PGD Synchronous Commit
======================

Commit scope kind: ``SYNCHRONOUS_COMMIT``

Overview
--------

PGD’s ``SYNCHRONOUS_COMMIT`` is a commit scope kind that works in a way
that is more like PostgreSQL’s `Synchronous commit <https://www.postgresql.org/docs/current/runtime-config-wal.html#GUC-SYNCHRONOUS-COMMIT>`_  option in its underlying
operation. Unlike the PostgreSQL option though, it is configured as a
commit scope and is easier to configure and interact with within PGD.

Unlike other commit scope kinds such as GROUP COMMIT and CAMO, the
transactions in a SYNCHRONOUS_COMMIT operation will not be transformed
into a two phase commit (2PC) transaction, but work more like a Postgres
Synchronous commit.

Example
-------

::

   SELECT bdr.add_commit_scope(
       commit_scope_name := example_sc_scope,
       origin_node_group := left_dc,
       rule := MAJORITY (left_dc) SYNCHRONOUS_COMMIT,
       wait_for_ready := true
   );

In this example, any node in the ``left_dc`` group, when this commit
scope is in use, will use ``SYNCHRONOUS_COMMIT`` to replicate changes to
the other nodes in the ``left_dc`` group. It will look for a majority of
nodes in the ``left_dc`` group to confirm that they have committed the
transaction.

Configuration
-------------

There are no parameters for ``SYNCHRONOUS_COMMIT`` and therefore no
configuration.

Confirmation
------------

.. csv-table::
  :header: Confirmation&nbsp;Level,PGD Synchronous Commit Handling
  :widths: 10,30
  :align: left
  :class: longtable

  `received`,A remote PGD node confirms the transaction once it's been fully received and is in in-memory write queue.
  `replicated`,Same behavior as `received`.
  `durable`,Confirms the transaction after all of its changes are flushed to disk. Analogous to `synchronous_commit = on` in legacy synchronous replication.
  `visible` (default),Confirms the transaction after all of its changes are flushed to disk and it's visible to concurrent transactions. Analogous to `synchronous_commit = remote_apply` in legacy synchronous replication.

Details
-------

Currently ``SYNCHRONOUS_COMMIT`` does not use the confirmation levels of
the commit scope rule syntax.