These settings control the behavior of the built-in streaming replication feature (see Section 26.2.5). Servers will be either a master or a standby server. Masters can send data, while standbys are always receivers of replicated data. When cascading replication (see Section 26.2.7) is used, standby servers can also be senders, as well as receivers. Parameters are mainly for sending and standby servers, though some parameters have meaning only on the master server. Settings may vary across the cluster without problems if that is required.
These parameters can be set on any server that is to send replication data to one or more standby servers. The master is always a sending server, so these parameters must always be set on the master. The role and meaning of these parameters does not change after a standby becomes the master.
max_wal_senders (integer)
       
       
        Specifies the maximum number of concurrent connections from standby
        servers or streaming base backup clients (i.e., the maximum number of
        simultaneously running WAL sender processes). The default is
        10.  The value 0 means
        replication is disabled.  Abrupt streaming client disconnection might
        leave an orphaned connection slot behind until a timeout is reached,
        so this parameter should be set slightly higher than the maximum
        number of expected clients so disconnected clients can immediately
        reconnect.  This parameter can only be set at server start.  Also,
        wal_level must be set to
        replica or higher to allow connections from standby
        servers.
       
When running a standby server, you must set this parameter to the same or higher value than on the master server. Otherwise, queries will not be allowed in the standby server.
max_replication_slots (integer)
       
       
         Specifies the maximum number of replication slots
         (see Section 26.2.6) that the server
         can support. The default is 10.  This parameter can only be set at
         server start.
         Setting it to a lower value than the number of currently
         existing replication slots will prevent the server from starting.
         Also, wal_level must be set
         to replica or higher to allow replication slots to
         be used.
        
wal_keep_segments (integer)
       
       
        Specifies the minimum number of past log file segments kept in the
        pg_wal
        directory, in case a standby server needs to fetch them for streaming
        replication. Each segment is normally 16 megabytes. If a standby
        server connected to the sending server falls behind by more than
        wal_keep_segments segments, the sending server might remove
        a WAL segment still needed by the standby, in which case the
        replication connection will be terminated.  Downstream connections
        will also eventually fail as a result.  (However, the standby
        server can recover by fetching the segment from archive, if WAL
        archiving is in use.)
       
        This sets only the minimum number of segments retained in
        pg_wal; the system might need to retain more segments
        for WAL archival or to recover from a checkpoint. If
        wal_keep_segments is zero (the default), the system
        doesn't keep any extra segments for standby purposes, so the number
        of old WAL segments available to standby servers is a function of
        the location of the previous checkpoint and status of WAL
        archiving.
        This parameter can only be set in the
        postgresql.conf file or on the server command line.
       
wal_sender_timeout (integer)
      
      Terminate replication connections that are inactive for longer than this amount of time. This is useful for the sending server to detect a standby crash or network outage. If this value is specified without units, it is taken as milliseconds. The default value is 60 seconds. A value of zero disables the timeout mechanism.
With a cluster distributed across multiple geographic locations, using different values per location brings more flexibility in the cluster management. A smaller value is useful for faster failure detection with a standby having a low-latency network connection, and a larger value helps in judging better the health of a standby if located on a remote location, with a high-latency network connection.
track_commit_timestamp (boolean)
      
      
        Record commit time of transactions. This parameter
        can only be set in postgresql.conf file or on the server
        command line. The default value is off.
       
These parameters can be set on the master/primary server that is to send replication data to one or more standby servers. Note that in addition to these parameters, wal_level must be set appropriately on the master server, and optionally WAL archiving can be enabled as well (see Section 19.5.3). The values of these parameters on standby servers are irrelevant, although you may wish to set them there in preparation for the possibility of a standby becoming the master.
synchronous_standby_names (string)
      
      
        Specifies a list of standby servers that can support
        synchronous replication, as described in
        Section 26.2.8.
        There will be one or more active synchronous standbys;
        transactions waiting for commit will be allowed to proceed after
        these standby servers confirm receipt of their data.
        The synchronous standbys will be those whose names appear
        in this list, and
        that are both currently connected and streaming data in real-time
        (as shown by a state of streaming in the pg_stat_replication
        view).
        Specifying more than one synchronous standby can allow for very high
        availability and protection against data loss.
       
