pg_receivexlog - Online in the Cloud

PROGRAM:

NAME

pg_receivexlog - stream transaction logs from a PostgreSQL server

SYNOPSIS

pg_receivexlog [option...]

DESCRIPTION

pg_receivexlog is used to stream the transaction log from a running PostgreSQL cluster.
The transaction log is streamed using the streaming replication protocol, and is written
to a local directory of files. This directory can be used as the archive location for
doing a restore using point-in-time recovery (see Section 24.3, “Continuous Archiving and
Point-in-Time Recovery (PITR)”, in the documentation).

pg_receivexlog streams the transaction log in real time as it's being generated on the
server, and does not wait for segments to complete like archive_command does. For this
reason, it is not necessary to set archive_timeout when using pg_receivexlog.

Unlike the WAL receiver of a PostgreSQL standby server, pg_receivexlog by default flushes
WAL data only when a WAL file is closed. The option --synchronous must be specified to
flush WAL data in real time.

The transaction log is streamed over a regular PostgreSQL connection and uses the
replication protocol. The connection must be made with a superuser or a user having
REPLICATION permissions (see Section 20.2, “Role Attributes”, in the documentation), and
pg_hba.conf must permit the replication connection. The server must also be configured
with max_wal_senders set high enough to leave at least one session available for the
stream.

If the connection is lost, or if it cannot be initially established, with a non-fatal
error, pg_receivexlog will retry the connection indefinitely, and reestablish streaming as
soon as possible. To avoid this behavior, use the -n parameter.

OPTIONS

-D directory
--directory=directory
Directory to write the output to.

This parameter is required.

--if-not-exists
Do not not error out when --create-slot is specified and a slot with the specified
name already exists.

-n
--no-loop
Don't loop on connection errors. Instead, exit right away with an error.

-s interval
--status-interval=interval
Specifies the number of seconds between status packets sent back to the server. This
allows for easier monitoring of the progress from server. A value of zero disables the
periodic status updates completely, although an update will still be sent when
requested by the server, to avoid timeout disconnect. The default value is 10 seconds.

-S slotname
--slot=slotname
Require pg_receivexlog to use an existing replication slot (see Section 25.2.6,
“Replication Slots”, in the documentation). When this option is used, pg_receivexlog
will report a flush position to the server, indicating when each segment has been
synchronized to disk so that the server can remove that segment if it is not otherwise
needed.

When the replication client of pg_receivexlog is configured on the server as a
synchronous standby, then using a replication slot will report the flush position to
the server, but only when a WAL file is closed. Therefore, that configuration will
cause transactions on the primary to wait for a long time and effectively not work
satisfactorily. The option --synchronous (see below) must be specified in addition to
make this work correctly.

--synchronous
Flush the WAL data to disk immediately after it has been received. Also send a status
packet back to the server immediately after flushing, regardless of --status-interval.

This option should be specified if the replication client of pg_receivexlog is
configured on the server as a synchronous standby, to ensure that timely feedback is
sent to the server.

-v
--verbose
Enables verbose mode.

The following command-line options control the database connection parameters.

-d connstr
--dbname=connstr
Specifies parameters used to connect to the server, as a connection string. See
Section 31.1.1, “Connection Strings”, in the documentation for more information.

The option is called --dbname for consistency with other client applications, but
because pg_receivexlog doesn't connect to any particular database in the cluster,
database name in the connection string will be ignored.

-h host
--host=host
Specifies the host name of the machine on which the server is running. If the value
begins with a slash, it is used as the directory for the Unix domain socket. The
default is taken from the PGHOST environment variable, if set, else a Unix domain
socket connection is attempted.

-p port
--port=port
Specifies the TCP port or local Unix domain socket file extension on which the server
is listening for connections. Defaults to the PGPORT environment variable, if set, or
a compiled-in default.

-U username
--username=username
User name to connect as.

-w
--no-password
Never issue a password prompt. If the server requires password authentication and a
password is not available by other means such as a .pgpass file, the connection
attempt will fail. This option can be useful in batch jobs and scripts where no user
is present to enter a password.

-W
--password
Force pg_receivexlog to prompt for a password before connecting to a database.

This option is never essential, since pg_receivexlog will automatically prompt for a
password if the server demands password authentication. However, pg_receivexlog will
waste a connection attempt finding out that the server wants a password. In some cases
it is worth typing -W to avoid the extra connection attempt.

pg_receivexlog can perform one of the two following actions in order to control physical
replication slots:

--create-slot
Create a new physical replication slot with the name specified in --slot, then exit.

--drop-slot
Drop the replication slot with the name specified in --slot, then exit.

Other options are also available:

-V
--version
Print the pg_receivexlog version and exit.

-?
--help
Show help about pg_receivexlog command line arguments, and exit.

ENVIRONMENT

This utility, like most other PostgreSQL utilities, uses the environment variables
supported by libpq (see Section 31.14, “Environment Variables”, in the documentation).

NOTES

When using pg_receivexlog instead of archive_command as the main WAL backup method, it is
strongly recommended to use replication slots. Otherwise, the server is free to recycle or
remove transaction log files before they are backed up, because it does not have any
information, either from archive_command or the replication slots, about how far the WAL
stream has been archived. Note, however, that a replication slot will fill up the server's
disk space if the receiver does not keep up with fetching the WAL data.

EXAMPLES

To stream the transaction log from the server at mydbserver and store it in the local
directory /usr/local/pgsql/archive:

$ pg_receivexlog -h mydbserver -D /usr/local/pgsql/archive

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