make unions a named type
------------------------
Key: AVRO-248
URL: https://issues.apache.org/jira/browse/AVRO-248
Project: Avro
Issue Type: New Feature
Components: spec
Reporter: Doug Cutting
Assignee: Doug Cutting
Fix For: 1.3.0
Unions are currently anonymous. However it might be convenient if they were
named. In particular:
- when code is generated for a union, a class could be generated that includes
an enum indicating which branch of the union is taken, e.g., a union of string
and int named Foo might cause a Java class like {code}
public class Foo {
public static enum Type {STRING, INT};
private Type type;
private Object datum;
public Type getType();
public String getString() { if (type==STRING) return (String)datum; else
throw ... }
public void setString(String s) { type = STRING; datum = s; }
....
}
{code} Then Java applications can easily use a switch statement to process
union values rather than using instanceof.
- when using reflection, an abstract class with a set of concrete
implementations can be represented as a union (AVRO-241). However, if one
wishes to create an array one must know the name of the base class, which is
not represented in the Avro schema. One approach would be to add an annotation
to the reflected array schema (AVRO-242) noting the base class. But if the
union itself were named, that could name the base class. This would also make
reflected protocol interfaces more consise, since the base class name could be
used in parameters return types and fields.
- Generalizing the above: Avro lacks class inheritance, unions are a way to
model inheritance, and this model is more useful if the union is named.
This would be an incompatible change to schemas. If we go this way, we should
probably rename 1.3 to 2.0. Note that AVRO-160 proposes an incompatible change
to data file formats, which may also force a major release.
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