Legacy synchronous replication using PGD
========================================

..  Warning Important::
   We highly recommend  :ref:`PGD Synchronous Commit <PGD Synchronous Commit>`  instead of legacy synchronous replication. !!!

Postgres provides :ref:`physical streaming replication <Monitoring through SQL>` 

(PSR), which is unidirectional but offers a :ref:`synchronous variant <Monitoring through SQL>`  .

For backward compatibility, PGD still supports configuring synchronous
replication with ``synchronous_commit`` and
``synchronous_standby_names`` . Consider using :ref:`Group commit <Group commit>`  or
 :ref:`PGD Synchronous Commit <PGD Synchronous Commit>` 

instead.

Unlike PGD replication options, PSR sync will persist first, replicating
after the WAL flush of commit record.

Usage
^^^^^

To enable synchronous replication using PGD, you need to add the
application name of the relevant PGD peer nodes to
``synchronous_standby_names`` . The use of ``FIRST x`` or ``ANY x``
offers some flexibility if this doesn’t conflict with the requirements
of non-PGD standby nodes.

Once you’ve added it, you can configure the level of synchronization per
transaction using ``synchronous_commit`` , which defaults to ``on`` .
This setting means that adding the application name to to
``synchronous_standby_names`` already enables synchronous replication.
Setting ``synchronous_commit`` to ``local`` or ``off`` turns off
synchronous replication.

Due to PGD applying the transaction before persisting it, the values
``on`` and ``remote_apply`` are equivalent for logical replication.

Comparison
^^^^^^^^^^

The following table summarizes what a client can expect from a peer node
replicated to after receiving a COMMIT confirmation from the origin node
the transaction was issued to. The Mode column takes on different
meaning depending on the variant. For PSR and legacy synchronous
replication with PGD, it refers to the ``synchronous_commit`` setting.

.. csv-table::
  :header: Variant,Mode,Received,Visible,Durable
  :widths: auto
  :align: left
  :class: longtable

  PSR Async,off (default),no,no,no
  PSR Sync,remote_write (2),yes,no,no (3)
  PSR Sync,on (2),yes,no,yes
  PSR Sync,remote_apply (2),yes,yes,yes
  PGD Legacy Sync (1),remote_write (2),yes,no,no
  PGD Legacy Sync (1),on (2),yes,yes,yes
  PGD Legacy Sync (1),remote_apply (2),yes,yes,yes

-  

   (1) Consider using Group Commit instead.\*

-  

   (2) Unless switched to local mode (if allowed) by setting
       ``synchronous_replication_availability`` to ``async'`` ,
       otherwise the values for the asynchronous PGD default apply.\*

-  

   (3) Written to the OS, durable if the OS remains running and only
       Postgres crashes.\*

Postgres configuration parameters
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The following table provides an overview of the configuration settings
that you must set to a non-default value (req) and those that are
optional (opt) but affecting a specific variant.

.. csv-table::
  :header: Setting (GUC),Group Commit,Lag Control,PSR,Legacy Sync
  :widths: auto
  :align: left
  :class: longtable

  synchronous_standby_names,n/a,n/a,req,req
  synchronous_commit,n/a,n/a,opt,opt
  synchronous_replication_availability,n/a,n/a,opt,opt

Migration to commit scopes
^^^^^^^^^^^^^^^^^^^^^^^^^^

You configure the Group Commit feature of PGD independent of
``synchronous_commit`` and ``synchronous_standby_names`` . Instead, the
``bdr.commit_scope`` GUC allows you to select the scope per transaction.
And instead of configuring ``synchronous_standby_names`` on each node
individually, Group Commit uses globally synchronized commit scopes.

..  Note ::
   While the grammar for `synchronous_standby_names`  and commit scopes looks similar, the former doesn't account for the origin node, but the latter does. Therefore, for example, `synchronous_standby_names = 'ANY 1 (..)'`  is equivalent to a commit scope of `ANY 2 (...)` . This choice makes reasoning about majority easier and reflects that the origin node also contributes to the durability and visibility of the transaction.  !!!