[proposal] Introduce custom logicalType

[ievgenii-shepeliuk]

Hello

Since Avro doesn't cover all the cases of type mapping - Chr.Avro already uses some conventions to map from Avro schema to C# classes, like for dates, durations etc.

This works fine in case when C# classes either manually created or schema is generated from C#.

But for other cases, e.g. when schema is provided by external entities (when working in polyglot ecosystem with C# / Java / Python) or in case when schema-first approach is chosen, i.e. when C# class is generated from schema during build - existing mapping approach can not be used.

So my proposal is to introduce some custom logical types - eg chr.avro.datetime, chr.avro.duration, chr.avro.ipaddress etc.
Those custom types then can be used to extend Chr.Avro code generator to create a proper corresponding C# type when generating C# code from Avro schema. Also, such logical types can be used by other languages as a organization-wide type hints for code generation.

As far as I understand, custom logical type are not contradicting Avro specification or ideology.

WDYT ?

[dstelljes]

Based on your examples, it seems that you’re primarily interested in cases where Chr.Avro prefers to use strings (as for ISO 8601 datetimes/durations). Is that accurate?

An alternative could be to use a custom property like the Java implementation:

{
  "type": "string",
  "avro.dotnet.type": "System.DateTime"
}

I think there are some advantages to that approach, one being that it’s unlikely to conflict with the spec later. For example, if the spec ever introduced a duration-iso8601 logical type (we can dream), we would avoid having two logical type options, possibly with their own subtle differences.

[ievgenii-shepeliuk]

1. For now, I've mentioned only Avro's string , but it also can be applied to other types: bytes, int etc.
2. The idea of using "avro.dotnet.type" looking great !

[ievgenii-shepeliuk]

@dstelljes are you interested in PR that would introduce this functionality ? I can start with a simple PR that introduces just one of hints.
I think it's relatively simple and only need a change in https://github.com/ch-robinson/dotnet-avro/blob/main/src/Chr.Avro.Codegen/CodeGenerator.cs

[dstelljes]

Absolutely; we’ll probably also want some changes to the serdes in 8.x, but code generation would be a great place to start.