Navigating Distributed PostgreSQL Options: pgLogical and Beyond

Traditional single-instance PostgreSQL deployments, while robust and reliable, fall short of meeting the stringent availability requirements of modern applications. This limitation has led to the development of distributed PostgreSQL solutions, with pgLogical replication being one of the pioneers of PostgreSQL active-active architecture.

The open-source pgLogical extension was designed to introduce logical replication capabilities beyond those that are natively available in core PostgreSQL. This tool allows users to enable selective replication of specific tables and bi-directional data flow between PostgreSQL instances. Since its inception, several new contenders have emerged as powerful solutions for distributed PostgreSQL management and the pgLogical project itself has ceased continuous development in its open source form (even though it remains available in maintenance mode for public usage).

Key Features of the Postgres pgLogical Extension

• Logical Replication: Unlike physical replication, pgLogical replication allows users to select specific tables for replication, making data synchronization more flexible and efficient.

• Bi-Directional Replication (BDR): The pgLogical extension introduced the ability for changes to flow in both directions between source and target databases—a fundamental requirement for active-active configurations.

• Replication Filtering: Administrators can apply filtering rules to determine which data changes should be replicated, adding another layer of flexibility for users that have to isolate data to meet data residency requirements.

• Independence from Physical Replication: pgLogical operates independently of physical replication mechanisms, allowing greater compatibility with different PostgreSQL configurations and versions.

Limitations of pgLogical Replication

While pgLogical laid the groundwork for active-active PostgreSQL deployments, it faces significant challenges when it comes to true bi-directional replication. The most critical issue is the lack of robust conflict resolution mechanisms—when two nodes simultaneously modify the same data, there exists no reliable way to determine which change should take precedence, which potentially compromises data integrity.

Additionally, error handling in conflict scenarios is limited, making it difficult for an administrator to identify and address issues that arise during replication. These limitations make pgLogical-based multi-master setups risky for production environments where data consistency and integrity is paramount.

Introducing pgEdge, an Alternative to pgLogical

pgEdge Enables Fully Open, Fully Distributed PostgreSQL

pgEdge emerged in late 2024 as a serverless distributed Postgres managed cloud service, delivering low latency and high availability in three minutes or less. The pgEdge Platform (for on-premises distributed PostgreSQL) as well as pgEdge Cloud (for deploying in the cloud) was largely inspired by the original capabilities of the pgLogical extension.

The goal changed from simply supporting logical streaming replication for PostgreSQL, using a publish/subscribe model (within pgLogical) to enabling globally distributed, edge applications that are always on, always available, and always fast, deployable across cloud regions and data centers.

How pgEdge Advances Fault Tolerant PostgreSQL at Scale

The Spock extension was written for the pgEdge project in order to enhance PostgreSQL scalability and transform how multi-master (active-active) replication is implemented in a PostgreSQL environment.  Crucial changes that differentiate Spock from pgLogical include:

• True Asynchronous Multi-Master Replication: Unlike pgLogical replication, Spock replication enables multiple nodes to safely accept writes simultaneously, enhancing performance, fault tolerance, and scalability.

  

• Advanced Conflict Resolution: Spock implements sophisticated conflict resolution mechanisms, including conflict-free delta-apply columns for numeric data. This approach ensures that conflicting changes to numeric values are intelligently resolved to maintain data consistency.

• Improved Error Handling: When conflicts do occur, Spock provides robust error handling capabilities.  When paired with reconciliation and repair features provided by the ACE (Active Consistency Engine) extension, pgEdge Postgres streamlines conflict resolution and error handling and reduces operational overhead for developers and administrators.

• Enhanced Monitoring and Management: Spock includes detailed metadata tables for tracking conflicts, replication status, and performance metrics, enabling administrators to make informed decisions about their distributed database environments.

  Working alongside Spock in a pgEdge cluster, ACE simplifies identifying and resolving data inconsistencies by providing functions that can compare your data on a granular level, by table, schema, or cluster and assist in repairs if needed.  Scheduled/automated data checks help keep an eye on your data around the clock, so any discrepancies are found fast!