        The name of a standby server for this purpose is the
        application_name setting of the standby, as set in the
        standby's connection information.  In case of a physical replication
        standby, this should be set in the primary_conninfo
        setting; the default is the setting of cluster_name
        if set, else walreceiver.
        For logical replication, this can be set in the connection
        information of the subscription, and it defaults to the
        subscription name.  For other replication stream consumers,
        consult their documentation.
       
This parameter specifies a list of standby servers using either of the following syntaxes:
[FIRST]num_sync(standby_name[, ...] ) ANYnum_sync(standby_name[, ...] )standby_name[, ...]
        where num_sync is
        the number of synchronous standbys that transactions need to
        wait for replies from,
        and standby_name
        is the name of a standby server.
        FIRST and ANY specify the method to choose
        synchronous standbys from the listed servers.
       
        The keyword FIRST, coupled with
        num_sync, specifies a
        priority-based synchronous replication and makes transaction commits
        wait until their WAL records are replicated to
        num_sync synchronous
        standbys chosen based on their priorities. For example, a setting of
        FIRST 3 (s1, s2, s3, s4) will cause each commit to wait for
        replies from three higher-priority standbys chosen from standby servers
        s1, s2, s3 and s4.
        The standbys whose names appear earlier in the list are given higher
        priority and will be considered as synchronous. Other standby servers
        appearing later in this list represent potential synchronous standbys.
        If any of the current synchronous standbys disconnects for whatever
        reason, it will be replaced immediately with the next-highest-priority
        standby. The keyword FIRST is optional.
       
        The keyword ANY, coupled with
        num_sync, specifies a
        quorum-based synchronous replication and makes transaction commits
        wait until their WAL records are replicated to at least
        num_sync listed standbys.
        For example, a setting of ANY 3 (s1, s2, s3, s4) will cause
        each commit to proceed as soon as at least any three standbys of
        s1, s2, s3 and s4
        reply.
       
        FIRST and ANY are case-insensitive. If these
        keywords are used as the name of a standby server,
        its standby_name must
        be double-quoted.
       
        The third syntax was used before PostgreSQL
        version 9.6 and is still supported. It's the same as the first syntax
        with FIRST and
        num_sync equal to 1.
        For example, FIRST 1 (s1, s2) and s1, s2 have
        the same meaning: either s1 or s2 is chosen
        as a synchronous standby.
       
        The special entry * matches any standby name.
       
There is no mechanism to enforce uniqueness of standby names. In case of duplicates one of the matching standbys will be considered as higher priority, though exactly which one is indeterminate.
         Each standby_name
         should have the form of a valid SQL identifier, unless it
         is *.  You can use double-quoting if necessary.  But note
         that standby_names are
         compared to standby application names case-insensitively, whether
         double-quoted or not.
        
        If no synchronous standby names are specified here, then synchronous
        replication is not enabled and transaction commits will not wait for
        replication.  This is the default configuration.  Even when
        synchronous replication is enabled, individual transactions can be
        configured not to wait for replication by setting the
        synchronous_commit parameter to
        local or off.
       
        This parameter can only be set in the postgresql.conf
        file or on the server command line.
       
vacuum_defer_cleanup_age (integer)
      
      
        Specifies the number of transactions by which VACUUM and
        HOT updates will defer cleanup of dead row versions. The
        default is zero transactions, meaning that dead row versions can be
        removed as soon as possible, that is, as soon as they are no longer
        visible to any open transaction.  You may wish to set this to a
        non-zero value on a primary server that is supporting hot standby
        servers, as described in Section 26.5.  This allows
        more time for queries on the standby to complete without incurring
        conflicts due to early cleanup of rows.  However, since the value
        is measured in terms of number of write transactions occurring on the
        primary server, it is difficult to predict just how much additional
        grace time will be made available to standby queries.
        This parameter can only be set in the postgresql.conf
        file or on the server command line.
       
        You should also consider setting hot_standby_feedback
        on standby server(s) as an alternative to using this parameter.
       
        This does not prevent cleanup of dead rows which have reached the age
        specified by old_snapshot_threshold.
       
