[jira] [Commented] (AVRO-1347) Improve name and alias matching for named schemas

["Doug Cutting (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/AVRO-1347?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13699247#comment-13699247 ] 

Doug Cutting commented on AVRO-1347:
------------------------------------

This is a fundamental change to alias semantics.  If we implement it, we'll need to add it to the specification too.

I'm not (yet) convinced this change is required.  The writer's schema is currently used minimally--one can generally replace writer schemas with their Parsing Canonical Form and everything would work the same.  The reader's schema is used to interpret the data, containing aliases and other annotations that influence the representation of that data.  In the above case, is there a reason that "foo" cannot be added to the aliases of "bar"?  The reader's schema is assumed to be malleable and the writer's is not.

Looking at the patch (should we choose to implement this), one potential bug is that one cannot specify an alias that has no namespace.  Probably null should be interpreted as the containing namespace and the empty string as no namespace.

Perhaps the annotation should be moved to AVRO-1341, independent of the change in alias semantics?

Also, the patch needs unit tests.
                
> Improve name and alias matching for named schemas
> -------------------------------------------------
>
>                 Key: AVRO-1347
>                 URL: https://issues.apache.org/jira/browse/AVRO-1347
>             Project: Avro
>          Issue Type: Improvement
>          Components: java
>            Reporter: Vincenz Priesnitz
>         Attachments: AVRO-1347.patch, AVRO-1347.patch
>
>
> When reading an avro file with a named schema, the aliases of the writers schema are not taken into account; only the aliases of the readers are matched against the writers name. Even if the writers aliases match the readers name, the schemas will not be matched.
> For example, the following two enum schemas will not be matched, even though they share a common alias. 
> {code}
> {
> 	"type"  : "enum",
> 	"name"  : "foo",
> 	"alias" : "CommonAlias",
> 	"symbols" : ["LEFT", "RIGHT"]
> }
> {code}
> {code}
> {
> 	"type"  : "enum",
> 	"name"  : "bar",
> 	"alias" : "CommonAlias",
> 	"symbols" : ["LEFT", "RIGHT"]
> }
> {code}
> In most cases, the DatumReader resolves records of different names or namespaces by matching their fields. 
> Unfortunately, there are some cases, where this sort of matching is not happening, but just the names are compared:
> * Other named nodes, like enums, fixed or fieldschemas are not matched this way. 
> * A record inside a union is also only matched by the full name. 
> The latter one is especially tricky, since two recordschemas that match structurally but differ in name or space, are interexchangable until they are put into an union, at which point an exception is thrown.
> I propose that two named schemas are matched, when they share a common name or alias. 
> I implemented said changes and added a java annotation @AvroAlias(alias, space) that allows one to add an alias to a record, enum or field.

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