WAL streaming

Barman can reduce the Recovery Point Objective (RPO) by allowing users to add continuous WAL streaming from a PostgreSQL server, on top of the standard archive_command strategy.

Barman relies on pg_receivewal , a utility that has been available from PostgreSQL 9.2 which exploits the native streaming replication protocol and continuously receives transaction logs from a PostgreSQL server (master or standby). Prior to PostgreSQL 10, pg_receivewal was named pg_receivexlog .

Important

Barman requires that pg_receivewal is installed on the same server. For PostgreSQL 9.2 servers, you need pg_receivexlog of version 9.2 installed alongside Barman. For PostgreSQL 9.3 and above, it is recommended to install the latest available version of pg_receivewal , as it is back compatible. Otherwise, users can install multiple versions of pg_receivewal / pg_receivexlog on the Barman server and properly point to the specific version for a server, using the path_prefix option in the configuration file.

In order to enable streaming of transaction logs, you need to:

1. setup a streaming connection as previously described 2. set the streaming_archiver option to on

The cron command, if the aforementioned requirements are met, transparently manages log streaming through the execution of the receive-wal command. This is the recommended scenario.

However, users can manually execute the receive-wal command:

barman receive-wal <server_name>

Note

The receive-wal command is a foreground process.

Transaction logs are streamed directly in the directory specified by the streaming_wals_directory configuration option and are then archived by the archive-wal command.

Unless otherwise specified in the streaming_archiver_name parameter, and only for PostgreSQL 9.3 or above, Barman will set application_name of the WAL streamer process to barman_receive_wal , allowing you to monitor its status in the pg_stat_replication system view of the PostgreSQL server.

Replication slots

Important

replication slots are available since PostgreSQL 9.4

Replication slots are an automated way to ensure that the PostgreSQL server will not remove WAL files until they were received by all archivers. Barman uses this mechanism to receive the transaction logs from PostgreSQL.

You can find more information about replication slots in the

PostgreSQL manual .

You can even base your backup architecture on streaming connection only. This scenario is useful to configure Docker-based PostgreSQL servers and even to work with PostgreSQL servers running on Windows.

Important

In this moment, the Windows support is still experimental, as it is not yet part of our continuous integration system.

How to configure the WAL streaming

First, the PostgreSQL server must be configured to stream the transaction log files to the Barman server.

To configure the streaming connection from Barman to the PostgreSQL server you need to enable the streaming_archiver , as already said, including this line in the server configuration file:

streaming_archiver = on

If you plan to use replication slots (recommended), another essential option for the setup of the streaming-based transaction log archiving is the slot_name option:

slot_name = barman

This option defines the name of the replication slot that will be used by Barman. It is mandatory if you want to use replication slots.

When you configure the replication slot name, you can manually create a replication slot for Barman with this command:

barman@backup$ barman receive-wal --create-slot pg
Creating physical replication slot barman on server pg
Replication slot barman created

Starting with Barman 2.10, you can configure Barman to automatically create the replication slot by setting:

create_slot = auto

Limitations of partial WAL files with recovery

The standard behaviour of pg_receivewal is to write transactional information in a file with .partial suffix after the WAL segment name.

Barman expects a partial file to be in the streaming_wals_directory of a server. When completed, pg_receivewal removes the .partial suffix and opens the following one, delivering the file to the archive-wal command of Barman for permanent storage and compression.

In case of a sudden and unrecoverable failure of the master PostgreSQL server, the .partial file that has been streamed to Barman contains very important information that the standard archiver (through PostgreSQL’s archive_command ) has not been able to deliver to Barman.

As of Barman 2.10, the get-wal command is able to return the content of the current .partial WAL file through the --partial/-P option. This is particularly useful in the case of recovery, both full or to a point in time. Therefore, in case you run a recover command with get-wal enabled, and without --standby-mode , Barman will automatically add the -P option to barman-wal-restore (which will then relay that to the remote get-wal command) in the restore_command recovery option.

get-wal will also search in the incoming directory, in case a WAL file has already been shipped to Barman, but not yet archived.