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CloudStack currently generates many API error messages through ad-hoc string construction, leading to inconsistencies, limited customization, and no clear support for localization.
This feature introduces a structured, key-based error messaging framework where the management server emits a stable error message key along with contextual metadata. Clients and operators can use these to present consistent, customizable, and localized error messages.

Goals

• Provide consistent and stable error messages using predefined keys.
• Allow operators to customize error message templates without rebuilding CloudStack.
• Enable client-side localization using error message keys and metadata.
• Support role-aware error messages, allowing more detailed messages for admin users.
• Maintain backward compatibility with existing API clients.

Non-Goals

• Converting all existing CloudStack errors in a single release.
• Introducing a full server-side localization framework for all languages.
• Changing existing API error codes or response structures beyond adding new fields.

High-Level Design

Error Message Keys and Metadata

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Instead of hard-coded strings, exceptions are raised with:

• an error message key
• optional metadata describing the error context

API Response Changes

API error responses include the following additional fields:

• errortextkey – the stable error message key
• errormetadata – a map of contextual values

The existing errortext field is preserved for backward compatibility.

Example:

{
	"deployvirtualmachineresponse": {
		"errorcode": 431,
		"cserrorcode": 4350,
		"errortext": "Unable to deploy Instance as the given service offering 'test' (ID: 13, UUID: ...) is inactive. Specify an active service offering.",
		"errortextkey": "vm.deploy.serviceoffering.inactive",
		"errormetadata": {
			"serviceOffering": "'test' (ID: 13, UUID: ...)"
		}
	}
}

Error Message Templates

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Admin-specific variants are supported by adding a .admin suffix to the key.

Example:

{
	"vm.deploy.serviceoffering.inactive":
	"Unable to deploy Instance as the given service offering {{serviceOffering}} is inactive. Specify an active service offering.",
	"vm.deploy.serviceoffering.inactive.admin":
	"Unable to deploy Instance as service offering {{serviceOffering}} is inactive. Please activate it or choose a different offering."
}

ErrorMessageResolver

A new component, ErrorMessageResolver, is responsible for resolving error message keys into user-facing messages.

Responsibilities:

• Load and cache error message templates from error-messages.json.
• Automatically reload templates when the file is modified.
• Select admin-specific templates when the caller is a root admin.
• Substitute placeholders using values from error metadata.
• Convert domain objects in metadata into user-friendly string representations.

Role-Aware Error Messages

The caller’s role is determined using CallContext.
If the caller is a root admin and an admin-specific template exists, it is used; otherwise, the default template is applied.

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The initial implementation applies this framework to selected validation paths in the VM deployment flow.
Additional flows and modules can be incrementally migrated in future releases.

Backward Compatibility

• Existing clients can continue using errortext as before.
• New clients may prefer errortextkey and errormetadata for localization and structured handling.
• No existing API fields are removed or modified.

Packaging and Operations

• error-messages.json is installed as a configuration file and not overwritten during upgrades.
• Operators may customize messages locally.
• In multi-management-server setups, operators must ensure the file is kept in sync across all nodes.

Testing Strategy

Unit tests for:

• template resolution and placeholder substitution
• admin vs non-admin template selection
• metadata object string conversion

Integration tests validating API responses include error message keys and metadata.

Future Enhancements

• Gradual migration of additional API flows to the new framework.
• Documentation for client-side localization using error message keys.
• Optional tooling for validating or reloading error message templates at runtime.