System functions v5

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Perform PGD management primarily by using functions you call from SQL. All functions in PGD are exposed in the bdr schema. Schema qualify any calls to these functions instead of putting bdr in the search_path.

Version information functions

`bdr.bdr_version`

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

`bdr.bdr_version_num`

This function retrieves the version number of the BDR extension 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 returns 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 information functions

`bdr.get_relation_stats`

Returns the relation information.

`bdr.get_subscription_stats`

Returns the current subscription statistics.

System and progress information parameters

PGD exposes some parameters that you can query using SHOW in psql or using PQparameterStatus (or equivalent) from a client application.

`bdr.local_node_id`

When you initialize a session, this is set to the node id the client is connected to. This allows an application to figure out the node it's connected to, even behind a transparent proxy.

It's also used with Connection pools and proxies.

`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. Combining it with bdr.wait_for_apply_queue, allows applications to perform causal reads across multiple nodes, that is, to wait until a transaction becomes remotely visible.

`transaction_id`

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

`bdr.is_node_connected`

Synopsis

bdr.is_node_connected(node_name name)

Returns boolean by checking if the walsender for a given peer is active on this node.

`bdr.is_node_ready`

Synopsis

bdr.is_node_ready(node_name name, span interval DEFAULT NULL)

Returns boolean by checking if the lag is lower than the given span or lower than the timeout for TO ASYNC otherwise.

Consensus function

`bdr.consensus_disable`

Disables the consensus worker on the local node until server restart or until it's reenabled using bdr.consensus_enable (whichever happens first).

Warning

Disabling consensus disables some features of PGD and affects availability of the EDB Postgres Distributed cluster if left disabled for a long time. Use this function only when working with Technical Support.

`bdr.consensus_enable`

Reenabled disabled consensus worker on local node.

`bdr.consensus_proto_version`

Returns currently used consensus protocol version by the local node.

Needed by the PGD group reconfiguration internal mechanisms.

`bdr.consensus_snapshot_export`

Synopsis

bdr.consensus_snapshot_export(version integer DEFAULT NULL)

Generate a new PGD consensus snapshot from the currently committed-and-applied state of the local node and return it as bytea.

By default, a snapshot for the highest supported Raft version is exported. But you can override that by passing an explicit version number.

The exporting node doesn't have to be the current Raft leader, and it doesn't need to be completely up to date with the latest state on the leader. However, bdr.consensus_snapshot_import() might not accept such a snapshot.

The new snapshot isn't automatically stored to the local node's bdr.local_consensus_snapshot table. It's only returned to the caller.

The generated snapshot might be passed to bdr.consensus_snapshot_import() on any other nodes in the same PGD node group that's behind the exporting node's Raft log position.

The local PGD consensus worker must be disabled for this function to work. Typical usage is:

 SELECT bdr.bdr_consensus_disable();
 \copy (SELECT * FROM bdr.consensus_snapshot_export()) TO 'my_node_consensus_snapshot.data'
 SELECT bdr.bdr_consensus_enable();

While the PGD consensus worker is disabled:

DDL locking attempts on the node fail or time out.
galloc sequences don't get new values.
Eager and CAMO transactions pause or error.
Other functionality that needs the distributed consensus system is disrupted. The required downtime is generally very brief.

Depending on the use case, it might be practical to extract a snapshot that already exists from the snapshot field of the bdr.local_consensus_snapshot table and use that instead. Doing so doesn't require you to stop the consensus worker.

`bdr.consensus_snapshot_import`

Synopsis

bdr.consensus_snapshot_import(IN snapshot bytea)

Import a consensus snapshot that was exported by bdr.consensus_snapshot_export(), usually from another node in the same PGD node group.

It's also possible to use a snapshot extracted directly from the snapshot field of the bdr.local_consensus_snapshot table on another node.

This function is useful for resetting a PGD node's catalog state to a known good state in case of corruption or user error.

