For a high-availability application backend, the best managed relational database services are usually the ones that give you:

• Automatic failover
• Multi-AZ / multi-zone deployment
• Backups and point-in-time restore
• Read replicas / horizontal read scaling
• Strong operational tooling
• Good latency and regional coverage

Here are the top options by cloud/platform:

1. Amazon Aurora (AWS)

Best overall for HA on AWS

• Compatible with MySQL and PostgreSQL
• Built for high availability with distributed storage and fast failover
• Supports Multi-AZ by design
• Can scale read traffic with replicas
• Strong performance and mature ecosystem

Pros

• Very strong uptime and recovery characteristics
• Good for mission-critical workloads
• Easy integration with AWS services

Cons

• More expensive than standard RDS in some cases
• Less portable than vanilla PostgreSQL/MySQL
• Some features differ from upstream engines

2. Google Cloud SQL / AlloyDB (GCP)

Best on GCP, especially for PostgreSQL

• Cloud SQL is the standard managed relational service for MySQL, PostgreSQL, and SQL Server
• AlloyDB is a high-performance PostgreSQL-compatible service with better scaling and HA than basic Cloud SQL

Pros

• Managed failover and backups
• Easy to use
• AlloyDB can outperform standard Postgres for demanding apps

Cons

• Cloud SQL is more basic than Aurora in some HA/scaling areas
• AlloyDB is PostgreSQL-focused, not general-purpose across engines

3. Azure SQL Database / Azure Database for PostgreSQL Flexible Server

Best on Azure

• Azure SQL Database is highly managed and HA-focused
• PostgreSQL Flexible Server supports zone-redundant HA
• Good integration with Azure platform tools

Pros

• Strong built-in HA
• Good managed operational experience
• Excellent if you’re already in Azure

Cons

• Tighter coupling to Azure-specific patterns
• PostgreSQL/MySQL options are not as feature-rich as some AWS/GCP counterparts in certain scenarios

4. Amazon RDS for PostgreSQL / MySQL

Best if you want standard engines with simpler managed HA

• Supports Multi-AZ deployments
• Easier than self-managing databases
• More portable than Aurora since it uses standard engine behavior

Pros

• Familiar PostgreSQL/MySQL
• Reliable HA options
• Simpler migration path from self-hosted databases

Cons

• Usually less performant/scalable than Aurora for demanding workloads
• Fewer advanced HA/storage features

5. CockroachDB Cloud

Best for globally distributed, strongly consistent SQL

• Not a traditional single-region relational DB service, but excellent for HA
• Multi-region by design
• Automatic replication and failover

Pros

• Very strong resilience
• Great for apps that need regional fault tolerance
• SQL interface with distributed architecture

Cons

• Different operational and performance tradeoffs from classic Postgres/MySQL
• Some SQL/features behave differently than traditional relational databases

6. YugabyteDB Managed

Best for distributed PostgreSQL-compatible workloads

• Designed for high availability and multi-region resilience
• PostgreSQL-compatible query layer

Pros

• Strong HA and fault tolerance
• Good for geo-distributed applications
• Cloud-managed offering available

Cons

• Not as simple as standard managed Postgres
• Compatibility and performance tradeoffs for some workloads

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Quick recommendations

Choose Aurora

if you’re on AWS and want the best balance of availability, performance, and managed operations.

Choose Cloud SQL / AlloyDB

if you’re on GCP and need a managed relational backend, especially for PostgreSQL.

Choose Azure SQL / PostgreSQL Flexible Server

if you’re on Azure and want strong managed HA with platform integration.

Choose CockroachDB Cloud or YugabyteDB

if your app needs multi-region active-active style resilience and can tolerate a distributed SQL model.

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What to look for in any HA relational DB service

When evaluating providers, check for:

• Multi-AZ or zone-redundant HA
• Automatic failover time
• Synchronous vs asynchronous replication
• Point-in-time recovery
• Backup retention
• Read replica support
• Maintenance window control
• Scaling limits
• Network isolation / private connectivity
• Encryption and IAM integration
• Connection pooling support

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Simple rule of thumb

• Standard web backend, single region: managed PostgreSQL/MySQL with Multi-AZ
• High traffic on AWS: Aurora
• Enterprise Azure app: Azure SQL or Azure PostgreSQL Flexible Server
• Need multi-region resilience: CockroachDB or YugabyteDB
• Need best Postgres performance on GCP: AlloyDB

If you want, I can also give you a ranked comparison table by cost, failover speed, scaling, and portability.