Model Registry Core Entities and Immutability Design

Core Concept
The registry has three entities: Model (logical grouping like "fraud-detection"), Version (immutable artifact from one training run), and Stage (pointer like dev/staging/prod). Versions never change; stages move between versions.
VERSION IDENTITY
Each version gets a content hash computed over the model binary. Same weights = same hash. Verify a loaded model by recomputing the hash—no ambiguity about "which v1.2.3 is this?"
VERSION METADATA
Each version stores: artifact location, model signature (input/output schema), training metadata (data snapshot, hyperparameters, git commit), evaluation metrics (accuracy, latency), and governance data (approver, timestamp, risk tier).
💡 Insight: The model signature defines the contract: which features, what types, what preprocessing. Mismatched signatures cause silent accuracy degradation.
WHY IMMUTABILITY
Once registered, a version cannot be modified—only deprecated. This prevents "worked yesterday" bugs where someone tweaks production models. Need changes? Create a new version. Rollback becomes trivial and audit straightforward.
STAGES AS POINTERS
Stages (dev, staging, prod) are mutable pointers to immutable versions. Promoting means moving the prod pointer from v1.23 to v1.24. Versions do not change. This enables atomic promotion and instant rollback.
⚠️ Trade-off: Immutability increases storage. Typical retention: keep last 10 prod versions, archive older, retain metadata indefinitely for compliance.

💡 Key Takeaways

✓Three core entities: Model (logical group), Version (immutable artifact), Stage (mutable pointer)

✓Version identity via content hash—same weights = same hash, guarantees what you registered is what loads

✓Model signature defines the contract: input features, types, preprocessing version

✓Immutability prevents worked-yesterday bugs; changes require new versions

✓Stages as pointers enable atomic promotion and instant rollback

📌 Interview Tips

1Explain that versions are immutable but stages are mutable pointers that move between versions

2Mention model signature as critical for preventing silent accuracy degradation

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