[jira] [Closed] (THRIFT-1477) Allow readFieldBegin() to pass back the field name instead of the field id

["James E. King III (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/THRIFT-1477?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

James E. King III closed THRIFT-1477.
-------------------------------------
    Resolution: Won't Do
      Assignee: James E. King III

Thrift matches fields by id, not by name.  This makes it possible to rename them over time without any binary incompatibility.  As such, any id to name mapping as [~bryanduxbury] suggests should be done externally.  Thanks.

> Allow readFieldBegin() to pass back the field name instead of the field id
> --------------------------------------------------------------------------
>
>                 Key: THRIFT-1477
>                 URL: https://issues.apache.org/jira/browse/THRIFT-1477
>             Project: Thrift
>          Issue Type: Improvement
>          Components: Java - Compiler
>            Reporter: Benjy Weinberger
>            Assignee: James E. King III
>            Priority: Minor
>
> [Apologies if this has been addressed in another issue. I couldn't find anything relevant on JIRA or the mailing list archives.]
> Background: I'm implementing a BSON protocol, in order to write Thrift messages to MongoDB (technically the protocol generates the object representation that the MongoDB driver expects, not a raw BSON string directly to the transport, but that's an unimportant detail here). 
> BSON, like JSON, naturally uses human-readable string field names. 
> When reading, the generated Thrift code (at least in Java) requires that readFieldBegin() pass back a TField with the id field set. It ignores the name field. Therefore the ids must appear in the stream. It's possible to contort these protocols to use ids instead of human-readable names (as TJSONProtocol does) but this isn't helpful in dealing with prior BSON or JSON data that we're trying to back-port into Thrift schemata.
> However, the generated read() method already knows how to map names to ids. So I propose allowing a TProtocol's readFieldBegin() method to pass back a TField with the name set and no id set (indicated, say, by id==-1), and let the read() method figure out the id to then switch on. 
> In some cases we could also allow the TField to omit the type information, which, again, is not naturally present in JSON. (BSON does embed type information, but its type system does not align fully with Thrift's, so it can't be used without further context). If the field is unknown, the only use for the type is for skipping the field value. But protocols like JSON and BSON can skip fields without this type information, since fields are delimited in the protocol in a type-independent way.
> Basically, what I propose is that readFieldBegin() be allowed to pass back just an id or just a name (and, for some protocols, no type information), since that is all read() needs in order to figure out how to read or skip the field. 
> I'm wondering what the Thrift elders think of this. Has it been discussed? Thanks!
> PS This does have the downside that if Thrift were to implement a pass-through feature for unrecognized fields (so that new messages read with old protocol versions will serialize back out with no loss) - we wouldn't be able to preserve fields for which we only had a name and no id. Or rather, we wouldn't be able to write them out to a protocol that requires ids, like the binary protocols. However this feature doesn't exist anyway, and I don't know if it's on the roadmap. 



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