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Durability

Durability#

EDB Postgres Distributed (PGD) offers a range of synchronous modes to complement its default asynchronous replication. These synchronous modes are configured through commit scopes; rules that define how operations are handled and when the system should consider a transaction committed.

Introducing#

  • PGD overview introduces the concepts and some of the essential terminology which is used when discussing synchronous commits.

  • Durability Terminology lists terms used around PGD’s durability options, including how nodes are referred to in replication.

  • Commit scopes is a more in-depth look at the structure of commit scopes and how to define them for your needs.

  • Commit scope rules looks at the syntax of and how to formulate a commit scope rule.

  • Comparing Durability Options compares how each option behaves.

Commit scope kinds#

  • Group commit focuses on the Group Commit option, where you can define a transaction as done when a group of nodes agrees it’s done.

  • CAMO or commit-at-most-once focuses on the Commit At Most Once option, in which applications take responsibility for verifying that a transaction has been committed before retrying. This ensures that their commits only happen at most once.

  • Lag Control looks at the commit scope mechanism which dynamically throttle nodes according to the slowest node and regulates how far out of sync nodes may go when a database node goes out of service.

  • PGD Synchronous Commit examines a commit scope mechanism which works in a similar fashion to legacy synchronous replication, but from within the commit scope framework.

Working with commit scopes#

  • Administering addresses how a PGD cluster with Group Commit in use should be managed.

  • Limitations lists the various combinations of durability options which are not currently supported or not possible. Do refer to this before deciding on a durability strategy.

  • Legacy synchronous replication using PGD shows how traditional Postgres synchronous operations can still be accessed under PGD.

  • Internal Timing of Operations compares legacy replication with PGD’s async and synchronous operations, especially the difference in the order by which transactions are flushed to disk or made visible.

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