These settings control the behavior of a standby server that is to receive replication data. Their values on the master server are irrelevant.
primary_conninfo (string)
        
        Specifies a connection string to be used for the standby server to connect with a sending server. This string is in the format described in Section 33.1.1. If any option is unspecified in this string, then the corresponding environment variable (see Section 33.14) is checked. If the environment variable is not set either, then defaults are used.
          The connection string should specify the host name (or address)
          of the sending server, as well as the port number if it is not
          the same as the standby server's default.
          Also specify a user name corresponding to a suitably-privileged role
          on the sending server (see
          Section 26.2.5.1).
          A password needs to be provided too, if the sender demands password
          authentication.  It can be provided in the
          primary_conninfo string, or in a separate
          ~/.pgpass file on the standby server (use
          replication as the database name).
          Do not specify a database name in the
          primary_conninfo string.
         
This parameter can only be set at server start. This setting has no effect if the server is not in standby mode.
primary_slot_name (string)
        
        
          Optionally specifies an existing replication slot to be used when
          connecting to the sending server via streaming replication to control
          resource removal on the upstream node
          (see Section 26.2.6).
          This parameter can only be set at server start.
          This setting has no effect if primary_conninfo is not
          set.
         
promote_trigger_file (string)
        
        
          Specifies a trigger file whose presence ends recovery in the
          standby.  Even if this value is not set, you can still promote
          the standby using pg_ctl promote or calling
          pg_promote.
          This parameter can only be set in the postgresql.conf
          file or on the server command line.
         
hot_standby (boolean)
      
      
        Specifies whether or not you can connect and run queries during
        recovery, as described in Section 26.5.
        The default value is on.
        This parameter can only be set at server start. It only has effect
        during archive recovery or in standby mode.
       
max_standby_archive_delay (integer)
      
      
        When Hot Standby is active, this parameter determines how long the
        standby server should wait before canceling standby queries that
        conflict with about-to-be-applied WAL entries, as described in
        Section 26.5.2.
        max_standby_archive_delay applies when WAL data is
        being read from WAL archive (and is therefore not current).
        If this value is specified without units, it is taken as milliseconds.
        The default is 30 seconds.
        A value of -1 allows the standby to wait forever for conflicting
        queries to complete.
        This parameter can only be set in the postgresql.conf
        file or on the server command line.
       
        Note that max_standby_archive_delay is not the same as the
        maximum length of time a query can run before cancellation; rather it
        is the maximum total time allowed to apply any one WAL segment's data.
        Thus, if one query has resulted in significant delay earlier in the
        WAL segment, subsequent conflicting queries will have much less grace
        time.
       
max_standby_streaming_delay (integer)
      
      
        When Hot Standby is active, this parameter determines how long the
        standby server should wait before canceling standby queries that
        conflict with about-to-be-applied WAL entries, as described in
        Section 26.5.2.
        max_standby_streaming_delay applies when WAL data is
        being received via streaming replication.
        If this value is specified without units, it is taken as milliseconds.
        The default is 30 seconds.
        A value of -1 allows the standby to wait forever for conflicting
        queries to complete.
        This parameter can only be set in the postgresql.conf
        file or on the server command line.
       
        Note that max_standby_streaming_delay is not the same as
        the maximum length of time a query can run before cancellation; rather
        it is the maximum total time allowed to apply WAL data once it has
        been received from the primary server.  Thus, if one query has
        resulted in significant delay, subsequent conflicting queries will
        have much less grace time until the standby server has caught up
        again.
       
wal_receiver_status_interval (integer)
      
      
       Specifies the minimum frequency for the WAL receiver
       process on the standby to send information about replication progress
       to the primary or upstream standby, where it can be seen using the
       pg_stat_replication
       view.  The standby will report
       the last write-ahead log location it has written, the last position it
       has flushed to disk, and the last position it has applied.
       This parameter's
       value is the maximum amount of time between reports.  Updates are
       sent each time the write or flush positions change, or at least as
       often as specified by this parameter.  Thus, the apply position may
       lag slightly behind the true position.
       If this value is specified without units, it is taken as seconds.
       The default value is 10 seconds.
       Setting this parameter to zero disables status updates completely.
       This parameter can only be set in
       the postgresql.conf file or on the server
       command line.
      
hot_standby_feedback (boolean)
      
      
        Specifies whether or not a hot standby will send feedback to the primary
        or upstream standby
        about queries currently executing on the standby. This parameter can
        be used to eliminate query cancels caused by cleanup records, but
        can cause database bloat on the primary for some workloads.
        Feedback messages will not be sent more frequently than once per
        wal_receiver_status_interval. The default value is
        off. This parameter can only be set in the
        postgresql.conf file or on the server command line.
       
