.. _wal_archiving:

WAL archiving
=============

Barman also offers additional features regarding WAL archiving.

.. _wal_archiving-wal-compression:

WAL compression
---------------

Barman can compress WAL files as they enter the Barman's WAL archive. This process is
handled automatically by ``barman cron`` or when the ``barman archive-wal`` command is
executed manually.

Compression is enabled via the ``compression`` option in the configuration file.
The option can use one of the following values:

* ``lz4``: for LZ4 compression (requires the ``lz4`` library to be installed);
* ``xz``: for XZ compression (uses Python's internal compression library);
* ``zstd``: for Zstandard compression (requires the ``zstandard`` library to be
  installed);
* ``gzip``: for Gzip compression (requires the ``gzip`` utility);
* ``pygzip``: for Gzip compression (uses Python's internal compression library);
* ``pigz``: for Pigz compression (requires the ``pigz`` utility);
* ``bzip2``: for Bzip2 compression (requires the ``bzip2`` utility);
* ``pybzip2``: for Bzip2 compression (uses Python's internal compression library);
* ``custom``: for custom compression, which requires you to set the following options
  as well: ``custom_compression_filter``, ``custom_decompression_filter``,
  ``custom_compression_magic``. Check :ref:`configuration-options-wals` for details.

.. note::
    When using options such as ``bzip2``, ``gzip`` and ``pigz`` Barman will fork a new
    process for compression.


.. _wal_archiving-synchronous-WAL-streaming:

Synchronous WAL streaming
-------------------------

Barman can also reduce the :term:`RPO` to zero, by collecting the transaction WAL files
like a synchronous standby server would.

To configure a scenario with :term:`RPO` zero, the Barman server must be configured to archive WALs via
a streaming connection and the receive-wal process has to be configured as a
synchronous connection to the Postgres server.

First, you need to retrieve the application name of the Barman receive-wal process with the 
``show-servers`` command:

.. code-block:: bash

  barman show-servers pg | grep streaming_archiver_name

Output:

.. code-block:: text

  streaming_archiver_name: barman_receive_wal

.. note::

  The application name Barman uses when starting the receive-wal process is configured
  with the ``streaming_archiver_name`` configuration option. The default value for this
  option is ``barman_receive_wal``.

Then the application name should be added to the ``synchronous_standby_names``
parameter in the ``postgresql.conf`` file:

.. code-block:: bash

  synchronous_standby_names = 'barman_receive_wal'


.. important::

  Barman with :term:`RPO` zero adds more security to your backups and gives you more
  recovery options. However, it should not be considered as a substitution of a
  real Postgres replica. Please read the `official Postgres documentation about
  "Syncronous Replication" <https://www.postgresql.org/docs/current/runtime-config-replication.html>`_
  for more information on this topic.

The Postgres server configuration needs to be reloaded for the changes to take effect.

If the server has been configured correctly, the ``barman replication-status`` command
should show the receive-wal process as a synchronous streaming client:

.. code-block:: bash

  barman replication-status pg

Output:

.. code-block:: text

  Status of streaming clients for server 'pg':
    Current xlog location on master: 0/9000098
    Number of streaming clients: 1

    1. #1 Sync WAL streamer
      Application name: barman_receive_wal
      Sync stage      : 3/3 Remote write
      Communication   : TCP/IP
      IP Address      : 139.59.135.32 / Port: 58262 / Host: -
      User name       : streaming_barman
      Current state   : streaming (sync)
      Replication slot: barman
      WAL sender PID  : 2501
      Started at      : 2016-09-16 10:33:01.725883+00:00
      Sent location   : 0/9000098 (diff: 0 B)
      Write location  : 0/9000098 (diff: 0 B)
      Flush location  : 0/9000098 (diff: 0 B)