You can import the snapshot if the importing node's apply_index is less than or equal to the snapshot-exporting node's commit_index when the snapshot was generated. (See bdr.get_raft_status().) A node that can't accept the snapshot because its log is already too far ahead raises an error and makes no changes. The imported snapshot doesn't have to be completely up to date, as once the snapshot is imported the node fetches the remaining changes from the current leader.

The PGD consensus worker must be disabled on the importing node for this function to work. See notes on bdr.consensus_snapshot_export() for details.

It's possible to use this function to force the local node to generate a new Raft snapshot by running:

SELECT bdr.consensus_snapshot_import(bdr.consensus_snapshot_export());

This approach might also truncate the Raft logs up to the current applied log position.

`bdr.consensus_snapshot_verify`

Synopsis

bdr.consensus_snapshot_verify(IN snapshot bytea)

Verify the given consensus snapshot that was exported by bdr.consensus_snapshot_export(). The snapshot header contains the version with which it was generated and the node tries to verify it against the same version.

The snapshot might have been exported on the same node or any other node in the cluster. If the node verifying the snapshot doesn't support the version of the exported snapshot, then an error is raised.

`bdr.get_consensus_status`

Returns status information about the current consensus (Raft) worker.

`bdr.get_raft_status`

Returns status information about the current consensus (Raft) worker. Alias for bdr.get_consensus_status.

`bdr.raft_leadership_transfer`

Synopsis

bdr.raft_leadership_transfer(IN node_name text, IN wait_for_completion boolean)

Request the node identified by node_name to be the Raft leader. The request can be initiated from any of the PGD nodes and is internally forwarded to the current leader to transfer the leadership to the designated node. The designated node must be an ACTIVE PGD node with full voting rights.

If wait_for_completion is false, the request is served on a best-effort basis. If the node can't become a leader in the bdr.raft_election_timeout period, then some other capable node becomes the leader again. Also, the leadership can change over the period of time per Raft protocol. A true return result indicates only that the request was submitted successfully.

If wait_for_completion is true, then the function waits until the given node becomes the new leader and possibly waits infinitely if the requested node fails to become Raft leader (for example, due to network issues). We therefore recommend that you always set a statement_timeout with wait_for_completion to prevent an infinite loop.

Utility functions

`bdr.wait_slot_confirm_lsn`

Allows you to wait until the last write on this session was replayed to one or all nodes.

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

If no slot name is passed, it waits until all PGD slots pass the LSN.

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

If a slot is dropped concurrently, the wait ends for that slot. If a node is currently down and isn't updating its slot, then the wait continues. You might want 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 PGD 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 PGD node to wait for the local application of certain transactions originating from a given PGD node. It returns 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, PGD provides a 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 which the peer waits. You can use this 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 were sent. In this case, transactions that are still in transit or buffered on the sender side aren't 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 to wait 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 by way of bdr.last_committed_lsn from the peer node.

`bdr.get_node_sub_receive_lsn`

You can use this function on a subscriber to get the last LSN that was received from the given origin. It can be either unfiltered or filtered to take into account only relevant LSN increments for transactions to be applied.

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 that's the source of the replication stream whose LSN is being retrieved.
committed —; The default (true) makes this function take into account only commits of transactions received rather than the last LSN overall. This includes actions that have no effect on the subscriber node.

`bdr.get_node_sub_apply_lsn`

You can use this function on a subscriber to get the last LSN that was 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 that's the source of the replication stream whose LSN is being retrieved.

`bdr.run_on_all_nodes`

Function to run a query on all nodes.

Warning

This function runs 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. Use caution when granting privileges to this function.

Synopsis

bdr.run_on_all_nodes(query text)

Parameters

query — Arbitrary query to execute.

Notes

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

This function doesn't 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 commands 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 might be blocked in a future release.

Example

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

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

This query returns 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 runs an arbitrary query on remote nodes with the privileges of the user used for the internode connections as specified in the node's DSN. Use caution 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 is executed.
query — Arbitrary query to execute.

