PostgreSQL for SQL Server DBAs - Can we implement a Distributed Availability Group in PostgreSQL Replication?
PostgreSQL does not have a native "Distributed Availability Group" feature like SQL Server. However, similar functionality can be implemented using logical replication, physical replication, or third-party tools for multi-cluster or distributed setups.
Below is a guide to implementing a distributed availability-like group in PostgreSQL, ensuring replication across clusters in different locations:
1. Define the Architecture
Distributed availability requires:
- Primary Cluster: The main cluster that handles writes.
- Replica Clusters: Secondary clusters in other locations (read-only or with logical replication for distributed writes).
2. Implementing the Distributed Availability Group
2.1 Logical Replication (Distributed Writes)
Logical replication allows row-level changes (inserts, updates, deletes) to replicate between clusters.
Steps:
- Enable Logical Replication:
- Modify postgresql.conf on both primary and replica clusters:
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wal_level = logical max_replication_slots = 10 max_wal_senders = 10 |
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- Restart the PostgreSQL server:
- Create a Publication on the Primary Cluster:
- On the primary cluster:
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CREATE PUBLICATION my_publication FOR ALL TABLES; |
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- Alternatively, select specific tables:
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CREATE PUBLICATION my_publication FOR TABLE transactions, customers; |
- Create a Subscription on the Replica Cluster:
- On the replica cluster:
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CREATE SUBSCRIPTION my_subscription CONNECTION 'host='PRIMARY-HOST' port=5432 dbname=mydb user=replication_user password=rep_password' PUBLICATION my_publication; |
- Verify Replication:
- Check the subscription status:
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SELECT * FROM pg_stat_subscription; |
2.2 Physical Replication (Read-Only Replicas)
Physical replication replicates entire database clusters at the block level.
Steps:
- Enable Physical Replication:
- Configure the primary cluster in postgresql.conf:
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wal_level = replica max_wal_senders = 5 max_replication_slots = 5 |
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- Create a replication user on the primary cluster:
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CREATE USER replication_user REPLICATION LOGIN PASSWORD 'rep_password'; |
- Set Up a Standby Cluster:
- On the replica server, initialize the data directory using pg_basebackup:
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pg_basebackup -h -D /var/lib/postgresql/data -U replication_user -Fp -Xs -P |
- Configure recovery.conf on the Replica:
- Set up the replica cluster for streaming:
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primary_conninfo = 'host= port=5432 user=replication_user password=rep_password' |
- Start the Replica Cluster:
- Start PostgreSQL on the replica server:
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sudo systemctl start postgresql |
3. Advanced Multi-Cluster Setup
3.1 Cross-Cluster Logical Replication
- Set up logical replication between multiple clusters for specific tables or workloads.
- Example: Cluster A replicates transactions to Cluster B, while Cluster B replicates analytics back to Cluster A.
3.2 Third-Party Tools for Distributed Availability Groups
- Bucardo: Multi-master replication tool for PostgreSQL.
- Use Bucardo for bi-directional replication and conflict resolution.
- Setup guide: Bucardo Documentation
- Citus: Distributes tables across nodes for high availability and distributed transactions.
- Setup guide: Citus Documentation
3.3 Connection Pooling Across Clusters
- Use a tool like pgpool-II or HAProxy to distribute connections and provide failover between primary and replica clusters.
4. Conflict Resolution in Distributed Writes
- Logical Replication Conflict Resolution:
- Implement triggers or custom logic to handle update conflicts between clusters.
- Bi-Directional Replication (BDR):
- Use PostgreSQL BDR for native conflict detection and resolution.
- Guide: BDR Documentation
5. Monitoring and Failover
5.1 Monitoring:
- Use pg_stat_replication to monitor physical replication:
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SELECT * FROM pg_stat_replication; |
- Use pg_stat_subscription for logical replication:
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SELECT * FROM pg_stat_subscription; |
5.2 Automated Failover:
- Use tools like Patroni or pg_auto_failover to automate failover between clusters.
6. Example Use Case
- Cluster A (Primary): Handles all write transactions.
- Cluster B (Replica): Processes read-heavy workloads using physical replication.
- Cluster C (Distributed Writes): Replicates specific tables back to Cluster A for analytics and reporting.
7. Summary
| Feature | Implementation |
|---|---|
| Logical Replication | Native PostgreSQL |
| Physical Replication | Streaming Replication |
| Multi-Master Replication | Bucardo or BDR |
| Distributed Transactions | Citus |
| Failover Automation | Patroni or pg_auto_failover |
This architecture provides a robust and distributed system similar to an availability group in PostgreSQL, ensuring high availability, scalability, and failover capabilities.
DISCLAIMER: The information provided in this article represents MinervaDB Inc.'s technical perspective based on our experience with PostgreSQL and SQL Server database technologies. While we strive to ensure accuracy, database architectures and implementations can vary significantly based on specific use cases and requirements. The comparisons and solutions discussed here may not be suitable for all environments. We recommend thorough testing and consultation with database experts before implementing architectural changes in your production environment. PostgreSQL and SQL Server are rapidly evolving technologies, and the features discussed may change with newer versions.
© 2024 Shiv Iyer and MinervaDB Inc. All rights reserved.
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