Eager conflict resolution v5

Eager conflict resolution (also known as Eager Replication) prevents conflicts by aborting transactions that conflict with each other with serializable errors during the COMMIT decision process.

You configure it using commit scopes as one of the conflict resolution options for Group Commit.

Usage

To enable Eager conflict resolution, the client needs to switch to a commit scope, which uses it at session level or for individual transactions as shown here:

BEGIN;

SET LOCAL bdr.commit_scope = 'eager_scope';

... other commands possible...

The client can continue to issue a COMMIT at the end of the transaction and let PGD manage the two phases:

COMMIT;

In this case, the eager_scope commit scope is defined something like this:

SELECT bdr.add_commit_scope(
    commit_scope_name := 'eager_scope',
    origin_node_group := 'top_group',
    rule := 'ALL (top_group) GROUP COMMIT (conflict_resolution = eager, commit_decision = raft) ABORT ON (timeout = 60s)',
    wait_for_ready := true
);

The commit scope group for the Eager conflict resolution rule can only be ALL or MAJORITY. Where ALL is used, the commit_decision setting must also be set to raft.

Error handling

Given that PGD manages the transaction, the client needs to check only the result of the COMMIT. This is advisable in any case, including single-node Postgres.

In case of an origin node failure, the remaining nodes eventually (after at least ABORT ON timeout) decide to roll back the globally prepared transaction. Raft prevents inconsistent commit versus rollback decisions. However, this requires a majority of connected nodes. Disconnected nodes keep the transactions prepared to eventually commit them (or roll back) as needed to reconcile with the majority of nodes that might have decided and made further progress.

Effects of Eager Replication in general

Increased abort rate

With single-node Postgres, or even with PGD in its default asynchronous replication mode, errors at COMMIT time are rare. The added synchronization step due to the use of a commit scope using eager for conflict resolution also adds a source of errors. Applications need to be prepared to properly handle such errors, usually by applying a retry loop.

The rate of aborts depends solely on the workload. Large transactions changing many rows are much more likely to conflict with other concurrent transactions.

Effects of MAJORITY and ALL node replication in general

Increased commit latency

Adding a synchronization step due to the use of a commit scope means more communication between the nodes, resulting in more latency at commit time. When ALL is used in the commit scope, this also means that the availability of the system is reduced, since any node going down causes transactions to fail.

If one or more nodes are lagging behind, the round-trip delay in getting confirmations can be large, causing high latencies. ALL or MAJORITY node replication adds roughly two network round trips (to the furthest peer node in the worst case). Logical standby nodes and nodes still in the process of joining or catching up aren't included but eventually receive changes.

Before a peer node can confirm its local preparation of the transaction, it also needs to apply it locally. This further adds to the commit latency, depending on the size of the transaction. This setting is independent of the synchronous_commit setting.

• On this page
• Usage
• Error handling
• Effects of Eager Replication in general
• Effects of MAJORITY and ALL node replication in general