Notes

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

This function doesn't go through normal replication. It uses direct client connection to all known nodes. By default, the connection is created with bdr.ddl_replication = off, to avoid replication issues when the same replicated DDL command is sent to multiple nodes.

Be careful when using this function since you risk breaking replication and causing inconsistencies between nodes. For global schema changes, use either transparent DDL replication or bdr.replicate_ddl_command() to replicate DDL.

`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 is executed.
query — Arbitrary query to execute.

Notes

This function doesn't go through normal replication. It uses direct client connection to all known nodes. By default, the connection is created with bdr.ddl_replication = off, to avoid replication issues when the same replicated DDL command is sent to multiple nodes.

Be careful when using this function since you risk breaking replication and causing inconsistencies between nodes in the group. For global schema changes, use either transparent DDL replication or bdr.replicate_ddl_command() to replicate DDL.

`bdr.global_lock_table`

This function acquires 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 lock.

Notes

This function acquires 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 to 30618 or lower.

`bdr.wait_for_xid_progress`

You can use this function to wait for the given transaction (identified by its XID) originated at the given node (identified by its 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 originated.
origin_topxid — XID of the transaction.
allnodes — If true then wait for the transaction to progress on all nodes. Otherwise wait only for the current node.

Notes

You can use the function only for those transactions that replicated a DDL command because only those transactions are tracked currently. If a wrong origin_node_id or origin_topxid is supplied, the function might wait forever or until statement_timeout occurs.

`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 the 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

`bdr.alter_node_kind`

PGD5 introduced a concept of Task Manager Leader node. The node is selected automatically by PGD, but for upgraded clusters, its important to set the node_kind properly for all nodes in the cluster. The user is expected to do this manually after upgrading to the latest PGD version by calling bdr.alter_node_kind() SQL function for each node.

Synopsis

bdr.alter_node_kind(node_name text,
                    node_kind text);

Parameters

node_name — Name of the node to change kind.
node_kind — Kind of the node, which can be one of: data, standby, witness, or subscriber-only.

`bdr.alter_subscription_skip_changes_upto`

Because logical replication can replicate across versions, doesn't replicate global changes like roles, and can replicate selectively, sometimes the logical replication apply process can encounter an error and stop applying changes.

Wherever possible, fix such problems by making changes to the target side. CREATE any missing table that's blocking replication, CREATE a needed role, GRANT a necessary permission, and so on. But occasionally a problem can't be fixed that way and it might be necessary to skip entirely over a transaction. Changes are skipped as entire transactions—all or nothing. To decide where to skip to, use log output to find the commit LSN, per the example that follows, or peek the change stream with the logical decoding functions.

Unless a transaction made only one change, you often need to manually apply the transaction's effects on the target side, so it's important to save the problem transaction whenever possible, as shown in the examples that follow.

It's possible to skip over changes without bdr.alter_subscription_skip_changes_upto by using pg_catalog.pg_logical_slot_get_binary_changes to skip to the LSN of interest, so this is a convenience function. It does do a faster skip, although it might bypass some kinds of errors in logical decoding.

This function works only on disabled subscriptions.

The usual sequence of steps is:

Identify the problem subscription and LSN of the problem commit.
Disable the subscription.
Save a copy of the transaction using pg_catalog.pg_logical_slot_peek_changes on the source node, if possible.
bdr.alter_subscription_skip_changes_upto on the target node.
Apply repaired or equivalent changes on the target manually, if necessary.
Reenable the subscription.

Warning

It's easy to make problems worse when using this function. Don't do anything unless you're certain it's the only option.

Synopsis

  bdr.alter_subscription_skip_changes_upto(
    subname text,
    skip_upto_and_including pg_lsn
  );

Example

Apply of a transaction is failing with an error, and you've determined that lower-impact fixes such as changes on the target side can't resolve this issue. You determine that you must skip the transaction.

