Is Go\'s `encoding/gob` Deterministic When Encoding Identical Objects?

The Deterministic Nature of Encoding/gob

Can we predict the consistency of the output produced by encoding/gob_encode() for two equal Go objects x and y, assuming no unconventional manipulations involving interfaces and maps?

Determinism in gob Encoding

Under typical conditions, yes, the encoding/gob package in Go exhibits deterministic behavior. When encoding two objects x and y of the same type and value, gob_encode() will consistently generate identical byte sequences. However, this consistency is not absolute.

Implications of Type Specifications

gob encoding uses a technique known as type specification to identify and describe the structure of the data being encoded. When encountering a previously unencountered data type, gob_encode() transmits type specifications along with the encoded data. However, subsequent encoding instances for the same data type omit the type specifications, instead referencing the previously transmitted specifications.

Consequences for Repeated Encoding

This type specification mechanism introduces a slight deviation from perfect determinism. The first encoding of a given data type will include type specifications, while subsequent encodings will not. As a result, the byte sequences generated for the first encoding and the following encodings may differ despite the underlying data being identical.

Exception: Maps and Non-Deterministic Order

It's important to note that deterministic encoding does not extend to situations involving maps. Maps in Go use a randomized iteration order, which means that the order of map keys will vary randomly. Consequently, gob_encode()'s output for maps may not be consistent, resulting in non-deterministic behavior.

Implications for Developer Confidence

In practical scenarios, developers can generally rely on the deterministic nature of encoding/gob for typical usage patterns. The presence of type specifications and the implications for map encoding need not be a significant concern unless explicit reproducibility is crucial. However, it's crucial to be aware of potential variations in the encoding/gob output across different Go versions for backward compatibility reasons.

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