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pg_receivewal
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pg_receivewal

pg_receivewal — stream write-ahead logs from aPostgres Pro server

Synopsis

pg_receivewal [option...]

Description

pg_receivewal is used to stream the write-ahead log from a runningPostgres Pro cluster. The write-ahead 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 (seeSection 24.3).

pg_receivewal streams the write-ahead log in real time as it's being generated on the server, and does not wait for segments to complete likearchive_command andarchive_library do. For this reason, it is not necessary to setarchive_timeout when usingpg_receivewal.

Unlike the WAL receiver of a Postgres Pro standby server,pg_receivewal 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. Sincepg_receivewal does not apply WAL, you should not allow it to become a synchronous standby whensynchronous_commit equalsremote_apply. If it does, it will appear to be a standby that never catches up, and will cause transaction commits to block. To avoid this, you should either configure an appropriate value forsynchronous_standby_names, or specifyapplication_name forpg_receivewal that does not match it, or change the value ofsynchronous_commit to something other thanremote_apply.

The write-ahead log is streamed over a regularPostgres Pro connection and uses the replication protocol. The connection must be made with a user havingREPLICATION permissions (seeSection 20.2) or a superuser, andpg_hba.conf must permit the replication connection. The server must also be configured withmax_wal_senders set high enough to leave at least one session available for the stream.

The starting point of the write-ahead log streaming is calculated whenpg_receivewal starts:

First, scan the directory where the WAL segment files are written and find the newest completed segment file, using as the starting point the beginning of the next WAL segment file.
If a starting point cannot be calculated with the previous method, and if a replication slot is used, an extraREAD_REPLICATION_SLOT command is issued to retrieve the slot'srestart_lsn to use as the starting point. This option is only available when streaming write-ahead logs fromPostgres Pro 15 and up.
If a starting point cannot be calculated with the previous method, the latest WAL flush location is used as reported by the server from anIDENTIFY_SYSTEM command.

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

In the absence of fatal errors,pg_receivewal will run until terminated by theSIGINT (Control+C) orSIGTERM signal.

Options

-Ddirectory --directory=directory

Directory to write the output to.

This parameter is required.

-Elsn --endpos=lsn

Automatically stop replication and exit with normal exit status 0 when receiving reaches the specified LSN.

If there is a record with LSN exactly equal tolsn, the record will be processed.

--if-not-exists

Do 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.

--no-sync

This option causespg_receivewal to not force WAL data to be flushed to disk. This is faster, but means that a subsequent operating system crash can leave the WAL segments corrupt. Generally, this option is useful for testing but should not be used when doing WAL archiving on a production deployment.

This option is incompatible with--synchronous.

-sinterval --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.

-Sslotname --slot=slotname

Requirepg_receivewal to use an existing replication slot (seeSection 25.2.6). When this option is used,pg_receivewal 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 ofpg_receivewal 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 ofpg_receivewal is configured on the server as a synchronous standby, to ensure that timely feedback is sent to the server.

-v --verbose

Enables verbose mode.

-Zlevel -Zmethod[:detail] --compress=level --compress=method[:detail]

Enables compression of write-ahead logs.

The compression method can be set togzip,lz4 (ifPostgres Pro was compiled with--with-lz4) ornone for no compression. A compression detail string can optionally be specified. If the detail string is an integer, it specifies the compression level. Otherwise, it should be a comma-separated list of items, each of the formkeyword orkeyword=value. Currently, the only supported keyword islevel.

If no compression level is specified, the default compression level will be used. If only a level is specified without mentioning an algorithm,gzip compression will be used if the level is greater than 0, and no compression will be used if the level is 0.

The suffix.gz will automatically be added to all filenames when usinggzip, and the suffix.lz4 is added when usinglz4.

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

-dconnstr --dbname=connstr

Specifies parameters used to connect to the server, as aconnection string; these will override any conflicting command line options.

This option is called--dbname for consistency with other client applications, but becausepg_receivewal doesn't connect to any particular database in the cluster, any database name included in the connection string will be ignored by the server. However, a database name supplied that way overrides the default database name (replication) for purposes of looking up the replication connection's password in~/.pgpass. Similarly, middleware or proxies used in connecting toPostgres Pro might utilize the name for purposes such as connection routing.

-hhost --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 thePGHOST environment variable, if set, else a Unix domain socket connection is attempted.

-pport --port=port

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

-Uusername --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

Forcepg_receivewal to prompt for a password before connecting to a database.

This option is never essential, sincepg_receivewal will automatically prompt for a password if the server demands password authentication. However,pg_receivewal 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_receivewal 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 thepg_receivewal version and exit.
-? --help: Show help aboutpg_receivewal command line arguments, and exit.

Exit Status

pg_receivewal will exit with status 0 when terminated by theSIGINT orSIGTERM signal. (That is the normal way to end it. Hence it is not an error.) For fatal errors or other signals, the exit status will be nonzero.

Environment

This utility, like most otherPostgres Pro utilities, uses the environment variables supported bylibpq (seeSection 32.15).

The environment variablePG_COLOR specifies whether to use color in diagnostic messages. Possible values arealways,auto andnever.

Notes

When usingpg_receivewal instead ofarchive_command orarchive_library as the main WAL backup method, it is strongly recommended to use replication slots. Otherwise, the server is free to recycle or remove write-ahead log files before they are backed up, because it does not have any information, either fromarchive_command orarchive_library 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.

pg_receivewal will preserve group permissions on the received WAL files if group permissions are enabled on the source cluster.

Examples

To stream the write-ahead log from the server atmydbserver and store it in the local directory/usr/local/pgsql/archive:

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