[AVRO-3408] Schema evolution with logical types

• Type: Improvement
• Status: Open
• Priority: Major
• Resolution: Unresolved
• Affects Version/s: 1.11.0
• Fix Version/s: 1.12.0
• Component/s: java
• Labels: pull-request-available

Language

java

• Assignee: Ivan Zemlyanskiy
• Reporter: Ivan Zemlyanskiy
• Created: 18/Feb/22 21:35
• Updated: 16/Feb/23 08:18

Description

Hello!

First of all, thank you for this project. I love Avro encoding from both technology and code culture points of view. 

I know you recommend migrating schema by adding a new field and removing the old one in the future, but please-please-please consider my case as well. 

In my company, we have some DTOs, and it's about 200+ fields in total that we encode with Avro and send over the network. About a third of them have type `java.math.BigDecimal`. At some point, we discovered we send them with a schema like

{
  "name":"performancePrice",
  "type":{
    "type":"string",
    "java-class":"java.math.BigDecimal"
  }
}

That's a kind of disaster for us cos we have pretty much a high load with ~2 million RPS.
So we start to think about migrating to something lighter than strings (no blame for choosing it as a default, I know BigDecimal has a lot of pitfalls, and string is the easiest way for encoding/decoding).
It was fine to make a standard precision for all such fields, so we found `Conversions.DecimalConversion` and decided at the end of the day we were going to use this logical type with a recommended schema like

    @Override
    public Schema getRecommendedSchema() {
        Schema schema = Schema.create(Schema.Type.BYTES);
        LogicalTypes.Decimal decimalType =
                LogicalTypes.decimal(MathContext.DECIMAL32.getPrecision(), DecimalUtils.MONEY_ROUNDING_SCALE);
        decimalType.addToSchema(schema);
        return schema;
    }

(we use `org.apache.avro.reflect.ReflectData`)

It all looks good and promising, but the question is how to migrate to such schema?
As I said, we have a lot of such fields, and migrating all of them with duplication fields with future removal might be painful and would cost us a considerable overhead.

I made some tests and found out if two applications register the same `BigDecimalConversion` but for one application the `getRecommendedSchema()` is like the method above and for another application the `getRecommendedSchema()` is

    @Override
    public Schema getRecommendedSchema() {
        Schema schema = Schema.create(Schema.Type.STRING);
        schema.addProp(SpecificData.CLASS_PROP, BigDecimal.class.getName());
        return schema;
    }

so they can easily read each other messages using SERVER schema.

So, I made two applications and wired them up with `ProtocolRepository`, `ReflectResponder` and all that stuff, I found out it doesn't work. Because `org.apache.avro.io.ResolvingDecoder` totally ignores logical types for some reason.
So as a result, one application specifically told "I encode this field as a byte array which supposed to be a logical type 'decimal' with precision N", but another application just tries to convert those bytes to a string and make a BigDecimal based on the result string. As a result, we got

java.lang.NumberFormatException: Character ' is neither a decimal digit number, decimal point, nor "e" notation exponential mark.

In my humble opinion, `org.apache.avro.io.ResolvingDecoder` should respect logical types in SERVER (ACTUAL) schema and use a corresponding conversion instance for reading values. In my example, I'd say it might be

ResolvingDecoder#readString() -> read the actual logical type -> find BigDecimalConversion instance -> conversion.fromBytes(readValueWithActualSchema()) -> conversion.toCharSequence(readValueWithConversion)

I'd love to read your opinion on all of that.
Thank you in advance for your time, and sorry for the long issue description.

Issue Links

• links to: GitHub Pull Request #1584