Roger created AVRO-2647:
---------------------------
Summary: specification does not fully specify semantics for unions
in default types
Key: AVRO-2647
URL: https://issues.apache.org/jira/browse/AVRO-2647
Project: Apache Avro
Issue Type: Bug
Components: doc
Reporter: Roger
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](https://github.com/actgardner/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.
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