Durability and performance options v5

Overview

EDB Postgres Distributed allows you to choose from several replication configurations based on your durability, consistency, availability, and performance needs using Commit Scopes.

In it's basic configuration, EDB Postgres Distributed will use asynchronous replication, however commit scopes can change both the default and the per transaction behavior. It's also possible to configure the legacy Postgres synchronous replication using standard synchronous_standby_names in a same way as the built-in physical or logical replication. Commit scopes however provide much more flexibility and control over the replication behavior.

The different synchronization settings affect three properties of interest to applications that are related but can all be implemented individually:

• Durability: Writing to multiple nodes increases crash resilience and allows you to recover the data after a crash and restart.
• Visibility: With the commit confirmation to the client, the database guarantees immediate visibility of the committed transaction on some sets of nodes.
• Conflict handling: Conflicts can be handled either optimistically post-commit, with conflicts being resolved when the transaction is replicated based on commit timestamps. Or they can be handled pessimistically pre-commit, where the client can rely on the transaction to eventually be applied on all nodes without further conflicts or get an abort directly informing the client of an error.

Commit Scopes allow two ways of controlling durability of the transaction:

• Group Commit. This option controls which and how many nodes have to reach a consensus before we consider transaction to be committable and at what stage of replication we can consider it committed. This also allows controlling the visibility ordering of the transaction.
• CAMO. Variant of Group Commit where the client is part of the consensus.
• Lag Control. This option controls how far behind can nodes be in terms of replication before allowing commit to proceed.

Postgres provides Physical Streaming Replication (PSR), which is unidirectional but offers a synchronous variant. For backward compatibility, BDR still supports configuring synchronous replication with synchronous_commit and synchronous_standby_names. See Legacy synchronous replication, but consider using Group Commit instead.

Terms and definitions

BDR nodes take different roles during the replication of a transaction. These are implicitly assigned per transaction and are unrelated even for concurrent transactions.

• The origin is the node that receives the transaction from the client or application. It's the node processing the transaction first, initiating replication to other BDR nodes, and responding back to the client with a confirmation or an error.

• A partner node is a BDR node expected to confirm transactions either according to Group Commit requirements.

• A commit group is the group of all BDR nodes involved in the commit, that is, the origin and all of its partner nodes, which can be just a few or all peer nodes.

Comparison

Most options for synchronous replication available to BDR allow for different levels of synchronization, offering different tradeoffs between performance and protection against node or network outages.

The following table summarizes what a client can expect from a peer node replicated to after having received 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 BDR, it refers to the synchronous_commit setting. And for Commit Scopes, it refers to the confirmation requirements of the commit scope configuration.

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

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

(3) Consider using Group Commit instead.

Reception ensures the peer operating normally can eventually apply the transaction without requiring any further communication, even in the face of a full or partial network outage. A crash of a peer node might still require retransmission of the transaction, as this confirmation doesn't involve persistent storage. All modes considered synchronous provide this protection.

Visibility implies the transaction was applied remotely. All other clients see the results of the transaction on all nodes, providing this guarantee immediately after the commit is confirmed by the origin node. Without visibility, other clients connected might not see the results of the transaction and experience stale reads.

Durability relates to the peer node's storage and provides protection against loss of data after a crash and recovery of the peer node. This can either relate to the reception of the data (as with physical streaming replication) or to visibility (as with Group Commit). The former eliminates the need for retransmissions after a crash, while the latter ensures visibility is maintained across restarts.

Internal timing of operations

For a better understanding of how the different modes work, it's helpful to realize PSR and BDR apply transactions differently.

With physical streaming replication, the order of operations is:

• Origin flushes a commit record to WAL, making the transaction visible locally.
• Peer node receives changes and issues a write.
• Peer flushes the received changes to disk.
• Peer applies changes, making the transaction visible locally.

With PGD, the order of operations is different:

• Origin flushes a commit record to WAL, making the transaction visible locally.
• Peer node receives changes into its apply queue in memory.
• Peer applies changes, making the transaction visible locally.
• Peer persists the transaction by flushing to disk.

For Group Commit, CAMO, and Eager, the origin node waits for a certain number of confirmations prior to making the transaction visible locally. The order of operations is:

• Origin flushes a prepare or precommit record to WAL.
• Peer node receives changes into its apply queue in memory.
• Peer applies changes, making the transaction visible locally.
• Peer persists the transaction by flushing to disk.
• Origin commits and makes the transaction visible locally.

The following table summarizes the differences.

Configuration

Configuring Commit Scopes, is done through SQL function just like other administration operations in PGD.

For example you might define basic Commit Scope which does Group Commit on majority of nodes in the example_group BDR group:

SELECT bdr.add_commit_scope(
  commit_scope_name := 'example_scope',
  origin_node_group := 'example_group',
  rule := 'ANY MAJORITY (example_group) GROUP COMMIT',
  wait_for_ready := true
);

You can then use the commit scope either by setting configuration variable (GUC) bdr.commit_scope either per transaction or globally to that commit scope.

BEGIN;
SET LOCAL bdr.commit_scope = 'example_scope';
...
COMMIT;

You can also set the default commit scope for given BDR group.

SELECT bdr.alter_node_group_option(
  node_group_name := 'example_group',
  config_key := 'default_commit_scope',
  config_value := 'example_scope'
);

Note that the default_commit_scope is checked in the group tree the given origin node belongs to from bottom to up. The default_commit_scope cannot be set to the special value local which means no commit scope this way, for that use the bdr.commit_scope configuration parameter.

Full details of the Commit Scope language with all the options are described in the Commit Scopes chapter.

Postgres configuration parameters

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

Planned shutdown and restarts

When using Group Commit with receive confirmations, take care with planned shutdown or restart. By default, the apply queue is consumed prior to shutting down. However, in the immediate shutdown mode, the queue is discarded at shutdown, leading to the stopped node "forgetting" transactions in the queue. A concurrent failure of the origin node can lead to loss of data, as if both nodes failed.

To ensure the apply queue gets flushed to disk, use either smart or fast shutdown for maintenance tasks. This approach maintains the required synchronization level and prevents loss of data.

Legacy synchronous replication using BDR

Usage

To enable synchronous replication using BDR, you need to add the application name of the relevant BDR peer nodes to synchronous_standby_names. The use of FIRST x or ANY x offers a some flexibility if this doesn't conflict with the requirements of non-BDR 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 to synchronous_standby_names already enables synchronous replication. Setting synchronous_commit to local or off turns off synchronous replication.

Due to BDR applying the transaction before persisting it, the values on and remote_apply are equivalent (for logical replication).

Migration to Commit Scopes

The Group Commit feature of BDR is configured 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 synchronous_standby_names configured on each node individually, Group Commit uses globally synchronized Commit Scopes.

• On this page
• Overview
• Terms and definitions
• Comparison
• Internal timing of operations
• Configuration
• Planned shutdown and restarts
• Legacy synchronous replication using BDR