An In-Depth Look at HDR, RSS, and SDS Licensing Modes

In the world of high-availability architectures, maintaining database secondary servers is essential for business continuity. However, the operational state of those secondaries, whether Warm, Hot, or Cold, carries significant financial implications. Understanding the “Passive vs. Active” threshold is key to balancing system resilience with budget compliance, as it directly impacts your licensing obligations and total cost of ownership.

What is HDR?

HDR is an Informix feature that continuously replicates logical log records from the primary server to a secondary server. This keeps the secondary in a "near-sync" state, ready to take over the workload should the primary server fail.

Warm Standby (HDR)

A Warm Standby is an instance that is powered on, has the Informix engine running, and is actively receiving and applying logs from the primary.

IBM Licensing Considerations:

• The Passive Advantage: If the secondary server is used strictly for High Availability (standing by to take over in case of a primary failure), it is considered Passive. Under many IBM Passport Advantage agreements, you do not need to pay for additional licenses for a passive standby, provided the primary is fully licensed.

• The Licensing Trigger: The moment you use the secondary for any actual workload, it becomes Active. In Informix, this includes:

• Read-Only Queries: If you enable UPDATABLE_SECONDARY or simply run reports/queries against the HDR secondary, it must be fully licensed.

• DML Operations: Any writes or deletes (if configured) trigger full licensing.

• Backup Exception: Performing a backup of the database from the standby server is generally permitted without triggering an "Active" status, allowing you to offload the backup overhead from the primary.

👉 Warm Standby = No cost if truly passive (idle); Full cost if used for queries or reporting.

Additionally, there is no fixed IBM licensing limit on the number of HDR secondary nodes from a licensing perspective. However, HDR topology is typically constrained to a 1:1 configuration (one primary to one HDR secondary) based on architectural design rather than licensing rules.

Cold Standby (HDR)

A Cold Standby is a secondary system where the software is installed, but the Informix engine is not running, or the server itself is powered off. It does not participate in continuous log replication.

IBM Licensing Considerations:

• Inactive State: No license is required while the engine is shut down.

• The "30-Day" Rule: IBM typically allows limited free use for disaster recovery testing (for regulatory purposes) for a defined, short duration.

• Activation: Once the engine is started and begins processing data or becomes the new primary, it must be fully licensed.

👉 Cold Standby = No cost while idle; Full cost when activated for production.

What About RSS (Remote Standalone Secondary)

RSS is designed for geographical distance and asynchronous replication. Unlike HDR, which is typically near synchronous one-to-one, you can have multiple RSS nodes globally.

IBM Licensing Considerations:

• Same Core Rule as HDR: If used purely as a disaster recovery target, RSS is typically covered under the primary’s license (subject to your specific licensing terms).

• The Scalability Factor: RSS is often used to scale read-only workloads across regions. This is where organizations frequently fall out of compliance. For example, if an RSS node in London is used for local reporting while the primary is in San Jose, that RSS node must be fully licensed.

• Delayed Apply: RSS supports delayed log application, which is useful for recovering from accidental data loss. However, the passive rule still applies - if no queries are run, it remains a backup node.

Note: There is no fixed IBM licensing limit on the number of RSS nodes. RSS supports a 1:N topology, but the number of deployable nodes is governed by Informix edition capabilities (WE, EE, AEE) and infrastructure constraints, not licensing count limits. In Workgroup Edition (WE), RSS is intended for basic HA and is practically limited to minimal configurations. Enterprise Edition (EE) and Advanced Enterprise Edition (AEE) are required for scalable RSS deployments (multiple nodes, read distribution, geo-scaling).

What about Licensing Metrics between pairs?

A common point of confusion is whether HDR or RSS secondary servers can be licensed differently from the primary.

Key Rule:
Once a secondary server is used for any workload (reporting, queries, applications), it becomes an active production node and must be fully licensed using the same metric as the primary (PVU, VPC, or Authorized User).

• Passive secondary (no activity) : No additional license required
• Active secondary (any workload) : Full licensing required (same metric as primary)

Not Permitted:
Mixing licensing metrics within the same Informix HA environment (e.g., VPC/PVU on primary and AUSI on RSS).

Example:
Primary licensed with 4 VPC.  Any active HDR/RSS must also be licensed using VPC (not a smaller AUSI subset, even for limited users).

Important:
Licensing applies to the entire deployment footprint, not per node or per use case. Attempting to license secondaries differently introduces compliance and audit risk.

The SDS Distinction (Shared Disk Secondary)

SDS servers rely on a shared storage layer rather than independent log shipping. Under IBM Informix licensing terms, it is critical to distinguish SDS from HDR and RSS.

• The Always-On Rule: Unlike HDR and RSS, SDS nodes are typically considered active resources because they share the primary’s disk and are often used for immediate workload distribution.

• Licensing Requirement: Unlike HDR, which allows for a passive "free" secondary, In almost all scenarios, SDS nodes must be fully licensed regardless of whether they are primarily used for failover. They are rarely eligible for the passive standby (zero cost) exception.

Summary

The financial efficiency of a high-availability strategy ultimately hinges on how usage is defined. An HDR or RSS server remains a cost-saving asset only as long as it functions strictly as an insurance policy.

There are no fixed licensing-based limits on the number of HDR or RSS nodes; however, topology and scale are governed by Informix edition capabilities. Additionally, once any secondary provides operational value, it must be licensed using the same metric as the primary. Partial or mixed metric licensing is not compliant.

The moment a secondary system provides any operational utility, whether for reporting, regional access, or data processing, it becomes an active production node requiring full licensing.

Anup Nair

Principal Technical Product Manager, IBM-Informix & IIUG Board of Directors

Disclaimer: The information provided in this article is current as of the date of publication and is intended for general informational purposes only. IBM reserves the right to modify, update, or withdraw product features, licensing terms, and policies at any time without notice. For the most accurate and up-to-date licensing guidance, always refer to official IBM documentation or consult IBM directly.