver_received_location	text	Location where the receiver receives LSNs
receiver_write_location	text	Location where the receiver writes LSNs
receiver_flush_location	text	Location where the receiver flushes LSNs
receiver_replay_location	text	Location where the receiver replays LSNs
sync_percent	text	Specifies the synchronization percentage.
sync_state	text	Synchronization state (asynchronous duplication, synchronous duplication, or potential synchronization)
sync_priority	integer	Priority of synchronous duplication (0 indicates asynchronization)
sync_most_available	text	Whether to block the active node when the synchronization on the standby node fails
channel	text	WALSender channel information

pg_stat_get_wal_receiver()

Description: Displays statistics about replication receiving threads on all WALs on the current DN.

Return type: record

The following information is returned:

**Table 4** pg_stat_get_wal_receiver()
Field	Type	Description
receiver_pid	integer	Current receiver PID
local_role	text	Local role
peer_role	text	Peer role
peer_state	text	Peer status
state	text	Current receiver's replication status
sender_sent_location	text	Location where the sender sends LSNs
sender_write_location	text	Location where the sender writes LSNs
sender_flush_location	text	Location where the sender flushes LSNs
sender_replay_location	text	Location where the sender replays LSNs
receiver_received_location	text	Location where the receiver receives LSNs
receiver_write_location	text	Location where the receiver writes LSNs
receiver_flush_location	text	Location where the receiver flushes LSNs
receiver_replay_location	text	Location where the receiver replays LSNs
sync_percent	text	Specifies the synchronization percentage.
channel	text	WALReceiver channel information

pg_stat_get_stream_replications()

Description: Displays information about all replication statistics on the current DN.

Return type: record

The following information is returned:

**Table 5** pg_stat_get_stream_replications()
Field	Type	Description
local_role	text	Local role
static_connections	integer	Connection statistics
db_state	text	Database status
detail_information	text	Detail information

Example:

SELECT * FROM pg_stat_get_stream_replications();
 local_role | static_connections | db_state | detail_information
------------+--------------------+----------+--------------------
 Normal     |                  0 | Normal   | Normal
(1 row)

pg_stat_xlog_space()

Description: Displays the Xlog space usage on the current DN.

Return type: record

The following information is returned:

**Table 6** pg_stat_xlog_space()
Column	Type	Description
xlog_files	bigint	Number of all identified xlog files in the pg_xlog directory, excluding the backup and archive_status subdirectories.
xlog_size	bigint	Total size (MB) of all identified xlog files in the pg_xlog directory, excluding the backup and archive_status subdirectories.
other_size	bigint	Total size (MB) of files in the backup and archive_status subdirectories of the pg_xlog directory.

Example:

SELECT * FROM pg_stat_xlog_space();
 xlog_files | xlog_size | other_size
------------+-----------+------------
         79 |      1264 |          0
(1 row)

pgxc_stat_xlog_space()

Description: Displays the Xlog space usage on all active DNs.

Return type: record

The following information is returned:

**Table 7** pgxc_stat_xlog_space()
Column	Type	Description
node_name	name	Node name
xlog_files	bigint	Number of all identified xlog files in the pg_xlog directory, excluding the backup and archive_status subdirectories.
xlog_size	bigint	Total size (MB) of all identified xlog files in the pg_xlog directory, excluding the backup and archive_status subdirectories.
other_size	bigint	Total size (MB) of files in the backup and archive_status subdirectories of the pg_xlog directory.

Example:

 SELECT * FROM  pgxc_stat_xlog_space();
  node_name   | xlog_files | xlog_size | other_size
--------------+------------+-----------+------------
 dn_6001_6002 |         73 |      1168 |          0
 dn_6003_6004 |         73 |      1168 |          0
 dn_6005_6006 |         73 |      1168 |          0
 cn_5003      |         79 |      1264 |          0
 cn_5001      |         72 |      1152 |          0
 cn_5002      |         73 |      1168 |          0
(6 rows)

Replication Functions

pg_create_logical_replication_slot('slot_name', 'plugin_name')

pg_create_physical_replication_slot ('slot_name', isDummyStandby)

pg_get_replication_slots()

pg_drop_replication_slot('slot_name')

pg_logical_slot_peek_changes('slot_name', 'LSN', upto_nchanges, 'options_name', 'options_value')

pg_logical_slot_get_changes('slot_name', 'LSN', upto_nchanges, 'options_name', 'options_value')

pg_replication_slot_advance ('slot_name', 'LSN')

pg_stat_get_data_senders()

pg_stat_get_wal_senders()

pg_stat_get_wal_receiver()

pg_stat_get_stream_replications()

pg_stat_xlog_space()

pgxc_stat_xlog_space()