The hosts discuss the various options for replication in PostgreSQL, including physical and logical replication. They explore offloading reads, implementing sticky reads, and using replicas for read-only parts of applications. They mention risks and downsides of using Postgres for replication, limitations and future improvements of logical replication, and differences between physical and logical replication in Postgres.
Replication is crucial for ensuring high availability, scaling resources, and providing additional data protection or recovery.
Physical replication is reliable and suitable for scenarios where all tables in the database are needed, while logical replication is recommended when the destination database differs from the source.
Deep dives
The Importance of Replication
Replication is needed in three main areas: ensuring high availability to minimize downtime, scaling resources by offloading reads to different servers, and as a means of additional data protection or recovery. Replication is crucial in cloud environments where downtime must be minimized, and it is also useful for scenarios where one machine is not sufficient to handle resource saturation. However, replication should not be relied upon solely as a backup solution, as it lacks the ability for point-in-time recovery or data restoration. Physical replication is a reliable and widely used method that involves replaying all changes made to the primary database on standby servers. Logical replication, on the other hand, is based on capturing wall records and is suited for use cases where the destination database differs from the source.
Choosing Between Physical and Logical Replication
Physical replication, which is reliable and battle-tested, is suitable for scenarios where all tables in the database are needed and when offloading reads to replicas is desired. It involves shipping wall records to standby nodes and providing high availability and scalability. However, it becomes challenging when using replicas for analytical queries due to potential pauses in replication and blocked vacuum operations on the primary. Logical replication, derived from physical replication, is recommended when the destination database differs from the source, such as during major version upgrades or when replicating specific tables to other systems like ClickHouse. Although logical replication has some limitations and performance challenges, ongoing improvements in Postgres are expected to address these issues.
Considerations for Managed Service Providers
Managed service providers like RDS and Google Cloud SQL have some limitations when it comes to replication. RDS now supports read queries on standby replicas and offers options for enabling high availability. However, there are constraints on the number of nodes allowed, and some nuances exist, like the behavior of logical slots in Patroni. Additionally, in cloud environments, it may be more cost-effective to rely on the managed service's built-in replication features rather than setting up physical replication manually. Companies like Attunity and FiveTran offer commercial solutions for replicating Postgres data, providing additional functionalities and ease of use for specific use cases.
Best Practices and Special Considerations
When using physical or logical replication, there are certain best practices and special considerations to keep in mind. For physical replication, it is vital to understand and plan for differences in behavior compared to the primary database, especially when dealing with subtransactions. Sticky reads can be implemented to ensure consistency for fast-paced applications and prevent users from seeing inconsistent or missing data. For logical replication, there are challenges with performance, scalability, and potential duplicates, especially when using third-party replication tools. However, logical replication allows for easy migration between different Postgres versions and different database systems. Overall, understanding the nuances of replication and applying best practices can help optimize performance and reliability.
Understanding RPO RTO, Choosing Between Wall-shipping and Streaming Replication, Implementing Sticky Reads, and Using Replicas for Read-only Parts of Applications
