Physical vs. Logical Replication in PostgreSQL: A Comprehensive Guide

Performance Comparison: Physical Replication vs. Logical Replication in PostgreSQL

Physical Replication Performance

One of the biggest advantages of physical replication is its performance. Since it replicates data at the block level, it is highly efficient and can handle large volumes of data quickly.

Physical replication directly transfers WAL files from the primary server to the standby server, minimizing the processing overhead.

• Data Transfer: WAL files are compact and efficient, making the replication process faster, especially for large or bulk data operations.
• Performance Impact: In most cases, physical replication imposes very little overhead on the primary server. It is ideal for high-performance scenarios where speed is critical.

Logical Replication Performance

While logical replication can be a bit slower than physical replication, it is still efficient for many use cases.

The process of decoding WAL entries into logical changes can introduce some additional overhead, especially in high-volume environments.

• Data Transfer: Logical replication involves converting the changes recorded in the WAL into logical operations. This extra layer of abstraction adds complexity and potential latency to the process.
• Performance Impact: In cases of frequent data modification, logical replication can incur a performance penalty due to the need to track and send changes at a logical level.

In summary, physical replication is generally better suited for performance-intensive operations, such as bulk data loads, whereas logical replication might introduce a bit more overhead due to the decoding of WAL records.

Version Compatibility: Physical vs. Logical Replication in PostgreSQL

Physical Replication Version Compatibility

Physical replication requires that both the primary and standby servers be running the same major version of PostgreSQL. This is because physical replication depends on the exact binary structure of the data, and any changes in the format between versions could lead to incompatibility.

• Version Requirement: Both servers must use the same version of PostgreSQL, including the same major version number (e.g., 12.x to 12.x). Minor version differences are typically acceptable.

Logical Replication Version Compatibility

One of the key advantages of logical replication is its version compatibility. Logical replication works by replicating changes at the logical level, which means that different major versions of PostgreSQL can be used between the primary and standby servers. This makes logical replication a useful tool for performing zero-downtime upgrades or migrating data between PostgreSQL versions without requiring a major version upgrade on the standby server.

• Version Flexibility: Logical replication supports cross-version replication, which is beneficial for upgrading PostgreSQL without interrupting services. For example, you can replicate data from PostgreSQL 11 to PostgreSQL 12, or even from one version of PostgreSQL to another in a rolling upgrade process.

Features and Flexibility of PostgreSQL Replication Methods

Physical Replication Features

Physical replication is simple and efficient, but it offers fewer customization options than logical replication. It is ideal for scenarios where the primary goal is to create a read-only copy of the database for high availability or disaster recovery.

• High Availability: Physical replication is a good choice for maintaining highly available systems where the primary server might go down unexpectedly. Standby servers can be automatically promoted to primary in the event of a failure.
• Cascading Replication: Physical replication supports cascading replication, where a secondary standby server can replicate data from another standby server, reducing the load on the primary server.

Logical Replication Features

Logical replication provides far more flexibility, as it allows you to replicate specific tables or schemas, and even apply custom filters to the data. You can choose which tables to replicate, apply data transformations, and even resolve conflicts when they occur.

• Selective Replication: With logical replication, you can selectively replicate specific tables, or subsets of data, making it ideal for scenarios where full database replication is unnecessary.
• Writable Standby: Logical replication enables the creation of writable standby nodes. This is particularly useful in multi-master replication scenarios, where both the primary and the standby servers can accept write operations.
• Custom Replication Logic: Logical replication allows for more advanced use cases, such as data transformation, filtering, and conflict resolution, which are not possible with physical replication.

Best Use Cases for Physical and Logical Replication in PostgreSQL

Physical Replication Use Cases

Physical replication is best suited for:

• High Availability: Ensuring that there is an exact copy of the primary server that can be used for failover in case of primary server failure.
• Disaster Recovery: Maintaining a disaster recovery site by replicating the entire database cluster to a standby server.
• Read-Only Replicas: Creating read-only replicas for reporting or analytical purposes without affecting the performance of the primary server.

Logical Replication Use Cases

Logical replication is ideal for:

• Selective Replication: Replicating only specific tables or databases, often for integration with other systems or reporting purposes.
• Cross-Version Replication: Enabling replication between different PostgreSQL versions during a rolling upgrade or migration process.
• Data Transformation and Integration: Replicating data to other systems or applying custom transformations.
• Multi-Master Replication: Enabling writeable standby nodes in multi-master replication setups.

PostgreSQL Replication Slots Explained

Physical Replication Slots

Physical replication slots are used to ensure that WAL files are not discarded by the primary server until they have been successfully received by all connected standby servers. This prevents data loss if a standby server becomes disconnected for a period of time.

• Slot Creation: Replication slots are created on the primary server, and each standby server is assigned its own slot.
• Purpose: The slot guarantees that the WAL files required by the standby are retained, preventing the risk of missing WAL records if the standby falls behind.

Logical Replication Slots

Logical replication slots are similar to physical replication slots but are used to track the changes that have been sent to a subscriber. They ensure that the logical changes are not lost, even if the subscriber is temporarily disconnected.

• Slot Creation: Logical replication slots are created for each logical replication stream and track the progress of changes sent to subscribers.
• Purpose: They ensure that no changes are lost and that the subscriber can resume replication from where it left off.

Conclusion: Choosing Between Physical and Logical Replication in PostgreSQL

The choice between physical and logical replication in PostgreSQL depends on the specific requirements of your application.

• Physical replication: is best suited for high availability, disaster recovery, and situations where an exact copy of the primary database is required. It is highly performant and reliable, but lacks the flexibility offered by logical replication.
• Logical replication: is ideal for scenarios that require more granular control over which data is replicated, as well as cross-version replication, data transformation, and integration with other systems. It also allows for writable standby nodes, enabling advanced multi-master replication setups.

Both methods are important tools in PostgreSQL’s replication arsenal, and understanding the differences between them will help you make the right choice for your specific use case.