Case 81: Serialization tag ID reassigned¶

Field	Value
Verdict	🔴 BREAKING
Category	Breaking
Platforms	Linux, macOS
Flags	ABI break
Detected `ChangeKind`s	`serialization_tag_changed`
Source files	browse on GitHub

Category: Payload ABI | Verdict: BREAKING

What breaks¶

DAAL's classic API persists models via daal::services::SerializationIface, which assigns each serializable class a uint64 tag ID. Files written by v1 embed these IDs; on load, a registry maps tag_id → factory.

If a maintainer reassigns an ID — e.g. swaps knn_model_tag and linear_regression_tag because they were "out of alphabetical order" — every existing saved model becomes silently corrupt:

A .dat file written by v1 starts with bytes encoding knn_model_tag = 0x1002.
Loaded against v2, the registry looks up 0x1002 and gets back the linear_regression factory.
Deserialization proceeds without any error (the bytes parse as a different but structurally similar class) and returns the wrong type.

There is no symbol change, no layout change, no link error, no load error. Every conventional ABI checker reports COMPATIBLE.

Why this is its own ChangeKind¶

This is a payload-level invariant — the binary contract between two different runs of the program separated by serialization. ChangeKinds today track types, symbols, and layouts; tag-ID reassignment leaves all three intact. A new SERIALIZATION_TAG_CHANGED ChangeKind gives this break a name and lets policy files require explicit acknowledgement of such changes.

How abicheck detects it¶

The new detector inspects exported variables and constants whose name matches a serialization-tag convention (configurable; defaults include *_serialization_tag, *_tag, kSerializationTag, SERIALIZATION_TAG, DAAL's *SerializationTag pattern). If two such constants in the same snapshot swap values between versions, a SERIALIZATION_TAG_CHANGED finding is emitted for each.

Code diff¶

// v1.h
constexpr std::uint64_t knn_model_tag         = 0x1002;
constexpr std::uint64_t linear_regression_tag = 0x1003;

// v2.h — values swapped
constexpr std::uint64_t knn_model_tag         = 0x1003;
constexpr std::uint64_t linear_regression_tag = 0x1002;

Real-world reference¶

daal::services::SerializationIface::getSerializationTag() returns an int that uniquely identifies the class for persistence. The implementation is sprinkled across cpp/daal/include/algorithms/*/_model.h files using DAAL_SERIALIZATION_TAG macros. The tag IDs are part of the persisted file format — changing them is on par with changing a wire protocol field.