[AVRO-2647] specification does not fully specify semantics for unions in default types

• Type: Bug
• Status: Open
• Priority: Minor
• Resolution: Unresolved
• Component/s: doc
• Assignee: Unassigned
• Reporter: Roger
• Created: 06/Dec/19 14:38
• Updated: 27/Apr/20 09:00

Description

Currently the specification does not make the semantics clear for union types within complex types clear. In particular, the spec talks about union fields, but leaves the semantics for unions in other contexts unspecified.

Here's an example which is undefined according to the current specification:

{
    "type": "record",
    "name": "R",
    "fields": [
        {
            "name": "F",
            "type": {
                "type": "array",
                "items": [
                    {
                        "type": "enum",
                        "name": "E1",
                        "symbols": ["A", "B"]
                    },
                    {
                        "type": "enum",
                        "name": "E2",
                        "symbols": ["B", "A", "C"]
                    }
                ]
            },
            "default": ["A", "B", "C"]
        }
    ]
}

By experiment, most implementations seem to have chosen the semantics that are documented in this PR.

In Java, the schema above is parsed without error, but when attempting to use the default value, it fails with a NullPointerException (trying to find the symbol C in E1). (Thanks for Ryan Skraba for this).

In gogen-avro it generates invalid code because it's assuming E1 but generating the symbol for "C" anyway.

FWIW at some point in the future, I believe that it would be nice to align the default value specification with the JSON encoding for Avro so there aren't two subtly different JSON encodings of an Avro value.

Issue Links

• links to: GitHub Pull Request #738