BDR System Functions v3.7

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BDR management is primarily accomplished via SQL-callable functions. All functions in BDR are exposed in the bdr schema. Any calls to these functions should be schema-qualified, rather than putting bdr in the search_path.

This page contains additional system functions that are not described in the other sections of the documentation.

Note that you cannot manipulate BDR-owned objects using pglogical functions; only using the following supplied functions.

Version Information Functions

bdr.bdr_edition

This function returns a textual representation of the BDR edition. BDR3 is distributed in either Standard Edition (SE) or Enterprise Edition (EE); this function can be used to check which of those is currently installed. Deprecated.

bdr.bdr_version

This function retrieves the textual representation of the BDR version that is currently in use.

bdr.bdr_version_num

This function retrieves a numerical representation of the BDR version that is currently in use. Version numbers are monotonically increasing, allowing this value to be used for less-than and greater-than comparisons.

The following formula is used to turn the version number consisting of major version, minor version and patch release into a single numerical value:

MAJOR_VERSION * 10000 + MINOR_VERSION * 100 + PATCH_RELEASE

System and Progress Information Parameters

BDR exposes some parameters that can be queried via SHOW in psql or using PQparameterStatus (or equivalent) from a client application. This section lists all such parameters BDR reports to.

bdr.local_node_id

Upon session initialization, this is set to the node id the client is connected to. This allows an application to figure out what node it is connected to even behind a transparent proxy.

It is also used in combination with CAMO.

bdr.last_committed_lsn

After every COMMIT of an asynchronous transaction, this parameter is updated to point to the end of the commit record on the origin node. In combination with bdr.wait_for_apply_queue, this allows applications to perform causal reads across multiple nodes, i.e. to wait until a transaction becomes remotely visible.

transaction_id

As soon as Postgres assigns a transaction id, this parameter is updated to show the transaction id just assigned, if CAMO is enabled.

Note

This is only available on EDB Postgres Extended.

Utility Functions

bdr.wait_slot_confirm_lsn

Allows the user to wait until the last write on this session has been replayed to one or all nodes.

Waits until a slot passes certain LSN. If no position is supplied, the current write position is used on the local node.

If no slot name is passed, it will wait until all BDR slots pass the LSN. This is a separate function from the one provided by pglogical so that we can only wait for slots registered for other BDR nodes, not all pglogical slots and, more importantly, not our BDR group slot.

The function polls every 1000ms for changes from other nodes.

If a slot is dropped concurrently the wait will end for that slot. If a node is currently down and is not updating its slot then the wait will continue. You may wish to set statement_timeout to complete earlier in that case.

Synopsis

bdr.wait_slot_confirm_lsn(slot_name text DEFAULT NULL, target_lsn pg_lsn DEFAULT NULL)

Parameters

slot_name - name of replication slot, or if NULL, all BDR slots (only)
target_lsn - LSN to wait for, or if NULL, use the current write LSN on the local node

bdr.wait_for_apply_queue

The function bdr.wait_for_apply_queue allows a BDR node to wait for the local application of certain transactions originating from a given BDR node. It will return only after all transactions from that peer node are applied locally. An application or a proxy can use this function to prevent stale reads.

For convenience, BDR provides a special variant of this function for CAMO and the CAMO partner node, see bdr.wait_for_camo_partner_queue.

In case a specific LSN is given, that's the point in the recovery stream from the peer to wait for. This can be used in combination with bdr.last_committed_lsn retrieved from that peer node on a previous or concurrent connection.

If the given target_lsn is NULL, this function checks the local receive buffer and uses the LSN of the last transaction received from the given peer node. Effectively waiting for all transactions already received to be applied. This is especially useful in case the peer node has failed and it's not known which transactions have been sent. Note that in this case, transactions that are still in transit or buffered on the sender side are not waited for.

Synopsis

bdr.wait_for_apply_queue(peer_node_name TEXT, target_lsn pg_lsn)

Parameters

peer_node_name - the name of the peer node from which incoming transactions are expected to be queued and which should be waited for. If NULL, waits for all peer node's apply queue to be consumed.
target_lsn - the LSN in the replication stream from the peer node to wait for, usually learned via bdr.last_committed_lsn from the peer node.

bdr.get_node_sub_receive_lsn

This function can be used on a subscriber to get the last LSN that has been received from the given origin. Either filtered to take into account only relevant LSN increments for transactions to be applied or unfiltered.

The difference between the output of this function and the output of bdr.get_node_sub_apply_lsn() measures the size of the corresponding apply queue.

