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https://issues.apache.org/jira/browse/AVRO-695?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12978470#action_12978470
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Doug Cutting commented on AVRO-695:
-----------------------------------
The current patch is a specification change, since it adds a new schema type,
"cycle". It is back-compatible, but not forward-compatible: implementations
that do not implement cycles would be unable to read data that contains cycle
schemas.
Instead, a schema for a cycle reference might be defined. For example, one
could define a org.apache.avro.cycles.CycleReference record containing a single
integer field. CycleReference would only be used in unions with types that it
may refer to. The DatumWriter would keep a IdentityHashMap<Object,Integer> of
records, maps and arrays written, adding an entry the first time an instance is
seen, and writing a CycleReference for subsequent occurrences in appropriate
unions. The DatumReader would then keep an array of all records, maps and
arrays that have been read and, when it reads a CycleReference in a union,
return a pointer to the indicated element of that array.
Adding cycles should not slow applications that do not require this feature.
They could be implemented in newly defined CycleDatumReader/Writer, or perhaps
GenericDatumReader and GenericDatumWriter could be modified to optionally
handle such cycles.
> 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: 1.4.1
> Reporter: Moustapha Cherri
> Fix For: 1.5.0
>
> Attachments: avro-1.4.1-cycle.patch.gz, avro-1.4.1-cycle.patch.gz
>
> 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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