In the error logs, find the commit record LSN to skip to, as in this example:

ERROR:  XX000: CONFLICT: target_table_missing; resolver skip_if_recently_dropped returned an error: table does not exist
CONTEXT:  during apply of INSERT from remote relation public.break_me in xact with commit-end lsn 0/300AC18 xid 131315
committs 2021-02-02 15:11:03.913792+01 (action #2) (effective sess origin id=2 lsn=0/300AC18)
while consuming 'I' message from receiver for subscription bdr_regression_bdrgroup_node1_node2 (id=2667578509)
on node node2 (id=3367056606) from upstream node node1 (id=1148549230, reporiginid=2)

In this portion of log, you have the information you need: the_target_lsn: 0/300AC18 the_subscription: bdr_regression_bdrgroup_node1_node2

Next, disable the subscription so the apply worker doesn't try to connect to the replication slot:

  SELECT bdr.alter_subscription_disable('the_subscription');

You can't skip only parts of the transaction: it's all or nothing. So we strongly recommend that you save a record of it by copying it out on the provider side first, using the subscription's slot name.

  \\copy (SELECT * FROM pg_catalog.pg_logical_slot_peek_changes('the_slot_name',
      'the_target_lsn', NULL, 'min_proto_version', '1', 'max_proto_version', '1',
      'startup_params_format', '1', 'proto_format', 'json'))
   TO 'transaction_to_drop.csv' WITH (FORMAT csv);

This example is broken into multiple lines for readability, but issue it in a single line. \copy doesn't support multi-line commands.

You can skip the change by changing peek to get, but bdr....skip_changes_upto does a faster skip that avoids decoding and outputting all the data:

  SELECT bdr.alter_subscription_skip_changes_upto('subscription_name',
      'the_target_lsn');

You can apply the same changes (or repaired versions of them) manually to the target node, using the dumped transaction contents as a guide.

Finally, reenable the subscription:

  SELECT bdr.alter_subscription_enable('the_subscription');

Global advisory locks

PGD supports global advisory locks. These locks are similar to the advisory locks available in PostgreSQL except that the advisory locks supported by PGD are global. They follow semantics similar to DDL locks. So an advisory lock is obtained by majority consensus and 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 only EXCLUSIVE locks are supported. 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's explicitly released before the end of the transaction. In this case, it becomes available as soon as it's released. Session-level advisory locks aren't currently supported.

Global advisory locks are reentrant. So if the same resource is locked three times, you must then unlock it three times for it to be released for use in other sessions.

`bdr.global_advisory_lock`

This function acquires an EXCLUSIVE lock on the provided object. If the lock isn't available, then it waits 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 releases a previously acquired lock on the application-defined source. The lock must have been 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 an 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 PGD 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 PGD Raft information returned from the view bdr.group_raft_details.

Synopsis

bdr.monitor_group_raft()

Parameters

node_group_name — the node group name that we want to check.

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 PGD 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.

`bdr.lag_control`

If Lag Control is enabled, this function shows information about the commit delay and number of nodes conforming to their configured lag measure for the local node and current database.

Synopsis

bdr.lag_control()

Output columns

commit_delay — Current runtime commit delay, in fractional milliseconds.
commit_delay_maximum — Configured maximum commit delay, in fractional milliseconds.
commit_delay_adjustment — Change to runtime commit delay possible during a sample interval, in fractional milliseconds.
conforming_nodes — Current runtime number of nodes conforming to lag measures.
conforming_nodes_minimum — Configured minimum number of nodes required to conform to lag measures, below which a commit delay adjustment is applied.
lag_bytes_threshold — Lag size at which a commit delay is applied, in kilobytes.
lag_bytes_maximum — Configured maximum lag size, in kilobytes.
lag_time_threshold — Lag time at which a commit delay is applied, in milliseconds.
lag_time_maximum — Configured maximum lag time, in milliseconds.
sample_interval — Configured minimum time between lag samples and possible commit delay adjustments, in milliseconds.

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