Synopsis

bdr.get_node_sub_receive_lsn(node_name name, committed bool default true)

Parameters

node_name - the name of the node which is the source of the replication stream whose LSN we are retrieving/
committed - the default (true) makes this function take into account only commits of transactions received, rather than the last LSN overall; including actions that have no effect on the subscriber node.

bdr.get_node_sub_apply_lsn

This function can be used on a subscriber to get the last LSN that has been received and applied from the given origin.

Synopsis

bdr.get_node_sub_apply_lsn(node_name name)

Parameters

node_name - the name of the node which is the source of the replication stream whose LSN we are retrieving.

bdr.run_on_all_nodes

Function to run a query on all nodes.

Warning

This function will run an arbitrary query on a remote node with the privileges of the user used for the internode connections as specified in the node's DSN. Caution needs to be taken when granting privileges to this function.

Synopsis

bdr.run_on_all_nodes(query text)

Parameters

query - arbitrary query to be executed.

Notes

This function will connect to other nodes and execute the query, returning a result from each of them in json format. Multiple rows may be returned from each node, encoded as a json array. Any errors, such as being unable to connect because a node is down, will be shown in the response field. No explicit statement_timeout or other runtime parameters are set, so defaults will be used.

This function does not go through normal replication, it uses direct client connection to all known nodes. By default, the connection is created with bdr.ddl_replication = off, since the command are already being sent to all of the nodes in the cluster.

Be careful when using this function since you risk breaking replication and causing inconsistencies between nodes. Use either transparent DDL replication or bdr.replicate_ddl_command() to replicate DDL. DDL may be blocked in a future release.

Example

It's useful to use this function in monitoring, for example in the following query:

SELECT bdr.run_on_all_nodes($$
	SELECT local_slot_name, origin_name, target_name, replay_lag_size
      FROM bdr.node_slots
     WHERE origin_name IS NOT NULL
$$);

...will return something like this on a two node cluster:

[
    {
        "dsn": "host=node1 port=5432 dbname=bdrdb user=postgres ",
        "node_id": "2232128708",
        "response": {
            "command_status": "SELECT 1",
            "command_tuples": [
                {
                    "origin_name": "node1",
                    "target_name": "node2",
                    "local_slot_name": "bdr_bdrdb_bdrgroup_node2",
                    "replay_lag_size": "0 bytes"
                }
            ]
        },
        "node_name": "node1"
    },
    {
        "dsn": "host=node2 port=5432 dbname=bdrdb user=postgres ",
        "node_id": "2058684375",
        "response": {
            "command_status": "SELECT 1",
            "command_tuples": [
                {
                    "origin_name": "node2",
                    "target_name": "node1",
                    "local_slot_name": "bdr_bdrdb_bdrgroup_node1",
                    "replay_lag_size": "0 bytes"
                }
            ]
        },
        "node_name": "node2"
    }
]

bdr.run_on_nodes

Function to run a query on a specified list of nodes.

Warning

This function will run an arbitrary query on remote nodes with the privileges of the user used for the internode connections as specified in the node's DSN. Caution needs to be taken when granting privileges to this function.

Synopsis

bdr.run_on_nodes(node_names text[], query text)

Parameters

node_names - text ARRAY of node names where query will be executed.
query - arbitrary query to be executed.

Notes

bdr.run_on_group

Function to run a query on a group of nodes.

Warning

Synopsis

bdr.run_on_group(node_group_name text, query text)

Parameters

node_group_name - name of node group where query will be executed.
query - arbitrary query to be executed.

Notes

bdr.global_lock_table

This function will acquire a global DML locks on a given table. See DDL Locking Details for information about global DML lock.

Synopsis

bdr.global_lock_table(relation regclass)

Parameters

relation - name or Oid of the relation to be locked.

Notes

This function will acquire the global DML lock independently of the ddl_locking setting.

The bdr.global_lock_table function requires UPDATE, DELETE, or TRUNCATE privilege on the locked relation, unless bdr.backwards_compatibility is set is set to 30618 or below.

bdr.wait_for_xid_progress

This function can be used to wait for the given transaction (identified by it's XID) originated at the given node (identified by it's node id) to make enough progress on the cluster. The progress is defined as the transaction being applied on a node and this node having seen all other replication changes done before the transaction is applied.

Synopsis

bdr.wait_for_xid_progress(origin_node_id oid, origin_topxid int4, allnodes boolean DEFAULT true)

Parameters

origin_node_id - node id of the node where the transaction was originated.
origin_topxid - XID of the transaction.
allnodes - if true then wait for the transaction to progress on all nodes. Otherwise only wait for the current node.

