Introduction
PostgreSQL is well known for its rock-solid replication features. For many workloads, physical replication works just fine. But as systems become more distributed and upgrade cycles more frequent, traditional primary–standby setups start to feel limiting.
Real-world architectures often need more flexibility:
- Replicating only specific tables
- Upgrading PostgreSQL versions with minimal downtime
- Sharing data across multiple systems
- Even enabling active-active architectures where writes can occur on multiple nodes, provided appropriate conflict-handling strategies are implemented.
This is where pglogical comes into play.
pglogical is a PostgreSQL extension that enables logical replication using a publish–subscribe model. It allows you to replicate exactly the data you want, where you want it, while keeping downtime and operational friction low.
What is pglogical?
pglogical is a logical replication system built on PostgreSQL’s logical decoding infrastructure. Instead of copying raw WAL blocks (as physical replication does), pglogical replicates logical changes, such as:
- INSERT
- UPDATE
- DELETE
- DDL changes are not automatically replicated by pglogical. Schema changes must typically be applied manually on subscriber nodes, although pglogical provides helper functions to assist with DDL synchronization
Data flows from a provider (source database) to one or more subscribers (target databases). This makes pglogical ideal for use cases that go beyond simple high availability.
pglogical replicates business data changes, not raw storage blocks.
How pglogical Works
Under the hood, pglogical uses PostgreSQL’s logical decoding infrastructure to extract changes from the Write-Ahead Log (WAL). Logical replication slots retain the required WAL and track replication progress, while the decoded changes are streamed to subscribers over replication connections.
The subscriber receives these changes and replays them, keeping selected data in sync with the provider.
This approach offers:
- Fine-grained control
- Schema- and table-level replication
- Flexible data distribution patterns
Why Choose pglogical?
While PostgreSQL’s built-in logical replication has matured significantly over recent releases, pglogical continues to offer advanced replication capabilities, flexible topologies, and migration features that are valuable in complex environments.
Compared to native replication methods, pglogical shines when you need:
- Table-level or schema-level replication
- Cross-version PostgreSQL replication
- Minimal-downtime migrations
- Logical data portability across environments
Common pglogical Replication Architectures
One of pglogical’s greatest strengths is its ability to support a wide variety of replication topologies. Let’s look at the most common patterns used in production environments.
1. One-Way Replication (Single Provider → Single Subscriber)
A simple, unidirectional setup where data flows from one provider to one subscriber.
Ideal for:
- Reporting databases
- Logical standbys
- Read scaling
This is the most common starting point for pglogical users.
2. One-to-Many Replication
A single provider replicates data to multiple subscribers.
Use cases:
- Multiple reporting systems
- Data sharing across teams
- Regional read replicas
This pattern allows the same data to power different workloads independently.
3. Many-to-One Replication
Multiple providers replicate data into a single central database.
Use cases:
- Data aggregation
- Centralized analytics platforms
- Multi-branch or multi-region consolidation
This architecture is especially useful for building unified reporting or analytics systems.
4. Bi-Directional (Multi-Master) Replication
Two databases replicate changes to each other, allowing writes on both sides.
Use cases:
- Active–active applications
- Geo-distributed systems
This setup requires careful conflict handling, well-defined ownership rules, and disciplined schema design.
5. Cascading Replication
A subscriber can also be configured as a provider for downstream nodes, enabling cascading-style logical replication topologies.
Use cases:
- Large-scale data distribution
- Reducing replication load on the primary provider
This approach improves scalability in complex environments.
6. Selective Table Replication
Only specific tables or schemas are replicated instead of the entire database.
Use cases:
- Microservices architectures
- Compliance-driven data sharing
- Partial environment synchronization
Selective replication is one of pglogical’s most powerful features.
7. Cross-Version Upgrade Replication
pglogical supports replication between different PostgreSQL major versions, making it useful for major-version upgrade migrations.
Use cases:
- Zero-downtime PostgreSQL upgrades
- Hardware or operating system migrations
This enables blue–green style upgrades with minimal risk.
8. Append-Only Workloads
pglogical is often used for append-only workloads, such as event streams, audit logs, and time-series data, where INSERT operations dominate, and updates or deletes are rare.
Use cases:
- Event-driven systems
- Audit logs
- Time-series workloads
This pattern works well for append-only datasets.
Where pglogical Fits Best
pglogical is particularly effective in environments where:
- Reporting and analytics workloads must be isolated from OLTP systems
- Multiple applications or teams need controlled access to shared data
- PostgreSQL upgrades must be performed without extended downtime
- Data must flow across regions, clouds, or infrastructure boundaries
In such scenarios, pglogical provides the control and flexibility that traditional replication models cannot.
These replication patterns allow PostgreSQL to evolve beyond traditional primary–standby deployments and support modern distributed architectures, data-sharing platforms, cross-version migrations, and active-active workloads.
What’s Next?
In Part 2, we’ll move from concepts to execution. We’ll walk through pglogical setup and configuration of bidirectional replication