Before diving into the specifics of database replication performance, it’s important to understand that AWS Aurora PostgreSQL and Azure Database for PostgreSQL Flexible Server represent two different architectural approaches to managed PostgreSQL services, each with unique strengths in handling replication workloads.
Overview of Replication Technologies
Aurora PostgreSQL Replication Architecture
Amazon Aurora PostgreSQL uses a distributed storage architecture that fundamentally changes how replication works compared to traditional PostgreSQL deployments. Aurora’s storage layer is designed to replicate data automatically across three Availability Zones, maintaining six copies of your data for high durability[4]. This architecture separates computation from storage, allowing read replicas to share the same underlying storage volume while maintaining their own independent buffer caches[5].
Azure PostgreSQL Flexible Server Replication Architecture
Azure Database for PostgreSQL Flexible Server employs PostgreSQL’s native replication mechanisms but enhances them with managed services capabilities. It relies on replication slots to manage write-ahead logs (WAL) segments between primary and replica servers[12]. This approach helps prevent replicas from going out of sync by ensuring WAL segments aren’t deleted before being processed by all replicas.
Replication Performance Findings
Replication Lag Metrics
The most critical performance indicator for database replication is replication lag - the time delay between a transaction being committed on the primary server and becoming visible on the replica.
Aurora PostgreSQL:
- Typically reports replication lag under 100 milliseconds in most operational scenarios[5]
- Recently introduced logical replication write-through cache reduces lag by up to 17x on pgbench workloads[10]
- The default write-through cache configuration delivers a 44% improvement in replication lag[1]
- Increasing the write-through cache size to 2GB further reduces lag by 59%[1]
Azure PostgreSQL Flexible Server:
- Azure documentation recommends setting alert rules for replication lags higher than 5 minutes, suggesting this as a threshold for concern[12]
- Less specific information is published regarding typical replication lag values
- Uses replication slots mechanism which can effectively manage WAL segments but may lead to storage issues if a replica remains inactive for extended periods[12]
Failover Performance
Failover speed is critical for maintaining database availability during unplanned outages.
Aurora PostgreSQL:
- Typically achieves failover times under 30 seconds in most scenarios[3]
- Quick detection of failures and efficient promotion of read replicas to new writer instances[3]
- Storage is continuously replicated across three Availability Zones, enabling rapid failover with minimal data loss[3][4]
Azure PostgreSQL Flexible Server:
- Configured with zone-redundant high availability, failover typically completes within 60-120 seconds[6]
- Provides a recovery point objective (RPO) of zero (no data loss)[6]
- Generally has decent but less consistent failover speeds compared to Aurora PostgreSQL[3]
Performance Optimization Features
Both platforms offer features to optimize replication performance, albeit with different approaches.
Aurora PostgreSQL:
- The write-through cache reduces storage I/O during logical decoding by caching recent WAL data[1][10]
- The ReplicaLag metric specifically measures the lag in page cache rather than data lag, as all instances share the same underlying storage[5]
- Distributed storage system provides high throughput for read-heavy workloads[11]
Azure PostgreSQL Flexible Server:
- Supports Premium SSDv2 storage designed for IO-intense workloads requiring sub-millisecond disk latencies[2]
- Offers automatic IOPS scaling through autoscale IOPS feature[2]
- Provides comprehensive built-in performance monitoring with metrics updated every minute[2]
Comparative Analysis
Architectural Differences Impacting Replication
The fundamental architectural differences between these services significantly impact replication performance:
Aurora PostgreSQL:
- Uses a shared storage model where all instances (both writer and read replicas) access the same underlying distributed storage volume[5]
- This shared storage architecture contributes to low replication lag since replicas directly read from the same storage layer[5]
- Read replicas maintain independent page caches, leading to minimal replication lag that primarily affects cache warming rather than data consistency[5]
Azure PostgreSQL Flexible Server:
- Relies more on traditional PostgreSQL replication mechanisms enhanced with cloud capabilities[12]
- Utilizes replication slots for managing transaction logs between primary and replicas[12]
- Requires careful monitoring to prevent storage issues caused by inactive replication slots accumulating WAL files[12]
Comparison Table: AWS Aurora PostgreSQL vs. Azure Database for PostgreSQL Flexible Server
| Feature | AWS Aurora PostgreSQL | Azure Database for PostgreSQL Flexible Server |
|---|---|---|
| Typical Replication Lag | <100ms in most cases[5] | Not explicitly stated; monitoring recommended for lags >5 minutes[12] |
| Replication Architecture | Shared distributed storage across all instances[5] | Traditional PostgreSQL replication with replication slots[12] |
| Failover Time | Typically <30 seconds[3] | 60-120 seconds with zone-redundant HA[6] |
| Data Durability | Six copies across three Availability Zones[4] | Zone-redundant HA with zero data loss (RPO=0)[6] |
| Performance Optimization | Write-through cache reducing lag by up to 17x[10] | Premium SSDv2 storage for IO-intense workloads[2] |
| Replication Lag Improvement | 44% improvement with default write-through cache; 59% with 2GB cache[1] | Not specifically documented |
| Cross-Region Replication | Supported, with minimal configuration required | Supported, with networking considerations for private networks[12] |
| Monitoring Capabilities | Built-in replication monitoring | Comprehensive performance monitoring with 30-day history[2] |
| Storage Impact | Minimal storage overhead for replication | Risk of storage growth due to WAL retention for replicas[12] |
Conclusion
AWS Aurora PostgreSQL generally demonstrates superior replication performance with significantly lower replication lag (typically <100ms) compared to Azure Database for PostgreSQL Flexible Server, where specific lag metrics are less clearly documented. This advantage stems primarily from Aurora’s innovative distributed storage architecture, which inherently reduces replication overhead.