Notes

The function can be used only for those transactions that have replicated a DDL command because only those transactions are tracked currently. If a wrong origin_node_id or origin_topxid is supplied, the function may wait forever or until statement_timeout is hit.

bdr.local_group_slot_name

Returns the name of the group slot on the local node.

Example

bdrdb=# SELECT bdr.local_group_slot_name();
 local_group_slot_name 
-----------------------
 bdr_bdrdb_bdrgroup

bdr.node_group_type

Returns the type of the given node group. Returned value is same as what was passed to bdr.create_node_group() when the node group was created, except normal is returned if the node_group_type was passed as NULL when the group was created.

Example

bdrdb=# SELECT bdr.node_group_type('bdrgroup');
 node_group_type
-----------------
 normal

Global Advisory Locks

BDR supports global advisory locks. These locks are very similar to the advisory locks available in PostgreSQL except that the advisory locks supported by BDR are global in nature. They follow semantics similar to DDL locks. So an advisory lock is obtained by majority consensus and hence can be used even if one or more nodes are down or lagging behind, as long as a majority of all nodes can work together.

Currently we only support EXCLUSIVE locks. So if another node or another backend on the same node has already acquired the advisory lock on the object, then other nodes or backends must wait for the lock to be released.

Advisory lock is transactional in nature. So the lock is automatically released when the transaction ends unless it is explicitly released before the end of the transaction, in which case it will be available as soon as it's released. Session level advisory locks are not currently supported.

Global advisory locks are re-entrant. So if the same resource is locked three times it must then be unlocked three times to be released for other sessions' use.

bdr.global_advisory_lock

This function acquires an EXCLUSIVE lock on the provided object. If the lock is not available, then it will wait until the lock becomes available or the bdr.global_lock_timeout is reached.

Synopsis

bdr.global_advisory_lock(key bigint)

parameters

key - the object on which an advisory lock is acquired.

Synopsis

bdr.global_advisory_lock(key1 integer, key2 integer)

parameters

key1 - first part of the composite key.
key2 - second part of the composite key.

bdr.global_advisory_unlock

This function released previously acquired lock on the application defined source. The lock must have been previously obtained in the same transaction by the application, otherwise an ERROR is raised.

Synopsis

bdr.global_advisory_unlock(key bigint)

parameters

key - the object on which advisory lock is acquired.

Synopsis

bdr.global_advisory_unlock(key1 integer, key2 integer)

parameters

key1 - first part of the composite key.
key2 - second part of the composite key.

Monitoring functions

bdr.monitor_group_versions

To provide a cluster-wide version check, this function uses BDR version information returned from the view bdr.group_version_details.

Synopsis

bdr.monitor_group_versions()

Notes

This function returns a record with fields status and message, as explained in Monitoring.

This function calls bdr.run_on_all_nodes().

bdr.monitor_group_raft

To provide a cluster-wide Raft check, this function uses BDR Raft information returned from the view bdr.group_raft_details.

Synopsis

bdr.monitor_group_raft()

Notes

This function returns a record with fields status and message, as explained in Monitoring.

This function calls bdr.run_on_all_nodes().

bdr.monitor_local_replslots

This function uses replication slot status information returned from the view pg_replication_slots (slot active or inactive) to provide a local check considering all replication slots except the BDR group slots.

Synopsis

bdr.monitor_local_replslots()

Notes

This function returns a record with fields status and message, as explained in Monitoring replication slots.

bdr.wal_sender_stats

If the decoding worker is enabled, this function shows information about the decoder slot and current LCR (logical change record) segment file being read by each WAL sender.

Synopsis

bdr.wal_sender_stats()

Output columns

pid — PID of the WAL sender (corresponds to pg_stat_replication's pid column).
is_using_lcr — Whether the WAL sender is sending LCR files. The next columns are NULL if is_using_lcr is FALSE.
decoder_slot_name — The name of the decoder replication slot.
lcr_file_name — The name of the current LCR file.

bdr.get_decoding_worker_stat

If the decoding worker is enabled, this function shows information about the state of the decoding worker associated with the current database. This also provides more granular information about decoding worker progress than is available via pg_replication_slots.

Synopsis

bdr.get_decoding_worker_stat()

Output columns

pid — The PID of the decoding worker (corresponds to the column active_pid in pg_replication_slots).
decoded_upto_lsn — LSN up to which the decoding worker read transactional logs.
waiting — Whether the decoding worker is waiting for new WAL.
waiting_for_lsn — The LSN of the next expected WAL.

Notes

For further details, see Monitoring WAL senders using LCR.

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Monitoring

Appendix B: Conflict Details