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https://issues.apache.org/jira/browse/AVRO-695?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14051686#comment-14051686
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Doug Cutting commented on AVRO-695:
-----------------------------------
I imagine a schema like the following, to permit potentially circular linked
lists:
{code}
{"type":"record", "name":"List", "fields":[
{"name":"nodeName", "type":"string"},
{"name":"next",
"type": ["null", "List",
{"type":"record","name":"org.apache.avro.CircularRef","fields":[{"name":"id","type":"int"}]
}] }] }
{code}
The writer would add an entry to an IdentityHashMap<Object,Integer> for every
sub-record it writes. Whenever it encounters a previously-written record, it
writes a ref instead. Similarly, the reader would add each records it reads to
an array, and when a ref is read, return the corresponding element of the array.
Such functionality should entirely be contained in CircularData (subclass of
ReflectData) and CircularDatumReader and CircularDatumWriter, with no
alterations to existing classes (except perhaps to expose functionality to
subclasses if needed).
> Cycle Reference Support
> -----------------------
>
> Key: AVRO-695
> URL: https://issues.apache.org/jira/browse/AVRO-695
> Project: Avro
> Issue Type: New Feature
> Components: spec
> Affects Versions: 1.7.6
> Reporter: Moustapha Cherri
> Attachments: avro-1.4.1-cycle.patch.gz, avro-1.4.1-cycle.patch.gz,
> avro_circular_references.zip, avro_circular_refs_2014_06_14.zip,
> circular_refs_and_nonstring_map_keys_2014_06_25.zip
>
> Original Estimate: 672h
> Remaining Estimate: 672h
>
> This is a proposed implementation to add cycle reference support to Avro. It
> basically introduce a new type named Cycle. Cycles contains a string
> representing the path to the other reference.
> For example if we have an object of type Message that have a member named
> previous with type Message too. If we have have this hierarchy:
> message
> previous : message2
> message2
> previous : message2
> When serializing the cycle path for "message2.previous" will be "previous".
> The implementation depend on ANTLR to evaluate those cycle at read time to
> resolve them. I used ANTLR 3.2. This dependency is not mandated; I just used
> ANTLR to speed thing up. I kept in this implementation the generated code
> from ANTLR though this should not be the case as this should be generated
> during the build. I only updated the Java code.
> I did not make full unit testing but you can find "avrotest.Main" class that
> can be used a preliminary test.
> Please do not hesitate to contact me for further clarification if this seems
> interresting.
> Best regards,
> Moustapha Cherri
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