Predefined commit scopes

Predefined commit scopes#

EDB Postgres Distributed (PGD) provides the following predefined commit scopes available for use:

local protect#

ASYNCHRONOUS COMMIT

The local protect commit scope is the default commit scope. It provides asynchronous commit with no durability guarantees. This means that transactions are considered committed as soon as they are written to the local node’s WAL, without waiting for any confirmation from other nodes in the cluster.

This commit scope is suitable for scenarios where high availability and low latency are more important than data durability. However, it does not provide any guarantees against data loss in case of node failures or network issues.

lag protect#

MAJORITY ORIGIN GROUP LAG CONTROL (max_lag_time = 30s, max_commit_delay = 10s)

The lag protect commit scope provides a durability guarantee based on the lag time of the majority origin group. It ensures that transactions are considered committed only when the lag time is within a specified limit (30 seconds in this case) and the commit delay is also within a specified limit (10 seconds in this case). This helps to prevent data loss in case of network issues or node failures.

This commit scope is useful in scenarios where data consistency and durability are important, but some latency is acceptable. It allows for a balance between performance and data safety by ensuring that transactions are not considered committed until they have been confirmed by the majority of nodes in the origin group within the specified lag and commit delay limits.

majority protect#

MAJORITY ORIGIN GROUP SYNCHRONOUS COMMIT

The majority protect commit scope provides a durability guarantee based on the majority origin group. It ensures that transactions are considered committed only when they are confirmed by the majority of nodes in the origin group. This helps to ensure data consistency and durability in case of node failures or network issues.

This commit scope is suitable for scenarios where data consistency and durability are critical, and it provides a higher level of protection against data loss compared to the local protect commit scope. However, it may introduce some latency due to the need for confirmation from multiple nodes before considering a transaction as committed.

adaptive protect#

MAJORITY ORIGIN GROUP SYNCHRONOUS COMMIT DEGRADE ON (timeout = 10s, require_write_lead = true) TO ASYNCHRONOUS COMMIT

The adaptive protect commit scope provides a more flexible durability guarantee. It allows transactions to be considered committed based on the majority origin group synchronous commit, but it can degrade to asynchronous commit if the transaction cannot be confirmed within a specified timeout (10 seconds in this case). This is useful in scenarios where network latency or node failures may cause delays in confirming transactions.

This commit scope is suitable for scenarios where data consistency and durability are important, but some flexibility is needed to handle potential delays. It provides a balance between performance and data safety by allowing transactions to be considered committed even if they cannot be confirmed by the majority of nodes within the specified timeout, while still providing a higher level of protection compared to the local protect commit scope.

Examples#

  • Set the lag protect commit scope for a transaction using the SET LOCAL command:

BEGIN;
SET LOCAL bdr.commit_scope = lag protect;
-- Your transaction statements here
COMMIT;

This ensures that the transaction is committed with the specified commit scope, providing the desired level of durability and availability.

  • Set the majority protect commit scope for the entire session using the SET command:

SET bdr.commit_scope = majority protect;

This ensures that all transactions executed in the session will use the specified commit scope, providing the desired level of durability and availability.

  • Set the adaptive protect commit scope for a PGD group using the bdr.alter_node_group_option() function:

SELECT bdr.alter_node_group_option(
        node_group_name:=mygroup,
        config_key:=default_commit_scope,
        config_value:=adaptive protect);

This ensures that all transactions executed in the specified PGD group will use the specified commit scope, providing the desired level of durability and availability, unless overridden by a session or transaction-level setting.

Contents