code-driven-cluster-migration

name	code-driven-cluster-migration
description	Step-by-step procedure for migrating a legacy Ember-based Matter server cluster in src/app/clusters/ to the code-driven pattern. Use this skill when converting an existing cluster. All rules from code-driven-cluster-development still apply; this skill adds the migration-specific workflow on top.

Code-Driven Cluster Migration

1. When to Invoke This Skill

Invoke this skill when you are converting an existing cluster under src/app/clusters/<folder>/ that does not yet contain a CodegenIntegration.cpp file (note: the folder naming convention is inconsistent — it may be <name>-server or simply <name>). If the cluster is already migrated, use code-driven-cluster-development instead.

All rules from code-driven-cluster-development apply to the converted cluster. This skill specifies the additional migration workflow: PR structure, backward compatibility, and integration wiring.

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2. The Three-PR Migration Workflow

Conversion must be split into at least three pull requests to ensure reviewability. Mixing these phases is a common cause for review rejection.

PR	Contents	Validation Method
1	File renames and compat-stub creation only.	git diff --color-moved
2	Code reordering / anonymous-namespace moves only.	git diff --color-moved=dimmed-zebra
3	Substantive conversion: new class, integration, tests, regen.	Functional review + ninja check

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3. Phase 0: Discovery & Strategy

3.1 Baseline & Map Blast Radius

• Inventory Attributes: Identify Mandatory (M), Optional (O), Fixed (F), and Nullable attributes from the cluster XML and generated Attributes.h.
• Map Consumers: Search examples/ for references to the legacy <name>-server.h or class <Name>Server. This determines your backward-compat strategy.

3.2 Backward-Compat Strategy

The standard approach preserves full include and API compatibility:

• CodegenIntegration.cpp: Keep any legacy public APIs alive here as thin forwarders to the new cluster class. This is the preferred location for all backward-compat shims.
• Legacy header (<name>-server.h): If it existed, update it to #include the new CodegenIntegration.h. This preserves include-path compatibility for application code without duplicating logic.
• Breaking changes: If direct Ember RAM writes (Attributes::X::Set) are used by apps and cannot be shimmed, you must update those apps in PR #3 and document the change in a cluster README.md.

3.3 Pick a Reference PR

Identify the "shape" of your cluster and study the corresponding reference PR's diff end-to-end.

Your cluster looks like...	Reference PR
Nullable measurement + min/max/tolerance	#71424 Relative Humidity
Mandatory list + delegate + commands	#43471 Actions
Multi-instance (per-endpoint state)	#43720 Closure Dimension
Singleton, node-scoped	#40422 Basic Information
Command-heavy with delegate	#42331 Chime
Writable scalar + features	#42968 Switch
Runtime-only (no Ember defaults)	#71552 Flow Measurement
Identify/timer-driven	#41232 Identify

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4. Implementation Phases

4.1 Phase 1: Renames (PR #1)

• Rename .cpp and .h to PascalCase (<Name>Cluster.cpp).
• Create a one-line compat stub at the old header path.
• Update BUILD.gn with renamed files. No logic changes.

4.2 Phase 2: Moves (PR #2)

• Move helpers to anonymous namespaces or group command handlers. No logic changes.

4.3 Phase 3: The Conversion (PR #3)

1. Strip Shell: Remove emberAf... calls and MatterPostAttributeChangeCallback.
2. Implement Class: Follow the patterns in code-driven-cluster-development.
3. Integration Layer: Create CodegenIntegration.cpp to bridge ZAP defaults to your new cluster.
4. ZAP/Build Updates:
   • Update src/app/common/templates/config-data.yaml (add to CodeDrivenClusters).
   • Update src/app/zap-templates/zcl/zcl.json (attributeAccessInterfaceAttributes).
5. Regeneration: Run scripts/tools/zap_regen_all.py and commit all generated files.

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5. Unit Testing with ClusterTester

The ClusterTester helper class (in src/app/server-cluster/testing/ClusterTester.h) is mandatory for testing code-driven clusters. It abstracts TLV handling, memory management for views, and fabric context.

5.1 Key Capabilities

• Automatic TLV Handling: Encodes C++ types for writes/commands and decodes for reads.
• Memory Safety: Maintains ownership of TLV data for views (like CharSpan or List) during the test scope.
• Type Safety: Uses generated types (e.g., Attributes::MyAttr::TypeInfo) to ensure spec alignment.

5.2 Core Patterns

• Attributes: Use tester.ReadAttribute(Id, outValue) and tester.WriteAttribute(Id, inValue).
• Commands: Use tester.Invoke(requestStruct). It returns an InvokeResult containing both status and response data.
• Side Effects:
  • Verify events via tester.GetNextGeneratedEvent().
  • Verify dirty markings (reporting) via tester.GetDirtyList().
• Fabric Scoping: Use tester.SetFabricIndex(idx) to simulate actions from specific fabrics.

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6. Ensuring Spec Compliance

Tests must not just "pass" — they must prove the cluster adheres to the Matter Specification and Test Plans.

6.1 Consult the Source of Truth

Use the matter-specification-access skill to obtain and read the latest specification and test plans.

• Validate Semantics: Ensure ReadAttribute and WriteAttribute handle constraints (nullable, min/max, fixed) exactly as defined in the spec.
• Mirror Test Plans: Use the .adoc test plans (e.g., src/cluster/<name>.adoc in chip-test-plans repo) as the blueprint for your unit tests. If a test plan requires a ConstraintError for a specific value, your unit test must assert exactly that.

6.2 Mandatory Test Coverage

• Attribute Constraints: Below min, at min, at max, above max.
• Nullable Handling: Null sentinel rejection (e.g., 0xFFFF for uint16) for nullable numeric types.
• Feature Gating: Verify that optional attributes/commands are only present when the corresponding FeatureMap bits are set (using Attributes() / AcceptedCommands()).

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7. Ember → Code-Driven Mapping Reference

Ember / Legacy Construct	Code-Driven Equivalent
emberAfReadAttribute	Member access + encoder.Encode
Attributes::X::Set	cluster->SetX(value) (Forwarded)
PreAttributeChangedCallback	Validation logic in WriteAttribute
PostAttributeChangeCallback	Automatic via SetAttributeValue
class FooServer : public AAI	class <Name>Cluster : public DefaultServerCluster

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8. Common Review Findings (Migration Specific)

1. Mixed PRs: Never combine renames with logic changes.
2. Missing Regen: Forgetting to commit files changed by zap_regen_all.py breaks the build for other apps.
3. Double Notification: Migrated clusters often have redundant NotifyAttributeChanged calls; use SetAttributeValue to avoid this.
4. Incorrect Fallbacks: When reading ZAP defaults in CodegenIntegration, ensure you handle read failures with safe, spec-compliant defaults.
5. Missing README: If you break backward compatibility, a README.md explaining the migration for app developers is mandatory.

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9. References

• Implementation Patterns: .agents/skills/code-driven-cluster-development/SKILL.md
• Migration Guide: docs/guides/migrating_ember_cluster_to_code_driven.md