## pg_receivewal

pg_receivewal — stream write-ahead logs from a Postgres Pro server

## Synopsis

`pg_receivewal` [`option`...]

## Description

pg_receivewal is used to stream the write-ahead log from a running Postgres 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 (see Section 25.3).

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 like archive_command does. For this reason, it is not necessary to set archive_timeout when using pg_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.

The write-ahead log is streamed over a regular Postgres Pro connection and uses the replication protocol. The connection must be made with a superuser or a user having `REPLICATION` permissions (see Section 21.2), 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_receivewal 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 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_receivewal to use an existing replication slot (see Section 26.2.6). 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 of pg_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 of pg_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.

`-Z `level``
`--compress=`level``

  Enables gzip compression of write-ahead logs, and specifies the compression level (0 through 9, 0 being no compression and 9 being best compression). The suffix `.gz` will automatically be added to all filenames.

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 34.1.1 for more information.

  The option is called `--dbname` for consistency with other client applications, but because pg_receivewal 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_receivewal to prompt for a password before connecting to a database.

  This option is never essential, since pg_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 the pg_receivewal version and exit.

`-?`
`--help`

  Show help about pg_receivewal command line arguments, and exit.

## Environment

This utility, like most other Postgres Pro utilities, uses the environment variables supported by libpq (see Section 34.14).

## Notes

When using pg_receivewal 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 write-ahead 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 write-ahead log from the server at `mydbserver` and store it in the local directory `/usr/local/pgsql/archive`:

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

## See Also

pg_basebackup