Durability in PGD Essential v6.0.1

By default PGD Essential uses asynchronous replication between its nodes, but it can be configured to use synchronous replication as well. This allows for a high degree of flexibility in terms of data durability and availability. Asynchronous replication offers lower latency and higher throughput, while synchronous replication provides stronger consistency guarantees at the cost of performance. PGD Essential allows you to choose the replication strategy through the use of commit scopes.

Commit Scopes

Commit scopes are a powerful feature of PGD Essential that allow you to control the durability and availability of your data. They enable you to specify the level of durability required for each transaction, allowing you to balance performance and consistency based on your application's needs. PGD Essential has four pre-defined commit scopes that you can use to control the durability of your transactions, among other things.

• local protect
• lag protect
• majority protect
• adaptive protect

The predefined commit scopes in PGD Essential are designed to provide a balance between performance and data safety. You cannot add, remove or modify a PGD Essential commit scope. In PGD Expanded, you can create and manage your own commit scopes, allowing for more flexibility and control over the durability guarantees.

local protect

This is the default commit scope for PGD Essential. 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.

lag protect

This commit scope 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.

majority protect

This 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.

adaptive protect

This 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.

For more information on commit scopes, see the Commit Scopes reference section and the Predefined Commit Scopes reference page.

Using Commit Scopes

To use commit scopes in PGD Essential, you can specify the desired commit scope when executing a transaction. This allows you to control the durability and availability of your data based on your application's needs. For example, you can use the lag protect commit scope for transactions that require a higher level of durability, while using the local protect commit scope for transactions that prioritize performance over durability.

Within a transaction

You can specify the commit scope for a transaction using the SET LOCAL command. For example, to use the lag protect commit scope for a transaction, you can execute the following commands:

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

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

For a session

You can also set the commit scope for the entire session using the SET command. For example, to set the majority protect commit scope for the entire session, you can execute the following command:

SET bdr.commit_scope = 'majority protect';

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

For a group

You can also set the default commit scope for a PGD group using the bdr.alter_node_group_option()function. For example, to set theadaptive protect` commit scope for a PGD group, you can execute the following command:

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

This will ensure 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.