Legacy synchronous replication using PGD v5.6
Important
We highly recommend PGD Synchronous Commit instead of legacy synchronous replication.
Postgres provides physical streaming replication (PSR), which is unidirectional but offers a synchronous variant.
For backward compatibility, PGD still supports configuring synchronous
replication with synchronous_commit and synchronous_standby_names. Consider
using Group Commit or Synchronous Commit
instead.
Unlike PGD replication options, PSR sync persists 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 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.
| Variant | Mode | Received | Visible | Durable |
|---|---|---|---|---|
| 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 affect a specific variant.
| Setting (GUC) | Group Commit | Lag Control | PSR | Legacy Sync |
|---|---|---|---|---|
| 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.