RDS Custom : The Concept, Benefits, Limitations , Use Cases

 

Amazon RDS Custom is a managed database service by AWS that provides greater control over the database and operating system than standard Amazon RDS. It is designed for applications that require custom configurations or access to the underlying OS and DB environment, which isn't possible with traditional RDS.

 Concept

Amazon RDS Custom is essentially a hybrid between Amazon RDS and a self-managed database on EC2:

• twtech gets Amazon RDS-like automation (backups, patching, monitoring).
• But also twtech has root OS and full DB instance access to customize settings or install additional software.

RDS Custom currently supports:

• RDS Custom for Oracle
• RDS Custom for SQL Server

It allows:

• Custom OS-level configurations
• Installation of additional software
• Access to database log and trace files
• Integration with legacy or third-party applications

Benefits

1. Root OS Access: SSH into the host and customize OS or database settings.
2. Customizations: Modify database parameters, install agents, use custom scripts, or patch manually.
3. Managed Automation: Get automated backups, monitoring, snapshots, and recovery.
4. Lift-and-Shift Compatibility: Ideal for migrating legacy databases with custom configurations.
5. License Flexibility: Bring-Your-Own-License (BYOL) support.

 Limitations

1. Fewer Engine Options: Only supports Oracle and SQL Server (as of now, May.28-2025).
2. Not Fully Managed: Since twtech has OS-level access, it is responsible for security, patching, and compliance for customizations.
3. Complexity: Adds operational overhead compared to standard RDS.
4. Limited High Availability: No Multi-AZ deployments (currently). twtech must manage HA and failover manually.
5. Cost: Higher than standard RDS because it uses dedicated EC2 instances (bare metal or Nitro-based).
6. Break Glass Responsibility: If twtech changes something that breaks automation, AWS will shift support responsibility back to twtech.

 Use Cases

Use Case	Why RDS Custom is Suitable
Legacy Database Migration	Migrate apps that require specific DB versions, patches, or configurations.
Custom Monitoring/Backup Tools	Install agents or tools not supported in standard RDS.
Third-Party Integrations	Required OS-level software or custom drivers.
Regulatory Compliance	Need access to trace files, audit logs, or custom hardening.
Custom Patch Management	twtech needs control over when and how patches are applied.
Oracle or SQL Server Advanced Features	Use features not supported in standard RDS editions.

 

twtech insights: When NOT to Use RDS Custom

• twtech doesn’t need OS-level access.
• twtech prefers fully managed, hands-off database solutions.
• twtech needs automated HA and failover out of the box.
• twtech application supports Aurora or open-source DBs ( then twtech would use Aurora/RDS instead).

 Summary

Feature	RDS	RDS Custom	EC2 Self-Managed
Fully Managed	✅	⚠️ Partial	❌
OS Access	❌	✅	✅
Custom Configs	❌	✅	✅
Automation	✅	✅ (with caveats)	❌
Responsibility	Low	Medium-High	High

RDS vs. RDS Custom

RDS: The entire database & the OS is managed by AWS.

RDS Custom: with full admin access, user can manage the OS & the database.