[jira] Updated: (AVRO-220) Dynamic schema resolution from writer to reader

["Scott Banachowski (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/AVRO-220?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Scott Banachowski updated AVRO-220:
-----------------------------------

    Status: Patch Available  (was: Open)

> Dynamic schema resolution from writer to reader
> -----------------------------------------------
>
>                 Key: AVRO-220
>                 URL: https://issues.apache.org/jira/browse/AVRO-220
>             Project: Avro
>          Issue Type: Improvement
>          Components: c++
>            Reporter: Scott Banachowski
>         Attachments: AVRO-220.patch
>
>
> This introduces a new feature for C++, which enables data written by one schema to be read by a different schema.
> Before parse time, it examines the two schemas, and assembles a list of instructions on how to parse the data, and how to translate the parsed data to the expected schema (or skip it, as appropriate).  The result of this operation is a sequence of instructions.
> At parse time, it basically follows the assembled list of instructions.  The instructions can be created once, and then reused to read multiple messages (analogous to assembling a program that does the parsing, and then executing this program when you want to read the data).
> A structure is used when assembling the instructions, that holds a mapping of memory locations for primitives (relative to the start of some structure).   This tells the parser where to copy the data to while parsing from the writer schema to a C++ structure. For non-primitive types, it stores the memory location for a functor that returns a destination address when invoked (for example, to add a map entry the functor will allocate the map entry, and return an address for the parser to write the actual data.  Then the instruction for the map data type is invoked to populate this address).  Setting up this offset structure is a chore, but using the code generator make it simple. The code generator correctly cranks out these structures for all the data types it generates. 

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