For applications where minimal replication lag is critical, Aurora PostgreSQL offers compelling advantages, particularly with its write-through cache feature that can reduce lag by up to 17x. However, Azure PostgreSQL Flexible Server provides a solid alternative with comparable durability guarantees (RPO=0), albeit with potentially longer failover times and less published information about typical replication lag performance.
Both platforms require monitoring and optimization to achieve optimal replication performance, with Aurora providing more automated features for lag reduction and Azure requiring more active management of replication slots to prevent storage issues.
References
Sources [1] Amazon Aurora PostgreSQL Adds Write-Through Cache to Improve … https://www.infoq.com/news/2023/03/aws-aurora-write-through-cache/ [2] [PDF] Azure Database PostgreSQL Flexible Server https://www.principledtechnologies.com/Microsoft/Azure-Database-PostgreSQL-Flexible-Server-0224.pdf [3] Aurora PostgreSQL VS Azure SQL Failover Speeds - The DBA Hub https://thedbahub.com/aurora-postgresql-vs-azure-sql-failover-speeds/ [4] Understand replication capabilities in Amazon Aurora PostgreSQL https://aws.amazon.com/blogs/database/understand-replication-capabilities-in-amazon-aurora-postgresql/ [5] Aurora PostgreSQL meaning of the ReplicationLag metric https://repost.aws/questions/QURDolpEIfTfWHzg8VJ5m3Gg/aurora-postgresql-meaning-of-the-replicationlag-metric [6] Azure database for PostgreSQL Flexible Server - RPO - Microsoft Q&A https://learn.microsoft.com/en-sg/answers/questions/1615380/azure-database-for-postgresql-flexible-server-rpo [7] Deep dive on Amazon Aurora and Amazon RDS for PostgreSQL … https://aws.amazon.com/blogs/database/is-amazon-rds-for-postgresql-or-amazon-aurora-postgresql-a-better-choice-for-me/ [8] Aurora Postgres latency seems poor compared to standard … - Reddit https://www.reddit.com/r/aws/comments/e5yk5b/aurora_postgres_latency_seems_poor_compared_to/ [9] what’s the promised value for PostgreSQL replication lag? - Stack … https://stackoverflow.com/questions/65446208/aws-rds-postgresql-whats-the-promised-value-for-postgresql-replication-lag [10] Achieve up to 17x lower replication lag with the new write-through … https://aws.amazon.com/blogs/database/achieve-up-to-17x-lower-replication-lag-with-the-new-write-through-cache-for-aurora-postgresql/ [11] Aurora vs. Postgres: A Comprehensive Comparison - Sprinkle Data https://www.sprinkledata.com/blogs/aurora-vs-postgres-a-comprehensive-comparison [12] Read replicas - Azure Database for PostgreSQL flexible server https://learn.microsoft.com/en-us/azure/postgresql/flexible-server/concepts-read-replicas [13] PostgreSQL vs. Aurora PostgreSQL: Choosing the Right Database … https://www.linkedin.com/pulse/postgresql-vs-aurora-choosing-right-database-your-aws-barry-o-connell-mgare [14] Migrate online from Amazon Aurora PostgreSQL to Azure Database … https://learn.microsoft.com/en-us/azure/postgresql/migrate/migration-service/tutorial-migration-service-aurora-online [15] Comparing On-Premise, AWS Aurora, Amazon RDS for PostgreSQL … https://virtual-dba.com/blog/choosing-the-right-postgresql-solution/ [16] Seamless Database Migration and Replication to AWS Aurora … https://www.striim.com/blog/aws-aurora-postgresql/ [17] Monitoring and metrics - Azure Database for PostgreSQL flexible … https://learn.microsoft.com/en-us/azure/postgresql/flexible-server/concepts-monitoring [18] Comparing Postgres Managed Services: AWS, Azure, GCP and … https://blog.peerdb.io/comparing-postgres-managed-services-aws-azure-gcp-and-supabase [19] Tuning PostgreSQL Replication Parameters in Amazon RDS and … https://dev.to/francotel/tuning-postgresql-replication-parameters-in-amazon-rds-and-aurora-3mjk [20] High availability (Reliability) in Azure Database for PostgreSQL https://learn.microsoft.com/en-us/azure/reliability/reliability-postgresql-flexible-server [21] Amazon Aurora vs. Microsoft Azure SQL Database vs. PostgreSQL … https://db-engines.com/en/system/Amazon+Aurora;Microsoft+Azure+SQL+Database;PostgreSQL [22] Replication with Amazon Aurora - AWS Documentation https://docs.aws.amazon.com/AmazonRDS/latest/AuroraUserGuide/Aurora.Replication.html [23] Working with read replicas for Amazon RDS for PostgreSQL https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_PostgreSQL.Replication.ReadReplicas.html [24] Azure Database for PostgreSQL flexible server vs. Single Server https://learn.microsoft.com/en-us/azure/postgresql/flexible-server/concepts-compare-single-server-flexible-server [25] 5 Common PostgreSQL Challenges in the Cloud: AWS RDS & Aurora https://www.enterprisedb.com/blog/5-common-postgresql-challenges-in-cloud-aws-rds-aurora-azure-database-dbaas-iaas [26] What to Look for if Your PostgreSQL Replication is Lagging https://severalnines.com/blog/what-look-if-your-postgresql-replication-lagging/ [27] Amazon Aurora vs. EDB Postgres vs. Microsoft Azure Cosmos DB … https://db-engines.com/en/system/Amazon+Aurora;EDB+Postgres;Microsoft+Azure+Cosmos+DB [28] Aurora Vs. RDS: Which AWS Database Solution Is Best? - CloudZero https://www.cloudzero.com/blog/aurora-vs-rds/