• Support for Partitioned Tables: Critical for geo-sharding applications, Spock can replicate partitioned tables, allowing data to be distributed across geographic regions and/or clouds while maintaining consistency.

• Data Residency and Privacy Features: For organizations dealing with privacy and data residency regulations, Spock offers the ability to link databases to specific countries and implement configurable rules for handling personally identifiable information (PII).

• Edge Computing Integration: With the rise of edge computing, Spock is designed to support built-in integration between distributed PostgreSQL and edge platforms like Cloudflare Workers, enabling data to be processed closer to users. Spock also has a relatively small footprint, so it can run on factory floor compute devices, including those based on the Raspberry Pi.

For many organizations, pgEdge's approach offers an attractive balance. By building on standard PostgreSQL while leveraging extended functionality with a suite of open-source extensions (Spock, ACE, Snowflake, and LOLOR) provides advanced active-active replication capabilities while maintaining full compatibility with the PostgreSQL ecosystem and minimizing migration overhead.

This makes pgEdge a nice all-in-one solution for active-active replication, following the open-source principles upon which PostgreSQL built its success. You get all of the power and resiliency of pure PostgreSQL with no vendor lock-in, designed with the horizontal scaling, fault tolerance, and high availability that comes with popular (and expensive) alternatives such as Oracle's GoldenGate.

Exploring the Broader Landscape of Distributed PostgreSQL Solutions

pgLogical is now in maintenance mode, with no new features planned, and no expectations for future development. As a result, new solutions have emerged to address the limitations of pgLogical, and to re-evaluate the original goals of the project so they meet modern business demands.

Today's distributed PostgreSQL market includes several options, each with its own approach and trade-offs:

Proprietary Forks and Hard Forks

Some vendors have created proprietary forks or "hard forks" of PostgreSQL to implement distributed functionality. While these solutions may offer certain advantages, they often deviate significantly from standard PostgreSQL, potentially limiting compatibility with the broader PostgreSQL ecosystem and creating problems such as vendor lock-in. Tools like PG Scorecard can be used to evaluate real compatibility with the PostgreSQL project in order to make more informed decisions.

Wire Protocol Compatible Systems

Unlike pgEdge, other products claim PostgreSQL compatibility by implementing the PostgreSQL wire protocol, but have different SQL syntax and semantics. These solutions allow an application to connect using standard PostgreSQL drivers, but often require significant code modifications and specialized training to address differences in SQL implementation.

Because pgEdge is 100% pure PostgreSQL, many applications can run on pgEdge without any modifications. pgEdge stays up-to-date with the latest PostgreSQL major and minor releases, ensuring that all the most recent enhancements in each release are available at or near release time.

Extension-Based Solutions

Products like pgEdge take an extension-based approach, implementing distributed functionality while maintaining full compatibility with standard PostgreSQL. This approach allows organizations to leverage the existing PostgreSQL ecosystem of extensions, tools, and talent while gaining the benefits of distributed architecture.

Choosing the Right Logical Replication Solution for Your Needs

When evaluating distributed PostgreSQL options, from pgLogical to pgEdge and beyond, your organization should consider several key factors:

• PostgreSQL Compatibility: How closely does the solution adhere to standard PostgreSQL? Will your existing applications, extensions, and tools continue to work without significant modifications?

• Code Migration Effort: What level of effort will be required to migrate existing applications to the new database system?

• Ecosystem Alignment: To what extent will you still be operating within and taking advantage of the wider PostgreSQL ecosystem?

• Architecture Type: Does the solution follow a CP (consistency prioritized) or AP (availability prioritized) architecture, and does this align with your application requirements?

• Version Support: How quickly does the solution incorporate support for new PostgreSQL major versions, ensuring you can benefit from the latest innovations?

Concerned about making the right choice for your use case? Take a close-up look at major solutions within the world of active-active Postgres with this detailed feature comparison.