leo-vital opened a new issue, #252:
URL: https://github.com/apache/avro-rs/issues/252
I have a struct like this:
```rust
#[derive(AvroSchema)]
struct Something {
f: String
}
```
Recently I have had a problem with the schema evolution using the `BACKWARD`
compatibility configuration in the Schema Registry. I first create an initial
schema version just fine, but then when I add a field whose type is
`Option<String>` to the struct above and try to encode it using
schema-registry-converter's `encode_struct` function, I see this error in the
Schema Registry:
```
io.confluent.kafka.schemaregistry.exceptions.IncompatibleSchemaException:
[{errorType:'READER_FIELD_MISSING_DEFAULT_VALUE', description:'The field
'field_name' at path '/fields/6' in the new schema has no default value and is
missing in the old schema', additionalInfo:'field_name'},...
```
Looking at it deeper, I noticed that the main problem seems to be the fact
that the canonical form does not include a default value. Even when I mark the
field with `#[avro(default="null")]` the same behavior can be observed. Even
though I can see that `T::get_schema()` returns a schema in which the field
contains a default value, when I get the schema's canonical form there is no
default value.
My understanding is that this is by design due to the STRIP rule
[here](https://github.com/apache/avro-rs/blob/d33d60014563cbe8d7b9050b9362e8fe89e1259a/avro/src/schema.rs#L2353).
But if that is the case, then how exactly can I get a retro-compatible schema
when adding an optional field? Is that simply not supported and I need to
manually create a schema version in the Schema Registry?
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