If cascaded replication is in use the feedback is passed upstream until it eventually reaches the primary. Standbys make no other use of feedback they receive other than to pass upstream.
        This setting does not override the behavior of
        old_snapshot_threshold on the primary; a snapshot on the
        standby which exceeds the primary's age threshold can become invalid,
        resulting in cancellation of transactions on the standby.  This is
        because old_snapshot_threshold is intended to provide an
        absolute limit on the time which dead rows can contribute to bloat,
        which would otherwise be violated because of the configuration of a
        standby.
       
wal_receiver_timeout (integer)
      
      
        Terminate replication connections that are inactive for longer
        than this amount of time. This is useful for
        the receiving standby server to detect a primary node crash or network
        outage.
        If this value is specified without units, it is taken as milliseconds.
        The default value is 60 seconds.
        A value of zero disables the timeout mechanism.
        This parameter can only be set in
        the postgresql.conf file or on the server
        command line.
       
wal_retrieve_retry_interval (integer)
      
      
        Specifies how long the standby server should wait when WAL data is not
        available from any sources (streaming replication,
        local pg_wal or WAL archive) before trying
        again to retrieve WAL data.
        If this value is specified without units, it is taken as milliseconds.
        The default value is 5 seconds.
        This parameter can only be set in
        the postgresql.conf file or on the server
        command line.
       
This parameter is useful in configurations where a node in recovery needs to control the amount of time to wait for new WAL data to be available. For example, in archive recovery, it is possible to make the recovery more responsive in the detection of a new WAL log file by reducing the value of this parameter. On a system with low WAL activity, increasing it reduces the amount of requests necessary to access WAL archives, something useful for example in cloud environments where the amount of times an infrastructure is accessed is taken into account.
recovery_min_apply_delay (integer)
      
      
        By default, a standby server restores WAL records from the
        sending server as soon as possible. It may be useful to have a time-delayed
        copy of the data, offering opportunities to correct data loss errors.
        This parameter allows you to delay recovery by a specified amount
        of time.  For example, if
        you set this parameter to 5min, the standby will
        replay each transaction commit only when the system time on the standby
        is at least five minutes past the commit time reported by the master.
        If this value is specified without units, it is taken as milliseconds.
        The default is zero, adding no delay.
       
It is possible that the replication delay between servers exceeds the value of this parameter, in which case no delay is added. Note that the delay is calculated between the WAL time stamp as written on master and the current time on the standby. Delays in transfer because of network lag or cascading replication configurations may reduce the actual wait time significantly. If the system clocks on master and standby are not synchronized, this may lead to recovery applying records earlier than expected; but that is not a major issue because useful settings of this parameter are much larger than typical time deviations between servers.
The delay occurs only on WAL records for transaction commits. Other records are replayed as quickly as possible, which is not a problem because MVCC visibility rules ensure their effects are not visible until the corresponding commit record is applied.
The delay occurs once the database in recovery has reached a consistent state, until the standby is promoted or triggered. After that the standby will end recovery without further waiting.
        This parameter is intended for use with streaming replication deployments;
        however, if the parameter is specified it will be honored in all cases
        except crash recovery.
        hot_standby_feedback will be delayed by use of this feature
        which could lead to bloat on the master; use both together with care.
        
          Synchronous replication is affected by this setting when synchronous_commit
          is set to remote_apply; every COMMIT
          will need to wait to be applied.
         
        This parameter can only be set in the postgresql.conf
        file or on the server command line.
       
These settings control the behavior of a logical replication subscriber. Their values on the publisher are irrelevant.
      Note that wal_receiver_timeout,
      wal_receiver_status_interval and
      wal_retrieve_retry_interval configuration parameters
      affect the logical replication workers as well.
     
max_logical_replication_workers (int)
      
      Specifies maximum number of logical replication workers. This includes both apply workers and table synchronization workers.
        Logical replication workers are taken from the pool defined by
        max_worker_processes.
       
The default value is 4.
max_sync_workers_per_subscription (integer)
      
      Maximum number of synchronization workers per subscription. This parameter controls the amount of parallelism of the initial data copy during the subscription initialization or when new tables are added.
Currently, there can be only one synchronization worker per table.
        The synchronization workers are taken from the pool defined by
        max_logical_replication_workers.
       
The default value is 2.